含泪刷128道面试题,50万字2022最新Android11位大厂面试专题

Posted 初一十五啊

tags:

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前言

面试专题前面的百度篇,腾讯篇,阿里篇,京东篇,bilibili篇,网易篇,字节篇,小红书,小米九大板块已经更新完了,还剩下2个专题~持续更新中。

1.12W字;2022最新Android11位大厂面试专题(一)百度篇

2.12W字;2022最新Android11位大厂面试专题(二)阿里篇

3.12W字;2022最新Android11位大厂面试专题(三)腾讯篇

4.面霸养成记;50万字Android面试文档(四五)字节,京东篇

5.面霸养成记;50万字Android面试文档(六七)网易,Bilibili篇

6.面霸养成记;50万字Android面试文档(八九)小米,小红书

一共50W字的文档,面试专题12W字只是一小部分,字数限制,分几篇更。

关注公众号:初一十五a 提前解锁 《整套50W字Android体系PDF》,让学习更贴近未来实战。

总共囊括

1.腾讯Android开发笔记(33W字)

2.2022最新Android十一位大厂面试专题(12W字)

3.音视频经典面试题(6W字)

共十一模块,今天来更新第十专题携程篇

九丶携程

1.Glide讲一讲

①基本使用流程

Glide最基本的使用流程就是下面这行代码,其它所有扩展的额外功能都是以其建造者链式调用的基础上增加的。

GlideApp.with(context).load(url).into(iv);

其中的GlideApp是注解处理器自动生成的,要使用GlideApp,必须先配置应用的AppGlideModule模块,里面可以为空配置,也可以根据实际情况添加指定配置。

@GlideModule
public class MyAppGlideModule extends AppGlideModule 

    @Override
    public void applyOptions(Context context, GlideBuilder builder) 
        // 实际使用中根据情况可以添加如下配置
        <!--builder.setDefaultRequestOptions(new RequestOptions().format(DecodeFormat.PREFER_RGB_565));-->
        <!--int memoryCacheSizeBytes = 1024 * 1024 * 20;-->
        <!--builder.setMemoryCache(new LruResourceCache(memoryCacheSizeBytes));-->
        <!--int bitmapPoolSizeBytes = 1024 * 1024 * 30;-->
        <!--builder.setBitmapPool(new LruBitmapPool(bitmapPoolSizeBytes));-->
        <!--int diskCacheSizeBytes = 1024 * 1024 * 100;-->
        <!--builder.setDiskCache(new InternalCacheDiskCacheFactory(context, diskCacheSizeBytes));-->
    


接下来,本文将针对Glide的最新源码版本V4.8.0对Glide加载网络图片的流程进行详细地分析与讲解,力争做到让读者朋友们知其然也知其所以然。

②GlideApp.with(context)源码详解

首先,用这份Glide框架图让我们对Glide的总体框架有一个初步的了解。

从GlideApp.with这行代码开始,内部主线执行流程如下。

GlideApp#with

return (GlideRequests) Glide.with(context);

Glide#with

return getRetriever(context).get(context);

return Glide.get(context).getRequestManagerRetriever();

// 外部使用了双重检锁的同步方式确保同一时刻只执一次Glide的初始化
checkAndInitializeGlide(context);

initializeGlide(context);

// 最终执行到Glide的另一个重载方法
initializeGlide(context, new GlideBuilder());

@SuppressWarnings("deprecation")
  private static void initializeGlide(@NonNull Context   context, @NonNull GlideBuilder builder) 
    Context applicationContext =     context.getApplicationContext();
    // 1、获取前面应用中带注解的GlideModule
    GeneratedAppGlideModule annotationGeneratedModule =     getAnnotationGeneratedGlideModules();
    // 2、如果GlideModule为空或者可配置manifest里面的标志为true,则获取manifest里面
    // 配置的GlideModule模块(manifestModules)。
    List<com.bumptech.glide.module.GlideModule>     manifestModules = Collections.emptyList();
    if (annotationGeneratedModule == null ||     annotationGeneratedModule.isManifestParsingEnabled(    )) 
      manifestModules = new   ManifestParser(applicationContext).parse();
    

    ...

    RequestManagerRetriever.RequestManagerFactory     factory =
        annotationGeneratedModule != null
            ? annotationGeneratedModule.getRequestManag    erFactory() : null;
    builder.setRequestManagerFactory(factory);
    for (com.bumptech.glide.module.GlideModule module :     manifestModules) 
      module.applyOptions(applicationContext, builder);
    
    if (annotationGeneratedModule != null) 
      annotationGeneratedModule.applyOptions(applicatio  nContext, builder);
    
    // 3、初始化各种配置信息
    Glide glide = builder.build(applicationContext);
    // 4、把manifestModules以及annotationGeneratedModule里面的配置信息放到builder
    // 里面(applyOptions)替换glide默认组件(registerComponents)
    for (com.bumptech.glide.module.GlideModule module :     manifestModules) 
      module.registerComponents(applicationContext,   glide, glide.registry);
    
    if (annotationGeneratedModule != null) 
      annotationGeneratedModule.registerComponents(appl  icationContext, glide, glide.registry);
    
    applicationContext.registerComponentCallbacks(glide    );
    Glide.glide = glide;

GlideBuilder#build

@NonNull
  Glide build(@NonNull Context context) 
    // 创建请求图片线程池sourceExecutor
    if (sourceExecutor == null) 
      sourceExecutor =   GlideExecutor.newSourceExecutor();
    

    // 创建硬盘缓存线程池diskCacheExecutor
    if (diskCacheExecutor == null) 
      diskCacheExecutor =   GlideExecutor.newDiskCacheExecutor();
    

    // 创建动画线程池animationExecutor
    if (animationExecutor == null) 
      animationExecutor =   GlideExecutor.newAnimationExecutor();
    

    if (memorySizeCalculator == null) 
      memorySizeCalculator = new   MemorySizeCalculator.Builder(context).build();
    

    if (connectivityMonitorFactory == null) 
      connectivityMonitorFactory = new   DefaultConnectivityMonitorFactory();
    

    if (bitmapPool == null) 
      // 依据设备的屏幕密度和尺寸设置各种pool的size
      int size =   memorySizeCalculator.getBitmapPoolSize();
      if (size > 0) 
        // 创建图片线程池LruBitmapPool,缓存所有被释放的bitmap
        // 缓存策略在API大于19时,为SizeConfigStrategy,小于为AttributeStrategy。
        // 其中SizeConfigStrategy是以bitmap的size和config为key,value为bitmap的HashMap
        bitmapPool = new LruBitmapPool(size);
       else 
        bitmapPool = new BitmapPoolAdapter();
      
    

    // 创建对象数组缓存池LruArrayPool,默认4M
    if (arrayPool == null) 
      arrayPool = new   LruArrayPool(memorySizeCalculator.getArrayPoolSiz  eInBytes());
    

    // 创建LruResourceCache,内存缓存
    if (memoryCache == null) 
      memoryCache = new   LruResourceCache(memorySizeCalculator.getMemoryCa  cheSize());
    

    if (diskCacheFactory == null) 
      diskCacheFactory = new   InternalCacheDiskCacheFactory(context);
    

    // 创建任务和资源管理引擎(线程池,内存缓存和硬盘缓存对象)
    if (engine == null) 
      engine =
          new Engine(
              memoryCache,
              diskCacheFactory,
              diskCacheExecutor,
              sourceExecutor,
              GlideExecutor.newUnlimitedSourceExecutor(  ),
              GlideExecutor.newAnimationExecutor(),
              isActiveResourceRetentionAllowed);
    
    
    RequestManagerRetriever requestManagerRetriever =
    new RequestManagerRetriever(requestManagerFactory);

    return new Glide(
        context,
        engine,
        memoryCache,
        bitmapPool,
        arrayPool,
        requestManagerRetriever,
        connectivityMonitorFactory,
        logLevel,
        defaultRequestOptions.lock(),
        defaultTransitionOptions);


Glide#Glide构造方法

Glide(...) 
    ...
    // 注册管理任务执行对象的类(Registry)
    // Registry是一个工厂,而其中所有注册的对象都是一个工厂员工,当任务分发时,
    // 根据当前任务的性质,分发给相应员工进行处理
    registry = new Registry();
    
    ...
    
    // 这里大概有60余次的append或register员工组件(解析器、编解码器、工厂类、转码类等等组件)
    registry
    .append(ByteBuffer.class, new ByteBufferEncoder())
    .append(InputStream.class, new StreamEncoder(arrayPool))
    
    // 根据给定子类产出对应类型的target(BitmapImageViewTarget / DrawableImageViewTarget)
    ImageViewTargetFactory imageViewTargetFactory = new ImageViewTargetFactory();
    
    glideContext =
        new GlideContext(
            context,
            arrayPool,
            registry,
            imageViewTargetFactory,
            defaultRequestOptions,
            defaultTransitionOptions,
            engine,
            logLevel);

RequestManagerRetriever#get

@NonNull
public RequestManager get(@NonNull Context context) 
  if (context == null) 
    throw new IllegalArgumentException("You cannot start a load on a null Context");
   else if (Util.isOnMainThread() && !(context instanceof Application)) 
    // 如果当前线程是主线程且context不是Application走相应的get重载方法
    if (context instanceof FragmentActivity) 
      return get((FragmentActivity) context);
     else if (context instanceof Activity) 
      return get((Activity) context);
     else if (context instanceof ContextWrapper) 
      return get(((ContextWrapper) context).getBaseContext());
    
  

  // 否则直接将请求与ApplicationLifecycle关联
  return getApplicationManager(context);

这里总结一下,对于当前传入的context是application或当前线程是子线程时,请求的生命周期和ApplicationLifecycle关联,否则,context是FragmentActivity或Fragment时,在当前组件添加一个SupportFragment(SupportRequestManagerFragment),context是Activity时,在当前组件添加一个Fragment(RequestManagerFragment)。

GlideApp#with小结

  1. 初始化各式各样的配置信息(包括缓存,请求线程池,大小,图片格式等等)以及glide对象。

  2. 将glide请求和application/SupportFragment/Fragment的生命周期绑定在一块。

with方法的执行流程

③load(url)源码详解

GlideRequest(RequestManager)#load

return (GlideRequest<Drawable>) super.load(string);

return asDrawable().load(string);

// 1、asDrawable部分
return (GlideRequest<Drawable>) super.asDrawable();

return as(Drawable.class);

// 最终返回了一个GlideRequest(RequestManager的子类)
return new GlideRequest<>(glide, this, resourceClass, context);

// 2、load部分
return (GlideRequest<TranscodeType>) super.load(string);

return loadGeneric(string);

@NonNull
private RequestBuilder<TranscodeType> loadGeneric(@Nullable Object model) 
    // model则为设置的url
    this.model = model;
    // 记录url已设置
    isModelSet = true;
    return this;

可以看到,load这部分的源码很简单,就是给GlideRequest(RequestManager)设置了要请求的mode(url),并记录了url已设置的状态。

load方法的执行流程

④into(iv)源码详解

真正复杂的地方要开始了。

RequestBuilder.into

 @NonNull
public ViewTarget<ImageView, TranscodeType>   into(@NonNull ImageView view) 
  Util.assertMainThread();
  Preconditions.checkNotNull(view);

  RequestOptions requestOptions =     this.requestOptions;
  if (!requestOptions.isTransformationSet()
      && requestOptions.isTransformationAllowed()
      && view.getScaleType() != null) 
    // Clone in this method so that if we use this   RequestBuilder to load into a View and then
    // into a different target, we don't retain the   transformation applied based on the previous
    // View's scale type.
    switch (view.getScaleType()) 
      // 这个RequestOptions里保存了要设置的scaleType,Glide自身封装了CenterCrop、CenterInside、
      // FitCenter、CenterInside四种规格。
      case CENTER_CROP:
        requestOptions =   requestOptions.clone().optionalCenterCrop();
        break;
      case CENTER_INSIDE:
        requestOptions =   requestOptions.clone().optionalCenterInside()  ;
        break;
      case FIT_CENTER:
      case FIT_START:
      case FIT_END:
        requestOptions =   requestOptions.clone().optionalFitCenter();
        break;
      case FIT_XY:
        requestOptions =   requestOptions.clone().optionalCenterInside()  ;
        break;
      case CENTER:
      case MATRIX:
      default:
        // Do nothing.
    
  

  // 注意,这个transcodeClass是指的drawable或bitmap
  return into(
      glideContext.buildImageViewTarget(view,     transcodeClass),
      /*targetListener=*/ null,
      requestOptions);

GlideContext#buildImageViewTarget

return imageViewTargetFactory.buildTarget(imageView, transcodeClass);

ImageViewTargetFactory#buildTarget

@NonNull
@SuppressWarnings("unchecked")
public <Z> ViewTarget<ImageView, Z>   buildTarget(@NonNull ImageView view,
    @NonNull Class<Z> clazz) 
  // 返回展示Bimtap/Drawable资源的目标对象
  if (Bitmap.class.equals(clazz)) 
    return (ViewTarget<ImageView, Z>) new   BitmapImageViewTarget(view);
   else if (Drawable.class.isAssignableFrom(clazz))     
    return (ViewTarget<ImageView, Z>) new   DrawableImageViewTarget(view);
   else 
    throw new IllegalArgumentException(
        "Unhandled class: " + clazz + ", try   .as*(Class).transcode(ResourceTranscoder)");
  

可以看到,Glide内部只维护了两种target,一种是BitmapImageViewTarget,另一种则是DrawableImageViewTarget,接下来继续深入。

RequestBuilder#into

private <Y extends Target<TranscodeType>> Y into(
      @NonNull Y target,
      @Nullable RequestListener<TranscodeType>   targetListener,
      @NonNull RequestOptions options) 
    Util.assertMainThread();
    Preconditions.checkNotNull(target);
    if (!isModelSet) 
      throw new IllegalArgumentException("You must call   #load() before calling #into()");
    

    options = options.autoClone();
    // 分析1.建立请求
    Request request = buildRequest(target,     targetListener, options);

    Request previous = target.getRequest();
    if (request.isEquivalentTo(previous)
        && !isSkipMemoryCacheWithCompletePreviousReques    t(options, previous)) 
      request.recycle();
      // If the request is completed, beginning again   will ensure the result is re-delivered,
      // triggering RequestListeners and Targets. If   the request is failed, beginning again will
      // restart the request, giving it another chance   to complete. If the request is already
      // running, we can let it continue running   without interruption.
      if (!Preconditions.checkNotNull(previous).isRunni  ng()) 
        // Use the previous request rather than the new     one to allow for optimizations like skipping
        // setting placeholders, tracking and     un-tracking Targets, and obtaining View     dimensions
        // that are done in the individual Request.
        previous.begin();
      
      return target;
    
    
    requestManager.clear(target);
    target.setRequest(request);
    // 分析2.真正追踪请求的地方
    requestManager.track(target, request);

    return target;


// 分析1
private Request buildRequest(
      Target<TranscodeType> target,
      @Nullable RequestListener<TranscodeType>   targetListener,
      RequestOptions requestOptions) 
    return buildRequestRecursive(
        target,
        targetListener,
        /*parentCoordinator=*/ null,
        transitionOptions,
        requestOptions.getPriority(),
        requestOptions.getOverrideWidth(),
        requestOptions.getOverrideHeight(),
        requestOptions);


// 分析1
private Request buildRequestRecursive(
      Target<TranscodeType> target,
      @Nullable RequestListener<TranscodeType>   targetListener,
      @Nullable RequestCoordinator parentCoordinator,
      TransitionOptions<?, ? super TranscodeType>   transitionOptions,
      Priority priority,
      int overrideWidth,
      int overrideHeight,
      RequestOptions requestOptions) 

    // Build the ErrorRequestCoordinator first if     necessary so we can update parentCoordinator.
    ErrorRequestCoordinator errorRequestCoordinator =     null;
    if (errorBuilder != null) 
      // 创建errorRequestCoordinator(异常处理对象)
      errorRequestCoordinator = new   ErrorRequestCoordinator(parentCoordinator);
      parentCoordinator = errorRequestCoordinator;
    

    // 递归建立缩略图请求
    Request mainRequest =
        buildThumbnailRequestRecursive(
            target,
            targetListener,
            parentCoordinator,
            transitionOptions,
            priority,
            overrideWidth,
            overrideHeight,
            requestOptions);

    if (errorRequestCoordinator == null) 
      return mainRequest;
    

    ...
    
    Request errorRequest =     errorBuilder.buildRequestRecursive(
        target,
        targetListener,
        errorRequestCoordinator,
        errorBuilder.transitionOptions,
        errorBuilder.requestOptions.getPriority(),
        errorOverrideWidth,
        errorOverrideHeight,
        errorBuilder.requestOptions);
    errorRequestCoordinator.setRequests(mainRequest,     errorRequest);
    return errorRequestCoordinator;


// 分析1
private Request buildThumbnailRequestRecursive(
      Target<TranscodeType> target,
      RequestListener<TranscodeType> targetListener,
      @Nullable RequestCoordinator parentCoordinator,
      TransitionOptions<?, ? super TranscodeType> transitionOptions,
      Priority priority,
      int overrideWidth,
      int overrideHeight,
      RequestOptions requestOptions) 
    if (thumbnailBuilder != null) 
      // Recursive case: contains a potentially recursive thumbnail request builder.
      
      ...

      ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
      // 获取一个正常请求对象
      Request fullRequest =
          obtainRequest(
              target,
              targetListener,
              requestOptions,
              coordinator,
              transitionOptions,
              priority,
              overrideWidth,
              overrideHeight);
      isThumbnailBuilt = true;
      // Recursively generate thumbnail requests.
      // 使用递归的方式建立一个缩略图请求对象
      Request thumbRequest =
          thumbnailBuilder.buildRequestRecursive(
              target,
              targetListener,
              coordinator,
              thumbTransitionOptions,
              thumbPriority,
              thumbOverrideWidth,
              thumbOverrideHeight,
              thumbnailBuilder.requestOptions);
      isThumbnailBuilt = false;
      // coordinator(ThumbnailRequestCoordinator)是作为两者的协调者,
      // 能够同时加载缩略图和正常的图的请求
      coordinator.setRequests(fullRequest, thumbRequest);
      return coordinator;
     else if (thumbSizeMultiplier != null) 
      // Base case: thumbnail multiplier generates a thumbnail request, but cannot recurse.
      // 当设置了缩略的比例thumbSizeMultiplier(0 ~  1)时,
      // 不需要递归建立缩略图请求
      ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
      Request fullRequest =
          obtainRequest(
              target,
              targetListener,
              requestOptions,
              coordinator,
              transitionOptions,
              priority,
              overrideWidth,
              overrideHeight);
      RequestOptions thumbnailOptions = requestOptions.clone()
          .sizeMultiplier(thumbSizeMultiplier);

      Request thumbnailRequest =
          obtainRequest(
              target,
              targetListener,
              thumbnailOptions,
              coordinator,
              transitionOptions,
              getThumbnailPriority(priority),
              overrideWidth,
              overrideHeight);

      coordinator.setRequests(fullRequest, thumbnailRequest);
      return coordinator;
     else 
      // Base case: no thumbnail.
      // 没有缩略图请求时,直接获取一个正常图请求
      return obtainRequest(
          target,
          targetListener,
          requestOptions,
          parentCoordinator,
          transitionOptions,
          priority,
          overrideWidth,
          overrideHeight);
    


private Request obtainRequest(
      Target<TranscodeType> target,
      RequestListener<TranscodeType> targetListener,
      RequestOptions requestOptions,
      RequestCoordinator requestCoordinator,
      TransitionOptions<?, ? super TranscodeType>   transitionOptions,
      Priority priority,
      int overrideWidth,
      int overrideHeight) 
    // 最终实际返回的是一个SingleRequest对象(将制定的资源加载进对应的Target
    return SingleRequest.obtain(
        context,
        glideContext,
        model,
        transcodeClass,
        requestOptions,
        overrideWidth,
        overrideHeight,
        priority,
        target,
        targetListener,
        requestListeners,
        requestCoordinator,
        glideContext.getEngine(),
        transitionOptions.getTransitionFactory());


从上源码分析可知,我们在分析1处的buildRequest()方法里建立了请求,且最多可同时进行缩略图和正常图的请求,最后,调用了requestManager.track(target, request)方法,接着看看track里面做了什么。

RequestManager#track

// 分析2
void track(@NonNull Target<?> target, @NonNull Request request) 
    // 加入一个target目标集合(Set)
    targetTracker.track(target);
    
    requestTracker.runRequest(request);

RequestTracker#runRequest

/**
* Starts tracking the given request.
*/
// 分析2
public void runRequest(@NonNull Request request) 
    requests.add(request);
    if (!isPaused) 
      // 如果不是暂停状态则开始请求
      request.begin();
     else 
      request.clear();
      if (Log.isLoggable(TAG, Log.VERBOSE)) 
        Log.v(TAG, "Paused, delaying request");
      
      // 否则清空请求,加入延迟请求队列(为了对这些请求维持一个强引用,使用了ArrayList实现)
      pendingRequests.add(request);
    

SingleRequest#begin

// 分析2
@Override
public void begin() 
  
  ...
  
  if (model == null) 
  
    ...
    // model(url)为空,回调加载失败
    onLoadFailed(new GlideException("Received null   model"), logLevel);
    return;
  

  if (status == Status.RUNNING) 
    throw new IllegalArgumentException("Cannot   restart a running request");
  

 
  if (status == Status.COMPLETE) 
    onResourceReady(resource,   DataSource.MEMORY_CACHE);
    return;
  

  status = Status.WAITING_FOR_SIZE;
  if (Util.isValidDimensions(overrideWidth, overrideHeight)) 
    // 当使用override() API为图片指定了一个固定的宽高时直接执行onSizeReady,
    // 最终的核心处理位于onSizeReady
    onSizeReady(overrideWidth, overrideHeight);
   else 
    // 根据imageView的宽高算出图片的宽高,最终也会走到onSizeReady
    target.getSize(this);
  

  if ((status == Status.RUNNING || status ==     Status.WAITING_FOR_SIZE)
      && canNotifyStatusChanged()) 
    // 预先加载设置的缩略图
    target.onLoadStarted(getPlaceholderDrawable());
  
  if (IS_VERBOSE_LOGGABLE) 
    logV("finished run method in " +   LogTime.getElapsedMillis(startTime));
  

requestManager.track(target, request)开始,最终会执行到SingleRequest#begin()方法的onSizeReady,可以猜到(因为后面只做了预加载缩略图的处理),真正的请求就是从这里开始的,咱们进去一探究竟~

SingleRequest#onSizeReady

// 分析2
@Override
public void onSizeReady(int width, int height) 
  stateVerifier.throwIfRecycled();
  
  ...
  
  status = Status.RUNNING;

  float sizeMultiplier =     requestOptions.getSizeMultiplier();
  this.width = maybeApplySizeMultiplier(width,     sizeMultiplier);
  this.height = maybeApplySizeMultiplier(height,     sizeMultiplier);

  ...
  
  // 根据给定的配置进行加载,engine是一个负责加载、管理活跃和缓存资源的引擎类
  loadStatus = engine.load(
      glideContext,
      model,
      requestOptions.getSignature(),
      this.width,
      this.height,
      requestOptions.getResourceClass(),
      transcodeClass,
      priority,
      requestOptions.getDiskCacheStrategy(),
      requestOptions.getTransformations(),
      requestOptions.isTransformationRequired(),
      requestOptions.isScaleOnlyOrNoTransform(),
      requestOptions.getOptions(),
      requestOptions.isMemoryCacheable(),
      requestOptions.getUseUnlimitedSourceGeneratorsP    ool(),
      requestOptions.getUseAnimationPool(),
      requestOptions.getOnlyRetrieveFromCache(),
      this);

  ...

终于看到Engine类了,感觉距离成功不远了,继续~

Engine#load

public <R> LoadStatus load(
    GlideContext glideContext,
    Object model,
    Key signature,
    int width,
    int height,
    Class<?> resourceClass,
    Class<R> transcodeClass,
    Priority priority,
    DiskCacheStrategy diskCacheStrategy,
    Map<Class<?>, Transformation<?>> transformations,
    boolean isTransformationRequired,
    boolean isScaleOnlyOrNoTransform,
    Options options,
    boolean isMemoryCacheable,
    boolean useUnlimitedSourceExecutorPool,
    boolean useAnimationPool,
    boolean onlyRetrieveFromCache,
    ResourceCallback cb) 
  
  ...

  // 先从弱引用中查找,如果有的话回调onResourceReady并直接返回
  EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
  if (active != null) 
    cb.onResourceReady(active,   DataSource.MEMORY_CACHE);
    if (VERBOSE_IS_LOGGABLE) 
      logWithTimeAndKey("Loaded resource from active     resources", startTime, key);
    
    return null;
  

  // 没有再从内存中查找,有的话会取出并放到ActiveResources(内部维护的弱引用缓存map)里面
  EngineResource<?> cached = loadFromCache(key,     isMemoryCacheable);
  if (cached != null) 
    cb.onResourceReady(cached,   DataSource.MEMORY_CACHE);
    if (VERBOSE_IS_LOGGABLE) 
      logWithTimeAndKey("Loaded resource from cache",     startTime, key);
    
    return null;
  

  EngineJob<?> current = jobs.get(key,     onlyRetrieveFromCache);
  if (current != null) 
    current.addCallback(cb);
    if (VERBOSE_IS_LOGGABLE) 
      logWithTimeAndKey("Added to existing load",     startTime, key);
    
    return new LoadStatus(cb, current);
  

  // 如果内存中没有,则创建engineJob(decodejob的回调类,管理下载过程以及状态)
  EngineJob<R> engineJob =
      engineJobFactory.build(
          key,
          isMemoryCacheable,
          useUnlimitedSourceExecutorPool,
          useAnimationPool,
          onlyRetrieveFromCache);

  // 创建解析工作对象
  DecodeJob<R> decodeJob =
      decodeJobFactory.build(
          glideContext,
          model,
          key,
          signature,
          width,
          height,
          resourceClass,
          transcodeClass,
          priority,
          diskCacheStrategy,
          transformations,
          isTransformationRequired,
          isScaleOnlyOrNoTransform,
          onlyRetrieveFromCache,
          options,
          engineJob);

  // 放在Jobs内部维护的HashMap中
  jobs.put(key, engineJob);

  // 关注点8 后面分析会用到
  // 注册ResourceCallback接口
  engineJob.addCallback(cb);
  // 内部开启线程去请求
  engineJob.start(decodeJob);

  if (VERBOSE_IS_LOGGABLE) 
    logWithTimeAndKey("Started new load", startTime,   key);
  
  return new LoadStatus(cb, engineJob);


public void start(DecodeJob<R> decodeJob) 
    this.decodeJob = decodeJob;
    // willDecodeFromCache方法内部根据不同的阶段stage,如果是RESOURCE_CACHE/DATA_CACHE则返回true,使用diskCacheExecutor,否则调用getActiveSourceExecutor,内部会根据相应的条件返回sourceUnlimitedExecutor/animationExecutor/sourceExecutor
    GlideExecutor executor =   
    decodeJob.willDecodeFromCache()
        ? diskCacheExecutor
        : getActiveSourceExecutor();
    executor.execute(decodeJob);

可以看到,最终Engine(引擎)类内部会执行到自身的start方法,它会根据不同的配置采用不同的线程池使用diskCacheExecutor/sourceUnlimitedExecutor/animationExecutor/sourceExecutor来执行最终的解码任务decodeJob。

DecodeJob#run

runWrapped();

private void runWrapped() 
    switch (runReason) 
      case INITIALIZE:
        stage = getNextStage(Stage.INITIALIZE);
        // 关注点1
        currentGenerator = getNextGenerator();
        // 关注点2 内部会调用相应Generator的startNext()
        runGenerators();
        break;
      case SWITCH_TO_SOURCE_SERVICE:
        runGenerators();
        break;
      case DECODE_DATA:
        // 关注点3 将获取的数据解码成对应的资源
        decodeFromRetrievedData();
        break;
      default:
        throw new IllegalStateException("Unrecognized     run reason: " + runReason);
    


// 关注点1,完整情况下,会异步依次生成这里的ResourceCacheGenerator、DataCacheGenerator和SourceGenerator对象,并在之后执行其中的startNext()
private DataFetcherGenerator getNextGenerator() 
    switch (stage) 
      case RESOURCE_CACHE:
        return new ResourceCacheGenerator(decodeHelper, this);
      case DATA_CACHE:
        return new DataCacheGenerator(decodeHelper, this);
      case SOURCE:
        return new SourceGenerator(decodeHelper, this);
      case FINISHED:
        return null;
      default:
        throw new IllegalStateException("Unrecognized     stage: " + stage);
    

SourceGenerator#startNext

// 关注点2
@Override
public boolean startNext() 
  // dataToCache数据不为空的话缓存到硬盘(第一执行该方法是不会调用的)
  if (dataToCache != null) 
    Object data = dataToCache;
    dataToCache = null;
    cacheData(data);
  

  if (sourceCacheGenerator != null &&     sourceCacheGenerator.startNext()) 
    return true;
  
  sourceCacheGenerator = null;

  loadData = null;
  boolean started = false;
  while (!started && hasNextModelLoader()) 
    // 关注点4 getLoadData()方法内部会在modelLoaders里面找到ModelLoder对象
    // (每个Generator对应一个ModelLoader),
    // 并使用modelLoader.buildLoadData方法返回一个loadData列表
    loadData =   helper.getLoadData().get(loadDataListIndex++);
    if (loadData != null
        && (helper.getDiskCacheStrategy().isDataCache  able(loadData.fetcher.getDataSource())
        || helper.hasLoadPath(loadData.fetcher.getDat  aClass()))) 
      started = true;
      // 关注点6 通过loadData对象的fetcher对象(有关注点3的分析可知其实现类为HttpUrlFetcher)的
      // loadData方法来获取图片数据
      loadData.fetcher.loadData(helper.getPriority(),     this);
    
  
  return started;

DecodeHelper#getLoadData

List<LoadData<?>> getLoadData() 
    if (!isLoadDataSet) 
      isLoadDataSet = true;
      loadData.clear();
      List<ModelLoader<Object, ?>> modelLoaders =   glideContext.getRegistry().getModelLoaders(model)  ;
      //noinspection ForLoopReplaceableByForEach to   improve perf
      for (int i = 0, size = modelLoaders.size(); i <   size; i++) 
        ModelLoader<Object, ?> modelLoader =     modelLoaders.get(i);
        // 注意:这里最终是通过HttpGlideUrlLoader的buildLoadData获取到实际的loadData对象
        LoadData<?> current =
            modelLoader.buildLoadData(model, width,     height, options);
        if (current != null) 
          loadData.add(current);
        
      
    
    return loadData;

HttpGlideUrlLoader#buildLoadData

@Override
public LoadData<InputStream> buildLoadData(@NonNull   GlideUrl model, int width, int height,
    @NonNull Options options) 
  // GlideUrls memoize parsed URLs so caching them     saves a few object instantiations and time
  // spent parsing urls.
  GlideUrl url = model;
  if (modelCache != null) 
    url = modelCache.get(model, 0, 0);
    if (url == null) 
      // 关注点5
      modelCache.put(model, 0, 0, model);
      url = model;
    
  
  int timeout = options.get(TIMEOUT);
  // 注意,这里创建了一个DataFetcher的实现类HttpUrlFetcher
  return new LoadData<>(url, new HttpUrlFetcher(url,     timeout));


// 关注点5
public void put(A model, int width, int height, B value) 
    ModelKey<A> key = ModelKey.get(model, width,     height);
    // 最终是通过LruCache来缓存对应的值,key是一个ModelKey对象(由model、width、height三个属性组成)
    cache.put(key, value);

从这里的分析,我们明白了HttpUrlFetcher实际上就是最终的请求执行者,而且,我们知道了Glide会使用LruCache来对解析后的url来进行缓存,以便后续可以省去解析url的时间。

HttpUrlFetcher#loadData

@Override
public void loadData(@NonNull Priority priority,
    @NonNull DataCallback<? super InputStream>   callback) 
  long startTime = LogTime.getLogTime();
  try 
    // 关注点6
    // loadDataWithRedirects内部是通过HttpURLConnection网络请求数据
    InputStream result =   loadDataWithRedirects(glideUrl.toURL(), 0, null,   glideUrl.getHeaders());
    // 请求成功回调onDataReady()
    callback.onDataReady(result);
   catch (IOException e) 
    if (Log.isLoggable(TAG, Log.DEBUG)) 
      Log.d(TAG, "Failed to load data for url", e);
    
    callback.onLoadFailed(e);
   finally 
    if (Log.isLoggable(TAG, Log.VERBOSE)) 
      Log.v(TAG, "Finished http url fetcher fetch in     " + LogTime.getElapsedMillis(startTime));
    
  


private InputStream loadDataWithRedirects(URL url, int redirects, URL lastUrl,
  Map<String, String> headers) throws IOException 
    
    ...

    urlConnection.connect();
    // Set the stream so that it's closed in cleanup to avoid resource leaks. See #2352.
    stream = urlConnection.getInputStream();
    if (isCancelled) 
      return null;
    
    final int statusCode = urlConnection.getResponseCode();
    // 只要是2xx形式的状态码则判断为成功
    if (isHttpOk(statusCode)) 
      // 从urlConnection中获取资源流
      return getStreamForSuccessfulRequest(urlConnection);
     else if (isHttpRedirect(statusCode)) 
    
      ...
      
      // 重定向请求
      return loadDataWithRedirects(redirectUrl, redirects + 1, url,   headers);
     else if (statusCode == INVALID_STATUS_CODE) 
      throw new HttpException(statusCode);
     else 
      throw new HttpException(urlConnection.getResponseMessage(),   statusCode);
    


private InputStream getStreamForSuccessfulRequest(HttpURLConnection urlConnection)
  throws IOException 
    if (TextUtils.isEmpty(urlConnection.getContentEncoding())) 
      int contentLength = urlConnection.getContentLength();
      stream = ContentLengthInputStream.obtain(urlConnection.getInputStr  eam(), contentLength);
     else 
      if (Log.isLoggable(TAG, Log.DEBUG)) 
        Log.d(TAG, "Got non empty content encoding: " +     urlConnection.getContentEncoding());
      
      stream = urlConnection.getInputStream();
    
    return stream;

HttpUrlFetcher#loadData方法的loadDataWithRedirects里面,Glide通过原生的HttpURLConnection进行请求后,并调用getStreamForSuccessfulRequest()方法获取到了最终的图片流。

DecodeJob#run

在我们通过HtttpUrlFetcherloadData()方法请求得到对应的流之后,我们还必须对流进行处理得到最终我们想要的资源。这里我们回到第10步DecodeJob#run方法的关注点3处,这行代码将会对流进行解码。

decodeFromRetrievedData();

接下来,继续看看他内部的处理。

private void decodeFromRetrievedData() 
    if (Log.isLoggable(TAG, Log.VERBOSE)) 
      logWithTimeAndKey("Retrieved data", startFetchTime,
          "data: " + currentData
              + ", cache key: " + currentSourceKey
              + ", fetcher: " + currentFetcher);
    
    Resource<R> resource = null;
    try 
      //  核心代码 
      // 从数据中解码得到资源
      resource = decodeFromData(currentFetcher, currentData,   currentDataSource);
     catch (GlideException e) 
      e.setLoggingDetails(currentAttemptingKey, currentDataSource);
      throwables.add(e);
    
    if (resource != null) 
      // 关注点8 
      // 编码和发布最终得到的Resource<Bitmap>对象
      notifyEncodeAndRelease(resource, currentDataSource);
     else 
      runGenerators();
    


 private <Data> Resource<R> decodeFromData(DataFetcher<?> fetcher, Data data,
  DataSource dataSource) throws GlideException 
    try 
      if (data == null) 
        return null;
      
      long startTime = LogTime.getLogTime();
      // 核心代码
      // 进一步包装了解码方法
      Resource<R> result = decodeFromFetcher(data, dataSource);
      if (Log.isLoggable(TAG, Log.VERBOSE)) 
        logWithTimeAndKey("Decoded result " + result, startTime);
      
      return result;
     finally 
      fetcher.cleanup();
    


@SuppressWarnings("unchecked")
private <Data> Resource<R> decodeFromFetcher(Data data, DataSource dataSource)
  throws GlideException 
    LoadPath<Data, ?, R> path = decodeHelper.getLoadPath((Class<Data>) data.getClass());
    // 核心代码
    // 将解码任务分发给LoadPath
    return runLoadPath(data, dataSource, path);


private <Data, ResourceType> Resource<R> runLoadPath(Data data, DataSource dataSource,
  LoadPath<Data, ResourceType, R> path) throws GlideException 
    Options options = getOptionsWithHardwareConfig(dataSource);
    // 将数据进一步包装
    DataRewinder<Data> rewinder =     glideContext.getRegistry().getRewinder(data);
    try 
      // ResourceType in DecodeCallback below is required for   compilation to work with gradle.
      // 核心代码
      // 将解码任务分发给LoadPath
      return path.load(
          rewinder, options, width, height, new   DecodeCallback<ResourceType>(dataSource));
     finally 
      rewinder.cleanup();
    

LoadPath#load

public Resource<Transcode> load(DataRewinder<Data> rewinder, @NonNull Options options, int width,
  int height, DecodePath.DecodeCallback<ResourceType> decodeCallback) throws GlideException 
List<Throwable> throwables = Preconditions.checkNotNull(listPool.acquire());
try 
  // 核心代码
  return loadWithExceptionList(rewinder, options, width, height, decodeCallback, throwables);
 finally 
  listPool.release(throwables);

private Resource<Transcode> loadWithExceptionList(DataRewinder<Data> rewinder,
      @NonNull Options options,
      int width, int height, DecodePath.DecodeCallback<ResourceType>   decodeCallback,
      List<Throwable> exceptions) throws GlideException 
    Resource<Transcode> result = null;
    //noinspection ForLoopReplaceableByForEach to improve perf
    for (int i = 0, size = decodePaths.size(); i < size; i++) 
      DecodePath<Data, ResourceType, Transcode> path =   decodePaths.get(i);
      try 
        // 核心代码
        // 将解码任务又进一步分发给DecodePath的decode方法去解码
        result = path.decode(rewinder, width, height, options,     decodeCallback);
       catch (GlideException e) 
        exceptions.add(e);
      
      if (result != null) 
        break;
      
    

    if (result == null) 
      throw new GlideException(failureMessage, new   ArrayList<>(exceptions));
    

    return result;

DecodePath#decode

public Resource<Transcode> decode(DataRewinder<DataType> rewinder,     int width, int height,
      @NonNull Options options, DecodeCallback<ResourceType> callback)   throws GlideException 
    // 核心代码
    // 继续调用DecodePath的decodeResource方法去解析出数据
    Resource<ResourceType> decoded = decodeResource(rewinder, width,     height, options);
    Resource<ResourceType> transformed =     callback.onResourceDecoded(decoded);
    return transcoder.transcode(transformed, options);


@NonNull
private Resource<ResourceType> decodeResource(DataRewinder<DataType>   rewinder, int width,
    int height, @NonNull Options options) throws GlideException 
  List<Throwable> exceptions =     Preconditions.checkNotNull(listPool.acquire());
  try 
    // 核心代码
    return decodeResourceWithList(rewinder, width, height, options,   exceptions);
   finally 
    listPool.release(exceptions);
  


@NonNull
private Resource<ResourceType>   decodeResourceWithList(DataRewinder<DataType> rewinder, int width,
    int height, @NonNull Options options, List<Throwable> exceptions)   throws GlideException 
  Resource<ResourceType> result = null;
  //noinspection ForLoopReplaceableByForEach to improve perf
  for (int i = 0, size = decoders.size(); i < size; i++) 
    ResourceDecoder<DataType, ResourceType> decoder = decoders.get(i);
    try 
      DataType data = rewinder.rewindAndGet();
      if (decoder.handles(data, options)) 
        // 获取包装的数据
        data = rewinder.rewindAndGet();
        // 核心代码 
        // 根据DataType和ResourceType的类型分发给不同的解码器Decoder
        result = decoder.decode(data, width, height, options);
      
     catch (IOException | RuntimeException | OutOfMemoryError e) 
      if (Log.isLoggable(TAG, Log.VERBOSE)) 
        Log.v(TAG, "Failed to decode data for " + decoder, e);
      
      exceptions.add(e);
    

    if (result != null) 
      break;
    
  

  if (result == null) 
    throw new GlideException(failureMessage, new   ArrayList<>(exceptions));
  
  return result;

可以看到,经过一连串的嵌套调用,最终执行到了decoder.decode()这行代码,decode是一个ResourceDecoder<DataType, ResourceType>接口(资源解码器),根据不同的DataTypeResourceType它会有不同的实现类,这里的实现类是ByteBufferBitmapDecoder,接下来让我们来看看这个解码器内部的解码流程。

ByteBufferBitmapDecoder#decode

/**
 * Decodes @link android.graphics.Bitmap Bitmaps from @link    java.nio.ByteBuffer ByteBuffers.
 */
public class ByteBufferBitmapDecoder implements     ResourceDecoder<ByteBuffer, Bitmap> 
  
  ...

  @Override
  public Resource<Bitmap> decode(@NonNull ByteBuffer source, int width,   int height,
      @NonNull Options options)
      throws IOException 
    InputStream is = ByteBufferUtil.toStream(source);
    // 核心代码
    return downsampler.decode(is, width, height, options);
  

可以看到,最终是使用了一个downsampler,它是一个压缩器,主要是对流进行解码,压缩,圆角等处理。

DownSampler#decode

public Resource<Bitmap> decode(InputStream is, int outWidth, int outHeight,
  Options options) throws IOException 
    return decode(is, outWidth, outHeight, options, EMPTY_CALLBACKS);


 @SuppressWarnings("resource", "deprecation")
public Resource<Bitmap> decode(InputStream is, int requestedWidth, int requestedHeight,
      Options options, DecodeCallbacks callbacks) throws IOException 
    Preconditions.checkArgument(is.markSupported(), "You must provide an     InputStream that supports"
        + " mark()");

    ...

    try 
      // 核心代码
      Bitmap result = decodeFromWrappedStreams(is, bitmapFactoryOptions,
          downsampleStrategy, decodeFormat, isHardwareConfigAllowed,   requestedWidth,
          requestedHeight, fixBitmapToRequestedDimensions, callbacks);
      // 关注点7   
      // 解码得到Bitmap对象后,包装成BitmapResource对象返回,
      // 通过内部的get方法得到Resource<Bitmap>对象
      return BitmapResource.obtain(result, bitmapPool);
     finally 
      releaseOptions(bitmapFactoryOptions);
      byteArrayPool.put(bytesForOptions);
    


private Bitmap decodeFromWrappedStreams(InputStream is,
      BitmapFactory.Options options, DownsampleStrategy downsampleStrategy,
      DecodeFormat decodeFormat, boolean isHardwareConfigAllowed, int requestedWidth,
      int requestedHeight, boolean fixBitmapToRequestedDimensions,
      DecodeCallbacks callbacks) throws IOException 
    
    // 省去计算压缩比例等一系列非核心逻辑
    ...
    
    // 核心代码
    Bitmap downsampled = decodeStream(is, options, callbacks, bitmapPool);
    callbacks.onDecodeComplete(bitmapPool, downsampled);

    ...

    // Bimtap旋转处理
    ...
    
    return rotated;


private static Bitmap decodeStream(InputStream is,     BitmapFactory.Options options,
      DecodeCallbacks callbacks, BitmapPool bitmapPool) throws   IOException 
    
    ...
    
    TransformationUtils.getBitmapDrawableLock().lock();
    try 
      // 核心代码
      result = BitmapFactory.decodeStream(is, null, options);
     catch (IllegalArgumentException e) 
      ...
     finally 
      TransformationUtils.getBitmapDrawableLock().unlock();
    

    if (options.inJustDecodeBounds) 
      is.reset();
    
    return result;

从以上源码流程我们知道,最后是在DownSampler的decodeStream()方法中使用了BitmapFactory.decodeStream()来得到Bitmap对象。然后,我们来分析下图片时如何显示的,我们回到步骤19的DownSampler#decode方法,看到关注点7,这里是将Bitmap包装成BitmapResource对象返回,通过内部的get方法可以得到Resource对象,再回到步骤15的DecodeJob#run方法,这是使用了notifyEncodeAndRelease()方法对Resource对象进行了发布。

DecodeJob#notifyEncodeAndRelease

private void notifyEncodeAndRelease(Resource<R> resource, DataSource     dataSource) 
 
    ...

    notifyComplete(result, dataSource);

    ...
    


private void notifyComplete(Resource<R> resource, DataSource     dataSource) 
    setNotifiedOrThrow();
    callback.onResourceReady(resource, dataSource);


从以上EngineJob的源码可知,它实现了DecodeJob.CallBack这个接口。

class EngineJob<R> implements DecodeJob.Callback<R>,
    Poolable 
    ...

EngineJob#onResourceReady

@Override
public void onResourceReady(Resource<R> resource, DataSource   dataSource) 
  this.resource = resource;
  this.dataSource = dataSource;
  MAIN_THREAD_HANDLER.obtainMessage(MSG_COMPLETE, this).sendToTarget();


private static class MainThreadCallback implements Handler.Callback

    ...

    @Override
    public boolean handleMessage(Message message) 
      EngineJob<?> job = (EngineJob<?>) message.obj;
      switch (message.what) 
        case MSG_COMPLETE:
          // 核心代码
          job.handleResultOnMainThread();
          break;
        ...
      
      return true;
    

从以上源码可知,通过主线程Handler对象进行切换线程,然后在主线程调用了handleResultOnMainThread这个方法。

@Synthetic
void handleResultOnMainThread() 
  ...

  //noinspection ForLoopReplaceableByForEach to improve perf
  for (int i = 0, size = cbs.size(); i < size; i++) 
    ResourceCallback cb = cbs.get(i);
    if (!isInIgnoredCallbacks(cb)) 
      engineResource.acquire();
      cb.onResourceReady(engineResource, dataSource);
    
  
 
  ...

这里又通过一个循环调用了所有ResourceCallback的方法,让我们回到步骤9处Engine#load方法的关注点8这行代码,这里对ResourceCallback进行了注册,在步骤8出SingleRequest#onSizeReady方法里的engine.load中,我们看到最后一个参数,传入的是this,可以明白,engineJob.addCallback(cb)这里的cb的实现类就是SingleRequest。接下来,让我们看看SingleRequestonResourceReady方法。

SingleRequest#onResourceReady

/**
 * A callback method that should never be invoked directly.
 */
@SuppressWarnings("unchecked")
@Override
public void onResourceReady(Resource<?> resource, DataSource   dataSource) 
  ...
  
  // 从Resource<Bitmap>中得到Bitmap对象
  Object received = resource.get();
  
  ...
  
  onResourceReady((Resource<R>) resource, (R) received, dataSource);


private void onResourceReady(Resource<R> resource, R resultDataSource dataSource) 

    ...

    try 
      ...

      if (!anyListenerHandledUpdatingTarget) 
        Transition<? super R> animation =
            animationFactory.build(dataSource, isFirstResource);
        // 核心代码
        target.onResourceReady(result, animation);
      
     finally 
      isCallingCallbacks = false;
    

    notifyLoadSuccess();

SingleRequest#onResourceReady方法中又调用了target.onResourceReady(result, animation)方法,这里的target其实就是我们在into方法中建立的那个BitmapImageViewTarget,看到BitmapImageViewTarget类,我们并没有发现onResourceReady方法,但是我们从它的子类ImageViewTarget中发现了onResourceReady方法,从这里继续往下看。

ImageViewTarget#onResourceReady

public abstract class ImageViewTarget<Z> extends ViewTarget<ImageView, Z>
implements Transition.ViewAdapter 

    ...

    @Override
    public void onResourceReady(@NonNull Z resource, @Nullable       Transition<? super Z> transition) 
      if (transition == null || !transition.transition(resource, this))   
        // 核心代码
        setResourceInternal(resource);
       else 
        maybeUpdateAnimatable(resource);
      
    
 
    ...
    
    private void setResourceInternal(@Nullable Z resource) 
        // Order matters here. Set the resource first to make sure that the         Drawable has a valid and
        // non-null Callback before starting it.
        // 核心代码
        setResource(resource);
        maybeUpdateAnimatable(resource);
    
    
    // 核心代码
    protected abstract void setResource(@Nullable Z resource);


这里我们在回到BitmapImageViewTargetsetResource方法中,终于看到Bitmap被设置到了当前的imageView上了。

public class BitmapImageViewTarget extends ImageViewTarget<Bitmap> 

    ...
    
    
    @Override
    protected void setResource(Bitmap resource) 
      view.setImageBitmap(resource);
    


到这里,我们的分析就结束了,从以上的分析可知,Glide将大部分的逻辑处理都放在了最后一个into方法中,里面经过了20多个分析步骤才将请求图片流、解码出图片,到最终设置到对应的imageView上。

⑤完整Glide加载流程图


可以看到,Glide最核心的逻辑都聚集在into()方法中,它里面的设计精巧而复杂,这部分的源码分析非常耗时,但是,如果你真真正正地去一步步去深入其中,你也许在Android进阶之路上将会有顿悟的感觉。

2.OKhttp拦截器

在这篇文章中,我们将学习如何使用 OkHttp 拦截器。我们还将看到我们可以使用它的真实用例以及我们如何使用它来充分利用它。在 Android 中,我们有很多可以使用 OkHttp 拦截器完成的用例。

今天,我们将通过以下部分来掌握它:

  1. 什么是拦截器?
  2. 拦截器的类型。
  3. 如何在 OkHttpClient 中添加拦截器?
  4. 创建拦截器。
  5. 使用拦截器的真实用例。

①什么是拦截器?

根据文档,拦截器是一种强大的机制,可以监视、重写和重试 API 调用。所以基本上,当我们做一些 API 调用时,我们可以监控调用或执行一些任务。

简单来说,Interceptor 就像机场安检过程中的安检人员。他们检查我们的登机牌,在上面盖章,然后让我们通过。

我们可以使用拦截器来做很多事情,例如,集中监控 API 调用。通常,我们需要为每个网络调用添加记录器,但是通过使用拦截器,我们可以集中添加一个记录器,这将适用于所有网络调用。另一个用例可以缓存网络调用的响应以构建离线优先应用程序,我们将在本博客后面详细了解它。

②拦截器的类型

我们有两种类型的拦截器,如下所示:

  • 应用程序拦截器:这些是添加在应用程序代码(我们编写的代码)和 OkHttp 核心库之间的拦截器。这些是我们使用addInterceptor() 添加的。
  • **网络拦截器:**这些是添加在 OkHttp 核心库和服务器之间的拦截器。这些可以使用addNetworkInterceptor() 添加到 OkHttpClient。

③如何在 OkHttpClient 中添加拦截器?

在构建OkHttpClient对象时,我们可以添加如下拦截器:

fun myHttpClient(): OkHttpClient 
        val builder = OkHttpClient().newBuilder()
            .addInterceptor(/*our interceptor*/)
        return builder.build()
    

在这里,在addInterceptor中,我们传递了我们创建的拦截器。现在,让我们看看如何创建拦截器。

3.1创建拦截器

要创建拦截器,我们需要通过实现 Interceptor 接口来创建一个类,如下所示:

class MyInterceptor : Interceptor 
    override fun intercept(chain: Interceptor.Chain): Response 
        /**
         * Our API Call will be intercepted here
         */
    

在这里,在intercept()中,我们可以在其中执行我们想要的任何操作。

要使用拦截器,我们可以像下面这样使用:

fun myHttpClient(): OkHttpClient 
    val builder = OkHttpClient().newBuilder()
        .addInterceptor(MyInterceptor())
    return builder.build()

我们可以在addInterceptor() 中添加 MyInterceptor。

现在,让我们讨论更多可以使用拦截器的真实用例。

3.2.使用拦截器的真实用例

以下是 Android 中的常见用例:

  • 集中记录错误

首先,我们需要创建ErrorInterceptor,如下所示:

class ErrorInterceptor : Interceptor 
    override fun intercept(chain: Interceptor.Chain): Response 

        val request: Request = chain.request()
        val response = chain.proceed(request)
        when (response.code()) 
            400 -> 
                //Show Bad Request Error Message
            
            401 -> 
                //Show UnauthorizedError Message
            

            403 -> 
                //Show Forbidden Message
            

            404 -> 
                //Show NotFound Message
            

            // ... and so on

        
        return response
    

  • 首先,我们从chain.request()获取请求**
  • 然后,我们通过在chain.proceed(request)中传递请求来获取服务器返回的响应
  • 现在,我们可以检查响应代码并执行操作。
  • 我们可以通过接口或使用 RxJava、EventBus 等将错误传递给视图。
  • 假设我们收到401 错误,即未经授权,那么我们可以执行一个操作来清除应用程序数据/注销用户或我们想要执行的任何操作。

现在,要在 OkHttpClient 中使用这个 ErrorInterceptor,我们可以像下面这样添加:

.addInterceptor(ErrorInterceptor())

这就是我们如何使用拦截器创建集中式错误记录器的方法。

OkHttp 有一个内置的记录器,对于调试非常有用。

**注意:**如果我们想记录 URL 重定向的详细信息,请考虑使用addNetworkInterceptor()在网络层使用拦截器。

  • 缓存响应

如果我们想缓存 API 调用的响应,这样如果我们再次调用 API,响应就会从 Cache 中出来。

假设我们有从客户端到服务器的 API 调用,并且从服务器启用了Cache-Control标头 ,那么 OkHttp Core 将尊重该标头并缓存从服务器发送的响应一段时间。

但是如果没有从服务器启用 Cache-Control 怎么办。我们仍然可以使用拦截器缓存来自 OkHttp 客户端的响应。

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