Picasso源码解析
Posted 流云易采
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一、Picasso使用:
gradle:
compile 'com.squareup.picasso:picasso:2.5.2'
使用:
Picasso.with(this).load("http://...../photo3.jpg").into(myImg);
自定义的使用方法:
Picasso picasso = new Picasso.Builder(this)
.memoryCache(new LruCache()) // 设置自定义的内存缓存
.addRequestHandler(requestHandler) // 设置自定义的RequestHandler
.defaultBitmapConfig(bitmapConfig) // 设置自定义的Bitmap Config
.downloader(okHttpDownloader) // 设置自定义的Downloader
.executor(executorService) // 设置自定义的线程池
.requestTransformer(transformer) // 设置自定义的Transformor
.listener(listener) // 添加Listener进行监听
.build();
picasso.load(File/resId/Uri/String)
.resize(width, height)
.centerInside()
.into(imageView);
二、源码分析
(一)Picasso.with进行一系列初始化
public static Picasso with(Context context)
if (singleton == null)
synchronized (Picasso.class)
if (singleton == null)
singleton = new Builder(context).build();
return singleton;
简单的多线程单例模式;同时使用Buidler模式来创建Picasso实例。
1、Picasso.Builder#build:
/** Start building a new @link Picasso instance. */
public Builder(Context context)
if (context == null)
throw new IllegalArgumentException("Context must not be null.");
this.context = context.getApplicationContext();
private Downloader downloader;
private ExecutorService service;
private Cache cache;
private Listener listener;
private RequestTransformer transformer;
private List<RequestHandler> requestHandlers;
private Bitmap.Config defaultBitmapConfig;
/** Create the @link Picasso instance. */
public Picasso build()
Context context = this.context;
// 为这一系列变量进行默认初始化
// Downloader执行实际的下载业务,返回Response
if (downloader == null)
downloader = Utils.createDefaultDownloader(context);
// 内存缓存,可以看到默认的是LruCache
if (cache == null)
cache = new LruCache(context);
// 线程池,执行网络请求的地方
if (service == null)
service = new PicassoExecutorService();
// request发送前进行处理
if (transformer == null)
transformer = RequestTransformer.IDENTITY;
// 用以统计
Stats stats = new Stats(cache);
// 进行Request以及Response的转发
Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);
// 创建一个Picasso
return new Picasso(context, dispatcher, cache, listener, transformer, requestHandlers, stats,
defaultBitmapConfig, indicatorsEnabled, loggingEnabled);
可以看到这里为许多重要类进行了默认初始化,后面获取图片使用到的都是这些默认的变量,当然这些变量也都可以进行自定义;
Downloader:默认实现是OkHttpDownloader,Picasso是对OkHttp的封装,它的网络请求以及对本地Cache的请求都是直接通过OkHttp来实现的;Downloader是实现本地磁盘查询,以及发起网络请求的类;
Cache:内存缓存;这里采用的LruCache,也可以自行定义。缓存的是进行网络请求后,大小等裁剪之后的图片资源。
PicassoExecutorService: Picasso之中的线程池,默认核心线程为3个,没有非核心线程;但也会根据手机当前的网络状态进行适时改变,比如WIFI状态的核心线程就为4个,而4G,3G,2G状态下的线程数分别为3,2,1个;
Stats:进行一些数据统计,比如图片下载命中率等。
RequestTransformer:用来预处理Request,必须修改域名等
Dispatcher:用以分发任务,它通过启动了一个DispatcherThread线程,然后创建一个用来处理消息的DispatcherHandler,该Handler的数据处理是在DispatcherThread中进行的。
RequestHandler: 后面将会看到,该对象用来持有一个具体的Request,所有Request最终其实都是由其对应的RequestHandler来进行处理的。
根据后面具体使用情况再做分析:
2、Picasso#load:
public RequestCreator load(String path)
if (path == null)
return new RequestCreator(this, null, 0);
if (path.trim().length() == 0)
throw new IllegalArgumentException("Path must not be empty.");
return load(Uri.parse(path));
public RequestCreator load(Uri uri)
return new RequestCreator(this, uri, 0);
这里创建一个RequestCreator
private final Picasso picasso;
private final Request.Builder data;
RequestCreator(Picasso picasso, Uri uri, int resourceId)
if (picasso.shutdown)
throw new IllegalStateException(
"Picasso instance already shut down. Cannot submit new requests.");
this.picasso = picasso;
this.data = new Request.Builder(uri, resourceId, picasso.defaultBitmapConfig);
data对应创建一个Request的Builder,这个Request中封装了了相应的请求信息,传入了请求加载图片的URI,已经resourceId,以及默认显示图片的配置信息。
进而一般调用into将图片加载到相应的控件中。
3、RequestCreator#into:
// RequestCreator.java
public void into(ImageView target)
into(target, null);
public void into(ImageView target, Callback callback)
long started = System.nanoTime();
// 盘算是否是在主线程,如果不是则会抛出异常
checkMain();
if (target == null)
throw new IllegalArgumentException("Target must not be null.");
// 判断reqeust是否合法,即存在URI或者对应的resId,否则会取消该请求
if (!data.hasImage())
picasso.cancelRequest(target);
if (setPlaceholder)
setPlaceholder(target, getPlaceholderDrawable());
return;
// 判断是否需要延期执行
if (deferred)
// 判断是否已经设置了宽高大小
if (data.hasSize())
throw new IllegalStateException("Fit cannot be used with resize.");
// 获取目标控件的宽高参数
int width = target.getWidth();
int height = target.getHeight();
// 表示当前控件并未加载到界面上(宽或高为0)
if (width == 0 || height == 0)
if (setPlaceholder)
setPlaceholder(target, getPlaceholderDrawable());
// 生成DeferredRequestCreator,加入相应队列进行处理
picasso.defer(target, new DeferredRequestCreator(this, target, callback));
return;
// 设置Request.Builder中的宽高参数大小
data.resize(width, height);
// 创建Request
Request request = createRequest(started);
// 获取request对应的key
String requestKey = createKey(request);
// 根据策略判断是够需要跳过读取MemoryCache
if (shouldReadFromMemoryCache(memoryPolicy)) // 尝试从MemoryCache中获取Bitmap
// 根据Requeskey来获取相应的Bitmap
Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
if (bitmap != null)
// 如果从MemoryCache中获取到相应的bitmap,则取消request
picasso.cancelRequest(target);
// 设置图片
setBitmap(target, picasso.context, bitmap, MEMORY, noFade, picasso.indicatorsEnabled);
if (picasso.loggingEnabled)
log(OWNER_MAIN, VERB_COMPLETED, request.plainId(), "from " + MEMORY);
// 调用回调callback的onSuccess函数
if (callback != null)
callback.onSuccess();
return;
// 如果从MemoryCache获取图片失败,或者根据缓存策略直接跳过读取MemoryCache,则设置默认图片
if (setPlaceholder)
setPlaceholder(target, getPlaceholderDrawable());
// 创建ImageViewAction,Action里面包含了一次请求所需要的所有信息
Action action =
new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
errorDrawable, requestKey, tag, callback, noFade);
// 将action入队列
picasso.enqueueAndSubmit(action);
整个图片加载流程和一般的三级缓存加载流程相似,下图已经介绍地很清晰:
上面主要是尝试从MemoryCache中获取对应Bitmap,如果获取失败,再创建Action,通过将Action入队列,然后从本地Cache以及网络上去获取该图片。
接着上面enqueueAndSubmit继续分析
4、Picasso#enqueueAndSubmit:
// Picasso.java
final Map<Object, Action> targetToAction;
void enqueueAndSubmit(Action action)
// 根据Action获取对应的Target
Object target = action.getTarget();
if (target != null && targetToAction.get(target) != action)
// This will also check we are on the main thread.
// 取消该target对应的以前的请求
cancelExistingRequest(target);
// 添加当前请求
targetToAction.put(target, action);
// 提交action
submit(action);
targetToAction是一个Map,它存储的是Target对应的Action,以Target为相应key值。
当新添加一个action时,首先尝试取消该Action对应的Target之前的Action请求,然后将新Action添加到Map中;
然后提交action请求;
先来看下取消一个Request的函数cancelExistingReqeust,常用的cancelReqeust也是直接调用该函数实现的:
// Picasso.java
final Map<Object, Action> targetToAction;
private void cancelExistingRequest(Object target)
// 判断是否在主线程中进行
checkMain();
// 从Map中移除该target对应action
Action action = targetToAction.remove(target);
// 如果该Target之前存在对应的action
if (action != null)
// 则调用该action的cancel函数进行取消
action.cancel();
// 并且调用Dispatcher进行分发cancel请求
dispatcher.dispatchCancel(action);
// 对于ImageView类型的Target
if (target instanceof ImageView)
ImageView targetImageView = (ImageView) target;
// 前面分析提到可能有需要延迟操作的请求,它是通过添加到DeferredRequestCreator来实现的,这里的需要将其cancel掉
DeferredRequestCreator deferredRequestCreator =
targetToDeferredRequestCreator.remove(targetImageView);
if (deferredRequestCreator != null)
deferredRequestCreator.cancel();
简单的cancel逻辑:
这里依然也需要先判断事件处理是否是在主线程中进行的;
然后尝试从targetToAction中删除该Target中对应的Action,如果该Action确实存在,则调用cancel函数取消该Action,并且调用Dispatcher来分发该cancel请求;
对应ImageView类型的target,前面分析中提到当需要defer即延迟的请求时,是通过创建DeferredReqeustCreator,并添加到一个Map中来实现的,这里同理应将其删除并cancel掉;
再来看看其cancel函数:
1)Action#cancel:
// ImageViewAction.java
class ImageViewAction extends Action<ImageView>
Callback callback;
@Override void cancel()
super.cancel();
if (callback != null)
callback = null;
// Action.java
abstract class Action<T>
boolean cancelled;
void cancel()
cancelled = true;
和Volley这些差不多,通过设置一个cancel标志来实现。
2) Dispatcher#dispatchCacnel
// Dispatcher.java
void dispatchCancel(Action action)
handler.sendMessage(handler.obtainMessage(REQUEST_CANCEL, action));
分发逻辑较为简单,使用Handler来实现的;
这里来具体来看下Dispatcher的定义,来确定handler的定义;Dispathcer是在创建Picasso时默认创建了一个Dispathcer实例,如下所示:
// Picasso.java
Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);
static final Handler HANDLER = new Handler(Looper.getMainLooper())
@Override
public void handleMessage(Message msg)
switch (msg.what)
...
而Dispatcher中的Handler并不是这个Handler,而是又做了一个封装:
this.handler = new DispatcherHandler(dispatcherThread.getLooper(), this);
// Dispatcher.java
private static class DispatcherHandler extends Handler
private final Dispatcher dispatcher;
public DispatcherHandler(Looper looper, Dispatcher dispatcher)
super(looper);
this.dispatcher = dispatcher;
@Override public void handleMessage(final Message msg)
switch (msg.what)
case REQUEST_SUBMIT:
Action action = (Action) msg.obj;
dispatcher.performSubmit(action);
break;
case REQUEST_CANCEL:
Action action = (Action) msg.obj;
dispatcher.performCancel(action);
break;
.......
default:
Picasso.HANDLER.post(new Runnable()
@Override public void run()
throw new AssertionError("Unknown handler message received: " + msg.what);
);
Dispathcer的作用是通过handler将相应的操作逻辑转换到自己Looper对应线程中去实现,然后调用performCancel去具体处理该Action;
下面接着来看Dispatcher#performCancel:
// Dispatcher.java
final Map<String, BitmapHunter> hunterMap;
final Set<Object> pausedTags;
final Map<Object, Action> pausedActions;
final Map<Object, Action> failedActions;
void performCancel(Action action)
// 获取Action对应的key
String key = action.getKey();
// 根据key获取对应的BitmapHunter
BitmapHunter hunter = hunterMap.get(key);
// 取消执行bitmapHunter
if (hunter != null)
hunter.detach(action);
if (hunter.cancel())
hunterMap.remove(key);
// 从暂停map中删除对应action
if (pausedTags.contains(action.getTag()))
pausedActions.remove(action.getTarget());
// 从失败map中删除对应Action
Action remove = failedActions.remove(action.getTarget());
逻辑较为简单,主要从各个可能存储该Action的集合中将其删除,如果已经有Bitmap在执行,则调用去cancel去处理。里面引出一个重要的类BitmapHunter,下面将会进行介绍。
如果没有从MemoryCache中获取到Bitmap(分两种情况,一直是根据policy直接跳过从MemoryCache中获取;另一种是MemoryCache中并没有对应的Bitmap缓存),这是将会来到submit(Action);
5、Picasso#submit:
// Picasso.java
void submit(Action action)
dispatcher.dispatchSubmit(action);
和前面类似。继续调用Dispatcher来分发submit事件,因此可以看出Dispatcher的主要作用是对Picasso(主线程)中发送的事件进行分发处理;
和dispatchCancel类似,接下来会来到performSubmit:
6、Dispatcher#performSubmit:
final Set<Object> pausedTags;
final Map<Object, Action> failedActions;
final Map<Object, Action> pausedActions;
final Map<String, BitmapHunter> hunterMap;
final ExecutorService service;
void performSubmit(Action action, boolean dismissFailed)
// 如果Action已经暂停
if (pausedTags.contains(action.getTag()))
pausedActions.put(action.getTarget(), action);
return;
// 获取对应的BitmapHunter
BitmapHunter hunter = hunterMap.get(action.getKey());
if (hunter != null) // 如果对应hunter不为null,表示前面有同样的请求在进行,因此这是可以合并请求
hunter.attach(action);
return;
// 判断是否已经关闭
if (service.isShutdown())
return;
// 创建BitmapHunter
hunter = forRequest(action.getPicasso(), this, cache, stats, action);
// 将Bitmap提交到线程池
hunter.future = service.submit(hunter);
// 保存Bitmap
hunterMap.put(action.getKey(), hunter);
if (dismissFailed)
failedActions.remove(action.getTarget());
具体的工作逻辑如下图:
当遇到相同的Action时,Picasso会通过BitmapHunter的Attach来合并相同的请求,避免同一个请求重复进行;简单看下attach:
1)Bitmap#attach:
// BitmapHunter.java
void attach(Action action)
boolean loggingEnabled = picasso.loggingEnabled;
Request request = action.request;
// 如果BitmapHunter对应的action仍未设置,则直接设置action返回
if (this.action == null)
this.action = action;
return;
// 创建一个保存相同Action的ArrayList
if (actions == null)
actions = new ArrayList<Action>(3);
actions.add(action);
// 获得Action的优先级
Picasso.Priority actionPriority = action.getPriority();
// 如果新添加Action的Priority优先级高于原来的优先级,则更新该Bitmaphunter的优先级
if (actionPriority.ordinal() > priority.ordinal())
priority = actionPriority;
可以看到合并相同请求,则是在BitmapHunter中维护一个Action列表actions;
当新添加Action的Priority优先级高于原来的优先级,则更新该BitmapHunter的优先级大小;
这里在来关注下Picasso中的优先级问题:
/**
* The priority of a request.
*
* @see RequestCreator#priority(Priority)
*/
public enum Priority
LOW,
NORMAL,
HIGH
Picasso中Priority是个枚举类,总共分为LOW,NORMAL,HIGH三个等级;而相对应地,Volley中Request的Priority则多了一个IMMEDIATE等级;顺便提下Volley中Reqeust可以根据Priority进行执行的原因是因为其实现了Comparable接口:
Volley#Reqeust:
public abstract class Request<T> implements Comparable<Request<T>>
public enum Priority
LOW,
NORMAL,
HIGH,
IMMEDIATE
@Override
public int compareTo(Request<T> other)
Priority left = this.getPriority();
Priority right = other.getPriority();
// High-priority requests are "lesser" so they are sorted to the front.
// Equal priorities are sorted by sequence number to provide FIFO ordering.
return left == right ?
this.mSequence - other.mSequence :
right.ordinal() - left.ordinal();
2)ExecutorService:
再来看下线程池ExecutorService,它是在Picasso创建之初Builder时就创建的,默认的实现为:
// Picasso.Builder
private ExecutorService service;
if (service == null)
service = new PicassoExecutorService();
2.1)来看PicassoExecutorService:
// PicassoExecutorService
class PicassoExecutorService extends ThreadPoolExecutor
// 默认的线程池的线程数量
private static final int DEFAULT_THREAD_COUNT = 3;
PicassoExecutorService()
super(DEFAULT_THREAD_COUNT, DEFAULT_THREAD_COUNT, 0, TimeUnit.MILLISECONDS,
new PriorityBlockingQueue<Runnable>(), new Utils.PicassoThreadFactory());
Picasso提供的是个默认核心线程数为3个,没有非核心线程(最大的线程数目也是为3),任务队列为PriorityBlockingQueue的线程池;
Picasso有个有趣的地方是,它可以根据当前应用的网络状态实时调整线程池中的线程数目:
2.2)PicassoExecutorService#adjustThreadCount:
// 根据网络状况调整线程池中线程的数量
void adjustThreadCount(NetworkInfo info)
if (info == null || !info.isConnectedOrConnecting())
setThreadCount(DEFAULT_THREAD_COUNT);
return;
switch (info.getType())
case ConnectivityManager.TYPE_WIFI:
case ConnectivityManager.TYPE_WIMAX:
case ConnectivityManager.TYPE_ETHERNET:
setThreadCount(4);
break;
case ConnectivityManager.TYPE_MOBILE:
switch (info.getSubtype())
case TelephonyManager.NETWORK_TYPE_LTE: // 4G
case TelephonyManager.NETWORK_TYPE_HSPAP:
case TelephonyManager.NETWORK_TYPE_EHRPD:
setThreadCount(3);
break;
case TelephonyManager.NETWORK_TYPE_UMTS: // 3G
case TelephonyManager.NETWORK_TYPE_CDMA:
case TelephonyManager.NETWORK_TYPE_EVDO_0:
case TelephonyManager.NETWORK_TYPE_EVDO_A:
case TelephonyManager.NETWORK_TYPE_EVDO_B:
setThreadCount(2);
break;
case TelephonyManager.NETWORK_TYPE_GPRS: // 2G
case TelephonyManager.NETWORK_TYPE_EDGE:
setThreadCount(1);
break;
default:
setThreadCount(DEFAULT_THREAD_COUNT);
break;
default:
setThreadCount(DEFAULT_THREAD_COUNT);
// 线程线程数量
private void setThreadCount(int threadCount)
setCorePoolSize(threadCount);
setMaximumPoolSize(threadCount);
即WIFI状态下线程数目为4,4G状态下为3,3G状态下为2,2G转态下线程池中只有一个线程;
再来看线程池中创建线程的Factory,看看Picasso的线程池中的线程有何特别之处:
2.3)PicassoThreadFactory:
static class PicassoThreadFactory implements ThreadFactory
@SuppressWarnings("NullableProblems")
public Thread newThread(Runnable r)
return new PicassoThread(r);
private static class PicassoThread extends Thread
public PicassoThread(Runnable r)
super(r);
@Override public void run()
Process.setThreadPriority(THREAD_PRIORITY_BACKGROUND);
super.run();
PicassoThread简单的Thread,线程优先级为后台线程;
再看service提交submit一个BitmapHunter时的操作逻辑:
2.4)PicassoExecutorService#submit:
// PicassoExecutorService
class PicassoExecutorService extends ThreadPoolExecutor
// 提交任务
@Override
public Future<?> submit(Runnable task)
PicassoFutureTask ftask = new PicassoFutureTask((BitmapHunter) task);
execute(ftask);
return ftask;
// 将BitmapHunter封装成PicassoFutureTask进行execute
private static final class PicassoFutureTask extends FutureTask<BitmapHunter>
implements Comparable<PicassoFutureTask>
private final BitmapHunter hunter;
public PicassoFutureTask(BitmapHunter hunter)
super(hunter, null);
this.hunter = hunter;
@Override
public int compareTo(PicassoFutureTask other)
Picasso.Priority p1 = hunter.getPriority();
Picasso.Priority p2 = other.hunter.getPriority();
// High-priority requests are "lesser" so they are sorted to the front.
// Equal priorities are sorted by sequence number to provide FIFO ordering.
return (p1 == p2 ? hunter.sequence - other.hunter.sequence : p2.ordinal() - p1.ordinal());
可以看到在submit中将BitmapHunter封装成一个FutureTask进行execute;
PicassoFutureTask可以实现对当前Runnbale运行结果的查看及控制;同期它提供了和Volley一个样的Priority一样的优先级机制。
回到前面performSubmit的执行逻辑,接下来会创建以及执行BitmapHunter,下面重点介绍BitmapHunter,它是一个任务具体执行的整体:
7、BitmapHunter:
1)Dispatcher#forReqeust:
// BitmapHunter.java
static BitmapHunter forRequest(Picasso picasso, Dispatcher dispatcher, Cache cache, Stats stats,
Action action)
// 根据Action获取对应Request
Request request = action.getRequest();
// 获取requestHandlers
List<RequestHandler> requestHandlers = picasso.getRequestHandlers();
// Index-based loop to avoid allocating an iterator.
//noinspection ForLoopReplaceableByForEach
// 查找使用能够处理该Request的requesthandler,如果有则传入BitmapHandler
for (int i = 0, count = requestHandlers.size(); i < count; i++)
RequestHandler requestHandler = requestHandlers.get(i);
if (requestHandler.canHandleRequest(request))
return new BitmapHunter(picasso, dispatcher, cache, stats, action, requestHandler);
// 使用默认的ERRORING_HANDLER来构造BitmapHunter
return new BitmapHunter(picasso, dispatcher, cache, stats, action, ERRORING_HANDLER);
先判断Picasso中是否已经设置了能够处理该Request的requestHandler,如果存在则传入BitmapHandler,如果不存在,则使用默认的ERRORING_HANDLER来构造BitmapHandler;
简单看下ERRORING_HANDLER:
// BitmapHunter.java
private static final RequestHandler ERRORING_HANDLER = new RequestHandler()
@Override public boolean canHandleRequest(Request data)
return true;
@Override public Result load(Request request, int networkPolicy) throws IOException
throw new IllegalStateException("Unrecognized type of request: " + request);
;
事实上处理Request请求主要是通过reqeustHandler的load函数来进行处理的,然后将处理后的结果封装成Result,然后返回;Picasso在其构造函数中提供了一些列的RequestHandler:
List<RequestHandler> allRequestHandlers =
new ArrayList<RequestHandler>(builtInHandlers + extraCount);
// ResourceRequestHandler needs to be the first in the list to avoid
// forcing other RequestHandlers to perform null checks on request.uri
// to cover the (request.resourceId != 0) case.
allRequestHandlers.add(new ResourceRequestHandler(context));
if (extraRequestHandlers != null)
allRequestHandlers.addAll(extraRequestHandlers);
allRequestHandlers.add(new ContactsPhotoRequestHandler(context));
allRequestHandlers.add(new MediaStoreRequestHandler(context));
allRequestHandlers.add(new ContentStreamRequestHandler(context));
allRequestHandlers.add(new AssetRequestHandler(context));
allRequestHandlers.add(new FileRequestHandler(context));
allRequestHandlers.add(new NetworkRequestHandler(dispatcher.downloader, stats));
requestHandlers = Collections.unmodifiableList(allRequestHandlers);
可以看到,按照不同的Request,Picasso提供了不同的RequestHandler进行处理,比如从资源中进行加载的则使用ResourceRequestHandler,从文件中加载则调用FileRequestHandler,从网络中直接下载使用NetworkRequestHandler等等。
下面将会看到这些RequestHandler的具体处理作用;
2)BitmapHunter:
// BitmapHunter.java
class BitmapHunter implements Runnable
/**
* Global lock for bitmap decoding to ensure that we are only are decoding one at a time. Since
* this will only ever happen in background threads we help avoid excessive memory thrashing as
* well as potential OOMs. Shamelessly stolen from Volley.
*/
private static final Object DECODE_LOCK = new Object();
@Override
public void run()
try
updateThreadName(data);
// 主要的执行逻辑在hunt方法中
result = hunt();
// 通过Dispatcher来分发运行之后的结果
if (result == null)
dispatcher.dispatchFailed(this);
else
dispatcher.dispatchComplete(this);
// 下面是进行运行过程中的一邪猎以往情况的处理
catch (Downloader.ResponseException e)
if (!e.localCacheOnly || e.responseCode != 504)
exception = e;
dispatcher.dispatchFailed(this);
catch (NetworkRequestHandler.ContentLengthException e)
exception = e;
dispatcher.dispatchRetry(this);
catch (IOException e)
exception = e;
dispatcher.dispatchRetry(this);
catch (OutOfMemoryError e)
StringWriter writer = new StringWriter();
stats.createSnapshot().dump(new PrintWriter(writer));
exception = new RuntimeException(writer.toString(), e);
dispatcher.dispatchFailed(this);
catch (Exception e)
exception = e;
dispatcher.dispatchFailed(this);
finally
Thread.currentThread().setName(Utils.THREAD_IDLE_NAME);
BitmapHunter实际上是一个Runnable,重点看其run中的执行逻辑;看到执行较为简单,都是通过hunt这个函数来实现的,获取到结果后,根据结果的成功与否,通过Dispatcher进行转发结果;
BitmapHunter中有一个成员变量较为有趣,DECODE_LOCK,这个变量在Volley中出现过,而且代码作者也说这是它从Volley中学习过来来,从名称中就可以看出,它是用来在解码图片时使用的锁,用来保证系统在同一时刻只有一个线程在解码图片;
2.1)Bitmap#hunt:
// BitmapHunter.java
final Request data;
Bitmap hunt() throws IOException
Bitmap bitmap = null;
// 这里还是先尝试从MemoryCache中去获取Bitmap
if (shouldReadFromMemoryCache(memoryPolicy))
bitmap = cache.get(key);
if (bitmap != null) // 如果获取成功,直接返回
stats.dispatchCacheHit(); // 统计命中信息
loadedFrom = MEMORY; // loadedFrom记录表示从Memory缓存中获取
return bitmap;
data.networkPolicy = retryCount == 0 ? NetworkPolicy.OFFLINE.index : networkPolicy;
// 通过RequestHandler.load来处理Request获取结果
RequestHandler.Result result = requestHandler.load(data, networkPolicy);
// 如果处理结果成功
if (result != null)
loadedFrom = result.getLoadedFrom();
exifRotation = result.getExifOrientation();
// 从结果中获取Bitmap
bitmap = result.getBitmap();
// If there was no Bitmap then we need to decode it from the stream.
// 如果Bitmap为null,则需要将Stream转化成Bitmap
if (bitmap == null)
InputStream is = result.getStream();
try
bitmap = decodeStream(is, data); // 将网络数据流decode成Bitmap
finally
Utils.closeQuietly(is);
if (bitmap != null)
// 更新统计信息
stats.dispatchBitmapDecoded(bitmap);
// 判断是否需要进行Transformation,即进行转换
if (data.needsTransformation() || exifRotation != 0)
// 这里体现了前面提到的DECODE_LOCK的作用,它保证在同一时刻,只有一个线程在执行图形变换
synchronized (DECODE_LOCK)
if (data.needsMatrixTransform() || exifRotation != 0)
bitmap = transformResult(data, bitmap, exifRotation);
if (data.hasCustomTransformations())
bitmap = applyCustomTransformations(data.transformations, bitmap);
if (bitmap != null)
stats.dispatchBitmapTransformed(bitmap);
return bitmap;
hunt获取Bitmap的流程也较为清晰,首先仍然进行一次尝试从MemoryCache中进行获取;
获取失败,通过requestHandler的load函数进行加载,加载返回的结果为Result;
然后判断获取到的结果是否已经是Bitmap,如果不是,则需要将其从Stream转化成Bitmap;
然后判断是否需要Transformation;使用Transformation可以实现对获取到的图片的转化,一般的使用方式为:
Picasso.with(this).load("").transform(new Transformation()
@Override
public Bitmap transform(Bitmap source)
// 进行图片转换
return null;
@Override
public String key()
return null;
).into(mImageView);
使用transform方法中添加Transformation进行图片处理,相应的图片处理开源库picasso-transformations源码地址已经集成了一些常用的Transformation可以直接进行使用;
transform的主要作用是将新创建Transformation对象添加到Request的transformations这个ArrayList中;
因而判断Request是否需要进行transform的方法:
// Request.java
boolean needsTransformation()
return needsMatrixTransform() || hasCustomTransformations();
// 判断是否需要矩阵变换
boolean needsMatrixTransform()
return hasSize() || rotationDegrees != 0;
// 即将Bitmap转化成ImageView适合的尺寸
public boolean hasSize()
return targetWidth != 0 || targetHeight != 0;
// 判断用户是否添加了相应的Transformation
boolean hasCustomTransformations()
return transformations != null;
然后根据需要,对图片进行转换;
这里转换时使用到了前面所提到的DECODE_LOCK,它保证在同一时刻,只有一个线程在执行图形变换操作;
流程梳理清楚了,下面接着来看通过requestHandler接着获取Bitmap的过程,这里通过NetworkRequestHandler为例,即从网络中获取;
2.2)NetworkRequestHandler#load:
// NetWorkRequestHandler.java
class NetworkRequestHandler extends RequestHandler
private final Downloader downloader;
@Override
public RequestHandler.Result load(Request request, int networkPolicy) throws IOException
// 可以看到真正处理网络请求的操作时通过downloader来实现的
Downloader.Response response = downloader.load(request.uri, request.networkPolicy);
// 下面处理response结果
if (response == null)
return null;
// 判断数据是从磁盘还是网络总获取的
Picasso.LoadedFrom loadedFrom = response.cached ? DISK : NETWORK;
Bitmap bitmap = response.getBitmap();
if (bitmap != null) // 如果可以直接获取到Bitmap,则将其封装成Result,返回
return new RequestHandler.Result(bitmap, loadedFrom);
// 否则获取其输入流InputStream
InputStream is = response.getInputStream();
if (is == null)
return null;
// Sometimes response content length is zero when requests are being replayed. Haven't found
// root cause to this but retrying the request seems safe to do so.
// 处理无效的Content长度为0的情况
if (loadedFrom == DISK && response.getContentLength() == 0)
Utils.closeQuietly(is);
// 抛出ContentLengthException,在Bitmap的run函数中会处理该异常,会调用retry进行重试
throw new ContentLengthException("Received response with 0 content-length header.");
if (loadedFrom == NETWORK && response.getContentLength() > 0)
stats.dispatchDownloadFinished(response.getContentLength());
// 封装成Result,返回
return new RequestHandler.Result(is, loadedFrom);
这里可以看到,真正处理请求的其实是Downloader这个实例,处理后返回的结果为Response;
Response中返回的结果可能有两种情况,一种是可能已经获取到了bitmap,一种可能是InputStream,这里需要分别进行处理,封装成Result,返回给BitmapHunter;
这里有个细节,当从磁盘中获取的结果Content的长度为0时,这里会抛出ContentLengthException异常,在BitmapHunter的run函数中会处理该异常,会调用retry进行重试;
downloader也是在Picasso初始化时就创建了的,如下所示:
if (downloader == null)
downloader = Utils.createDefaultDownloader(context);
2.3)Downloader:
// Utils.java
static Downloader createDefaultDownloader(Context context)
try
Class.forName("com.squareup.okhttp.OkHttpClient");
return OkHttpLoaderCreator.create(context);
catch (ClassNotFoundException ignored)
return new UrlConnectionDownloader(context);
可以看到这里提供了两种实现,一种是如果系统中已经添加了OkhttpClient依赖,则使用Class.forName加载OkHttpClient类时,会加载成功,这时使用OkHttpLoaderCreator进行创建,也就是下面的网络请求通过okHttp来实现;否则是使用android中默认提供的UrlConnection;这里一句OkHttp来进行分析;
Picasso源码解析之Lrucache算法源码解析