Volley框架的基本解读

Posted 2022-12-02 zero-27

在前面六篇博客中，我们已经将Volley框架主线全部走完，今天我们来讲讲Volley框架中两个十分重要的角色Request与Response，这两个类之所以放到最后来讲，一方面对Volley没有一个清晰的认识，可能会比较难懂，另方面好戏不都是压轴的吗？

我们先看Request的源码：

public abstract class Request<T> implements Comparable<Request<T>>

它是一个抽象类，并实现了Comparable用于比较排序，还记得RequestQueue中的mCacheQueue与mNetworkQueue这两个优先级队列吗？实现Comparable接口就是为了能在队列中排序，而从实现某些Request的优先请求。

我们还有先从构造方法开始看起：

public Request(String url, Response.ErrorListener listener) 
        this(Method.DEPRECATED_GET_OR_POST, url, listener);
    

这个构造方法有两个参数，分别是URL与错误回调，它调用了另一个构造方法，Method是Request的一个内部接口，我们看看：

/**
     * Supported request methods.
     * 支持的请求方式
     */
    public interface Method 
        int DEPRECATED_GET_OR_POST = -1;
        int GET = 0;
        int POST = 1;
        int PUT = 2;
        int DELETE = 3;
    

可见内部定义了四种支持的请求方式，DEPRECATED_GET_OR_POST这个大家可以无视，我也不是很理解，接着看：

public Request(int method, String url, Response.ErrorListener listener) 
        mMethod = method;
        mUrl = url;
        mErrorListener = listener;
        setRetryPolicy(new DefaultRetryPolicy());

        mDefaultTrafficStatsTag = TextUtils.isEmpty(url) ? 0: Uri.parse(url).getHost().hashCode();
    

这里为我们创建了一个重试策略并绑定在request中，mDefaultTrafficStatsTag是用于流量监控的唯一标识，通过URL产生。

Request内部定义了很多公共方法，比较重要的有这么几个，我们一起来看看：

/**
     * Returns true if this request has been canceled.
     * 
     * 是否取消该request
     */
    public boolean isCanceled() 
        return mCanceled;
    

/**
     * Mark this request as canceled.  No callback will be delivered.
     * 取消请求
     */
    public void cancel() 
        mCanceled = true;
    

在之前的源码解析中我们经常可以看到isCanceled，现在我们知道它仅仅只是做了一个标识而已。

/**
     * Returns the cache key for this request.  By default, this is the URL.
     * 获取缓存的Key，就是URL
     */
    public String getCacheKey() 
        return getUrl();
    

/**
     * Returns the URL of this request.
     * 请求URL
     */
    public String getUrl() 
        return mUrl;
    

getCacheKey该方法在缓存解析的时候出现过，缓存的Key就是URL。

/**
     * Returns a Map of parameters to be used for a POST or PUT request.  Can throw
     * @link AuthFailureError as authentication may be required to provide these values.
     *
     * <p>Note that you can directly override @link #getBody() for custom data.</p>
     *
     * @throws AuthFailureError in the event of auth failure
     * 
     * 空请求体，子类可以复写该方法，如果子类需要的话
     */
    protected Map<String, String> getParams() throws AuthFailureError 
        return null;
    

public byte[] getBody() throws AuthFailureError 
        Map<String, String> params = getParams();
        if (params != null && params.size() > 0) 
            return encodeParameters(params, getParamsEncoding());
        
        return null;
    

private byte[] encodeParameters(Map<String, String> params, String paramsEncoding) 
        StringBuilder encodedParams = new StringBuilder();
        try 
            for (Map.Entry<String, String> entry : params.entrySet()) 
                encodedParams.append(URLEncoder.encode(entry.getKey(), paramsEncoding));
                encodedParams.append('=');
                encodedParams.append(URLEncoder.encode(entry.getValue(), paramsEncoding));
                encodedParams.append('&');
            
            return encodedParams.toString().getBytes(paramsEncoding);
         catch (UnsupportedEncodingException uee) 
            throw new RuntimeException("Encoding not supported: " + paramsEncoding, uee);
        
    

该方法用于POST请求，添加请求体。

/**
     * Priority values.  Requests will be processed from higher priorities to
     * lower priorities, in FIFO order.
     * 
     * 优先级
     */
    public enum Priority 
        LOW,
        NORMAL,
        HIGH,
        IMMEDIATE
    

/**
     * Returns the @link Priority of this request; @link Priority#NORMAL by default.
     * 
     * 获取优先级，默认正常
     */
    public Priority getPriority() 
        return Priority.NORMAL;
    

Request的优先级，当你有急切请求需求的时候，就可以设置它。

/**
     * Mark this request as having a response delivered on it.  This can be used
     * later in the request's lifetime for suppressing identical responses.
     * 
     * 标记分发事件完毕
     */
    public void markDelivered() 
        mResponseDelivered = true;
    

它同样是以个标记。

@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.
        // 如果优先级相等，按序列号排序，序列号高则放在后面
        return left == right ?
                this.mSequence - other.mSequence :
                right.ordinal() - left.ordinal();
    

这里是重点，request的比较，优先级优先，如果优先级相同，再比较序列号，序列号由RequestQueue中AtomicInteger类，我们调用add方法时生成并绑定在request中，AtomicInteger的incrementAndGet方法可以生成单调递增的序列号。

/**
     * Gets a sequence number.
     * 
     * 生成单调递增的序列号
     */
    public int getSequenceNumber() 
        return mSequenceGenerator.incrementAndGet();
    

Request有两个抽象方法，

/**
     * Subclasses must implement this to parse the raw network response
     * and return an appropriate response type. This method will be
     * called from a worker thread.  The response will not be delivered
     * if you return null.
     * @param response Response from the network
     * @return The parsed response, or null in the case of an error
     * 
     * 解析response
     */
    abstract protected Response<T> parseNetworkResponse(NetworkResponse response);

/**
     * Subclasses must implement this to perform delivery of the parsed
     * response to their listeners.  The given response is guaranteed to
     * be non-null; responses that fail to parse are not delivered.
     * @param response The parsed response returned by
     * @link #parseNetworkResponse(NetworkResponse)
     * 
     * 分发Response
     */
    abstract protected void deliverResponse(T response);

在说之前，我找个具体子类先看看，依然还是StringRequest：

@Override
    protected Response<String> parseNetworkResponse(NetworkResponse response) 
        String parsed;
        try 
            parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
         catch (UnsupportedEncodingException e) 
            parsed = new String(response.data);
        
        return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));
    

@Override
    protected void deliverResponse(String response) 
        mListener.onResponse(response);
    

一目了然，deliverResponse就是负责成功回调，因为在request只定义了公共的错误回调，成功回调交由子类自己去实现，而parseNetworkResponse则负责解析NetworkResponse，每个不同的子类解析出不同的返回类型，如这里的String，值得一提的是，这个方法之所以会定义在Request中是因为Request的泛型，只有Request知道它请求想让Response返回的是什么。