上一篇的同步流程分析（http://www.reibang.com/p/343118d0bc19），這次來研究下異步的流程。
在這之前，先補充下纬向，Dispatcher(調(diào)度器)检诗，是保存同步和異步Call的地方胰柑，負責執(zhí)行異步AsyncCall（就是一個子線程）缭黔。

先來看看這段實現(xiàn)異步請求的最簡潔代碼：

       //異步
        OkHttpClient asynClient = new OkHttpClient();
        Request asynRequest = new Request.Builder().url("http://......").build();
        asynClient.newCall(asynRequest).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {

            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {

            }
        });

這段代碼，前奏準備都跟同步的一樣践险，我們來看看關(guān)鍵代碼asynClient.newCall(asynRequest).enqueue（callback）；
還記得嗎，上一篇里面分析過newCall捏境，會創(chuàng)建一個call接口的實現(xiàn)類RealCall對象于游，enqueue()字面上是加入隊列的意思，還是再看看call的定義吧：

public interface Call extends Cloneable {
    ......
     Response execute() throws IOException;
     void enqueue(Callback responseCallback);
    ......
}

不熟悉realCall的可以先去看看okhttp同步流程源碼分析（http://www.reibang.com/p/343118d0bc19）垫言，既然realCall是call的實現(xiàn)類贰剥，很好，我們?nèi)ealCall看看enqueue的實現(xiàn)：

  @Override public void enqueue(Callback responseCallback) {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
  }

之前筷频，同步流程中蚌成，realCall直接調(diào)用execute()，在里面通過各種攔截器之后獲取到最終數(shù)據(jù)凛捏，然而担忧，在異步流程中，看上面最后一行代碼坯癣，入隊列的是一個AsyncCall瓶盛，成功，失敗回調(diào)的接口對象作為AsyncCall的參數(shù)示罗，在 研究client.dispatcher().enqueue()：之前惩猫，我們先看看AsyncCall到底是啥：

 //RealCall.java
 ......
 final class AsyncCall extends NamedRunnable {
   .....
       AsyncCall(Callback responseCallback) {
     super("OkHttp %s", redactedUrl());
     this.responseCallback = responseCallback;
   }
   ......
@Override protected void execute() {
     boolean signalledCallback = false;
     try {
       Response response = getResponseWithInterceptorChain();
       if (retryAndFollowUpInterceptor.isCanceled()) {
         signalledCallback = true;
         responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
       } else {
         signalledCallback = true;
         responseCallback.onResponse(RealCall.this, response);
       }
     } catch (IOException e) {
       if (signalledCallback) {
         // Do not signal the callback twice!
         Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
       } else {
         eventListener.callFailed(RealCall.this, e);
         responseCallback.onFailure(RealCall.this, e);
       }
     } finally {
       client.dispatcher().finished(this);
     }
   }
}

AsyncCall是RealCall的內(nèi)部類，并且AsyncCall繼承了NamedRunnable類蚜点，這個類里面有Runable字眼轧房，execute()也是重寫父類的方法，里面的實現(xiàn)跟同步里面獲取數(shù)據(jù)是一致的绍绘，都是通過重重攔截器奶镶，最終獲取到數(shù)據(jù)，只是處理起來陪拘，異步里面通過接口來回調(diào)而已厂镇。咱再看看NamedRunnable這個類：

/**
 * Runnable implementation which always sets its thread name.
 */
public abstract class NamedRunnable implements Runnable {
  protected final String name;

  public NamedRunnable(String format, Object... args) {
    this.name = Util.format(format, args);
  }

  @Override public final void run() {
    String oldName = Thread.currentThread().getName();
    Thread.currentThread().setName(name);
    try {
      execute();
    } finally {
      Thread.currentThread().setName(oldName);
    }
  }

  protected abstract void execute();
}

NamedRunnable竟然是個抽象類，execute()抽象方法在AsyncCall實現(xiàn)左刽，在run()里面調(diào)用剪撬。我們知道，多線程的實現(xiàn)方式：實現(xiàn)Runnable接口悠反、繼承Thread類残黑，從這里，我們知道AsyncCall其實就是一個Runnable的子類斋否，可以理解為AsyncCall就是一個子線程梨水。我們再回頭來看client.dispatcher().enqueue(AsyncCall(...)):

  //Dispatcher.java
  public final class Dispatcher {

  private int maxRequests = 64;
  private int maxRequestsPerHost = 5;
  ......
  /** Ready async calls in the order they'll be run. */
  private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
  /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
  ......
  synchronized void enqueue(AsyncCall call) {
    if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
      runningAsyncCalls.add(call);
      executorService().execute(call);
    } else {
      readyAsyncCalls.add(call);
    }
  }
  ......
}

上面已經(jīng)把關(guān)鍵代碼貼出，有兩個隊列茵臭，一個是正在運行的子線程隊列runningAsyncCalls疫诽，一個是待執(zhí)行的請求隊列readyAsyncCalls，
enqueue里面，這個判斷： 正在運行的子線程 < 64 &&單個host最大同時執(zhí)行的子線程 < 5,就把call添加到runningAsyncCalls隊列中奇徒，并且通過線程池來執(zhí)行這個call雏亚，否則就添加到readyAsyncCalls隊列。
可以摩钙，這很異步罢低，線程池都出來了，看看這個線程池：

   //Dispatcher.java
    if (executorService == null) {
      executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
          new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));
    }
    return executorService;
  }

上面我們知道正在執(zhí)行的AsyncCall會添加到runningAsyncCalls隊列胖笛，那子線程任務(wù)執(zhí)行完了后總要移除吧网持，我們回到上面的AsyncCall內(nèi)部的execute()方法，看最后面那幾行长踊，finally里面的client.dispatcher().finished(this)功舀，this就是當前的AsyncCall對象：

Dispatcher.java
  /** Used by {@code Call#execute} to signal completion. */
  void finished(RealCall call) {
    finished(runningSyncCalls, call, false);
  }
private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
    int runningCallsCount;
    Runnable idleCallback;
    synchronized (this) {
      if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
      if (promoteCalls) promoteCalls();
      runningCallsCount = runningCallsCount();
      idleCallback = this.idleCallback;
    }

    if (runningCallsCount == 0 && idleCallback != null) {
      idleCallback.run();
    }
  }

看到了吧，calls.remove(call)身弊，這里面移除辟汰，這個判斷，如果是同步的話阱佛，就直接拋出異常帖汞，同步流程中，并沒有加入隊列的操作瘫絮，只有異步的時候才會執(zhí)行后續(xù)的操作涨冀。

看來要好好分析下攔截器了填硕，主要的獲取數(shù)據(jù)的操作全特么在這里面麦萤。