细节点 — 2

接着上面的细节点，我们在调用了watch方法后，是不是马上就会执行泄漏的检查？？

答案是否定的。

源码如下：

private void ensureGoneAsync(final long watchStartNanoTime, final KeyedWeakReference reference) {
    watchExecutor.execute(new Retryable() {
      @Override public Retryable.Result run() {
        return ensureGone(reference, watchStartNanoTime);
      }
    });
  }

watch里面会调用这个方法，而这个方法又转给了watchExecutor。

watchExecutor的实现类是AndroidWatchExecutor：

public final class AndroidWatchExecutor implements WatchExecutor {

  static final String LEAK_CANARY_THREAD_NAME = "LeakCanary-Heap-Dump";
  private final Handler mainHandler;
  private final Handler backgroundHandler;
  private final long initialDelayMillis;
  private final long maxBackoffFactor;

  public AndroidWatchExecutor(long initialDelayMillis) {
    mainHandler = new Handler(Looper.getMainLooper());
    HandlerThread handlerThread = new HandlerThread(LEAK_CANARY_THREAD_NAME);
    handlerThread.start();
    backgroundHandler = new Handler(handlerThread.getLooper());
    this.initialDelayMillis = initialDelayMillis;
    maxBackoffFactor = Long.MAX_VALUE / initialDelayMillis;
  }

  @Override public void execute(Retryable retryable) {
    if (Looper.getMainLooper().getThread() == Thread.currentThread()) {
      waitForIdle(retryable, 0);
    } else {
      postWaitForIdle(retryable, 0);
    }
  }

  void postWaitForIdle(final Retryable retryable, final int failedAttempts) {
    mainHandler.post(new Runnable() {
      @Override public void run() {
        waitForIdle(retryable, failedAttempts);
      }
    });
  }

  void waitForIdle(final Retryable retryable, final int failedAttempts) {
    // This needs to be called from the main thread.
    Looper.myQueue().addIdleHandler(new MessageQueue.IdleHandler() {
      @Override public boolean queueIdle() {
        postToBackgroundWithDelay(retryable, failedAttempts);
        return false;
      }
    });
  }

  void postToBackgroundWithDelay(final Retryable retryable, final int failedAttempts) {
    long exponentialBackoffFactor = (long) Math.min(Math.pow(2, failedAttempts), maxBackoffFactor);
    long delayMillis = initialDelayMillis * exponentialBackoffFactor;
    backgroundHandler.postDelayed(new Runnable() {
      @Override public void run() {
        Retryable.Result result = retryable.run();
        if (result == RETRY) {
          postWaitForIdle(retryable, failedAttempts + 1);
        }
      }
    }, delayMillis);
  }
}

IdleHandler

IdleHandler是在MessageQueue中没有新的message执行，Looper就进入了idle状态，这时就会执行IdleHandler回调。

换句话说，IdleHandler就是 优先级别较低的 Message，只有当 Looper 没有消息要处理时才得到处理。而且，内部的 queueIdle() 方法若返回 true，表示该任务一直存活，每次 Looper 进入 Idle 时就执行；反之，如果返回 false，则表示只会执行一次，执行完后丢弃。

因此，watch的调用不是马上执行，它会等到主线程空闲了，没其他事情可做的时候，向后台发送一个延时消息，进行对象是否回收的检查。