QueueInterrogator.java

package com.google.android.apps.common.testing.ui.espresso.base;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static com.google.common.base.Throwables.propagate;

import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.os.MessageQueue;
import android.os.SystemClock;
import android.util.Log;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;

/**
 * Isolates the nasty details of touching the message queue.
 */
final class QueueInterrogator {

  enum QueueState { EMPTY, TASK_DUE_SOON, TASK_DUE_LONG, BARRIER };
  
  private static final String TAG = "QueueInterrogator";

  private static final Method messageQueueNextMethod;
  private static final Field messageQueueHeadField;
  private static final int LOOKAHEAD_MILLIS = 15;

  private final Looper interrogatedLooper;
  private volatile MessageQueue interrogatedQueue;

  static {
    Method nextMethod = null;
    Field headField = null;
    try {
      nextMethod = MessageQueue.class.getDeclaredMethod("next");
      nextMethod.setAccessible(true);

      headField = MessageQueue.class.getDeclaredField("mMessages");
      headField.setAccessible(true);
    } catch (IllegalArgumentException e) {
      nextMethod = null;
      headField = null;
      Log.e(TAG, "Could not initialize interrogator!", e);
    } catch (NoSuchFieldException e) {
      nextMethod = null;
      headField = null;
      Log.e(TAG, "Could not initialize interrogator!", e);
    } catch (NoSuchMethodException e) {
      nextMethod = null;
      headField = null;
      Log.e(TAG, "Could not initialize interrogator!", e);
    } catch (SecurityException e) {
      nextMethod = null;
      headField = null;
      Log.e(TAG, "Could not initialize interrogator!", e);
    } finally {
      messageQueueNextMethod = nextMethod;
      messageQueueHeadField = headField;
    }
  }

  QueueInterrogator(Looper interrogatedLooper) {
    this.interrogatedLooper = checkNotNull(interrogatedLooper);
    checkNotNull(messageQueueHeadField);
    checkNotNull(messageQueueNextMethod);
  }

  // Only for use by espresso - keep package private.
  Message getNextMessage() {
    checkThread();

    if (null == interrogatedQueue) {
      initializeQueue();
    }

    try {
      return (Message) messageQueueNextMethod.invoke(Looper.myQueue());
    } catch (IllegalAccessException e) {
      throw propagate(e);
    } catch (IllegalArgumentException e) {
      throw propagate(e);
    } catch (InvocationTargetException e) {
      throw propagate(e);
    } catch (SecurityException e) {
      throw propagate(e);
    }
  }

  QueueState determineQueueState() {
    // may be called from any thread.

    if (null == interrogatedQueue) {
      initializeQueue();
    }
    synchronized (interrogatedQueue) {
      try {
        Message head = (Message) messageQueueHeadField.get(interrogatedQueue);
        if (null == head) {
          // no messages pending - AT ALL!
          return QueueState.EMPTY;
        }
        if (null == head.getTarget()) {
          // null target is a sync barrier token.
          return QueueState.BARRIER;
        } else {
          long headWhen = head.getWhen();
          long nowFuz = SystemClock.uptimeMillis() + LOOKAHEAD_MILLIS;

          if (nowFuz > headWhen) {
            return QueueState.TASK_DUE_SOON;
          } else {
            return QueueState.TASK_DUE_LONG;
          }
        }
      } catch (IllegalAccessException e) {
        throw propagate(e);
      }
    }
  }

  private void initializeQueue() {
    if (interrogatedLooper == Looper.myLooper()) {
      interrogatedQueue = Looper.myQueue();
    } else {
      Handler oneShotHandler = new Handler(interrogatedLooper);
      FutureTask<MessageQueue> queueCapture = new FutureTask<MessageQueue>(
          new Callable<MessageQueue>() {
            @Override
            public MessageQueue call() {
              return Looper.myQueue();
            }
          });
      oneShotHandler.postAtFrontOfQueue(queueCapture);
      try {
        interrogatedQueue = queueCapture.get();
      } catch (ExecutionException ee) {
        throw propagate(ee.getCause());
      } catch (InterruptedException ie) {
        throw propagate(ie);
      }
    }
  }

  private void checkThread() {
    checkState(interrogatedLooper == Looper.myLooper(), "Calling from non-owning thread!");
  }
}