android的MessageQueue.IdleHandler

MessageQueue内部有个IdleHandler接口，具体定义如下：

/**
* Callback interface for discovering when a thread is going to block
* waiting for more messages.
*/
public static interface IdleHandler {
/**
* Called when the message queue has run out of messages and will now
* wait for more.  Return true to keep your idle handler active, false
* to have it removed.  This may be called if there are still messages
* pending in the queue, but they are all scheduled to be dispatched
* after the current time.
*/
boolean queueIdle();
}

简而言之，就是在looper里面的message暂时处理完了，这个时候会回调这个接口，返回false，那么就会移除它，返回true就会在下次message处理完了的时候继续回调。

下面先通过个例子来直观看下IdleHandler的执行时机。

public class IdleHandleActivity extends Activity {

public static final String TAG = "IdleHandleActivity";

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.d(TAG, "onCreate");
setContentView(R.layout.activity_idle_handler);

Looper.myQueue().addIdleHandler(new MessageQueue.IdleHandler() {
@Override
public boolean queueIdle() {
Log.d(TAG, "IdleHandler post");
return false;
}
});

new Handler().post(new Runnable() {
@Override
public void run() {
Log.d(TAG, "Handler post1");
}
});
new Handler().post(new Runnable() {
@Override
public void run() {
Log.d(TAG, "Handler post2");
}
});
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
Log.d(TAG, "Handler delay post");
}
}, 500);
}

@Override
protected void onStart() {
super.onStart();
Log.d(TAG, "onStart");
}

@Override
protected void onResume() {
super.onResume();
Log.d(TAG, "onResume");
}
}

public class MyView extends TextView {

......

@Override
protected void onFinishInflate() {
super.onFinishInflate();
Log.d(IdleHandleActivity.TAG, getText() + " onFinishInflate");
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
Log.d(IdleHandleActivity.TAG, getText() + " onMeasure");
}

@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
Log.d(IdleHandleActivity.TAG, getText() + " onLayout");
}

@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
Log.d(IdleHandleActivity.TAG, getText() + " onDraw");
}
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context="com.outman.example.androidtest.MainActivity">

<com.outman.example.androidtest.MyView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="MyView!"/>

</LinearLayout>

执行结果：

04-08 17:30:33.994 16452-16452/ D/IdleHandleActivity: onCreate
04-08 17:30:34.038 16452-16452/ D/IdleHandleActivity: MyView! onFinishInflate
04-08 17:30:34.068 16452-16452/ D/IdleHandleActivity: onStart
04-08 17:30:34.073 16452-16452/ D/IdleHandleActivity: onResume
04-08 17:30:34.089 16452-16452/ D/IdleHandleActivity: Handler post1
04-08 17:30:34.089 16452-16452/ D/IdleHandleActivity: Handler post2
04-08 17:30:34.097 16452-16452/ D/IdleHandleActivity: MyView! onMeasure
04-08 17:30:34.106 16452-16452/ D/IdleHandleActivity: MyView! onMeasure
04-08 17:30:34.107 16452-16452/ D/IdleHandleActivity: MyView! onLayout
04-08 17:30:34.125 16452-16452/ D/IdleHandleActivity: MyView! onDraw
04-08 17:30:34.136 16452-16452/ D/IdleHandleActivity: IdleHandler post
04-08 17:30:34.569 16452-16452/ D/IdleHandleActivity: Handler delay post

通过执行结果可以看到

1. IdleHandler的回调在，所有Handler执行完MessageQueue中的内容，再执行

2. IdleHandler的回调在，Activity执行完onResume方法，再执行

3. IdleHandler的回调在，初始化完View后，即执行完onDraw后，再执行

在Glide和LeakCanary中都有IdleHandler的使用

Glide中

// Responsible for cleaning up the active resource map by remove weak references that have been cleared.
private static class RefQueueIdleHandler implements MessageQueue.IdleHandler {
private final Map<Key, WeakReference<EngineResource<?>>> activeResources;
private final ReferenceQueue<EngineResource<?>> queue;

public RefQueueIdleHandler(Map<Key, WeakReference<EngineResource<?>>> activeResources,
ReferenceQueue<EngineResource<?>> queue) {
this.activeResources = activeResources;
this.queue = queue;
}

@Override
public boolean queueIdle() {
ResourceWeakReference ref = (ResourceWeakReference) queue.poll();
if (ref != null) {
activeResources.remove(ref.key);
}

return true;
}
}

LeakCanary中

private void showToast(final FutureResult<Toast> waitingForToast) {
mainHandler.post(new Runnable() {
@Override public void run() {
final Toast toast = new Toast(context);
toast.setGravity(Gravity.CENTER_VERTICAL, 0, 0);
toast.setDuration(Toast.LENGTH_LONG);
LayoutInflater inflater = LayoutInflater.from(context);
toast.setView(inflater.inflate(R.layout.leak_canary_heap_dump_toast, null));
toast.show();
// Waiting for Idle to make sure Toast gets rendered.
Looper.myQueue().addIdleHandler(new MessageQueue.IdleHandler() {
@Override public boolean queueIdle() {
waitingForToast.set(toast);
return false;
}
});
}
});
}

参考 https://mp.weixin.qq.com/s/KpeBqIEYeOzt_frANoGuSg