[android小知识]removeCallbacks与postDelayed的恩怨史（removeCallbacks所谓的无效, View和Handler 的postDelayed）...

最新推荐文章于 2024-10-31 14:07:55 发布

最新推荐文章于 2024-10-31 14:07:55 发布 · 1.4k 阅读

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#移动开发 #java

本文详细介绍了如何使用Android中的postDelayed方法实现任务的延迟执行，并解释了如何通过removeCallbacks方法取消延迟任务。同时，文章提供了实际代码示例，展示了在不同情况下如何正确地使用这些方法。

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常见的.postDelayed(runnable, time)方法就是常见的延迟处理的方法之一，postDelayed(mRun,time);当想要需要取消的时候，调用removeCallbacks(mRun);即可，就可以取消postDelayed的方法里面的runnable,当然如果里面runnable已经执行了，那就不可能撤回或者中止了。

http://blog.youkuaiyun.com/bluewindtalker/article/details/51984300

下面是布局文件

<span style="font-size:18px;"> <Button
                android:id="@+id/btn_demo_removecallback_start"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="开始postdelayed" />

            <Button
                android:id="@+id/btn_demo_removecallback_stop"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="测试removecallback" />

            <TextView
                android:id="@+id/tv_demo_removecallback_test_onAnimationEnd"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="测试removecallback runnable效果" /></span>

以下是测试代码。

<span style="font-size:18px;">final TextView mRemovecallbackTV = (TextView)findViewById(R.id.tv_demo_removecallback_test_onAnimationEnd);

         mRun = new Runnable() {
            @Override
            public void run() {
                if(DirectionMarqueeDemoActivity.this.isFinishing()){
                    BlueLog.e(TAG,"DirectionMarqueeDemoActivity  isFinishing ");
                    mMarqueeAnimationTV.clearAnimation();
                    mRemovecallbackTV.removeCallbacks(mRun);
                }
                BlueLog.e(TAG, "Runnable   " + DirectionMarqueeDemoActivity.this);
                mRemovecallbackTV.postDelayed(this, 3000);
            }
        };

        findViewById(R.id.btn_demo_removecallback_start).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mRemovecallbackTV.postDelayed(mRun, 3000);
                BlueLog.e(TAG, " postDelayed " );
            }
        });
        findViewById(R.id.btn_demo_removecallback_stop).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mRemovecallbackTV.removeCallbacks(mRun);
                BlueLog.e(TAG, "removeCallbacks ");
            }
        });</span>

其中mRun为Activity的成员变量。

运行的时候我们发现

即使activity 结束了调用了onDestory方法后这里仍然运行，问题在于代码中，先removeCallBacks,再postDelayed,这里加一个return就好，

正确的代码大家就在removeCallBacks加入return即可，下面是logcat的打印

所以如要要循环调用postDelayed的时候，一样要进行判断是否所属的activity已经退出了。将结束方法一定要体现出来

而postDelayed(runnable, time)有view.postDelayed(mRun, time);和new Handler().postDelayed(mRun,time);这两种，

对于view.postDelayed我们通过源码发现

 public boolean postDelayed(Runnable action, long delayMillis) {
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            return attachInfo.mHandler.postDelayed(action, delayMillis);
        }
        // Assume that post will succeed later
        ViewRootImpl.getRunQueue().postDelayed(action, delayMillis);
        return true;
    }

return attachInfo.mHandler.postDelayed(action, delayMillis);最后进入了Handler的postDelayed

public final boolean postDelayed(Runnable r, long delayMillis)
    {
        return sendMessageDelayed(getPostMessage(r), delayMillis);
    }

而ViewRootImpl.getRunQueue().postDelayed(action, delayMillis);进入ViewRootImpl.java中，

static final class RunQueue {
        private final ArrayList<HandlerAction> mActions = new ArrayList<HandlerAction>();

        void post(Runnable action) {
            postDelayed(action, 0);
        }

        void postDelayed(Runnable action, long delayMillis) {
            HandlerAction handlerAction = new HandlerAction();
            handlerAction.action = action;
            handlerAction.delay = delayMillis;

            synchronized (mActions) {
                mActions.add(handlerAction);
            }
        }

        void removeCallbacks(Runnable action) {
            final HandlerAction handlerAction = new HandlerAction();
            handlerAction.action = action;

            synchronized (mActions) {
                final ArrayList<HandlerAction> actions = mActions;

                while (actions.remove(handlerAction)) {
                    // Keep going
                }
            }
        }

        void executeActions(Handler handler) {
            synchronized (mActions) {
                final ArrayList<HandlerAction> actions = mActions;
                final int count = actions.size();

                for (int i = 0; i < count; i++) {
                    final HandlerAction handlerAction = actions.get(i);
                    handler.postDelayed(handlerAction.action, handlerAction.delay);
                }

                actions.clear();
            }
        }

最后仍然执行了handler.postDelayed(handlerAction.action, handlerAction.delay)，所以两个内容是相差无几的。

其中HandlerAction来自于view调用invalidate时，父View传入的。