Android 中的 Handler 同步分割栏

深藏阁楼爱情的钟 2022-05-24 05:39 153阅读 0赞

#### Handler异步实现控制Message的处理的，异步线程可以在Handler中设置异步卡子，设置好了以后，当前Handler的同步Message都不再执行，直到异步线程将卡子去掉。 ####

### 一、postSyncBarrier 设置同步卡 ###

*  方法是隐藏的需要反射获取
 *  API版本<23方法位于Looper中，API版本>23位于  
    MessageQueue中
 *  作用是为Handler设置卡子，过滤掉同步消息，同步消息等待执行

### 二、removeSyncBarrier 移除同步卡 ###

*  方法是隐藏的需要反射获取
 *  API版本<23方法位于Looper中，API版本>23位于  
    MessageQueue中
 *  作用是为Handler移除卡子，放开同步消息过滤，同步消息继续执行。

package qin.xue.imageviewscaletype.activity;
    
    import android.app.Activity;
    import android.os.Bundle;
    import android.os.Handler;
    import android.os.HandlerThread;
    import android.os.Looper;
    import android.os.MessageQueue;
    import android.util.Log;
    
    import java.lang.reflect.Method;
    
    import qin.xue.imageviewscaletype.R;
    
    /** * Created by qinxue on 2018/5/21. */
    
    public class SyncBarrierActivity extends Activity { 
        private static final String TAG = "SyncBarrierActivity";
    
        private Handler handler;
        private Handler workHanlder;
        private HandlerThread workHandlerThread;
        private Method methodPostSyncBarrier;
        private Looper looper;
        private int token;
        private int count = 0;
        private long start = 0;
        private long end = 0;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.sync_barrier_activity_layout);
            workHandlerThread = new HandlerThread("work_thread");
            workHandlerThread.start();
            looper = workHandlerThread.getLooper();
            workHanlder = new Handler(looper);
            handler = new Handler();
            workHanlder.post(runnable);
    
            handler.post(new Runnable() {
                @Override
                public void run() {
                    start = System.currentTimeMillis();
                    Log.i(TAG, "设置同步卡 时间：" + start);
                    postSyncBarrier();        //设置同步卡
                }
            });
    
            handler.postDelayed(new Runnable() {
                @Override
                public void run() {
                    end = System.currentTimeMillis();
                    Log.i(TAG, "移除同步卡 时间：" + end + "耗时：" + (end - start));
                    removeSyncBarrier();    //5秒后移除同步卡
                }
            }, 5000);
        }
    
        //postSyncBarrier 和 removeSyncBarrier 方法是否在MessageQueue类中（api23及以上）
        private static boolean methodInQueue() {
            return android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.M;
        }
    
    
        private Runnable runnable = new Runnable() {
            @Override
            public void run() {
                count++;
                Log.i(TAG, "count: " + count + "time: " + System.currentTimeMillis()); //同步打印数据
                workHanlder.postDelayed(runnable, 100);
            }
        };
    
        /** * 移除卡子 */
        private void removeSyncBarrier() {
            if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.M) {
                removeSyncBarrier(looper.getQueue());
            } else {
                removeSyncBarrier(looper);
            }
        }
    
        private void removeSyncBarrier(Object obj) {
            try {
                Method method = MessageQueue.class.getMethod("removeSyncBarrier", int.class);
                method.invoke(obj, token);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    
        /** * 添加卡子 */
        private void postSyncBarrier() {
            if (methodInQueue()) {
                postSyncBarrier(looper.getQueue());
            } else {
                postSyncBarrier(looper);
            }
        }
    
        private void postSyncBarrier(Object obj) {
            try {
                methodPostSyncBarrier = obj.getClass().getMethod("postSyncBarrier");
                token = (int) methodPostSyncBarrier.invoke(obj);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    
    }

### 三、结果展示 ###

05-21 17:44:22.926 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 1time: 1526895862926
    05-21 17:44:22.947 12715-12715/qin.xue.imageviewscaletype I/SyncBarrierActivity: 设置同步卡 时间：1526895862947
    05-21 17:44:27.931 12715-12715/qin.xue.imageviewscaletype I/SyncBarrierActivity: 移除同步卡 时间：1526895867930耗时：4983
    05-21 17:44:27.932 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 2time: 1526895867932
    05-21 17:44:28.033 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 3time: 1526895868033
    05-21 17:44:28.134 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 4time: 1526895868134
    05-21 17:44:28.236 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 5time: 1526895868235
    05-21 17:44:28.336 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 6time: 1526895868336
    05-21 17:44:28.437 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 7time: 1526895868437
    05-21 17:44:28.538 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 8time: 1526895868538
    05-21 17:44:28.639 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 9time: 1526895868639
    05-21 17:44:28.739 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 10time: 1526895868739
    05-21 17:44:28.841 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 11time: 1526895868841
    05-21 17:44:28.941 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 12time: 1526895868941
    05-21 17:44:29.043 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 13time: 1526895869043
    05-21 17:44:29.144 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 14time: 1526895869144
    05-21 17:44:29.244 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 15time: 1526895869244
    05-21 17:44:29.345 12715-12732/qin.xue.imageviewscaletype I/SyncBarrierActivity: count: 16time: 1526895869345

### 四、代码解释 ###

*  代码中首先加了使用主线程设置异步线程workHandler中的异步卡子，导致workhandler中的Message不在执行，仅保留待执行。
 *  然后在5s之后在主线程移除了异步线程workHandler中的卡子，workhandler中的Message继续执行下去。

### 五、源码分析 ###

#### 我这里是6.0的源码 ####

##### postSyncBarrier #####

public int postSyncBarrier() {
            return postSyncBarrier(SystemClock.uptimeMillis());
        }
    
        private int postSyncBarrier(long when) {
            // Enqueue a new sync barrier token.
            // We don't need to wake the queue because the purpose of a barrier is to stall it.
            synchronized (this) {
                final int token = mNextBarrierToken++;
                final Message msg = Message.obtain();
                msg.markInUse();
                msg.when = when;
                msg.arg1 = token;
    
                Message prev = null;
                Message p = mMessages;
                if (when != 0) {
                    while (p != null && p.when <= when) {
                        prev = p;
                        p = p.next;
                    }
                }
                if (prev != null) { // invariant: p == prev.next
                    msg.next = p;
                    prev.next = msg;
                } else {
                    msg.next = p;
                    mMessages = msg;
                }
                return token;
            }
        }

\-这里可以看到这个卡子msg，新建了一个msg并根据时间插入到消息队列中，而且这个msg没有设置target

##### removeSyncBarrier #####

public void removeSyncBarrier(int token) {
            // Remove a sync barrier token from the queue.
            // If the queue is no longer stalled by a barrier then wake it.
            synchronized (this) {
                Message prev = null;
                Message p = mMessages;
                while (p != null && (p.target != null || p.arg1 != token)) {
                    prev = p;
                    p = p.next;
                }
                if (p == null) {
                    throw new IllegalStateException("The specified message queue synchronization "
                            + " barrier token has not been posted or has already been removed.");
                }
                final boolean needWake;
                if (prev != null) {
                    prev.next = p.next;
                    needWake = false;
                } else {
                    mMessages = p.next;
                    needWake = mMessages == null || mMessages.target != null;
                }
                p.recycleUnchecked();
    
                // If the loop is quitting then it is already awake.
                // We can assume mPtr != 0 when mQuitting is false.
                if (needWake && !mQuitting) {
                    nativeWake(mPtr);
                }
            }
        }

\-（p.target != null || p.arg1 != token）这里如果arg1 == token或者target为null，那么这是一个卡子。需要移除掉 ，代码如下。 prev.next = p.next;

##### next() #####

*  在MessageQueue中还有这个方法，获取消息队列中的下一个Message

if (msg != null && msg.target == null) {
                        // Stalled by a barrier.  Find the next asynchronous message in the queue.
                        do {
                            prevMsg = msg;
                            msg = msg.next;
                        } while (msg != null && !msg.isAsynchronous());
                    }

摘取部分代码，循环便利整个消息队列，如果target为null，那么直接利用isAsynchronous()找到下一个异步的message，跳过同步消息，设置执行时间。  
大致就是这样。

### Handler中还有阻塞机制 ###

\-如果发先下一条消息执行时间过长，会阻塞掉线程Looper循环的。在C++层，下次再研究下。

[转载自原文][Link 1]

[Link 1]: https://my.oschina.net/youranhongcha/blog/492591