Android Handler 从使用到进阶

1.Handler

  • 只要是开发Android的同学,Handler这个经常都会看到,也会使用到,本文章就做个笔记。
  • Android规定了只能在主线程更新UI,那子线程想更新UI,在操作完成后,就可用通过Handler发消息,然后在主线程更新UI了。其实可以理解为生产者-消费者模式,发送消息,取出消息并处理。
  • Android系统源码中,Android的消息机制中,大量使用Handler,所以了解Handler非常的有必要。
  • 下图是一个消息发送的简易流程,一各个步骤分析。


    Handler

2.Handler简单使用

2.1 发送消息

  • 最基本的使用就是各种sendMessage,带不带参数,是否延迟等。
  • 发送消息方法非常多,根据自己需求选择,所有发送消息最后都是
  • enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,long uptimeMillis)
发送消息

2.2 使用

  • 这样直接使用会有内存泄漏风险,后面说。
  • Handler创建有两种,一个是在构造方法传CallBack,一个是重写类的 handleMessage() 方法。
    private static final int MESSAGA_TEST_1 = 1;

    /**
     * 主线程有初始化好Looper,所以在主线程处理消息
     */
    private Handler mHandler = new Handler(new Handler.Callback() {
        @Override
        public boolean handleMessage(@NonNull Message msg) {
            switch (msg.what) {
                case MESSAGA_TEST_1:
                    if (tvTest != null) {
                        //传递消息obj
                        tvTest.setText((String) msg.obj);
                    }
                    break;
            }
            return false;
        }
    });

 
    private Handler mHandler2 = new Handler() {
        @Override
        public void handleMessage(@NonNull Message msg) {
            switch (msg.what) {
                case MESSAGA_TEST_1:
                    if (tvTest != null) {
                        //传递消息obj
                        tvTest.setText((String) msg.obj);
                    }
                    break;
            }
        }
    };

    private void onClick() {
        //当点击事件执行,就会在子线程发送消息更新textview
        new Thread(new Runnable() {
            @Override
            public void run() {
                clickEndMessage();
            }
        }).start();
    }
  

    /**
     * 可以在子线程发送
     */
    private void clickEndMessage() {
        //obtain享元模式
        Message message = Message.obtain(mHandler);
        //what相当于标记
        message.what = MESSAGA_TEST_1;
        //obj传数据
        message.obj = new String("baozi");
        //普通发送message
        mHandler.sendMessage(message);
        .
        .
        .
        //发送标记,不带其他内容,内部会封装成只有标记的Message对象
        mHandler.sendEmptyMessage();
        //尾部带有Delayed,发送延迟消息,单位毫秒
        mHandler.sendMessageDelayed(message, 1000);
        //尾部带有AtTime,发送消息的时间跟Delayed差别就是Delayed是执行的当前时间+传进去的时间,AtTime就传进去的绝对时间
        mHandler.sendMessageAtTime();
        //在队列头插入消息
        mHandler.sendMessageAtFrontOfQueue(message);
    }


    @Override
    protected void onDestroy() {
        if(mHandler!=null){
            //关闭activity时,移除消息
            mHandler.removeMessages(MESSAGA_TEST_1);
            mHandler = null;
        }
        super.onDestroy();
    }

2.3 view.post()

  • 比如view,post()、postDelayed() 方法,可以延迟五秒后更新UI,实际就是使用Handler。
tvTest.postDelayed(new Runnable() {
    @Override
    public void run() {
        tvTest.setText("5s");
    }
},5*1000);

/**
 * View 源码
 */
public boolean postDelayed(Runnable action, long delayMillis) {
    final AttachInfo attachInfo = mAttachInfo;
    if (attachInfo != null) {
        return attachInfo.mHandler.postDelayed(action, delayMillis);
    }

    // Postpone the runnable until we know on which thread it needs to run.
    // Assume that the runnable will be successfully placed after attach.
    getRunQueue().postDelayed(action, delayMillis);
    return true;
}

2.4 runOnUiThread

  • 经常用的 runOnUiThread() 方法也是用Handler。
runOnUiThread(new Runnable() {
    @Override
    public void run() {
        //更新UI
    }
});  

/**
 * Activity 源码
 */
public final void runOnUiThread(Runnable action) {
    if (Thread.currentThread() != mUiThread) {
        mHandler.post(action);
    } else {
        action.run();
    }
}

3.子线程中使用

3.1 子线程直接创建Handler错误

  • 子线程不能直接创建Handler,会报异常,因为Looper还没创建,而主线程默认就初始化好Looper
  • 应该先Looper.
    private Handler handler2;

    /**
     * 子线程
     */
    private void thread() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                handler2 = new Handler(new Handler.Callback() {
                    @Override
                    public boolean handleMessage(@NonNull Message msg) {
                        return false;
                    }
                });
            }
        }).start();
    }
  • 提示的错误。


    子线程创建Handler

3.2 主线程默认初始化Looper

  • ActivityThread 类就能主线程找到Looper初始化,Looper.prepareMainLooper();
    public static void main(String[] args) {
        .
        .
        Looper.prepareMainLooper();
        .
        .
          
        ActivityThread thread = new ActivityThread();
        .
        .
    }

3.3 Handler构造方法查看

  • 构造方法中可以看到 mLooper = Looper.myLooper(); 获取的 mLooper 为null,就报上面的那个异常了。
    public Handler(@Nullable Callback callback, boolean async) {
        .
        .
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread " + Thread.currentThread()
                        + " that has not called Looper.prepare()");
        }
        .
        .
    }

3.4 子线程正确的创建

  • 先执行,Looper.prepare(); 初始化Looper,然后调用loop()方法,再创建Handler。
  • 每个线程Looper.prepare();只能调用一次,否则会报错。
    private Handler handler2;

    /**
     * 子线程
     */
    private void thread() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                Looper.prepare();
                Looper looper = Looper.myLooper();
                looper.loop();

                handler2 = new Handler(looper, new Handler.Callback() {
                    @Override
                    public boolean handleMessage(@NonNull Message msg) {
                        return false;
                    }
                });
            }
        }).start();
    }

4.Message

  • Message就是一个存放消息的类,是一个链表结构。

4.1 基本参数

public final class Message implements Parcelable {
    //用于handler标记处理的
    public int what;
    //可以传递的int参数1
    public int arg1;
    //可以传递的int参数2
    public int arg2;
    //可以传递的obj参数
    public Object obj;

    //执行时间
    public long when;
    //传递的bundle
    Bundle data;
    //Message绑定的Handler
    Handler target;
    //Handler.post()时传的callback
    Runnable callback;
    //链表结构
    Message next;

4.2 享元模式obtain()

  • obtain() 可以重用Message,减少开销提高性能。
  • 菜鸟教程-享元模式
    /**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }

4.3 回收recycle()

  • 如果发送的延迟消息,或者消息在执行,就会报错,一般我们不用调用recycle方法。
    /**
     * Return a Message instance to the global pool.
     * 

* You MUST NOT touch the Message after calling this function because it has * effectively been freed. It is an error to recycle a message that is currently * enqueued or that is in the process of being delivered to a Handler. *

*/ public void recycle() { if (isInUse()) { if (gCheckRecycle) { throw new IllegalStateException("This message cannot be recycled because it " + "is still in use."); } return; } recycleUnchecked(); }
  • Message用完并不是内存回收,只是把里面的内容清空,等下次复用。
  • 这个链表也不是无限的,最多就50个节点
    private static final int MAX_POOL_SIZE = 50;

    @UnsupportedAppUsage
    void recycleUnchecked() {
        // Mark the message as in use while it remains in the recycled object pool.
        // Clear out all other details.
        flags = FLAG_IN_USE;
        what = 0;
        arg1 = 0;
        arg2 = 0;
        obj = null;
        replyTo = null;
        sendingUid = UID_NONE;
        workSourceUid = UID_NONE;
        when = 0;
        target = null;
        callback = null;
        data = null;

        synchronized (sPoolSync) {
            //最多50个
            if (sPoolSize < MAX_POOL_SIZE) {
                next = sPool;
                sPool = this;
                sPoolSize++;
            }
        }
    }

5.MessageQueue

  • 这是一个阻塞队列。
  • 队列是在不停的for循环的,是一个死循环,那不就一直占用着cpu?所以就有了native的方法,处理休眠唤醒。

5.1 MessageQueue每个线程只有一个

  • 消息队列每个线程只有一个,跟Looper绑定在一起,在分析Looper时会一起分析。

5.2 消息入队

  • 前面我们知道Handler所有消息入队最后都是调用 enqueueMessage(Message msg, long when)
  • 判断插入消息的位置,还判断是否唤醒操作。
    boolean enqueueMessage(Message msg, long when) {
        //handler为空就报异常
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        //加锁
        synchronized (this) {
            //消息正在使用会报错
            if (msg.isInUse()) {
                throw new IllegalStateException(msg + " This message is already in use.");
            }
            //判断线程是否存活
            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w(TAG, e.getMessage(), e);
                msg.recycle();
                return false;
            }

            //标记正在使用
            msg.markInUse();
            msg.when = when;
            //链表头
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                //如果队列为空,或者消息延迟时间为0,或者延迟时间小于mMessage的,就插入在头部
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                //唤醒队列
                needWake = mBlocked;
            } else {            
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                
                //插入队列中间。通常,除非队列的开头有障碍并且消息是队列中最早的异步消息,否则我们不必唤醒事件队列。
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                //在中间插入,根据时间位置插入
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            //是否唤醒
            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

5.3 消息出队

  • 消息出队其实应该放在 Looper.loop() 里面分析更合适,这里先写,后面结合 loop() 一起看会更好。
  • 前面 return null; 看注释的意思是,如果looper已经退出和释放,就返回null。
  • 这里无限循环,就是一定要取到消息,有消息,阻塞时间为消息的执行时间减去当前时间,如果没消息就阻塞, nativePollOnce(ptr, nextPollTimeoutMillis)
  • 这 next() 方法只有一个地方返回msg,关注这里就行了。
    @UnsupportedAppUsage
    Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }
            //没消息取就阻塞
            nativePollOnce(ptr, nextPollTimeoutMillis);

            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages;
                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());
                }
                if (msg != null) {
                    if (now < msg.when) {
                        // Next message is not ready.  Set a timeout to wake up when it is ready.
                        //阻塞时间为消息的执行时间减去当前时间
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // Got a message.
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        //只有这里返回msg
                        return msg;
                    }
                } else {
                    //没有更多消息,设置为-1,阻塞
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                .
                .
        }
    }

5.4 退出

  • 主线程是不能退出。
  • 传入的 safe 处理,分别是移除消息未执行的消息,和移除全部消息。
    void quit(boolean safe) {
        //主线程是不能退出的
        if (!mQuitAllowed) {
            throw new IllegalStateException("Main thread not allowed to quit.");
        }

        synchronized (this) {
            if (mQuitting) {
                return;
            }
            mQuitting = true;
            
            //移除消息
            if (safe) {
                removeAllFutureMessagesLocked();
            } else {
                removeAllMessagesLocked();
            }

            // We can assume mPtr != 0 because mQuitting was previously false.
            nativeWake(mPtr);
        }
    }
    /**
     * 移除全部消息
     */
    private void removeAllMessagesLocked() {
        Message p = mMessages;
        while (p != null) {
            Message n = p.next;
            p.recycleUnchecked();
            p = n;
        }
        mMessages = null;
    }

    /**
     * 移除未执行的消息,正在运行的等待完成再回收
     */
    private void removeAllFutureMessagesLocked() {
        final long now = SystemClock.uptimeMillis();
        Message p = mMessages;
        if (p != null) {
            
            if (p.when > now) {
                //如果执行时间还未到,即未执行的消息,移除回收
                removeAllMessagesLocked();
            } else {
                //等待在执行的消息执行完再回收移除
                Message n;
                for (;;) {
                    n = p.next;
                    if (n == null) {
                        return;
                    }
                    if (n.when > now) {
                        break;
                    }
                    p = n;
                }
                p.next = null;
                do {
                    p = n;
                    n = p.next;
                    p.recycleUnchecked();
                } while (n != null);
            }
        }
    }

6.Looper

  • Handler要负责发送消息,MessageQueue消息队列存放消息,Looper就是负责消息循环。
  • 商场的扶手电梯大家应该都知道吧,可以把扶手电梯的电机看成Looper,一梯一梯看成消息队列MessageQueue,坐电梯的人看成Message,就这样不停的循环,把消息送去处理。

6.1 ThreadLocal

  • Android 开发也要掌握的Java知识 -ThreadLocal 可以看下ThreadLocal原理。
  • Looper就是用到了ThreadLocal,Looper内部直接就有一个,还定义成static,final了,也就意味着Android里面获取的ThreadLocal只有这一个。
    @UnsupportedAppUsage
    static final ThreadLocal sThreadLocal = new ThreadLocal();

6.2 初始化prepare(),为何只能调用一次

  • 在prepare()方法可以看到,如果初始化过,就调用就会报错了,所以每个线程最多只有一个Looper,但Handler可以有很多个。
    /** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    public static void prepare() {
        prepare(true);
    }

    private static void prepare(boolean quitAllowed) {
        //每个线程只能有一个looper
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        //初始化后设置给sThreadLocal
        sThreadLocal.set(new Looper(quitAllowed));
    }

6.3 绑定当前线程,创建消息对列

  • 创建消息对列,Looper绑定到当前Thread。
  • quitAllowed消息队列是否可销毁,主线程的是不可销毁的,子线程默认是可销毁。
    private Looper(boolean quitAllowed) {
        //创建消息队列
        mQueue = new MessageQueue(quitAllowed);
        //绑定当前线程
        mThread = Thread.currentThread();
    }

6.4 拿到当前线程的looper

    /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

6.5 loop()

  • 首先拿到当前线程的looper,如果没有prepare()那就是空,报异常。
  • 然后就是不停的循环,取出消息,再处理消息。
  • Android中为什么主线程不会因为Looper.loop()里的死循环卡死?
  /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
        //拿到当前线程的looper,如果没有prepare()那就是空,报异常
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        if (me.mInLoop) {
            Slog.w(TAG, "Loop again would have the queued messages be executed"
                    + " before this one completed.");
        }
      .
      .
      .

        for (;;) {
            //不停的取消息
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            .
            .
            try {
                //处理消息,msg.target就是绑定的handler
                msg.target.dispatchMessage(msg);
                if (observer != null) {
                    observer.messageDispatched(token, msg);
                }
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } catch (Exception exception) {
                if (observer != null) {
                    observer.dispatchingThrewException(token, msg, exception);
                }
                throw exception;
            } finally {
                ThreadLocalWorkSource.restore(origWorkSource);
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            .
            .
            //回收消息
            msg.recycleUnchecked();
        }
    }

6.6 退出

  • 退出 Looper 其实就是调用 MessageQueue 的退出,传的 safe 不同而已,那这两段注释说啥。
  • 传 false ,就是说后面发送消息都不会再处理了,发送消息全部都失败,而且该方法不安全,建议使用 quitSafely()。
  • 传 true,跟上面效果差不多,就是MessageQueue的逻辑,移除未执行的消息,正在运行的等待完成。
    /**
     * Quits the looper.
     * 

* Causes the {@link #loop} method to terminate without processing any * more messages in the message queue. *

* Any attempt to post messages to the queue after the looper is asked to quit will fail. * For example, the {@link Handler#sendMessage(Message)} method will return false. *

* Using this method may be unsafe because some messages may not be delivered * before the looper terminates. Consider using {@link #quitSafely} instead to ensure * that all pending work is completed in an orderly manner. *

* * @see #quitSafely */ public void quit() { mQueue.quit(false); } /** * Quits the looper safely. *

* Causes the {@link #loop} method to terminate as soon as all remaining messages * in the message queue that are already due to be delivered have been handled. * However pending delayed messages with due times in the future will not be * delivered before the loop terminates. *

* Any attempt to post messages to the queue after the looper is asked to quit will fail. * For example, the {@link Handler#sendMessage(Message)} method will return false. *

*/ public void quitSafely() { mQueue.quit(true); }

7.Handler

7.1 消发送息

  • Handler 发送消息最后都是执行 enqueueMessage()
    enqueueMessage
  • 这就做了 Message 的 target 指向 handler 自己。
  • 消息入队。
  • mAsynchronous 默认就是false , 构造方法我们基本也不会去动这参数,默认handler 是同步的。
    private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
            long uptimeMillis) {
        //消息的target就是handler
        msg.target = this;
        msg.workSourceUid = ThreadLocalWorkSource.getUid();
        //设置异步
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        //消息入队
        return queue.enqueueMessage(msg, uptimeMillis);
    }

7.2 处理消息

  • Looper.loop() 里面 msg.target.dispatchMessage(msg); 就是让 Handler 处理消息。
  • 这里可以看到三种,第一种是 Handler 发送消息用的 post , 就会把 Callback 封装到 Message ,走 Message 自己的 Callback 。
  • 第二种是Handler 创建时,重写了 Callback,这是有返回值得,如果为 true ,就会再执行下面的 handleMessage 方法。
  • 第三种就是没传 Callback ,就执行 Handler 自己的 handleMessage ,但要重写该方法。
    /**
     * Handle system messages here.
     */
    public void dispatchMessage(@NonNull Message msg) {
        //Message 内部的callback,就是Handler,post()方法封装在Message内部的
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            //如果Handler创建时传Callback就执行这里
            if (mCallback != null) {
                //如果返回值为false 就结束
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            //如果mCallback为null,或者上面返回值为true,就执行这里
            handleMessage(msg);
        }
    }
    
     /**
     * Message 内部的callback,就是Handler,post()方法封装在Message内部的
     */
    private static void handleCallback(Message message) {
        message.callback.run();
    }
    
    /**
     * 创建时如果传Callback,就执行
     */
    public interface Callback {
        /**
         * @param msg A {@link android.os.Message Message} object
         * @return True if no further handling is desired
         */
        boolean handleMessage(@NonNull Message msg);
    }
    
    /**
     * 创建时如果没传Callback,而是重写了该方法
     */
    public void handleMessage(@NonNull Message msg) {
    }

8.内存泄漏

  • 面试的时候贼喜欢问这个问题!!!
  • 转 小题大做 | 内存泄漏简单问,你能答对吗

9.HandlerThread

  • HandlerThread 是谷歌封装好 Looper 的 Thread ,子线程如果需要Handler推荐使用
  • 这里就是保证在使用Handler之前,Looper 一定准备好。
public class HandlerThread extends Thread {
    int mPriority;
    int mTid = -1;
    Looper mLooper;
    private @Nullable Handler mHandler;

    public HandlerThread(String name) {
        super(name);
        mPriority = Process.THREAD_PRIORITY_DEFAULT;
    }
    
    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        //获得锁创建looper
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }
     
    /**
     * This method returns the Looper associated with this thread. If this thread not been started
     * or for any reason isAlive() returns false, this method will return null. If this thread
     * has been started, this method will block until the looper has been initialized.
     * @return The looper.
     */
    public Looper getLooper() {
        if (!isAlive()) {
            return null;
        }
 
        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
           //如果线程存活而且looper为空,就等待让出锁,直到looper创建
            while (isAlive() && mLooper == null) {
                try {
                    wait();
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }
}

参考文档

文章写完了,发现别人有更好的,想了想还是把自己写的发出去吧。。。
Android消息机制1-Handler(Java层)
小题大做 | 内存泄漏简单问,你能答对吗
又又又又攒了一个月的Android面试题

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