Handler、Looper、Message、MessageQu

2020-03-19  本文已影响0人  ArcherZang
  1. Message是什么
    Message implements Parcelable
    其构造方法是没有参数,注释也推荐我们使用obtain()构建
    通过重载static方法obtain和recycleUnchecked知道其有几个重要的参数
    集合平时的使用以及注释,其中大部分参数我们知道他的用途
    说说不常见的 flags标记当前message状态,包括是否支持异步
    when什么时候发送//TO do
    gCheckRecycle判断是否在回收正在使用的messsage时抛出异常IllegalStateException小于LOLLIPOP不能抛
    最重要的4个静态变量:
    sPoolSync是对象锁,虽然是Object,synchronized使用在链式加减、取出和新增的时候使用
    sPoolSize当前回收池大小
    MAX_POOL_SIZE是回收池最大值
    sPool当前链式最顶层和Message对象内部的next组成了全局唯一的回收池(范围是当前进程)
    应该是message在android内部使用很频繁,而java不是立马进行垃圾回收的所以建立一个回收池;
    如果不是使用obtain()就让回收池失去了意义。
    int flags /*package*/
    long when /*package*/
    public int what
    public int arg1
    public int arg2
    Bundle data
    Runnable callback
    Message next
    Handler target
    /**
    * An arbitrary object to send to the recipient.  When using
    * {@link Messenger} to send the message across processes this can only
    * be non-null if it contains a Parcelable of a framework class (not one
    * implemented by the application).   For other data transfer use
    * {@link #setData}.
    *
    * <p>Note that Parcelable objects here are not supported prior to
    * the {@link android.os.Build.VERSION_CODES#FROYO} release.
    */
    public Object obj;
    /**
    * Optional Messenger where replies to this message can be sent.  The
    * semantics of exactly how this is used are up to the sender and
    * receiver.
    */
    public Messenger replyTo
    /**
    * Optional field indicating the uid that sent the message.  This is
    * only valid for messages posted by a {@link Messenger}; otherwise,
    * it will be -1.
    */
    public int sendingUid = -1

    public static final Object sPoolSync = new Object();
    private static Message sPool;
    private static int sPoolSize = 0;
    private static final int MAX_POOL_SIZE = 50;

    private static boolean gCheckRecycle = true;
  1. Handler
    因为不是数据载体,所以不能直接像上面一样分析
    通常我们会new一个Handler,所以我们先分析构造方法
    不带Looper的构造方法最终会走到Handler(Callback callback, boolean async)
    带Looper最终会走到Handler(Looper looper, Callback callback, boolean async)
    先说一下他们的共同点callback和async没有参数默认值null和false;
    区别:
    不带looper:mLooper = Looper.myLooper();mQueue = mLooper.mQueue;
    带looper:mLooper = looper;mQueue = looper.mQueue;
    那么Looper.myLooper()从Looper的sThreadLocal.get()获取;这里有一个前提是不带looper的必须在UI线程创建;
    而sThreadLocal.get()给出了最好的答案,mLooper == null会抛出异常;
    那么是谁在UI线程中创建了myLooper()
    /**
    * Use the {@link Looper} for the current thread with the specified callback interface
    * and set whether the handler should be asynchronous.
    *
    * Handlers are synchronous by default unless this constructor is used to make
    * one that is strictly asynchronous.
    *
    * Asynchronous messages represent interrupts or events that do not require global ordering
    * with respect to synchronous messages.  Asynchronous messages are not subject to
    * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
    *
    * @param callback The callback interface in which to handle messages, or null.
    * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
    * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
    *
    * @hide
    */
    public Handler(Callback callback, boolean async) {
       if (FIND_POTENTIAL_LEAKS) {
           final Class<? extends Handler> klass = getClass();
           if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                   (klass.getModifiers() & Modifier.STATIC) == 0) {
               Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                   klass.getCanonicalName());
           }
       }

       mLooper = Looper.myLooper();
       if (mLooper == null) {
           throw new RuntimeException(
               "Can't create handler inside thread " + Thread.currentThread()
                       + " that has not called Looper.prepare()");
       }
       mQueue = mLooper.mQueue;
       mCallback = callback;
       mAsynchronous = async;
    }

    /**
    * Use the provided {@link Looper} instead of the default one and take a callback
    * interface in which to handle messages.  Also set whether the handler
    * should be asynchronous.
    *
    * Handlers are synchronous by default unless this constructor is used to make
    * one that is strictly asynchronous.
    *
    * Asynchronous messages represent interrupts or events that do not require global ordering
    * with respect to synchronous messages.  Asynchronous messages are not subject to
    * the synchronization barriers introduced by conditions such as display vsync.
    *
    * @param looper The looper, must not be null.
    * @param callback The callback interface in which to handle messages, or null.
    * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
    * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
    *
    * @hide
    */
    public Handler(Looper looper, Callback callback, boolean async) {
      mLooper = looper;
      mQueue = looper.mQueue;
      mCallback = callback;
      mAsynchronous = async;
    }
  1. Looper.prepareMainLooper();
    这个方法在ActivityThread的Main方法中调用(//TODO回头贴链接)
    prepare(false)方法里调用了sThreadLocal.set(new Looper(false))
    new Looper(false)新建了MessageQueue要注意UI线程false,同时mThread = Thread.currentThread();
    然后设置sMainLooper = myLooper(),sMainLooper一般sdk用来检测当前线程是否是主线程即UI线程;
    所以我们总结一下handler的创建方式:
    主线程:Handler handler = new Handler()不带looper的三种方法都可以
    Handler handler = new Handler(Looper.sMainLooper)带looper的三种方法都可以
    其他线程:Looper.prepare();Handler handler = new Handler()不带looper的三种方法都可以,不要忘了前面那句
    Handler handler = new Handler(Looper.prepare())带looper的三种方法都可以

    public static void prepareMainLooper() {
         prepare(false);
         synchronized (Looper.class) {
             if (sMainLooper != null) {
                 throw new IllegalStateException("The main Looper has already been prepared.");
             }
             sMainLooper = myLooper();
         }
     }
    
     private static void prepare(boolean quitAllowed) {
            if (sThreadLocal.get() != null) {
                throw new RuntimeException("Only one Looper may be created per thread");
            }
         sThreadLocal.set(new Looper(quitAllowed));
     }
    
     private Looper(boolean quitAllowed) {
         mQueue = new MessageQueue(quitAllowed);
         mThread = Thread.currentThread();
     }
    
  2. handler.sendMessage()

    handler.post(Runnable r)所有类似的post方法都是调用getPostMessage方法封装成message发送
    handler.sendEmptyMessage(int what)所有类似的sendEmptyMessage方法都是调用Message msg = Message.obtain()生成msg后把所有参数都赋值给新生成的msg后发送
    关于sendMessage这里有4种方法:
    前3个是往下调用关系,sendMessageAtTime和sendMessageAtFrontOfQueue都直接调用enqueueMessage:

    • —> sendMessage(Message msg),调用下一个方法参数delayMillis=0
    • —> sendMessageDelayed(Message msg, long delayMillis)调用下一个方法参数uptimeMillis=SystemClock.uptimeMillis() + delayMillis
    • —> sendMessageAtTime(Message msg, long uptimeMillis)
    • sendMessageAtFrontOfQueue(Message msg)是直接放到队列前面
      ——————————————————————————————这里分割
    • —> enqueueMessage(queue, msg, uptimeMillis),最终都走了这个方法只不过sendMessageAtFrontOfQueue的参数uptimeMillis=0

    还有一个特殊点的message.sendToTarget(),因为他的target是Handler所以还是走sendMessage(Message msg)

     public void sendToTarget() {
         target.sendMessage(this);
     }
    
  3. enqueueMessage()
    // Handler.java
    当前Handler是否支持异步,如果支持修改message的flags属性,
    然后加入队列,这里的queue来自looper

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }
    
  4. enqueueMessage(Message msg, long when)
    // MessageQueue.java
    mQuitting默认false,如果为true回收message,并且返回false;
    正常流程

    • 标记message在使用,将message.when重置为传入的when
    • (1)如果p == null || when == 0 || when < p.when,队列第一个messsage不是空,参数message的when=0且小于第一个messsage的when
    • (2)否则p!=null && when>0&&when < p.when,
      needWake必定为false,因为加入队列的message必定target不为空,
      无线for循环pre、p和p.next迭代获取p如果为空或者加入队列message的when小于p.when,就让msg.next = p以及pre.next = msg
    • 如果needWake=true,nativeWake(mPtr)注释说可以假定mPtr != 0;
      此方法的调用应该是来自native层,当然不排除package内的调用,因为java层正常添加的message都会有target,
    boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }
    
        synchronized (this) {
            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;
            }
    
            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                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.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                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;
            }
    
            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }
    
  5. Looper.loop()
    这个方法在ActivityThread的Main方法最后调用
    主要看无限for循环

    • 当前线程Looprt队列的next()获取Message msg
    • if (msg == null) return退出
    • mSlowDispatchThresholdMs 最大分发时间,log时间
      mSlowDeliveryThresholdMs 最大发送时间delivery time - post time,log时间
      thresholdOverride默认值0,通过adb设置
      这些不是很重要打log用
    • msg.target.dispatchMessage(msg);
    • msg.recycleUnchecked()
    public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;
    
        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();
    
        // Allow overriding a threshold with a system prop. e.g.
        // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
        final int thresholdOverride =
                SystemProperties.getInt("log.looper."
                        + Process.myUid() + "."
                        + Thread.currentThread().getName()
                        + ".slow", 0);
    
        boolean slowDeliveryDetected = false;
    
        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }
    
            // This must be in a local variable, in case a UI event sets the logger
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }
    
            final long traceTag = me.mTraceTag;
            long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
            long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
            if (thresholdOverride > 0) {
                slowDispatchThresholdMs = thresholdOverride;
                slowDeliveryThresholdMs = thresholdOverride;
            }
            final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
            final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
    
            final boolean needStartTime = logSlowDelivery || logSlowDispatch;
            final boolean needEndTime = logSlowDispatch;
    
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
    
            final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
            final long dispatchEnd;
            try {
                msg.target.dispatchMessage(msg);
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (logSlowDelivery) {
                if (slowDeliveryDetected) {
                    if ((dispatchStart - msg.when) <= 10) {
                        Slog.w(TAG, "Drained");
                        slowDeliveryDetected = false;
                    }
                } else {
                    if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                            msg)) {
                        // Once we write a slow delivery log, suppress until the queue drains.
                        slowDeliveryDetected = true;
                    }
                }
            }
            if (logSlowDispatch) {
                showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
            }
    
            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }
    
            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }
    
            msg.recycleUnchecked();
        }
    }
    
  6. Message next()
    final long ptr = mPtr
    如果ptr==0 直接返回空
    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    以下无限for循环:

    • 如果nextPollTimeoutMillis != 0,调用Binder.flushPendingCommands()

    • nativePollOnce(ptr, nextPollTimeoutMillis)
      阻塞nextPollTimeoutMillis毫秒,>0到达时间继续往下执行,0继续执行,-1永远停止

    • synchronized (this) 同步代码块--开始-------------

      • 设置 now = SystemClock.uptimeMillis();
        设置 Message prevMsg = null;
        设置 Message msg = mMessages;

      • IF 如果msg!=null并且msg.target==null,
        dowhile循环,
        do:会设置prevMsg = msg,msg = msg.next;
        while:msg != null && !msg.isAsynchronous(),就是msg为空或者异步退出当前循环,
        msg是否异步取决与Handler是否有设置默认是false,如果没有handler就是msg自身的值;这个过程

      • 1 . IF 如果msg不为空:
        (1)if 如果now < msg.when:
        计算nextPollTimeoutMillis取msg.when - now和 Integer.MAX_VALUE之间最小值
        (2)else :
        mBlocked = false
        如果prevMsg != null,prevMsg.next = msg.next
        如果prevMsg == null,mMessages = msg.next;并且清空msg.next,然后标记msg.markInUse(),最后返回,因为回收的msg会进入回收池

      • 2 . ELSE :
        nextPollTimeoutMillis = -1

      • 如果mQuitting=true,调用dispose(),dispose()内部执行如果mPtr != 0销毁底层队列且mPtr = 0,完成dispose()方法后返回null;

      • 如果pendingIdleHandlerCount < 0 && (mMessages == null || now < mMessages.when)设置pendingIdleHandlerCount = mIdleHandlers.size();
        这是为了第一次循环时队列中没有msg或者msg没有到执行时间,因为pendingIdleHandlerCount默认-1

      • 如果pendingIdleHandlerCount <= 0那么mBlocked = true并且跳过本次循环;

        因为此时nextPollTimeoutMillis = -1,那么调用nativePollOnce(ptr, nextPollTimeoutMillis)就会阻塞

      • 如果IdleHandler[] mPendingIdleHandlers = null,就new一个数组,数组大小在pendingIdleHandlerCount和mIdleHandlers.size()直接选择最大值

      • 将ArrayList<IdleHandler> mIdleHandlers转成数组加入mPendingIdleHandlers

    • synchronized (this) 同步代码块--结束-------------

    • 内部for循环int i = 0; i < pendingIdleHandlerCount; i++
      迭代数组IdleHandler[] mPendingIdleHandlers
      每次keep默认值为false
      每次keep = idler.queueIdle(),获取当前数组item,然后把item的queueIdle()返回值给keep
      如果keep=flase,同步从数组移出当前item

      源码里重写IdleHandler类时都会返回false,因为如果返回ture,在loop空闲时会一直调用mIdleHandlers里的元素,这样cpu就得不到休息了

    • pendingIdleHandlerCount = 0;
      nextPollTimeoutMillis = 0;如果nativePollOnce(ptr, nextPollTimeoutMillis),不会停止会继续走下去

    无限for循环结束

    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();
            }
    
            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.
                        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();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }
    
                // Process the quit message now that all pending messages have been handled.
                if (mQuitting) {
                    dispose();
                    return null;
                }
    
                // If first time idle, then get the number of idlers to run.
                // Idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }
    
                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }
    
            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler
    
                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf(TAG, "IdleHandler threw exception", t);
                }
    
                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }
    
            // Reset the idle handler count to 0 so we do not run them again.
            pendingIdleHandlerCount = 0;
    
            // While calling an idle handler, a new message could have been delivered
            // so go back and look again for a pending message without waiting.
            nextPollTimeoutMillis = 0;
        }
    }
    
    /**
    * 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();
    }
    
  7. msg.target.dispatchMessage(msg);
    指向Handler内部的dispatchMessage(Message msg)

    • IF msg.callback不等于空就调用
    • ELSE
      • IF mCallback != null
        如果mCallback.handleMessage(msg)=true,就return
      • 调用handleMessage(msg)
    /**
    * Handle system messages here.
    */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
    
  8. handleCallback(msg)

    如果message有callback优先处理,且不会经过handler的callback和handleMessage

    private static void handleCallback(Message message) {
        message.callback.run();
    }
    
    Message
    Runnable callback;
    
  9. mCallback.handleMessage(msg)

    如果没有给msg设置callback

    就调用handler的mCallback的handleMessage

    ​ 如果返回true就不调用handleMessage(msg),直接return

    public interface Callback {
        /**
        * @param msg A {@link android.os.Message Message} object
        * @return True if no further handling is desired
        */
        public boolean handleMessage(Message msg);
    }
    
  1. handleMessage(msg)

    子类重写用来处理消息

    如果msg.callback=null或者mCallback=null或者mCallback.handleMessage返回false

    就会调用,handler内部是空实现

  2. msg.recycleUnchecked()

    除了flags = FLAG_IN_USE和sendingUid = -1

    其他全部置为空或者0

    回收池增加,同时将Message持有的msg给当前对象的next,并且将当前对象置顶

    /**
    * Recycles a Message that may be in-use.
    * Used internally by the MessageQueue and Looper when disposing of queued Messages.
    */
    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 = -1;
        when = 0;
        target = null;
        callback = null;
        data = null;
    
        synchronized (sPoolSync) {
            if (sPoolSize < MAX_POOL_SIZE) {
                next = sPool;
                sPool = this;
                sPoolSize++;
            }
        }
    }
    
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