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Android异步消息机制

简介:
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目录介绍

  • 1.Handler的常见的使用方式
  • 2.如何在子线程中定义Handler
  • 3.主线程如何自动调用Looper.prepare()
  • 4.Looper.prepare()方法源码分析
  • 5.Looper中用什么存储消息
  • 6.Handler发送消息如何运作
  • 7.Looper.loop()方法源码分析
  • 8.runOnUiThread如何实现子线程更新UI
  • 9.Handler的post方法和view的post方法
  • 10.得出部分结论

好消息

  • 博客笔记大汇总【16年3月到至今】,包括Java基础及深入知识点,Android技术博客,Python学习笔记等等,还包括平时开发中遇到的bug汇总,当然也在工作之余收集了大量的面试题,长期更新维护并且修正,持续完善……开源的文件是markdown格式的!同时也开源了生活博客,从12年起,积累共计47篇[近20万字],转载请注明出处,谢谢!
  • 链接地址:https://github.com/yangchong211/YCBlogs
  • 如果觉得好,可以star一下,谢谢!当然也欢迎提出建议,万事起于忽微,量变引起质变!
  • 00.Android异步消息机制

    • 如何在子线程中定义Handler,主线程如何自动调用Looper.prepare(),Looper.prepare()方法源码分析,Looper中用什么存储消息,Looper.loop()方法源码分析,runOnUiThread如何实现子线程更新UI等等
  • 01.Handler消息机制

    • 为什么不允许在子线程中访问UI,Handler消息机制作用,避免子线程手动创建looper,ActivityThread源码分析,ActivityThread源码分析,Looper死循环为什么不会导致应用卡死,会消耗大量资源吗?

1.Handler的常见的使用方式

  • handler机制大家都比较熟悉呢。在子线程中发送消息,主线程接受到消息并且处理逻辑。如下所示

    • 一般handler的使用方式都是在主线程中定义Handler,然后在子线程中调用mHandler.sendXx()方法,这里有一个疑问可以在子线程中定义Handler吗?
    public class MainActivity extends AppCompatActivity {
    
        private TextView tv ;
    
        /**
         * 在主线程中定义Handler,并实现对应的handleMessage方法
         */
        public static Handler mHandler = new Handler() {
            @Override
            public void handleMessage(Message msg) {
                if (msg.what == 101) {
                    Log.i("MainActivity", "接收到handler消息...");
                }
            }
        };
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            tv = (TextView) findViewById(R.id.tv);
            tv.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    new Thread() {
                        @Override
                        public void run() {
                            // 在子线程中发送异步消息
                            mHandler.sendEmptyMessage(1);
                        }
                    }.start();
                }
            });
        }
    }

2.如何在子线程中定义Handler

  • 直接在子线程中创建handler,看看会出现什么情况?

    • 运行后可以得出在子线程中定义Handler对象出错,难道Handler对象的定义或者是初始化只能在主线程中?其实不是这样的,错误信息中提示的已经很明显了,在初始化Handler对象之前需要调用Looper.prepare()方法
    tv.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            new Thread() {
                @Override
                public void run() {
                    Handler mHandler = new Handler() {
                        @Override
                        public void handleMessage(Message msg) {
                            if (msg.what == 1) {
                                Log.i(TAG, "在子线程中定义Handler,接收并处理消息");
                            }
                        }
                    };
                }
            }.start();
        }
    });
    • 如何正确运行。在这里问一个问题,在子线程中可以吐司吗?答案是可以的,只不过又条件,详细可以看这篇文章02.Toast源码深度分析

      • 这样程序已经不会报错,那么这说明初始化Handler对象的时候我们是需要调用Looper.prepare()的,那么主线程中为什么可以直接初始化Handler呢?难道是主线程创建handler对象的时候,会自动调用Looper.prepare()方法的吗?
    tv.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            new Thread() {
                @Override
                public void run() {
                    Looper.prepare();
                    Handler mHandler = new Handler() {
                        @Override
                        public void handleMessage(Message msg) {
                            if (msg.what == 1) {
                                Log.i(TAG, "在子线程中定义Handler,接收并处理消息");
                            }
                        }
                    };
                    Looper.loop();
                }
            }.start();
        }
    });

3.主线程如何自动调用Looper.prepare()

  • 首先直接可以看在App初始化的时候会执行ActivityThread的main方法中的代码,如下所示

    • 可以看到Looper.prepare()方法在这里调用,所以在主线程中可以直接初始化Handler了。
    public static void main(String[] args) {
        //省略部分代码
        Looper.prepareMainLooper();
        ActivityThread thread = new ActivityThread();
        thread.attach(false);
        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }
        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }
  • 并且可以看到还调用了:Looper.loop()方法,可以知道一个Handler的标准写法其实是这样的

    Looper.prepare();
    Handler mHandler = new Handler() {
       @Override
       public void handleMessage(Message msg) {
          if (msg.what == 101) {
             Log.i(TAG, "在子线程中定义Handler,并接收到消息");
           }
       }
    };
    Looper.loop();

4.Looper.prepare()方法源码分析

  • 源码如下所示

    • 可以看到Looper中有一个ThreadLocal成员变量,熟悉JDK的同学应该知道,当使用ThreadLocal维护变量时,ThreadLocal为每个使用该变量的线程提供独立的变量副本,所以每一个线程都可以独立地改变自己的副本,而不会影响其它线程所对应的副本。
    public static void prepare() {
        prepare(true);
    }
    
    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));
    }
  • 思考:Looper.prepare()能否调用两次或者多次

    • 如果运行,则会报错,并提示prepare中的Excetion信息。由此可以得出在每个线程中Looper.prepare()能且只能调用一次
    //这里Looper.prepare()方法调用了两次
    Looper.prepare();
    Looper.prepare();
    Handler mHandler = new Handler() {
       @Override
       public void handleMessage(Message msg) {
           if (msg.what == 1) {
              Log.i(TAG, "在子线程中定义Handler,并接收到消息。。。");
           }
       }
    };
    Looper.loop();

5.Looper中用什么存储消息

  • 先看一下下面得源代码

    • 看Looper对象的构造方法,可以看到在其构造方法中初始化了一个MessageQueue对象。MessageQueue也称之为消息队列,特点是先进先出,底层实现是单链表数据结构
    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();
    }
  • 得出结论

    • Looper.prepare()方法初始话了一个Looper对象并关联在一个MessageQueue对象,并且一个线程中只有一个Looper对象,只有一个MessageQueue对象。

6.Handler发送消息如何运作

  • 首先看看构造方法

    • 可以看出在Handler的构造方法中,主要初始化了一下变量,并判断Handler对象的初始化不应再内部类,静态类,匿名类中,并且保存了当前线程中的Looper对象。
    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 that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }
  • 看handler.sendMessage(msg)方法

    • 关于下面得源码,是步步追踪,看enqueueMessage这个方法,原来msg.target就是Handler对象本身;而这里的queue对象就是我们的Handler内部维护的Looper对象关联的MessageQueue对象。
    handler.sendMessage(message);
    
    //追踪到这一步
    public final boolean sendMessage(Message msg){
        return sendMessageDelayed(msg, 0);
    }
    
    
    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
    }
    
    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }
  • 看MessageQueue对象的enqueueMessage方法

    • 看到这里MessageQueue并没有使用列表将所有的Message保存起来,而是使用Message.next保存下一个Message,从而按照时间将所有的Message排序
    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;
    }

7.Looper.loop()方法源码分析

  • 看看里面得源码,如下所示

    • 看到Looper.loop()方法里起了一个死循环,不断的判断MessageQueue中的消息是否为空,如果为空则直接return掉,然后执行queue.next()方法
    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;
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();
        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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            final long end;
            try {
                msg.target.dispatchMessage(msg);
                end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (slowDispatchThresholdMs > 0) {
                final long time = end - start;
                if (time > slowDispatchThresholdMs) {
                    Slog.w(TAG, "Dispatch took " + time + "ms on "
                            + Thread.currentThread().getName() + ", h=" +
                            msg.target + " cb=" + msg.callback + " msg=" + msg.what);
                }
            }
            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();
        }
    }
  • 看queue.next()方法源码

    • 大概的实现逻辑就是Message的出栈操作,里面可能对线程,并发控制做了一些限制等。获取到栈顶的Message对象之后开始执行:msg.target.dispatchMessage(msg)
    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;
        }
    }
  • 那么msg.target是什么呢?通过追踪可以知道就是定义的Handler对象,然后查看一下Handler类的dispatchMessage方法:

    • 可以看到,如果我们设置了callback(Runnable对象)的话,则会直接调用handleCallback方法
    • 在初始化Handler的时候设置了callback(Runnable)对象,则直接调用run方法。
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
    
    private static void handleCallback(Message message) {
        message.callback.run();
    }

8.runOnUiThread如何实现子线程更新UI

  • 看看源码,如下所示

    • 如果msg.callback为空的话,会直接调用我们的mCallback.handleMessage(msg),即handler的handlerMessage方法。由于Handler对象是在主线程中创建的,所以handler的handlerMessage方法的执行也会在主线程中。
    • 在runOnUiThread程序首先会判断当前线程是否是UI线程,如果是就直接运行,如果不是则post,这时其实质还是使用的Handler机制来处理线程与UI通讯。
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
    
    @Override
    public final void runOnUiThread(Runnable action) {
        if (Thread.currentThread() != mUiThread) {
            mHandler.post(action);
        } else {
            action.run();
        }
    }

9.Handler的post方法和view的post方法

  • Handler的post方法实现很简单,如下所示

    mHandler.post(new Runnable() {
        @Override
        public void run() {
    
        }
    });
    
    public final boolean post(Runnable r){
       return  sendMessageDelayed(getPostMessage(r), 0);
    }
  • view的post方法也很简单,如下所示

    • 可以发现其调用的就是activity中默认保存的handler对象的post方法
    public boolean post(Runnable action) {
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            return attachInfo.mHandler.post(action);
        }
        ViewRootImpl.getRunQueue().post(action);
        return true;
    }
    
    public void post(Runnable action) {
        postDelayed(action, 0);
    }
    
    public void postDelayed(Runnable action, long delayMillis) {
        final HandlerAction handlerAction = new HandlerAction(action, delayMillis);
    
        synchronized (this) {
            if (mActions == null) {
                mActions = new HandlerAction[4];
            }
            mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction);
            mCount++;
        }
    }

10.得出部分结论

  • 得出得结论如下所示

    • 1.主线程中定义Handler对象,ActivityThread的main方法中会自动创建一个looper,并且与其绑定。如果是子线程中直接创建handler对象,则需要手动创建looper。不过手动创建不太友好,需要手动调用quit方法结束looper。这个后面再说
    • 2.一个线程中只存在一个Looper对象,只存在一个MessageQueue对象,可以存在N个Handler对象,Handler对象内部关联了本线程中唯一的Looper对象,Looper对象内部关联着唯一的一个MessageQueue对象。
    • 3.MessageQueue消息队列不是通过列表保存消息(Message)列表的,而是通过Message对象的next属性关联下一个Message从而实现列表的功能,同时所有的消息都是按时间排序的。

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