Android的消息机制

基础知识 Android知识点

概述

• 消息驱动模式，所有外部消息，转为内部的消息，在主线程中分发处理。
• 每个线程最多有一个消息队列（Looper）
• 相关类，:Looper,Handler,Message,MessageQueue

Looper

消息循环处理器，内部会创建一个MessageQueue，来存放所有消息，主线程是自动创建的，新建的线程需要自己新建一个Looper对象（HandlerThread）。

public final class Looper {
    // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
    private static Looper sMainLooper;  // guarded by Looper.class
    final MessageQueue mQueue;  // 消息队列，存放消息
    final Thread mThread;   // 所在线程
    // Handler创建前，持有的Looper必须prepare
    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));
    }
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            // 返回new的唯一一个ThreadLocal<Looper>，线程独有内存
            sMainLooper = myLooper();
        }
    }
    public static Looper getMainLooper() {
        synchronized (Looper.class) {
            return sMainLooper;
        }
    }
    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();
        // 无限循环
        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                return;
            }
            // ...省略输出Log
            msg.target.dispatchMessage(msg);
            // ...
            msg.recycleUnchecked();
        }
    }
    public static Looper myLooper() {
        return sThreadLocal.get();
    }
    public static MessageQueue myQueue() {
        return myLooper().mQueue;
    }
    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }
    public boolean isCurrentThread() {
        return Thread.currentThread() == mThread;
    }
    public void quit() {
        mQueue.quit(false);
    }
    public void quitSafely() {
        mQueue.quit(true);
    }
    // ...
}

prepare() : 多次调用，会产生异常。
loop()：无限循环

Handler

负责Message的发送和回调处理

• 支持发送Message
• 支持发送Runnable

public class Handler {
    public interface Callback {
        public boolean handleMessage(Message msg);
    }
    // 必须实现才能处理消息
    public void handleMessage(Message msg) {
    }
    // 由Looper的MessageQueue中的Message持有的Handler对象调用
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
    public Handler() {
        this(null, false);
    }
    public Handler(Callback callback) {
        this(callback, false);
    }
    public Handler(Looper looper) {
        this(looper, null, false);
    }
    public Handler(Looper looper, Callback callback) {
        this(looper, callback, false);
    }
    public Handler(boolean async) {
        this(null, async);
    }
    public Handler(Callback callback, boolean async) {
        mLooper = Looper.myLooper();
        // 没准备Looper会抛异常
        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;
    }
    // 发送消息（就是把Message添加到MessageQueue中）
    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);
    }
    final Looper mLooper;
    final MessageQueue mQueue;
    final Callback mCallback;
    final boolean mAsynchronous;
    IMessenger mMessenger;
    // ...
}

MessageQueue

消息队列，存放Message，Message是一个链表

public final class MessageQueue {
    // True if the message queue can be quit.
    private final boolean mQuitAllowed;
    @SuppressWarnings("unused")
    private long mPtr; // used by native code
    Message mMessages;
    private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
    private SparseArray<FileDescriptorRecord> mFileDescriptorRecords;
    private IdleHandler[] mPendingIdleHandlers;
    private boolean mQuitting;
}

Message

public final class Message implements Parcelable {
    // 唯一标识
    public int what;
    public int arg1;
    public int arg2;
    public Object obj;
    public Messenger replyTo;
    public int sendingUid = -1;
    /*package*/ static final int FLAG_IN_USE = 1 << 0;
    /** If set message is asynchronous */
    /*package*/ static final int FLAG_ASYNCHRONOUS = 1 << 1;
    /** Flags to clear in the copyFrom method */
    /*package*/ static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE;
    /*package*/ int flags;
    /*package*/ long when;
    /*package*/ Bundle data;
    // 消息中持有Handler
    /*package*/ Handler target;
    /*package*/ Runnable callback;
    // sometimes we store linked lists of these things
    /*package*/ Message next;
    private 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;
}

概述

需要注意的是线程默认是没有Looper的，我们在子线程使用Handler必须先创建Looper否则会发生异常。
至于主线程为什么可以直接使用Handler呢?那是因为主线程在入口的main方法中就已经帮我们创建了一个Looper对象，并开启了一个Looper循环，帮我构建好了这样一个消息消息循环的环境。

总结

在主程序的入口main方法中进行了主线程Looper的创建以及Handler的创建，以及将改Looper与主线程绑定。然后通过Looper.loop方法进行消息的循环，不断的从消息队列（在初始化Looper的构造函数中进行了MessageQueue的初始化）取出消息，然后交给Message所持有的Handler来处理，Handler通过调用dispatchMessage()方法来处理消息。从而形成了整个消息系统机制。注意：因为我们一般使用Handler都是在主线程中，不用考虑Looper的创建，因为刚才说了，启动程序时候默认给我们创建了一个Looper对象，所在在这个环境下我们可以自由使用Handler,但是如果我们要在子线程中使用Handler就必须先通过Looper.prepare()方法创建一个Looper对象，然后创建handler对象然后通过Looper.loop()方法实Loop循环，不断的处理消息。

HandlerThread

在子线程中创建自己的Handler和Looper，独立的消息处理系统。

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;
    }
    public HandlerThread(String name, int priority) {
        super(name);
        mPriority = priority;
    }
    protected void onLooperPrepared() {
    }
    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }
    public Looper getLooper() {
        if (!isAlive()) {
            return null;
        }
        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) {
                try {
                    wait();
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }
    // 可以获取Handler，来分发消息，然后在子线程的消息循环中处理
    public Handler getThreadHandler() {
        if (mHandler == null) {
            mHandler = new Handler(getLooper());
        }
        return mHandler;
    }
    public boolean quitSafely() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quitSafely();
            return true;
        }
        return false;
    }
    public int getThreadId() {
        return mTid;
    }
}

• HandlerThread将loop转到子线程中处理，可分担MainLooper的工作量，降低了主线程的压力，使主界面更流畅。
• 开启一个线程起到多个线程的作用。处理任务是串行执行，按消息发送顺序进行处理。HandlerThread本质是一个线程，在线程内部，代码是串行处理的。
• 但是由于每一个任务都将以队列的方式逐个被执行到，一旦队列中有某个任务执行时间过长，那么就会导致后续的任务都会被延迟处理。
• HandlerThread拥有自己的消息队列，它不会干扰或阻塞UI线程。
• 对于网络IO操作，HandlerThread并不适合，因为它只有一个线程，需要排队一个一个等候执行。