Android学习感悟之消息机制

本篇Android消息机制的原理,包括四个方面ThreadLocal、MessageQueue、Looper和Handler,会通过消息机制流程来分析理解。

简介

由于所有UI操作都必须在主线程中完成,所以我们有时候在子线程得到的数据后需要转化到主线程去更改UI,而这就是Android提供消息机制的原因。这里有个细节,为什么UI操作必须要在主线程完成呢?其中最主要的原因是我们的View并不是线程安全的。这又有一个问题了,为什么不给View加锁呢?因为它会让View的访问变得复杂,而且会降低UI的访问效率,锁是会阻塞某些线程的执行,而使用单线程就可以避免这两个问题,然后使用Handler转换线程也不麻烦。

下面就进入正题,其实消息机制最主要的就是Handler、Looper以及MessageQueue,我简单的理了个消息机制的流程图,如下:

Android学习感悟之消息机制_第1张图片

下面我们会根据这个流程去解读源码。

消息机制流程源码分析

Looper.prepare()

看了上边的流程图,发现我们自己在主线程中使用handler好像没有直接操作过Looper呀?其实在界面创建之前系统已经给我们初始化了Looper,源码在ActivityThread中的main方法中,如下:

public static void main(String[] args) {

    Looper.prepareMainLooper();

    ActivityThread thread = new ActivityThread();
    thread.attach(false);

    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }

    Looper.loop();

}

其中去掉了些这里不涉及的代码,可以看到其实主线程还是走了这个流程的,其中有一点不一样的地方,是主线程的looper是单独存起来的,来看源码:

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();
}

MessageQueue(boolean quitAllowed) {
    mQuitAllowed = quitAllowed;
    mPtr = nativeInit();
}

第一步prepare主要就做了两个事,在threadLocal中设置Looper,Looper中初始化MessageQueue;这里有个知识点,ThreadLocal是如何让不同线程的Looper都不一样的,如果不看系统的实现方式,我们也能大体想到,使用一个Map去存,key就是线程,value就是Looper,这样我们就能保证不同线程Looper不一样,且同一线程只有一个Looper了,这里就深入一步查看一下ThreadLocal的源码:

public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
    table = new Entry[INITIAL_CAPACITY];
    int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
    table[i] = new Entry(firstKey, firstValue);
    size = 1;
    setThreshold(INITIAL_CAPACITY);
}

Entry(ThreadLocal k, Object v) {
    super(k);
    value = v;
}

private void set(ThreadLocal key, Object value) {

    // We don't use a fast path as with get() because it is at
    // least as common to use set() to create new entries as
    // it is to replace existing ones, in which case, a fast
    // path would fail more often than not.

    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (len-1);

    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {
        ThreadLocal k = e.get();

        if (k == key) {
            e.value = value;
            return;
        }

        if (k == null) {
            replaceStaleEntry(key, value, i);
            return;
        }
    }

    tab[i] = new Entry(key, value);
    int sz = ++size;
    if (!cleanSomeSlots(i, sz) && sz >= threshold)
        rehash();
}

这段代码是刚才ThreadLocal保存Looper所调用的代码,大体含义还是比较简单,就是存入一个Entry数组中,每一项都有自己的key和value,其中索引是用一个hashCode和这个数组的长度进行&运算的来的。这样就会让查询变得便利,其实HashMap就是类似的储存方式。

然后再看看ThreadLocal的get方法:

public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
    ThreadLocalMap.Entry e = map.getEntry(this);
    if (e != null)
        return (T) e.value;
    }
    return setInitialValue();
}

private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return value;
}

ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

private Entry getEntry(ThreadLocal key) {
    int i = key.threadLocalHashCode & (table.length - 1);
    Entry e = table[i];
    if (e != null && e.get() == key)
        return e;
    else
        return getEntryAfterMiss(key, i, e);
}

private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
    Entry[] tab = table;
    int len = tab.length;

    while (e != null) {
        ThreadLocal k = e.get();
        if (k == key)
            return e;
        if (k == null)
            expungeStaleEntry(i);
        else
            i = nextIndex(i, len);
        e = tab[i];
    }
    return null;
}

这样看来,其实ThreadLocal的功能就是类似HashMap一样,存取都是通过key,数据结构使用数组,所以是通过key的hashCode与数组长度的到。

这样对ThreadLocal应该就有了一定的了解了。

下一步。

创建Handler

创建Handler的方式有很多,但是最终不外乎两种,传入Looper和不传入Looper;如下:

不传Looper

public Handler(Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) {
        final Class 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;
}

传入Looper

public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

其作用在于在handler中的得到Looper对象以及其MessageQueue,然后另外还有回调等数据。便于用户直接操作这一个类就可以了。

Looper.loop()

直接上代码看看该方法做了哪些操作

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;

    for (;;) {
        Message msg = queue.next(); // might block
        if (msg == null) {
            return;
        }
        
        try {
            msg.target.dispatchMessage(msg);
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }

        msg.recycleUnchecked();
    }
}

Message next() {

    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;
            }
           //其他代码...
        }
    }
}

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();
}

public void handleMessage(Message msg) {
}

去掉了一些其他代码,可以看到loop这个方法是有一个死循环,只有在MeesageQueue的next返回null时,才会结束Looper的循环;而MessageQueue的next方法也是一个死循环,只有当mQuitting为true时返回null,其他时候都在循环中,当有要处理的消息时,就返回该Message,然后交给相应的Handler去处理,里边会判断callback是否为空,来判断是post的消息还是send的消息,然后再分别处理,而send的消息需要我们自己重写handleMessage方法去实现消息处理,而具体发送消息会在下文说到。

这里边涉及到的MessageQueue的数据结构其实是使用链式存储,具体的方式下文再分解,这里先知道大体流程。

Handler发送消息

Handler发送消息有两种方式:

  • sendMessage(Message)
  • post(Runnable)

其实他们两个方法最后都是转化成Mesasge去实现,来直接看源码。

public final boolean sendMessage(Message msg) {
    return sendMessageDelayed(msg, 0);
}

public final boolean sendMessageDelayed(Message msg, long delayMillis) {
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

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);
}

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;
}

下面是post的源码,也就在第一步在构造Message对象,然后后边就一样了。

public final boolean post(Runnable r) {
    return sendMessageDelayed(getPostMessage(r), 0);
}

private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

这里的post和send消息都还有其他的一些方法,比如延时发送,比如发送一个空消息,等等但是归根结底都是调用了enqueueMessage方法,来加入到MessageQueue中。

然后Looper的loop方法在需要处理的时候就能得到该消息,并处理。而这其中最主要的是,需要理解怎么加入到MessageQueue的。

所以我们需要先理解其存储结构,可以看到,Message.next属性,也是Message类型的,所以可以猜到应该是使用了链式存储,所以它并不是一个队列。

再来看一下enqueueMessage方法,这里跟一下代码:

当是第一条Message传入时,可以知道mMessages为空,所以进入if,然后让传入的msg.next = p, mMessages = msg,即当前消息的后一条时空,mMessages为第一条消息;

假设第一条的when是5,这时候第二条消息进入,when是10;所以会进入else,然后再进入循环遍历后加入到链表的最后;

这时候第三条消息进入,when是8;这时候还是会第三条消息的when比第一条消息的when大,所以还是会进入else,循环之后,发现不用加到最后,因为第三条的when小于第二条的when,就break了,这时候就插入到了第二条。

这时候链表的顺序就为:消息1->消息3->消息3。

最后再提供一个WeakHandler的源码,它能避免内存泄漏,原理是采用了弱引用。

import android.os.Handler;
import android.os.Message;

import net.arvin.afbaselibrary.listeners.IWeakHandler;

import java.lang.ref.WeakReference;

public class WeakHandler extends Handler {
    private WeakReference mActivity;

    public WeakHandler(IWeakHandler activity) {
        mActivity = new WeakReference<>(activity);
    }

    @Override
    public void handleMessage(Message msg) {
        if (mActivity != null) {
            IWeakHandler weakHandleInterface = mActivity.get();
            if (weakHandleInterface != null) {
                weakHandleInterface.handleMessage(msg);
            }
        }
    }

}

//回调接口IWeakHandler

import android.os.Message;

public interface IWeakHandler {
    void handleMessage(Message msg);
}

至此,整个消息的流程就基本分析完了。

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