Handler消息机制

为什么不允许子线程访问UI？UI线程不是线程安全的，多线程并发访问会出问题。为什么不加锁呢？首先加锁机制会使UI访问逻辑变复杂，其次锁机制降低UI访问效率，因为锁机制会阻塞某些线程的执行

1.Handler无参构造函数

    public Handler() {        this(null, false);    }    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;    }

无参构造函数中生成以下两个重要的对象mLooper和mQueue
mLooper是什么呢？

    public static Looper myLooper() {        return sThreadLocal.get();    }

mLooper是一个ThreadLocal对象。ThreadLocal是一个线程内部的存储类，通过它可以在指定线程中存储数据（后面详细介绍）
mQueue是一个MessageQueue对象，看名字就知道这是用来存储Message的Queue。不过，真的是队列么？其实不是，查看源码就知道，里面是一个ArrayList

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;    //*********************************在这里，看到了么？？？？？？？？？？    private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();    private IdleHandler[] mPendingIdleHandlers;    private boolean mQuitting;    // Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.    private boolean mBlocked;    。。。。。。。。}

2.如何发送消息的

Handler中有好多发送消息的函数，最终都会调用这个函数

    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);    }

接下来我们看enqueueMessage

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {        msg.target = this;        if (mAsynchronous) {            msg.setAsynchronous(true);        }        return queue.enqueueMessage(msg, uptimeMillis);    }

接下来转到MessageQueue的enqueueMessage

    boolean enqueueMessage(Message msg, long when) {    。。。。。。        synchronized (this) {            if (mQuitting) {                IllegalStateException e = new IllegalStateException(                        msg.target + " sending message to a Handler on a dead thread");                Log.w("MessageQueue", 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();                //******************************************看到这个想到啥？链表的添加操作                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;    }

3.如何接收消息

    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 (;;) {            //*******************************************************************无限循环，寻找是否有消息            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 (false) Log.v("MessageQueue", "Returning message: " + msg);                        //*******************************************如果有消息那就返回并从单链表中移除                        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("MessageQueue", "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;        }    }

4.如何让消息循环

Android主线程是ActivityThread（E:\adt\sdk\sources\android-23\android\app）。主线程入口方法是

    public static void main(String[] args) {        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");        SamplingProfilerIntegration.start();        // CloseGuard defaults to true and can be quite spammy.  We        // disable it here, but selectively enable it later (via        // StrictMode) on debug builds, but using DropBox, not logs.        CloseGuard.setEnabled(false);        Environment.initForCurrentUser();        // Set the reporter for event logging in libcore        EventLogger.setReporter(new EventLoggingReporter());        AndroidKeyStoreProvider.install();        // Make sure TrustedCertificateStore looks in the right place for CA certificates        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());        TrustedCertificateStore.setDefaultUserDirectory(configDir);        Process.setArgV0("<pre-initialized>");        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"));        }        // End of event ActivityThreadMain.        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);        Looper.loop();        throw new RuntimeException("Main thread loop unexpectedly exited");    }

注意其中三句话

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

先看prepareMainLooper（就生成一个sMainLooper）

    public static void prepareMainLooper() {        prepare(false);        synchronized (Looper.class) {            if (sMainLooper != null) {                throw new IllegalStateException("The main Looper has already been prepared.");            }            sMainLooper = myLooper();        }    }

再看loop函数

    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();        for (;;) {            //********************************next寻找是否有消息，如果没有就阻塞。有的话就dispatchMessage            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            Printer logging = me.mLogging;            if (logging != null) {                logging.println(">>>>> Dispatching to " + msg.target + " " +                        msg.callback + ": " + msg.what);            }            msg.target.dispatchMessage(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();        }    }

再看一下dispatchMessage

    public void dispatchMessage(Message msg) {        if (msg.callback != null) {            handleCallback(msg);        } else {            if (mCallback != null) {                if (mCallback.handleMessage(msg)) {                    return;                }            }            handleMessage(msg);        }    }

看，出现了熟悉的handleMessage了吧

5.ThreadLoacal介绍

ThreadLocal是一个线程内部的存储类，通过它可以在指定线程中存储数据。等取数据的时候就会从各自的线程中取一个数组
先来个小例子

public class MainActivity extends Activity {    private ThreadLocal<String> mThreadLocal = new ThreadLocal<String>();    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        mThreadLocal.set("Main");        Log.e("Main", mThreadLocal.get());        new Thread(new Runnable() {            @Override            public void run() {                mThreadLocal.set("Thread1");                Log.e("Thread1", mThreadLocal.get());            }        }).start();        new Thread(new Runnable() {            @Override            public void run() {                mThreadLocal.set("Thread2");                Log.e("Thread2", mThreadLocal.get());            }        }).start();    }}

结果是

为什么呢？这要从get，set函数说起

    public void set(T value) {        //*************************关键就在这里的currentThread，这样就保证了维护了数据的副本，彼此互不干扰        Thread currentThread = Thread.currentThread();        Values values = values(currentThread);        if (values == null) {            values = initializeValues(currentThread);        }        values.put(this, value);    }    public T get() {        // Optimized for the fast path.        //************************get的时候也是这样，先找到对应的Thread，在找到数据的副本        Thread currentThread = Thread.currentThread();        Values values = values(currentThread);        if (values != null) {            Object[] table = values.table;            int index = hash & values.mask;            if (this.reference == table[index]) {                return (T) table[index + 1];            }        } else {            values = initializeValues(currentThread);        }        return (T) values.getAfterMiss(this);    }

6.总结

主线程先通过这三句话开启循环

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

此时因为没有消息导致next阻塞。
然后我们自己new Handler。
以后每次sendMessage的时候就添加进MessageQueue里面。此时由于有Message了，loop中的next不会阻塞就能取到Message，就调用dispatchMessage
然后就调用我们重写的handleMessage方法完成消息的发送

如果不是主线程而是子线程就用如下三句开启循环

Looper.prepare();Handler handler = new Handler();Looper.loop();