Handler机制

ThreadLocal主要方法介绍

ThreadLocal在Handler机制中用于存取Looper对象，并保证Looper对象与线程一一对应

/**     * Sets the current thread's copy of this thread-local variable     * to the specified value.  Most subclasses will have no need to     * override this method, relying solely on the {@link #initialValue}     * method to set the values of thread-locals.     *     * @param value the value to be stored in the current thread's copy of     *        this thread-local.     */    public void set(T value) {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null)            map.set(this, value);        else            createMap(t, value);    }

/**     * Returns the value in the current thread's copy of this     * thread-local variable.  If the variable has no value for the     * current thread, it is first initialized to the value returned     * by an invocation of the {@link #initialValue} method.     *     * @return the current thread's value of this thread-local     */    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();    }

Looper循环器主要方法介绍

/***将Looper 设置到 ThreadLocal 中，注意不管在哪个线程这里必须先调用Looper.prepare（），当然主线程在ActivityThread中已经先调用了，这个在下文将会展示*/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));    }

/** *创建消息队列 *获取当前线程 *这里保证当前Thread当前MessageQueue当前Looper一一对应，这里也印证了Only one Looper may be created per thread */private Looper(boolean quitAllowed) {        mQueue = new MessageQueue(quitAllowed);        mThread = Thread.currentThread();    }

 /**   *Looper循环遍历 MessageQueue 针对每一个正常消息最终会走到 msg.target.dispatchMessage(msg);   *实际上msg.target就是Handler对象，所以这里将消息交由Handler处理   *   * Run the message queue in this thread. Be sure to call   * {@link #quit()} to end the loop.   */   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 (;;) {            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;            if (traceTag != 0) {                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));            }            try {                msg.target.dispatchMessage(msg);            } finally {                if (traceTag != 0) {                    Trace.traceEnd(traceTag);                }            }            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();        }    }

ActivityThread main()方法是Android程序Java的程序入口是静态函数，我们看到下面 Looper.prepareMainLooper();

public static void main(String[] args) {            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());            Security.addProvider(new AndroidKeyStoreProvider());            // 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"));            }            Looper.loop();            throw new RuntimeException("Main thread loop unexpectedly exited");        }

    /**     *我们看到prepareMainLooper方法调用了prepare（），至此我们知道主线程调用了prepare（）     * Initialize the current thread as a looper, marking it as an     * application's main looper. The main looper for your application     * is created by the Android environment, so you should never need     * to call this function yourself.  See also: {@link #prepare()}     */     public static void prepareMainLooper() {        prepare(false);        synchronized (Looper.class) {            if (sMainLooper != null) {                throw new IllegalStateException("The main Looper has already been prepared.");            }            sMainLooper = myLooper();        }    }

Handler主要方法介绍

Handler主要用于发送消息并处理消息

1、发送消息
sendEmptyMessageAtTime(int what, long uptimeMillis)
sendMessage(Message msg)
postDelayed(Runnable r, long delayMillis)
…
最终都会调用sendMessageAtTime()

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);    }

我们看到sendMessageAtTime()最终调用enqueueMessage()，这里的enqueueMessage方法的主要操作其实就是向MessageQueue中插入一条数据。

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

2、处理消息
在enqueueMessage()中将消息放入消息队列

/**     * 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);        }    }public interface Callback {          public boolean handleMessage(Message msg);      }   private static void handleCallback(Message message) {          message.callback.run();      }  

MessageQueue主要方法介绍

MessageQueue作为Message存储的一个单链表（注意：MessageQueue虽然翻译过来是消息队列，但是它的内部存储结构并不是真正的队列，而是采用单链表的数据结构来存储消息列表），重要的是两个方法，enqueueMessage和next

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;    }

next返回这条消息给Looper

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;        }    }

可以发现next是是一个无限循环的方法，唯一跳出循环的条件是取出MessageQueue中的msg,然后return msg。如果MessageQueue 中没有消息，那么next方法将一直阻塞在这里。当有新消息到来时，next方法会返回这条消息并将其从MessageQueue中删除。

Message主要方法介绍

以下是Message的主要成员变量
public int what;//标识哪条消息

//消息携带的参数
public int arg1; //arg1 and arg2 are lower-cost alternatives to using
public int arg2;//arg1 and arg2 are lower-cost alternatives to using
public Object obj;
Bundle data;
long when;

Handler target;//注意这里target是Handler对象
Runnable callback;
Message next;// sometimes we store linked lists of these things

Message主要方法
obtain()
obtain(Message orig)
obtain(Handler h)
obtain(Handler h, Runnable callback)
…
我们看到基本上是获取消息的方法，我们在使用Message时尽量通过这些方法获取，减小内存开销。