Android之Handler和Loooper源码分析

1、handler在主线程和子线程互相通信(子线程和子线程的通信)简单使用

我们使用handler,可以实现主线程和子线程之间的相互通信,然后子线程和子线程之间的通信,如果不清楚,基本用法请先参考我的这篇博客

Android之用Handler实现主线程和子线程互相通信以及子线程和子线程之间的通信  http://blog.csdn.net/u011068702/article/details/75577005

2、handler在主线程为什么不需要调用Looper.prepare()

我们看下Looper.java这个类,它在安卓android.os包下,我们看这个类的一开始的注释

  * <p>This is a typical example of the implementation of a Looper thread,
  * using the separation of {@link #prepare} and {@link #loop} to create an
  * initial Handler to communicate with the Looper.
  *
  * <pre>
  *  class LooperThread extends Thread {
  *      public Handler mHandler;
  *
  *      public void run() {
  *          Looper.prepare();
  *
  *          mHandler = new Handler() {
  *              public void handleMessage(Message msg) {
  *                  // process incoming messages here
  *              }
  *          };
  *
  *          Looper.loop();
  *      }
  *  }</pre>

很明显,在一个线程里面需要使用Handler之前需要Looper.prepare(),但是我们平时在主线程更新UI的时候,为什么没有看到这行代码呢?

我们看下ActivityThread.java这个类,它在安卓包名android.app目录下,我们知道ActivityThread.java这个类是安卓程序的入口,我们看下main函数的代码

    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");
    }

我们可以看到有Looper.prepareMainLooper()函数,我们点击进去

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

然后看到了prepare(false)函数,我们再点击这个函数

    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));
    }

我们可以看到这里就调用prepare函数,所以主线程不需要调用Looper.prepare()函数,然后我们也可以看到这里有行这个代码

        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }

在Looper.java类中,我们的Looper保存在ThreadLocal里面

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

用ThreadLocal修饰的变量,可以理解为只有当前线程可以改变这个参数,其它线程不可以改变这个参数,如果你对ThreadLocal不清楚,单独可以先看下我这篇博客的简单使用

java之ThreadLocal简单使用总结    http://blog.csdn.net/u011068702/article/details/75770226

        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }

上面的代码写得很清楚了,如果当前的sThreadLocal对象里面有个Looper对象,那么就会抛出异常,而且英文也提示了,所以,一个线程为什么只能有一个Looper对象的原因,所以如果在程序里面,每个线程调用2次Looper.prepare()就会报错,我们再看这行代码

        sThreadLocal.set(new Looper(quitAllowed));

点击Looper的构造函数

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

里面构建了一个MessageQueue对象,上面我们分析一个线程只有一个Looper对象,那么Looper对象只构建一个,也就意味着MessageQueue对象也只构建一次,所以一个线程也只有一个MessageQueue对象的原因。

在main函数里面,也调用了Looper.loop()函数,后面分析这个方法

3、分析Handler发送消息

我们先看下Handler.java的构造方法,这个类在安卓 android.os这个目录下面

    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;
    }

这里看出初始化变量,然后保存了当前线程中的Looper对象。

发送消息的时候我们一般都这样写
handler.sendMessage(message) 

所以我们点击 sendMessage方法看下

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

再点击sendMessageDelayed(msg, 0);

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

再点击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);
    }

再点击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);
    }

这里msg.target就是Handler对象本身,再点击queue.enqueueMessage(msg, uptimeMillis)(在MessageQueue这个类里面)

 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("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();
                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;
    }

可以看出MessageQueue从而按照时间将所有的Message排序

然后我们不是最后还调用了Looper.loop()函数吗?点击进去

/**
     * 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
            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();
        }
    }

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,点击进去

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 (false) Log.v("MessageQueue", "Returning message: " + msg);
                        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;
        }
    }

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,然后就执行这个函数

 msg.target.dispatchMessage(msg);

我么知道msg.tartget对象就是handler对象,我们点击dispatchMessage(msg)函数

    /**
     * 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);
        }
    }

我们可以知道msg.callback  != null的时候,就执行了handleCallback(msg)

    private static void handleCallback(Message message) {
        message.callback.run();
    }

意味着这个Runable执行 run方法

然后还有就是也可能会执行到这里来

            handleMessage(msg);

点击进去,

    /**
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }

我们一般在主线程这样写接收消息

    public Handler mHandlerCToP = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            switch(msg.what) {
                case 0:
                    break;
                default:
                    break;
            }
        }
    };

也就意味着把handleMessage()方法重写了,所以我们的代码,有地方发送消息,loop()不断分发消息,当收到了,如果接收到了,我们重写handleMessage就会掉到这个地方来,得到我需要的数据。

4、分析runOnUiThread方法和Handler.post方法和view的post方法

1、分析runOnUiThread()方法

我们一般在子线程调用这个方法也可以来更新UI

        runOnUiThread(new Runnable() {
            @Override
            public void run() {

            }
        });

点击进去

    public final void runOnUiThread(Runnable action) {
        if (Thread.currentThread() != mUiThread) {
            mHandler.post(action);
        } else {
            action.run();
        }
    }

再点击mHandler.post(action) 方法

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

点击进去

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

看到了吗?其实最后还是到了发送消息这里,一开始我们不是分析了Looper.loop()里面的dispatchMessage()方法吗?

我么知道msg.tartget对象就是handler对象,我们点击dispatchMessage(msg)函数

    /**
     * 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);
        }
    }

很明显程序,会走handleCallback(msg);所以会调到handler.java里面的这个方法

    private static void handleCallback(Message message) {
        message.callback.run();
    }

这样就执行了这个Runnable

2、分析handler.post()方法

上面那个函数内部有handler.post()这个方法,已分析

3、分析view.post()方法

点击view.post()方法

    public boolean post(Runnable action) {
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            return attachInfo.mHandler.post(action);
        }
        // Assume that post will succeed later
        ViewRootImpl.getRunQueue().post(action);
        return true;
    }

可以发现其调用的就是activity中默认保存的handler对象的post方法

(0)

相关推荐