Handler实现原理和流程分析

Handler对象

0x01. 使用场景:

-0x1.1. 线程间通信
Handler最常见的是在Activity中实现工作线程和UI线程的通信功能.比如在异步线程的回调中不能直接处理UI,那么则必须通知UI,让主线程去处理UI相关的业务逻辑.

-0x1.2. 工作线程中事件的有序业务逻辑处理

HandlerThread+Handler来实现一个工作线程中有序的多任务处理框架.

0x02. 用法示例

下面代码示例包含Handler的声明,对象创建, 事件模拟, 事件处理, handler对象的销毁,一个完整的使用流程和规范.

public class MainActivity extends AppCompatActivity {
    private static final String TAG = MainActivity.class.getSimpleName();
    public HandlerWork handlerWork;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    //1.创建handler对象
        handlerWork = new HandlerWork(this);

    //2.模拟异步消息事件
        Message msg = Message.obtain();
        msg.obj = "onCreate() >>>>>>";
        handlerWork.sendMessage(msg);
    }

    @Override
    protected void onDestroy(){
        super.onDestroy();
        Message msg = Message.obtain();
        msg.obj = "onDestroy() >>>>>>";
        handlerWork.sendMessage(msg);

    //4.清理引用对象
        if (null != handlerWork){
            handlerWork.removeCallbacksAndMessages(null);
            handlerWork = null;
        }
    }

    //0.静态内部类声明
    public static class HandlerWork extends Handler{
        MainActivity activityWR;
        HandlerWork(MainActivity activity){
            activityWR = new WeakReference<>(activity).get();
        }

        @Override
        public void handleMessage(Message msg) {
            Log.i(TAG, "handleMessage: msg:"+msg);
        //3.这里处理业务逻辑
        }
    }
}

0x03. 源码分析

• 0x03.1. 声明

根据实际情况声明类可以为静态内部类或者外部类.

 public static class HandlerWork extends Handler{
        MainActivity activityWR;
        HandlerWork(MainActivity activity){
            activityWR = new WeakReference<>(activity).get();
        }
    //省略...

• 0x03.2. 注册
  把当前对象注册到Handler里面.

handlerWork = new HandlerWork(this);

• 0x03.3. 触发事件

    Message msg = Message.obtain();
        msg.obj = "onCreate() >>>>>>";
        handlerWork.sendMessage(msg);

看看sendMessage的调用了sendMessageDelayed方法.

    /**
     * Pushes a message onto the end of the message queue after all pending messages
     * before the current time. It will be received in {@link #handleMessage},
     * in the thread attached to this handler.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

继续跟踪到sendMessageAtTime方法.

    /**
     * Enqueue a message into the message queue after all pending messages
     * before (current time + delayMillis). You will receive it in
     * {@link #handleMessage}, in the thread attached to this handler.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

添加消息到消息队列中.

    /**
     * Enqueue a message into the message queue after all pending messages
     * before the absolute time (in milliseconds) uptimeMillis.
     * The time-base is {@link android.os.SystemClock#uptimeMillis}.
     * Time spent in deep sleep will add an additional delay to execution.
     * You will receive it in {@link #handleMessage}, in the thread attached
     * to this handler.
     * 
     * @param uptimeMillis The absolute time at which the message should be
     *         delivered, using the
     *         {@link android.os.SystemClock#uptimeMillis} time-base.
     *         
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    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);
    }

这里可以看到msg被加到消息队列(mQueue), msg的target对象赋值为当前的Handler对象.

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

mQueue是哪里来的?我们看看Handler的默认构造函数中做了什么事情?

    /**
     * Default constructor associates this handler with the {@link Looper} for the
     * current thread.
     *
     * If this thread does not have a looper, this handler won't be able to receive messages
     * so an exception is thrown.
     */
    public Handler() {
        this(null, false);
    }

接着继续,这里看到mLooper是从Looper.myLooper()中获取的,mQueue是取的当前mLooper中的mQueue.

    /**
     * Use the {@link Looper} for the current thread with the specified callback interface
     * and set whether the handler should be asynchronous.
     *
     * Handlers are synchronous by default unless this constructor is used to make
     * one that is strictly asynchronous.
     *
     * Asynchronous messages represent interrupts or events that do not require global ordering
     * with respect to synchronous messages.  Asynchronous messages are not subject to
     * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
     *
     * @param callback The callback interface in which to handle messages, or null.
     * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
     * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
     *
     * @hide
     */
    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;
    }

到目前为止,我们已经清楚的看到消息Message是怎么组建并发送到消息队列中的.

• 0x03.4. 监听

frameworks/base/core/android/os/MessageQueue.java中enqueueMesage的实现:

    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;
    }

• 0x03.5. 触发

frameworks/base/core/android/os/Looper.java

    /**
     * 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);//这里就是消息拍发的逻辑.根据当前消息中的target对象,调用它的dispatchMessage方法.
            } 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();
        }
    }

但是没有看到Looper是在哪里初始化的? 主线程中的message是如何派发的?

frameworks/base/core/android/app/ActivityThread.java

    final ApplicationThread mAppThread = new ApplicationThread();
    final Looper mLooper = Looper.myLooper();
    final H mH = new H();
    //省略...
    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());

        // Make sure TrustedCertificateStore looks in the right place for CA certificates
        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
        TrustedCertificateStore.setDefaultUserDirectory(configDir);

        Process.setArgV0("");

        Looper.prepareMainLooper();//在这里就初始化了主线程的Looper

        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(); //这里进入Looper的消息循环

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

• 0x03.6. 回调

frameworks/base/core/android/os/Handler.java

    /**
     * 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);
        }
    }

重载handleMessage的接口实现,处理自己的业务逻辑.

    @Override
        public void handleMessage(Message msg) {
            Log.i(TAG, "handleMessage: >>>>>> msg:"+msg);
        }

0x04. 总结

这里根据代码流程走了一遍主线程中Message的处理逻辑,从消息的发送,加入到队列,消息派发,回调处理等,让我们使用Handler的时候只要明白handler中这个消息是如何流转的.