Handler简析

Handler简析

主线程Looper创建

ActivityThread main()→ActivityThread Looper.prepareMainLooper()（创建全局唯一主线程Looper对象）→Looper prepare()→Looper sThreadLocal.set(new Looper(quitAllowed))

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); //创建Looper的同时创建全局唯一主线程消息队列MessageQueue
    mThread = Thread.currentThread();
}

Activity
new Handler()→重写handleMessage(Message msg)

 public Handler(@Nullable 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();  //获取Looper
     if (mLooper == null) {
         throw new RuntimeException(
             "Can't create handler inside thread " + Thread.currentThread()
                     + " that has not called Looper.prepare()");
     }
     mQueue = mLooper.mQueue;  //获取MessageQueue
     mCallback = callback;
     mAsynchronous = async;
 }

//从全局线程池取出主线程Looper
public static @Nullable Looper myLooper() {
    return sThreadLocal.get();
}

发送消息
Activity handler.sendMessage(message)（开始发送消息）→queue.enqueueMessage(msg, uptimeMillis)（将消息放入消息队列）

private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
        long uptimeMillis) {
    msg.target = this;
    msg.workSourceUid = ThreadLocalWorkSource.getUid();

    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

取消息
ActivityThread main()→ Looper.loop()（Looper轮询方法）→Handler dispatchMessage(Message msg)回调接口或方法

public static void loop() {
    final Looper me = myLooper(); //获取Looper对象
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue;  //获取消息队列

    // 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();

    // Allow overriding a threshold with a system prop. e.g.
    // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
    final int thresholdOverride =
            SystemProperties.getInt("log.looper."
                    + Process.myUid() + "."
                    + Thread.currentThread().getName()
                    + ".slow", 0);

    boolean slowDeliveryDetected = false;

    for (;;) {
        Message msg = queue.next(); //从消息队列中取出消息
        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);
        }
        // Make sure the observer won't change while processing a transaction.
        final Observer observer = sObserver;

        final long traceTag = me.mTraceTag;
        long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
        long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
        if (thresholdOverride > 0) {
            slowDispatchThresholdMs = thresholdOverride;
            slowDeliveryThresholdMs = thresholdOverride;
        }
        final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
        final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);

        final boolean needStartTime = logSlowDelivery || logSlowDispatch;
        final boolean needEndTime = logSlowDispatch;

        if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
            Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
        }

        final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
        final long dispatchEnd;
        Object token = null;
        if (observer != null) {
            token = observer.messageDispatchStarting();
        }
        long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
        try {
            msg.target.dispatchMessage(msg);  //调用handler方法
            if (observer != null) {
                observer.messageDispatched(token, msg);
            }
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } catch (Exception exception) {
            if (observer != null) {
                observer.dispatchingThrewException(token, msg, exception);
            }
            throw exception;
        } finally {
            ThreadLocalWorkSource.restore(origWorkSource);
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        if (logSlowDelivery) {
            if (slowDeliveryDetected) {
                if ((dispatchStart - msg.when) <= 10) {
                    Slog.w(TAG, "Drained");
                    slowDeliveryDetected = false;
                }
            } else {
                if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                        msg)) {
                    // Once we write a slow delivery log, suppress until the queue drains.
                    slowDeliveryDetected = true;
                }
            }
        }
        if (logSlowDispatch) {
            showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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();
    }
}


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

来源：CSDN

作者：qq_33117489

链接：https://blog.csdn.net/qq_33117489/article/details/104053270