《Android 开发艺术探索》- 第十章内容总结

11 分钟 读完 (约 1646 字)

Android 的消息机制主要就是 Handler 机制，Handler 的主要应用场景就是在子线程中更新 UI，在 ViewRootImpl 中有一个检测当前线程的函数：

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void checkThread() {
    if (mThread != Thread.currentThread()) {
        throw new CalledFromWrongThreadException(
                "Only the original thread that created a view hierarchy can touch its views.");
    }
}

其中 mThread 就是主线程，只要更新 UI 的线程不是主线程就会抛出这个让人熟悉的异常

有时候还有用到 Activity#runOnUiThread 这个函数在子线程更新 UI，看一下该函数源码可以发现其内部实现同样采用了 Handler，如果当前线程是主线程就直接执行，如果不是的话需要 post 到消息池中：

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

Handler 还包含了其他两部分内容，分别是：MessageQueue 和 Looper

MessageQueue

MessageQueue 通常也叫做消息池，其中有两个关键函数，enqueueMessage 处理插入数据的操作，next 函数处理获取以及移除消息的操作，先看一下 enqueueMessage 的源码：

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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;
}

相比其他类的源码，还算比较简单的了，只从该函数的源码中就可以看得出来 MessageQueue 的数据结构是一个单链表，MessageQueue 主要作用是用来存取数据的，链表对于存取操作性能优于其他数据结构，再来看一下 next 源码：

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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;
    // 从这里可以看出这是一个无限循环
    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;
    }
}

从源码中可以看出 next 函数中有一个无限循环，如果消息池中不存在消息了，next 函数就会一直阻塞

Looper

分析 Looper 前需要了解另外一个类 ThreaLocal，这个类为线程提供了存储局部变量的能力，每个线程都有自己独立初始化的变量副本，下面一个例子能帮助理解：

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val threadLocal: ThreadLocal<String> = ThreadLocal()
threadLocal.set("main")
println(Thread.currentThread().name + " threadLocal: " + threadLocal.get())
thread(name = "thread_001") {
    threadLocal.set("thread_001")
    println(Thread.currentThread().name + " threadLocal: " + threadLocal.get())
}

thread(name = "thread_002") {
    println(Thread.currentThread().name + " threadLocal: " + threadLocal.get())
}

执行结果：

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main threadLocal: main
thread_001 threadLocal: thread_001
thread_002 threadLocal: null

在理解了 ThreadLocal 之后再来分析 Looper，首先看一下 Looper 的创建过程：

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public static void prepare() {
    prepare(true);
}

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);
    mThread = Thread.currentThread();
}

其中 sThreadLocal 就是 ThreadLocal 类的实例，在构造函数中又会初始化一个 MessageQueue 实例，让 Looper 运行的关键是 loop 函数：

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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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;

        final long traceTag = me.mTraceTag;
        if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
            Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
        }
        final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
        final long end;
        try {
            msg.target.dispatchMessage(msg);
            end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        if (slowDispatchThresholdMs > 0) {
            final long time = end - start;
            if (time > slowDispatchThresholdMs) {
                Slog.w(TAG, "Dispatch took " + time + "ms on "
                        + Thread.currentThread().getName() + ", h=" +
                        msg.target + " cb=" + msg.callback + " msg=" + msg.what);
            }
        }

        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();
    }
}

可以看出该函数和 MessageQueue#next 一样，内部也是一个无限循环，当 next 返回 null 时才会退出该循环，当调用了 quit 函数后 next 函数返回 null，其中 Looper 中有两个函数 quit、quitSafely 都调用了 MessageQueue#quit，其中 quitSafely 是等消息池中所有消息执行完之后才退出，而 quit 则是立即退出

在 loop 中有这么一行代码 msg.target.dispatchMessage(msg)，这行代码是处理消息的关键。其中 msg.target 就是当前的 Handler 对象，且该操作是在 Looper 中执行，所以不管 Handler 在任何地方发送消息，最终消息都会在创建 Handler 的线程中执行

#读书笔记
Java 锁总结 《Java 并发编程实战》

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