五分钟深入理解 Handler

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Handler大家最熟悉不过,每次重新回顾Handler都有不同的感觉,学到新的东西。现分享一些自己对Handler的理解。

Handler发送消息

  • 首先Handler通过sendMessage方法将Message发送到MessageQueue消息列对中。
public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }
  • 点击查看源码
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);
    }

大体意思是获得消息队列,开始如入队列。这里mQueue为Handler的成员变量,Handler里并没有说明这个对象从哪里来的。先继续往下分析。

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

msg.target = this;将Handler类这个对象赋值给Message的target变量。这就说明在入队列的时候Message对应的Handler都是绑定好的。可以有多个Handler。之后Looper循环取消息的时候会用到。

  • Handler分析到这儿,下面进入MessageQueue
 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;
    }

Message prev;
for (;;) {
prev = p;
p = p.next;
一个死循环,底层是链表结构,先进先出的底层原理。消息列队会按顺序执行。值得重点的是这行代码 if (needWake) {
nativeWake(mPtr);
}
nativeWake指的是唤醒Looper消息泵,开始取消息。

  • 进入Looper分析源码
    Looper中是消息泵 内部启动一个死循环泵消息。没有消息会休眠,有消息放入MessageQueue中的时候会唤醒。俩个重要的方法,prepare(), loop();
 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));
    }

prepare方法中,sThreadLocal为ThreadLocal的对象,
static final ThreadLocal< Looper > sThreadLocal = new ThreadLocal< Looper >();类似一个集合,底层hashmap。
在thread线程开始前调用get方法,Looper只能实例化一次
如果不为空则抛出”Only one Looper may be created per thread”异常。然后set方法保存looper

public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

set方法中通过获取当前线程保存到ThreadLocalMap当中

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();
        }
    }

Loop方法中首先final Looper me = myLooper();获取当前的looper,mylooper通过ThreadLocal的get方法得到。

  public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

get方法同理也是通过当前线程及map获取looper

 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();
    }

if (me == null) {
throw new RuntimeException(“No Looper; Looper.prepare() wasn’t called on this thread.”);
}
注意到如果在子线程Handler没有调用looper.prepare(),就会抛出这个异常。
之后MessageQueue中顺序取消息,重点是下面代码
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}

looper中msg.target是绑定的Handler , handler分发消息,处理消息。这样一个流程完成

public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
而CallBack接口中的**handleMessage**则是回调处理消息的方法。

 public interface Callback {
        public boolean handleMessage(Message msg);
    }

最后我们解决一下前面提到的Handler中的mQueue来源

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

mQueue是在looper构造器创建初始化的,
以Handler的默认构造器为例,构造器里 mLooper = Looper.myLooper();获取looper, mQueue = mLooper.mQueue;获取了消息队列。

 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;
    }

小结

整个Handler流程从发送Message到handleMessage,经历了入队,Loop消息出队列,再用之前绑定的Handler分发消息。像是一个走了一个圈又回归到最初,这样的思想很好的解决了异步处理耗时,并及时通知UI更新,避免了anr。并且我们知道Handler是哪个线程的由Looper决定,需要注意的是Handler的默认构造是获得的主线程的looper。所以不需要looper.prepare(),系统帮我们做好了。就这些吧,文章短小,但还是比较精简的能快速理解原理。水平有限,难免有不足,望大家能给予斧正。