一言沐、Handler用處

Handler有兩個主要用法：

• 計劃在將來某個時間點處理Message和Runnable
• 在不同線程里將一個動作加入Handler所對應的隊列去執(zhí)行

二、成員變量

Handler有4個不可變成員變量：消息隊列mQueue漠其、消息隊列所屬mLooper狠鸳、可選Handler回調(diào)mCallback耸别、可選異步標志mAsynchronous

final MessageQueue mQueue;
final Looper mLooper;
final Callback mCallback;
final boolean mAsynchronous;

三涮因、構(gòu)造方法

如果線程已經(jīng)有Looper，那么Handler可以使用下面的構(gòu)造方法坷虑。

public Handler() {
    this(null, false);
}

public Handler(Callback callback) {
    this(callback, false);
}
// 不知道這個異步是不是和指令重排序有關(guān)
public Handler(boolean async) {
    this(null, async);
}

public Handler(Callback callback, boolean async) {
    // 判斷是否匿名類甲馋、本地類、成員類迄损，并判斷修飾符是否是static摔刁，不是打出就警告信息，避免內(nèi)存泄漏
    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());
        }
    }
    // 主動獲取Handler所在線程的Looper
    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;
}

帶Looper形參的構(gòu)造方法海蔽，傳入的Looper不能為空共屈。通常和Looper.getMainLooper()合用。

public Handler(Looper looper) {
    this(looper, null, false);
}

public Handler(Looper looper, Callback callback) {
    this(looper, callback, false);
}

public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

四党窜、把Runnable封裝成Message

這主要作用是把r賦值給msg.callback拗引，把token賦值給m.obj。因為下一個代碼塊就使用到這個方法幌衣，所以拿到前面先說矾削。

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

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

五、Post和Send

注意下面4個方法：

• 前兩個方法封裝形參Runnable豁护，方法名組成是 post()
• 后兩個方法形參是msg或msg.what哼凯，方法名組成是 sendMessage()
• 沒有方法形參既有Runnable，又有msg
• 方法名帶Delayed可設置延遲時間楚里，帶EmptyMessage為創(chuàng)建空消息
• 這4個方法的共同點是都調(diào)用了sendMessageDelayed()断部，返回這個調(diào)用的結(jié)果

public final boolean post(Runnable r) {
   return sendMessageDelayed(getPostMessage(r), 0); //delayMillis = 0 
}
// 時間單位毫秒，如:delayMillis = 1000
public final boolean postDelayed(Runnable r, long delayMillis) {
    return sendMessageDelayed(getPostMessage(r), delayMillis);
}

public final boolean sendMessage(Message msg) {
    return sendMessageDelayed(msg, 0);
}
// msg.what用16進制班缎，如:0x01
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
    Message msg = Message.obtain();
    msg.what = what;
    return sendMessageDelayed(msg, delayMillis);
}

SystemClock.uptimeMillis()是從開機到現(xiàn)在的毫秒數(shù)蝴光，不包括手機睡眠的時間。

postAtTime()重載方法調(diào)用了sendMessageAtTime()达址。

public final boolean postAtTime(Runnable r, long uptimeMillis){
    return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}

public final boolean postAtTime(Runnable r, Object token, long uptimeMillis){
    return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}

sendEmptyMessage()調(diào)sendEmptyMessageDelayed()

sendEmptyMessageDelayed()又和sendEmptyMessageAtTime最終調(diào)用sendMessageAtTime()蔑祟。

public final boolean sendEmptyMessage(int what) {
    return sendEmptyMessageDelayed(what, 0);
}

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

public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
    Message msg = Message.obtain();
    msg.what = what;
    return sendMessageAtTime(msg, uptimeMillis);
}

總而言之，上面所有post和send都終結(jié)在sendMessageAtTime()沉唠，而sendMessageAtTime()僅負責把消息送進消息隊列中疆虚，然后給一個具體執(zhí)行時間點梭冠。

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);
}

消息默認是放在消息隊列的隊尾處蓝丙。返回成功代表成功進入隊列，不代表消息會被調(diào)度荷愕。

一般情況下纱扭，消息隊列都會等待所有消息完成才退出牍帚。但如果手動關(guān)閉消息隊列，那滯留在消息隊列的消息不會得到處理乳蛾，然后消息被丟棄暗赶，這是進入消息隊列卻不一定能調(diào)度的主要原因鄙币。

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

消息也可以放在消息隊列的對頭優(yōu)先執(zhí)行，不過這兩個方法只能在非常特殊的情況下采取用蹂随，因為順序問題和未知副作用很容易導致隊列后方消息的饑餓十嘿。

public final boolean postAtFrontOfQueue(Runnable r){
    return sendMessageAtFrontOfQueue(getPostMessage(r));
}

public final boolean sendMessageAtFrontOfQueue(Message msg) {
    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, 0);
}

六、調(diào)度和三種消息回調(diào)

6.1 消息調(diào)度

當消息到達預定執(zhí)行時間岳锁，消息所在的Looper就會調(diào)用msg.target.dispatchMessage(msg)

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

6.2 三種消息回調(diào)方式

(1) 首先dispatchMessage(msg)嘗試執(zhí)行消息體的msg.callback绩衷。不過由于上面有EmptyMessage一類方法的存在，所以msg.callback可能為空而不執(zhí)行激率。

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

(2) msg.callback不行就看看Handler自己有沒有mCallback咳燕。

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

例子: 創(chuàng)建Handler時可以實現(xiàn)這個回調(diào)，支持操作主線程UI

Handler handler = new Handler(new Handler.Callback() {
    @Override
    public boolean handleMessage(Message msg) {
        return false;
        Toast.makeText(Activity.this,"handleMessage override",Toast.LENGTH_SHORT).show();
    }
});

(3) 如果上兩個回調(diào)都不存在乒躺，就只能寄托于我們自己重寫的方法招盲。舉個例子：

@Override
public void handleMessage(Message msg) {
    super.handleMessage(msg);
    switch(msg.what){
        case START_ACTIVITY:
            Intent intent = new Intent(Activity.this, MainActivity.class);
            Activity.this.startActivity(intent);
            Activity.this.finish();
            break;
        case TOAST_SHORT_SHOW:
            Toast.makeText(Activity.this, "Toast", Toast.LENGTH_SHORT).show();
            break;
    }
}

七、移除隊列消息

根據(jù)消息身份what嘉冒、消息Runnable或msg.obj移除隊列中對應的消息曹货。調(diào)用那個要根據(jù)你選用什么而定。例如發(fā)送msg讳推，就用同一個msg.what作為參數(shù)顶籽。都調(diào)用MessageQueue.removeMessages，具體在MessageQueue的源碼閱讀里面說银觅。

public final void removeCallbacks(Runnable r) {
    mQueue.removeMessages(this, r, null);
}

public final void removeCallbacks(Runnable r, Object token) {
    mQueue.removeMessages(this, r, token);
}

public final void removeMessages(int what) {
    mQueue.removeMessages(this, what, null);
}

public final void removeMessages(int what, Object object) {
    mQueue.removeMessages(this, what, object);
}

public final void removeCallbacksAndMessages(Object token) {
    mQueue.removeCallbacksAndMessages(this, token);
}

八礼饱、查找對應消息

上面是移除，這里是查看有沒有對應的消息

public final boolean hasMessages(int what) {
    return mQueue.hasMessages(this, what, null);
}

public final boolean hasMessages(int what, Object object) {
    return mQueue.hasMessages(this, what, object);
}

public final boolean hasCallbacks(Runnable r) {
    return mQueue.hasMessages(this, r, null);
}

九设拟、阻塞非安全執(zhí)行

如果當前執(zhí)行線程是Handler的線程慨仿，Runnable會被立刻執(zhí)行久脯。否則把它放在消息隊列中一直等待執(zhí)行完畢或者超時纳胧。超時后這個任務還是在隊列中，在后面的某個時刻它仍然會執(zhí)行帘撰，很有可能造成死鎖跑慕，所以盡量不要用它。

這個方法使用場景是Android初始化一個WindowManagerService摧找，因為WindowManagerService不成功核行，其他組件就不允許繼續(xù)，所以使用阻塞的方式直到完成蹬耘。

public final boolean runWithScissors(final Runnable r, long timeout) {
    if (r == null) {
        throw new IllegalArgumentException("runnable must not be null");
    }
    if (timeout < 0) {
        throw new IllegalArgumentException("timeout must be non-negative");
    }

    if (Looper.myLooper() == mLooper) {
        r.run();
        return true;
    }
    // 一個阻塞的隊列
    BlockingRunnable br = new BlockingRunnable(r);
    return br.postAndWait(this, timeout);
}

IMessenger mMessenger;  // IPC

private static final class BlockingRunnable implements Runnable {
    private final Runnable mTask;
    private boolean mDone;

    public BlockingRunnable(Runnable task) {
        mTask = task;
    }

    @Override
    public void run() {
        try {
            mTask.run();
        } finally {
            synchronized (this) {
                mDone = true;
                notifyAll();
            }
        }
    }

    public boolean postAndWait(Handler handler, long timeout) {
        if (!handler.post(this)) {
            return false;
        }

        synchronized (this) {
            if (timeout > 0) {
                final long expirationTime = SystemClock.uptimeMillis() + timeout;
                while (!mDone) {
                    long delay = expirationTime - SystemClock.uptimeMillis();
                    if (delay <= 0) {
                        return false; // timeout
                    }
                    try {
                        wait(delay);
                    } catch (InterruptedException ex) {
                    }
                }
            } else {
                while (!mDone) {
                    try {
                        wait();
                    } catch (InterruptedException ex) {
                    }
                }
            }
        }
        return true;
    }
}

十芝雪、獲取消息名

獲取消息里Handler的類名，或消息msg.what的16進制值

public String getMessageName(Message message) {
    if (message.callback != null) {
        return message.callback.getClass().getName();
    }
    return "0x" + Integer.toHexString(message.what);
}

獲取空消息體的重載方法

public final Message obtainMessage() {
    return Message.obtain(this);
}

public final Message obtainMessage(int what) {
    return Message.obtain(this, what);
}

public final Message obtainMessage(int what, Object obj) {
    return Message.obtain(this, what, obj);
}

public final Message obtainMessage(int what, int arg1, int arg2) {
    return Message.obtain(this, what, arg1, arg2);
}

public final Message obtainMessage(int what, int arg1, int arg2, Object obj) {
    return Message.obtain(this, what, arg1, arg2, obj);
}

十一综苔、其他

剩下這個方法關(guān)于跨進程通訊的Messager惩系，在AIDL中使用位岔。

private final class MessengerImpl extends IMessenger.Stub {
    public void send(Message msg) {
        msg.sendingUid = Binder.getCallingUid();
        Handler.this.sendMessage(msg);
    }
}