最近在看android的looper這個類與android的訊息佇列的處理有一定的緊密關係，今天寫一些關於這個類的幾個常用的方法，主要是起到自己鞏固學習的作用，方便以後自己的檢視。

1、prepare()  原始碼如下

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));
}

    先是判斷了一下sThreadLocal是否為空，不為空時報出異常否側講一個Looper例項放入其中。這也可以從中看出不可以多次的去呼叫這個方法，第一次呼叫後sThreadLocal中就已經是有值的了，如果再次呼叫就會報錯。

2、getMainLooper() 原始碼如下

public static Looper getMainLooper() {
    synchronized (Looper.class) {
        return sMainLooper;
    }
}

3、loop() 原始碼如下

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
Printer logging = me.mLogging;
        if (logging != null) {
            logging.println(">>>>> Dispatching to " + msg.target + " " +
                    msg.callback + ": " + msg.what);
        }

        msg.target.dispatchMessage(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();
    }
}

4、Looper() 原始碼如下

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

5、myLooper 原始碼如下

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

6、quit() 原始碼如下

public void quit() {
    mQueue.quit(false);
}

   停止looper。