Looper判断当前线程是否是主线程

1.判断方法

可以借助Looper来判断当前线程是否是主线程：

代码：

public boolean isMainThread() {    return Looper.getMainLooper() == Looper.myLooper();}

getMainLooper()得到的是整个application的主线程。myLooper()得到的是当前线程。

2.Looper

官方文档里说明：

Class used to run a message loop for a thread. Threads by default do not have a message loop associated with them; to create one, call prepare() in the thread that is to run the loop, and then loop() to have it process messages until the loop is stopped.

官方推荐使用例子：

class LooperThread extends Thread {    public Handler mHandler;    public void run() {        Looper.prepare();        mHandler = new Handler() {            public void handleMessage(Message msg) {                // process incoming messages here            }        };        Looper.loop();    }}

2.1.Looper中变量声明

// sThreadLocal.get() will return null unless you've called prepare().static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();private static Looper sMainLooper;  // guarded by Looper.classfinal MessageQueue mQueue;final Thread mThread;private Printer mLogging;private long mTraceTag;

sThreadLocal每个使用该变量的线程都会将已创建的Looper存储到其中，并在prepare的时候调用判断，是否已经创建过Looper

2.2.prepare

public static void prepare() {    prepare(true);}private static void prepare(boolean quitAllowed) {    if (sThreadLocal.get() != null) {//每个线程只能有一个looper        throw new RuntimeException("Only one Looper may be created per thread");    }    sThreadLocal.set(new Looper(quitAllowed));}

2.3.构造方法

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

创建了消息队列并绑定了当前线程

2.4.loop方法

public static void loop() {    final Looper me = myLooper(); //返回当前线程绑定的looper    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();    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.isTagEnabled(traceTag)) {            Trace.traceBegin(traceTag, msg.target.getTraceName(msg));        }        try {//有消息就分发            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();    }}

首先调用myLooper方法获取当前looper：

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

接着拿到该looper对应的消息队列me.mQueue。然后就无限循环从队列里取消息Message，取到消息后就调用msg.target.dispatchMessage(msg)方法进行消息处理。

消息处理完之后要进行回收，msg.recycleUnchecked()

3.总结

一个线程对应一个Looper，一个Looper对应一个MessageQueue，Looper调用loop()开始通过消息队列的next()方法来无限循环获取消息并处理。