类描述
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();
}
}
Looper线程实现的经典例子，使用prepare和looper方法分开创建一个初始handler来与Looper来通信。
此类包含基于MessageQueue设置和管理事件循环所需的代码。 影响队列状态的API应在MessageQueue或Handler上定义，而不是在Looper本身上定义。 例如，空闲处理程序和同步屏障在队列中定义，而准备线程，循环和退出在循环中定义。
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal sThreadLocal = new ThreadLocal();
如果不调用prepare方法，sThreadLocal.get()方法将返回空，也就是在myLooper()静态方法返回为空，my Looper是在Handler构造时候调用，如果为null，将会抛出异常.
- prepare() 方法
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);
}
在当前线程初始化一个looper，在创建handler时启动looper。确保在调用prepare方法后，调用loop方法，并使用quit方法退出。
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));
}
每个线程只能绑定一个Looper，否则会抛异常。
创建一个新的looper实例，保存在sThreadLocal
- 构造
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
在创建Looper实例的时候，创建一个Message Queue和当前线程，
也是就Looper绑定了当前线程和一个Message Queue。quitAllowed默认为true。
- prepareMainLooper()
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
调用prepare()，quitAllowed参数为false，校验sMainLooper是否已经存在，如果已经存在，会抛异常，最后调用 sMainLooper = myLooper()，到目前为止，还没有体现出来Main这个意思。
看看方法描述怎么将：将当前线程初始化一个Looper，并标记成mainLooper，main Looper是在android应用环境创建的，所以不需要调用这个方法。
还是没看出来，先放这里。
- getMainLooper()
/**
* Returns the application's main looper, which lives in the main thread of the application.
*/
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
返回应用程序的mainLooper，位于应用程序的主线程主。
又是这个sMainLooper。不行了 我必须看看它到底怎么就办成main的。妈的看来不是在looper类中显示出来的。还得先放一放。
- myLooper()
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
返回与当前线程相关联的Looper，如果线程未与Looper绑定，返回null。sThreadLocal.get()这玩意一会在搞它。
-myQueue
/**
* Return the {@link MessageQueue} object associated with the current
* thread. This must be called from a thread running a Looper, or a
* NullPointerException will be thrown.
*/
public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}
这个mQueue就是初始化Looper时绑定的mQueue。必须从Looper的线程调用，否则会出险空指针一场。
- isCurrentThread
/**
* Returns true if the current thread is this looper's thread.
*/
public boolean isCurrentThread() {
return Thread.currentThread() == mThread;
}
判断当前的线程是否是looper的线程
- setMessageLogging
/**
* Control logging of messages as they are processed by this Looper. If
* enabled, a log message will be written to printer
* at the beginning and ending of each message dispatch, identifying the
* target Handler and message contents.
*
* @param printer A Printer object that will receive log messages, or
* null to disable message logging.
*/
public void setMessageLogging(@Nullable Printer printer) {
mLogging = printer;
}
控制由这个Looper处理的消息记录，如果启用将在每个消息分派的开始和结束时，将日志消息写入 printer 中，用以识别目标的Handler和Message。
- quit
/**
* Quits the looper.
*
* Causes the {@link #loop} method to terminate without processing any
* more messages in the message queue.
*
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
*
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates. Consider using {@link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
*
*
* @see #quitSafely
*/
public void quit() {
mQueue.quit(false);
}
退出Looper 真正的操作是调用 Message Queue中的quit方法。
描述：终止loop方法，不会在处理消息队列(MessageQueue)中其它消息。
在退出Looper之后，任何尝试将Message加入到消息队列都将失败，消息队列不会接受任何消息，handler.sendMessage将返回false。
使用这个方法可能是不安全的，因为有一些消息可能不会在循环终止之前派发。可以使用quitSafely（）方法来确保有所的待处理的Handler都以有序的方式完成。
- quitSafely()
/**
* Quits the looper safely.
*
* Causes the {@link #loop} method to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* However pending delayed messages with due times in the future will not be
* delivered before the loop terminates.
*
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
*
*/
public void quitSafely() {
mQueue.quit(true);
}
和quit方法比起来就差了一个参数，true表示安全退出，
一旦消息队列的素有消息都处理完毕，就会立即退出loop方法。
但是延迟发送的消息会在looper终止之前传递。
退出looper之后任何，不允许任何Message发送到Message Queue
- loop()最主要的方法了
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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.isTagEnabled(traceTag)) {
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();
}
}
上面的一些方法只要知道是干啥的 又是作用就行，最主要的方法就是这个loop方法。
调用loop方法首先去调用myLooper()获取当前线程绑定的Looper，
没有肯定不行，抛异常，之前也都说过，一个线程绑定一个Looper，一个Message Queue（prepare-->new Looper()），然后获取到绑定的Message Queue，接下来是一个死循环,从消息队列中取出Message，这里如果没有消息，线程就会阻塞在这里，如果Message 返回 null，结束方法，如果有Message，继续往下走，
mLogging控制由这个Looper处理的消息记录，如果启用将在每个消息分派的开始和结束时，将日志消息写入 printer 中。最后把Message释放，并放入到回收池中。
总结
Looper类的作用：
Looper绑定一个线程和消息队列（new Looper()体现），loop方法开启死循环，不停的从MessageQueue中获取Message，并把消息根据Target分发出去。使用getMainLooper可以获取到主线程Looper在MainThread#Main中初始化调用。
未知问题解决
- prepareMainLooper
prepareMainLooper静态方法，实在ActivityThread#main方法中调用，Looper也是在这里调用的。在app创建的时候开始创建这些东西。
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
为什么开启死循环looper不会卡死
参考文章：
https://www.zhihu.com/question/34652589