对于一位Android开发者来说,对
Handler
、
Looper
、
Message
三个乖宝贝应该再熟悉不过了,这里我们先简单介绍下这三者的关系,之后再用
Looper.loop
方法做点有意思的事情,加深对运行循环的理解。
一、源码理解Handler
、Looper
、Message
通常我们在使用Handler
时会在主线程中new出一个Handler
来接收消息,我们来看下Handler
源码:
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}
在源码注释中说到默认的构造方法创建Handler
,会从当前线程中取出Looper
,如果当前线程没有Looper
,这个Handler
不能够接收到消息并会抛出异常。
我们继续点进去:
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());
}
}
//获取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;
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();//从ThreadLocal中获取
}
既然Looper
是从ThreadLocal
中获取的,那必然有时机要存进去,我们看下Looper
是什么时候存进去的:
/** 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);
}
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));
}
也就是说我们在调用Looper. prepare
方法时会创建Looper
并存入ThreadLocal
中,注意默认quitAllowed
参数都为true,也就是默认创建的Looper
都是可以退出的,我们可以点进去看看:
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
//进去MessageQueue.java
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
⚠️注意:MessageQueue
的成员变量mQuitAllowed
,在调用Looper.quit
方法时会进入MessageQueue
对mQuitAllowed
进行判断,可以简单看下源码,后面会再说到:
//MessageQueue.java
void quit(boolean safe) {
//如果mQuitAllowed为false,也就是不允许退出时会报出异常
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
看到这里我们应该是有疑问的,
- 第一个疑问:默认我们调用
Looper.prepare
方法时mQuitAllowed
变量都为true的,那它什么时候为false?又是被如何设为false的?
- 第二个疑问:我们在创建
Handler
时,并没有往ThreadLocal
中存Looper
,而却直接就取出了ThreadLocal
中的Looper
,那么这个Looper
是什么时候创建并存入的?
这里就要说到ActivityThread
中main
方法了。Zygote进程孵化出新的应用进程后,会执行ActivityThread
类的main
方法。在该方法里会先准备好Looper
和消息队列,并将Looper
存入ThreadLocal
中,然后调用attach
方法将应用进程绑定到ActivityManagerService
,然后进入loop循环,不断地读取消息队列里的消息,并分发消息。
//ActivityThread
public static void main(String[] args) {
SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
Process.setArgV0("");
//创建主线程的阻塞队列
Looper.prepareMainLooper();
// 创建ActivityThread实例
ActivityThread thread = new ActivityThread();
//执行初始化
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
AsyncTask.init();
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
//开启循环
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
我们看下开启的loop循环吧:
/**
* 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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
//注意这里 msg.target为发送msg的Handler
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();
}
}
Looper.loop
方法内部是个死循环(for(;;))。queue.next();
是从阻塞队列里取走头部的Message,当没有Message时主线程就会阻塞。view绘制,事件分发,activity启动,activity的生命周期回调等等都是一个个的Message,系统会把这些Message插入到主线程中唯一的queue中,所有的消息都排队等待主线程的执行。
回过来我们捋一下思路,首先,我们在主线程中创建了Handler
,在Handler
的构造方法中会判断是否创建了Looper
,由于在ActivityThread.main
方法中我们初始化了Looper
并将其存入ThreadLocal
中,所以可以正常创建Handler
。(而如果不是在主线程中创建Handler
,则需要在创建之前手动调用Looper.prepare
方法。)在Looper
的构造方法中创建了MessageQueue
消息队列用于存取Message。然后,Handler.sendMessage
发送消息,在queue.enqueueMessage(msg, uptimeMillis)
方法中将Message存入MessageQueue
中,并最终在Loop.loop
循环中取出消息调用msg.target.dispatchMessage(msg);
也就是发送消息的Handler
的dispatchMessage
方法处理消息,在dispatchMessage
最终调用了handleMessage(msg);
方法。这样我们就可以正常处理发送到主线程的消息了。
二、用Looper搞事情
- 异步任务时阻塞线程,让程序按需要顺序执行
- 判断主线程是否阻塞
- 防止程序异常崩溃
1. 异步任务时阻塞线程,让程序按需要顺序执行
在处理异步任务的时候,通常我们会传入回调来处理请求成功或者失败的逻辑,而我们通过Looper
处理消息机制也可以让其顺序执行,不使用回调。我们来看下吧:
String a = "1";
public void click(View v){
new Thread(new Runnable() {
@Override
public void run() {
//模拟耗时操作
SystemClock.sleep(2000);
a = "22";
mHandler.post(new Runnable() {
@Override
public void run() {
mHandler.getLooper().quit();
}
});
}
}).start();
try{
Looper.loop();
}catch (Exception e){
}
Toast.makeText(getApplicationContext(),a,Toast.LENGTH_LONG).show();
}
当点击按钮的时候我们开启线程处理耗时操作,之后调用Looper.loop();
方法处理消息循环,也就是说主线程又开始不断的读取queue中的Message并执行。这样当执行mHandler.getLooper().quit();
时会调用MessageQueue
的quit
方法:
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
...
}
这个就到了之前我们分析的变量mQuitAllowed
,主线程不允许退出,这里会抛出异常,而最终这段代码是在Looper.loop
方法中获取消息调用msg.target.dispatchMessage
执行的,我们将Looper.loop
的异常给捕获住了,从而之后代码继续执行,弹出Toast。
2. 判断主线程是否阻塞
一般来说,Loop.loop
方法中会不断取出Message,调用其绑定的Handler在UI线程进行执行主线程刷新操作。
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
//注意这里 msg.target为发送msg的Handler
msg.target.dispatchMessage(msg);
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
也就是这里,基本上可以说msg.target.dispatchMessage(msg);
我们可以根据这行代码的执行时间来判断UI线程是否有耗时操作。
在msg.target.dispatchMessage(msg);
前后,分别有logging
判断并打印>>>>> Dispatching to
和<<<<< Finished to
的log,我们可以设置logging
并打印相应时间,基本就可以判断消耗时间。
Looper.getMainLooper().setMessageLogging(new Printer() {
private static final String START = ">>>>> Dispatching";
private static final String END = "<<<<< Finished";
@Override
public void println(String x) {
if (x.startsWith(START)) {
//开始
}
if (x.startsWith(END)) {
//结束
}
}
});
3. 防止程序异常崩溃
既然主线程异常事件最终都是在Looper.loop
调用中发生的,那我们在Looper.loop
方法中将异常捕获住,那主线程的异常也就不会导致程序异常了:
private Handler mHandler = new Handler();
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.layout_test);
mHandler.post(new Runnable() {
@Override
public void run() {
while (true){
try{
Looper.loop();
}catch (Exception e){
}
}
}
});
}
public void click2(View v){
int a = 1/0;//除数为0 运行时报错
}
主线程的所有异常都会从我们手动调用的Looper.loop
处抛出,一旦抛出就会被try{}catch
捕获,这样主线程就不会崩溃了。此原理的开源项目:Cockroach,有兴趣可以看下具体实现。