Looper
文档这么介绍这个类的
- 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
- {@link #prepare} in the thread that is to run the loop, and then
- {@link #loop} to have it process messages until the loop is stopped.
通常在线程中是这样使用
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();
}
首先调用了 Looper.prepare() 接着调用了Looper.loop();，具体来看看这两个函数
继续看看 Lopper 中的主要成员变量
// sThreadLocal.get() will return null unless you've called prepare().
//在prepare 的时候 set 的值
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper; // guarded by Looper.class
//消息队列
final MessageQueue mQueue;
final Thread mThread;
而ThreadLocal类为每一个线程都维护了自己独有的变量拷贝。每个线程都拥有了自己独立的一个变量，竞争条件被彻底消除了，那就没有任何必要对这些线程进行同步，它们也能最大限度的由CPU调度，并发执行。并且由于每个线程在访问该变量时，读取和修改的，都是自己独有的那一份变量拷贝，变量被彻底封闭在每个访问的线程中，并发错误出现的可能也完全消除了。对比前一种方案，这是一种以空间来换取线程安全性的策略。
具体 可参看这里
prepare()
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));
}
prepare 比较简单，只是初始化了 sThreadLocal的值。一个 Looper 只能有一个 ThreadLoacl 对象，当sThreadLocal.get() != null抛出异常
ThreadLocal对象保证每个线程中的 Looper 实例互不相同
** 一句话概括 prepare()，对于每个 thread 准备一个 Looper 变量，并保证每个Looper在每个线程中独一无二**
loop()
public static void loop() {
//这里得到的 Looper，就是prepare 中 set 的，一个线程对应一个 Looper
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();
//循环从 MessageQueue 取出消息执行，MessageQueue在 Looper 的构造函数中被创建
for (;;) {
// 如果没有消息则阻塞。
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);
}
//MessageQueue中 取出的 msg，交给target.dispatchMessage 处理
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.recycle();
}
}
mylooper()
public static Looper myLooper() {
return sThreadLocal.get();
}
直接返回ThreadLocal中存储的 Looper 变量。
然后取出 looper 中的 Queue，并且循环取出队列中的 message，交给msg.target.dispatchMessage(msg);处理
target 是 handler 后面会讲到
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
//post 的回调
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
handleMessage(msg)是一个空方法，我们在创建 handler 实例时，复写这个方法。然后根据msg.what进行消息处理
Handler
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());
}
}
//通过Looper.myLooper()获取了当前线程保存的Looper实例， mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
// 获取了这个Looper实例中保存的MessageQueue（消息队列），这样就保证了handler的实例与我们Looper实例中MessageQueue关联上了。
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
同样，通过Looper.myLooper()获取当前线程中的 Looper 对象，并且得到其中的 Queue
所有 sendMessage、sendMessageDelayed、sendEmptyMessageDelayed等，最终都调用
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//Looper的loop方法会取出每个msg然后交给msg,target.dispatchMessage(msg)去处理消息
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
最终会将msg 加入到 queue 的队列中
MessageQueue
boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {
throw new AndroidRuntimeException("Message must have a target.");
}
synchronized (this) {
if (mQuitting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
}
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
Handler post
eg
mHandler.post(new Runnable()
{
@Override
public void run()
{
//do something 在 handler 线程
}
});
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
在getPostMessage中，得到了一个Message对象，然后将我们创建的Runable对象作为callback属性，赋值给了此message.
产生一个Message对象，可以new ，也可以使用Message.obtain()方法；两者都可以，但是更建议使用obtain方法，因为Message内部维护了一个Message池用于Message的复用，避免使用new 重新分配内存。
最终还是会调用enqueueMessage
在 dispathMessage 的过程中
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
在getPostMessage中，将post 中的Runnable赋值给了Message.callback
所以，会执行handleCallback(msg)
private static void handleCallback(Message message) {
message.callback.run();
}
即执行 Post 中的Runnable
其实这个Runnable并没有创建什么线程，而是发送了一条消息
总结
到此，这个流程已经解释完毕，让我们首先总结一下
- 首先Looper.prepare()在本线程中保存一个Looper实例，然后该实例中保存一个MessageQueue对象；因为Looper.prepare()在一个线程中只能调用一次，所以MessageQueue在一个线程中只会存在一个。
- Looper.loop()会让当前线程进入一个无限循环，不端从MessageQueue的实例中读取消息，然后回调msg.target.dispatchMessage(msg)方法。
- Handler的构造方法，会首先得到当前线程中保存的Looper实例，进而与Looper实例中的MessageQueue想关联。
- Handler的sendMessage方法，会给msg的target赋值为handler自身，然后加入MessageQueue中。
- 在构造Handler实例时，我们会重写handleMessage方法，也就是msg.target.dispatchMessage(msg)最终调用的方法。
- Activity的启动代码中，已经在当前UI线程调用了Looper.prepare()和Looper.loop()方法。