一、使用详解
（1）Handler使用
//创建一个带有Looper的线程
class LooperThread extends Thread{
@Override
public void run() {
Looper.prepare();
Looper.loop();
}
}
//在主线程中创建，自动绑定主线程Looper
private Handler uiHandler = new Handler() {
//重写Handler的处理消息的方法handleMessage()
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
switch (msg.what){
case 1:
break;
}
}
};
//获取子线程实例
LooperThread looperThread = new LooperThread();
//开启子线程
looperThread.start();
//获取子线程Looper
Looper loop = looperThread .getLooper();
//手动绑定子线程Looper
private Handler mHandler = new Handler(loop) {
//重写Handler的处理消息的方法handleMessage()
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
switch (msg.what){
case 1:
break;
}
}
};
//发送消息
Message message = new Message();
message.what = 1;
message.obj = "result";
uiHandler.sendMessage(message )
mHandler.sendMessage(message )
（2）Handler构造方法
- Handler()：构造函数将通过调用Looper.myLooper()获取当前线程绑定的Looper对象，将该Looper对象保存到名为mLooper的成员字段中。
- Handler(Looper looper)：直接将该Looper保存到名为mLooper的成员字段中。
- Handler(Callback callback)：构造函数传递了Callback对象，Callback是Handler中的内部接口，需要实现其内部的handleMessage方法。
- Handler(Looper looper, Callback callback)
处理Message消息，通过实现Handler.Callback的handleMessage方法或重写Handler本身的handleMessage方法
多线程实现：向Thread的post函数传入一个Runnable对象者重写Thread本身的run方法。
二、源码解析
（1）Handler源码
Handler的创建
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class 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());
}
}
//获取当前线程的Looper对象
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
//获取当前Looper的消息队列MessageQueue对象
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
Handler.sendMessage发送消息
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
//Handler所绑定的消息队列MessageQueue
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) {
//将Message的target绑定为当前的Handler
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//通过queue.enqueueMessage(msg, uptimeMillis)我们将Message放入到消息队列中。
return queue.enqueueMessage(msg, uptimeMillis);
}
Handler.dispatchMessage发送消息到Handler
//派发消息到对应的Handler实例。根据传入的msg作出对应的操作
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
//使用了post发送消息，则执行handleCallback方法，回调Runnable复写的run方法
handleCallback(msg);
} else {
//使用了sendMessage发送消息，则执行handleMessage，回调复写的handleMessage
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
（2）Looper创建源码
Looper.prepare()创建Looper，当前线程和Looper就进行了双向的绑定
//Looper对象中通过sThreadLocal就可以找到其绑定的线程
static final ThreadLocal sThreadLocal = new ThreadLocal();
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
//1个线程中只能对应1个Looper实例
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//创建Looper对象存放在ThreadLocal变量中
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
//创建消息队列对象
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper.loop()循环获取MessageQueue中消息Message
public static void loop() {
//获取当前线程所绑定的Looper
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//获取当前线程所关联的消息队列
final MessageQueue queue = me.mQueue;
//code...
//消息循环
for (;;) {
//从消息队列中取出消息
Message msg = queue.next(); // might block
//若取出的消息为空，则线程阻塞
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
//code...
try {
//Message所关联的Handler通过dispatchMessage方法让Handler处理该Message
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
//code...
//释放消息占据的资源
msg.recycleUnchecked();
}
}
Looper类还提供了一些有用的方法
//获取当前线程的Looper
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
//获取looper对象所属线程
public Thread getThread() {
return mThread;
}
//结束looper循环
public void quit() {
// 创建一个空的message，它的target为NULL，表示结束循环消息
Message msg = Message.obtain();
// 发出消息
mQueue.enqueueMessage(msg, 0);
}
（3）MessageQueue源码
MessageQueue.enqueueMessage将一个Message放入到消息队列MessageQueue中
boolean enqueueMessage(Message msg, long when) {
//code...
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
//判断消息队列里有无消息
if (p == null || when == 0 || when < p.when) {
//消息队列无消息将当前插入的消息作为队头，若此时消息队列处于等待状态，则唤醒
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
//消息队列里有消息，则根据消息创建的时间 插入到队列中
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;
}
MessageQueue.next从消息队列MessageQueue中阻塞式地取出一个Message
Message next() {
//code...
//确定消息队列中是否还有消息。从而决定消息队列应处于出队消息状态还是等待状态
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
//若是nextPollTimeoutMillis为-1，此时消息队列处于等待状态
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
//从消息队列中取出消息：按创建Message对象的时间顺序
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
//消息队列中已无消息，则将nextPollTimeoutMillis参数设为-1。下次循环时，消息队列则处于等待状态
nextPollTimeoutMillis = -1;
}
//code...
}
//code...
}
}
（4）Message源码
//Message内部维护了一个Message池，用于Message消息对象的复用
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
//若池内无消息对象可复用，则还是用关键字new创建
return new Message();
}