Android 消息机制
Android的消息机制主要指Handler的运行机制,Handler的运行需要底层的MessageQueue和Looper支撑
MessageQueue就是消息队列,它内部存储了一组消息,以队列的形式对外提供插入和删除的工作,但内部存储结构不是真正的队列,而是采用单链表的数据结构来存储消息列表
Looper就是消息循环,它会以无限循环的形式去查找是否有新消息,有的话就处理消息,否则就一直等待
ThreadLocal被使用在Looper中,它并不是线程,它的作用是可以在每个线程中存储数据,Handler创建的时候需要当前线程的Looper来构造消息循环系统,就会使用到ThreadLocal来获取当前线程的Looper
Android 消息机制源码分析
Android的主线程就是ActivityThread,主线程的入口方法为main,源码如下:
public static void main(String[] args)
...
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();// 1.创建消息循环Looper
ActivityThread thread = new ActivityThread();
thread.attach(false);
if(sMainThreadHanlder == null){
sMainThreadHanler = thread.getHandler(); //UI线程的Handler
}
AsyncTask.init();
...
Looper.loop(); // 2.执行消息循环
throw new RuntimeException("Main thread loop unexp0ectedly exited");
主线程中通过Looper.prepareMainLooper()创建了一个消息队列,最后执行Looper.loop()来启动消息循环
首先来看下Looper.prepareMainLooper()方法:
public static Looper myLooper(){
return sThreadLocal.get();
}
//设置UI线程的Looper
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
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));
}
prepareMainLooper()方法中调用了prepare()方法,在这个方法中创建了一个Looper对象,并且设置给sThreadLocal,最后将创建完的Looper对象传递给sMainLooper
Looper构造方法:
private Looper(boolean quitAllowed){
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
MessageQueue消息队列在Looper的构造方法中创建并关联,并将当前线程的对象保存起来
然后来看下Looper.loop()方法:
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; // 1.获取消息队列
...
for (;;) { // 2.死循环,即消息循环
Message msg = queue.next(); // 3.获取消息(might block)
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
try {
msg.target.dispatchMessage(msg); //4.处理消息
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
msg.recycleUnchecked(); //回收消息
}
}
loop()方法其实就是建立一个死循环,然后从消息队列中逐个取出消息,最后处理消息的过程
用于去除消息的queue.next()源码:
Message next() {
...
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
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;
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.
nextPollTimeoutMillis = -1;
}
...
}
...
}
}
queue.next()是一个无限循环,如果消息队列中没有消息,那么会进行阻塞,当有新消息来时next方法会返回这条消息并将其从单链表中移除
msg.target.dispatchMessage(msg)消息处理机制:
msg是Message类型,源码如下:
public final class Message implements Parcelable{
Handler target; //target处理
Runnable callback; //Runnable类型的callback
Message next; //下一条消息,消息队列是链式存储的
...
}
可以看到target是Handler类型,实际上就是调用Message内部关联handler的dispatchMessage(msg)方法
//消息处理函数,子类覆写
public void handleMessage(Message msg){
}
private final void handleCallback(Message message){
message.callback.run();
}
//分发消息
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
dispatchMessage是一个分发的方法,对应handler的两种消息发送方法,handleCallback对应post(Runnable callback),handleMessage对应sendMessage
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;
}
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) {
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);
}
可以看到在post(Runnable r)时,会将Runnable包装成Message对象,并将Runnable对象设置给Message对象的callback字段,post与sendMessage最终都会调用sendMessageDelayed(msg,time)方法,最后调用enqueueMessage(queue, msg, uptimeMillis)方法
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
最终就是将消息追加到Messagequeue中
看一下queue.enqueueMessage(msg, uptimeMillis)的源码:
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
...
msg.markInUse();
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;
}
enqueueMessage中主要操作其实就是单链表的插入操作,然后我们可以看到使用了synchronized锁,在next方法中同样也使用了synchronized锁,这样就保证了在插入或取出消息时的数据同步性
最后来看一下Handler的构造方法:
public Handler(Callback callback, boolean async) {
...
mLooper = Looper.myLooper(); //获取Looper
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; //获取消息队列
mCallback = callback;
mAsynchronous = async;
}
Handler会在内部通过Looper.myLooper()来获取Looper对象,并且通过mLooper.mQueue来获取消息队列,如果myLooper()中无法从ThreadLocal获取到Looper对象的话就会抛出异常,这也就解释了在没有Looper的子线程中创建Handler会引发异常的原因
参考:
Android 开发进阶从小工到专家
Android 开发艺术探索
https://blog.csdn.net/lmj623565791/article/details/38377229/