前言
这篇文章打算讲handler原理，我相信handler原理已经被很多大佬写透过的东西，但是我想从一个不同的角度来写，从实践来了解handler原理。希望这篇文章能带给你来点收获。
Handler介绍
handler是Android一套消息传递机制，像在Andorid里AsyncTask、IntentService、activity生命周期控制等...都存在handler的身影，handler机制中有四个很重要的对象：
- Handler 负责消息的发送和处理
- MessageQueue 消息队列（虽然听名字数据结构像是队列，但是实际上是单向链表）
- Message 传递的消息
- Looper 负责消息的轮询，并且将消息发送给持有该消息的handler（一个线程只能有一个looper）
1、实现handler
首先我们看下handler源码的构造函数
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public boolean handleMessage(Message msg);
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
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());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
callback参数：回调handlerMessag ，我们将在handler所在的线程收到Message looper参数：消息泵，handler将会与这个looper绑定，mQueue是一个Messagequeue，Messagequeue是looper的成员变量 ，可以看的出来如果没有looper将会报错 。 async:是否是异步消息
这里我们模仿源码实现简易handler类
public class Handler {
private final MessageQueue mQueue;
private Looper mLooper;
public Handler(){
Looper looper=Looper.myLooper();
mLooper=looper;
mQueue=looper.mQueue;
}
//发送消息
public void sendMessage(Message message){
message.target=this;
mQueue.enqueueMessage(message);
}
//处理消息
public void handleMessage(Message message) {
}
}
2、实现Looper
我们以Looper.myLooper()作为入口看下Looper的源码
/** 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));
}
/**
* 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();
}
可以看的出如果要获得looper最终是要调prepare(boolean quitAllowed)这个函数，这里再次证明一个线程只能创建一个looper，这里就会有小伙伴问我在主线程明明没有调用Looper.prepare啊，其实在主线程ActivityThread已经自动帮我创建了，所以在其他线程需要Looper的话均需要手动调用Looper.prepare();
那Looper又是怎么作为消息泵，并且将消息传递给handler呢？我们来看Looper.loop的源码（有剪切）
/**
* 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;
...
...
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
try {
msg.target.dispatchMessage(msg);
...
}
...
可以看得出来Looper的消息轮询是一个死循环，且不断的调用 Message msg = queue.next() 从MessageQueue取消息，然后调用 msg.target.dispatchMessage(msg);这里的target即handler，这样将消息传递给handler了；
这里我们模仿源码实现简易Looper
public class Looper {
//用于存放在该线程中的looper，确保一条线程只有一个looper,
//当使用ThreadLocal维护变量时，ThreadLocal为每个使用该变量的线程提供独立的变量副本，
//所以每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本
private static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<>();
MessageQueue mQueue;
private Looper(){
mQueue =new MessageQueue();
}
public static Looper myLooper(){
return sThreadLocal.get();
}
public static void prepare(){
if (null!=sThreadLocal.get()){
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}
public static void loop(){
Looper looper=myLooper();
if (looper==null){
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
MessageQueue queue=looper.mQueue;
for (;;){
Message next=queue.next();
if (next==null||next.target==null){
continue;
}
next.target.handleMessage(next);
}
}
public void quit(){
mQueue.quit();
}
}
3、实现MessageQueue
我就以上述queue.nxet()为切入点进入源码看一下
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
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;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
这里看似很繁琐，其实可以大致流程为 nativePollOnce(ptr, nextPollTimeoutMillis);执行Native的消息机制，该方法会一直等待Native消息，其中 timeOutMillis参数为超时等待时间。如果为-1，则表示无限等待，直到有事件发生为止。如果值为0，则无需等待立即返回，所以主线程一直轮询是不会一直消耗cpu性能的，也不会造成卡顿，因为一有消息就会被唤醒。如果想进一步了解native消息机制可以看深入理解Java Binder和MessageQueue 接下来就是看 if (msg != null && msg.target == null) 是否有屏障消息，如果有就过滤掉同步消息直接执行就近的异步消息，代码再往下看就是时间是否到了消息等待的时候，到了则返回，接着往下看，这里涉及到空闲任务，当没有消息的时候，会执行一些空闲任务，例如GC，空闲任务执行完后nextPollTimeoutMillis又会重新置为0。
那MessageQueue里是怎么存消息的呢，当调用Handler发送消息的时候，不管是调用sendMessage,sendEmptyMessage,sendMessageDelayed还是其他发送一系列方法。最终都会调用 sendMessageAtTime(Message msg, long uptimeMillis),然而这个方法最后会调用enqueueMessage(Message msg, long when)。
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);
}
boolean enqueueMessage(Message msg, long when) {
...省略
synchronized (this) {
...省略
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;
}
消息队列里没有消息或者无需等待、小于消息头的等待时间的时候直接将消息放在头部，否则将消息插入合适的位置（比较等待时间），如果触发needWake那么就会直接唤醒native的消息，这里的mptr是NativeMessageQueue对象然后返回的地址。
这里我们模仿实现简易MessageQueue
public class MessageQueue {
//消息链表
Message mMessage;
private boolean isQuit;
public void enqueueMessage(Message message){
synchronized (this){
if (isQuit){
return;
}
Message p=mMessage;
if (null==p){
mMessage=message;
}else {
Message prev;
for (;;){
prev =p;
p=p.next;
if (null==p){
break;
}
}
prev.next=message;
}
notify();//唤醒，模拟 nativeWake(mPtr);
}
}
public Message next(){
synchronized (this){
Message message;
for (;;){
if (isQuit){
return null;
}
message=mMessage;
if (null!=message){
break;
}try {
wait(); //等待唤醒 模拟 nativePollOnce(ptr, nextPollTimeoutMillis);
}catch (InterruptedException e ){
e.printStackTrace();
}
}
mMessage=mMessage.next;
return message;
}
}
public void quit(){
synchronized (this){
isQuit=true;
Message message=this.mMessage;
while (null!=message){
Message next=message.next;
message.recycle();
message=next;
}
notify();
}
}
}
4、实现Message
Message部分源码
/*package*/ Handler target;
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
/*package*/ Message next;
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
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;
}
}
return new Message();
}
这里的target就是handler这样就可以同过message.target.handlMessage() 去让handler处理消息了，这里还有一个next,这样他在messageQueue里就可以产生一个消息的单向链表。这里还有一个变量池，有兴趣的小伙伴可以自行去了解下message的复用
这里我们模仿实现简易Message
public class Message {
int what;
Object object;
Message next;
Handler target;
public void recycle(){
object=null;
next=null;
target=null;
}
}
这次我们分析和模拟handler机制算是完成了，让我们来看一下效果吧。
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Looper.prepare(); //手动调用我们自己prepare方法
final Handler handler=new Handler(){
@Override
public void handleMessage(Message message) { //处理消息
Log.i("info1", "handleMessage: "+message.what);
}
};
new Thread() {
@Override
public void run() {
Message message=new Message();
message.what=45;
handler.sendMessage(message); //子线程里发送消息
}
}.start();
Looper.loop(); //开启轮询
}
2020-01-02 16:40:38.862 22050-22050/? I/info1: handleMessage: 45
