Handler消息机制与AsyncTask深度解析-从源码带你看handler

handler在我们Android开发过程中是最长用到的处理消息及其他事务的,其消息机制相信也被大家所熟知 .接下来深入带大家看一下handler的消息机制和AsyncTask的解析

Handler消息机制

Hanlder有四大核心类

Message

Hanldre

Messagqueue

Looper

Message的实例

Message msg = new Message(); Message.obtain();//取队列的下一个消息 public static Message obtain() { synchronized (mPoolSync) { if (mPool
!= null) { Message m = mPool; mPool = m.next; m.next = null; return m; } } return new Message(); }

Hanldre的实例

实例化Hanlder

public Handler() {
....
//得到一个Looper的实例
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//得到消息队列
mQueue = mLooper.mQueue;
mCallback = null;
}

public static final Looper myLooper() {
return (Looper)sThreadLocal.get();
}

public static final void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());//实例化Looper
}

Messagqueue的实例，当Looper被创建的时候，同时也创建一个对应的消息队列

private Looper() {
mQueue = new MessageQueue();
mRun = true;
mThread = Thread.currentThread();
}

Looper的实例，当prepare方法被调用的时候，Looper被实例化

public static final void prepare() { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created
per thread"); } sThreadLocal.set(new Looper());//实例化Looper }

public static final void prepareMainLooper() {
prepare();//调用了prepare方法，其实就是实例化Looper
setMainLooper(myLooper());
if (Process.supportsProcesses()) {
myLooper().mQueue.mQuitAllowed = false;
}
}

谁调用prepareMainLooper方法();当Activity被创建的时候，创建一个对应的线程ActivityThread.java

public static final void main(String[] args) {
SamplingProfilerIntegration.start();

Process.setArgV0("<pre-initialized>");
//调用prepareMainLooper
Looper.prepareMainLooper();
if (sMainThreadHandler == null) {
sMainThreadHandler = new Handler();
}

ActivityThread thread = new ActivityThread();
thread.attach(false);

if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}

Looper.loop();//轮询器开始工作

}

Looper.loop()方法的工作原理，代码如下：？

public static final void loop() {

Looper me = myLooper();

MessageQueue queue = me.mQueue;

while (true) {

Message msg = queue.next(); // might block

//if (!me.mRun) {

//    break;

//}

if (msg != null) {

if (msg.target == null) {

// No target is a magic identifier for the quit message.

return;

}

if (me.mLogging!= null) me.mLogging.println(

">>>>> Dispatching to " + msg.target + " "

+ msg.callback + ": " + msg.what

);

msg.target.dispatchMessage(msg);

if (me.mLogging!= null) me.mLogging.println(

"<<<<< Finished to    " + msg.target + " "

+ msg.callback);

msg.recycle();

}

}

}

AsyncTask原理

1.伪代码

new AsyncTask<String ,String ,String>(){

//2.在主线程执行，做一条准备操作；
public void onPreExecute(){

}
//3.运行在子线程中，做一些耗时的操作，比如联网请求数据。
public String doInBackground(String ...params){

return null;

}
//4.result就是doInBackground方法返回的值，做一些释放资源的操作。
public void onPostExecute(String result){

}

}.execute(String ...params);//1.在主线程中执行这个方法，要开始执行异步任务了；

2.模仿AsyncTask的原理，自定义类似于AsyncTask工具类

3.查看AsyncTask源码execute 和构造方法

在AsyncTask中onPreExecute方法代码的实现

protected void onPreExecute() {
}

protected abstract Result doInBackground(Params... params);

protected void onPostExecute(Result result) {
}

查看AsyncTask的构造方法

public AsyncTask() {
//WorkerRunnable继承或者实现什么 实现Callable接口
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
return doInBackground(mParams);
}
};

//FutureTask继承或者实现什么 实现RunnableFuture接口 ；
//RunnableFuture继承两个接口 Runnable, Future
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
Message message;
Result result = null;

try {
result = get();
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occured while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));
message.sendToTarget();
return;
} catch (Throwable t) {
throw new RuntimeException("An error occured while executing "
+ "doInBackground()", t);
}

message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(AsyncTask.this, result));
message.sendToTarget();
}
};
}

//callable 是WorkerRunnable
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
sync = new Sync(callable);
}

//callable 是WorkerRunnable
Sync(Callable<V> callable) {
this.callable = callable;
}

execute()方法的执行，重点,在主线程中执行

public final AsyncTask<Params, Progress, Result> execute(Params... params) {
....

onPreExecute();//主线程

mWorker.mParams = params;
sExecutor是什么 线程池ThreadPoolExecutor
sExecutor.execute(mFuture);mFuture是FutureTask//当这个代码执行的时候会执行 FutureTask的run

return this;
}

private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(COREPOOLSIZE,
MAXIMUM_POOLSIZE,
KEEPALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);

FutureTask的run方法代码如下

//1.调用innerRun

public void run() {

sync.innerRun();

}

void innerRun() {

if (!compareAndSetState(READY, RUNNING))

return;

runner = Thread.currentThread();

if (getState() == RUNNING) { // recheck after setting thread

V result;

try {

//2.调用了call方法；callable是WorkerRunnable

result = callable.call();

} catch (Throwable ex) {

setException(ex);

return;

}

//4.设置值

set(result);

} else {

releaseShared(0); // cancel

}

}

mWorker = new WorkerRunnable<Params, Result>() {

//3.调用call

public Result call() throws Exception {

Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);

return doInBackground(mParams);//在子线程

}

};

protected void set(V v) {

//5.

sync.innerSet(v);

}

void innerSet(V v) {

for (;;) {

int s = getState();

if (s == RAN)

return;

if (s == CANCELLED) {

// aggressively release to set runner to null,

// in case we are racing with a cancel request

// that will try to interrupt runner

releaseShared(0);

return;

}

if (compareAndSetState(s, RAN)) {

result = v;

releaseShared(0);

//6.

done();

return;

}

}

}

mFuture = new FutureTask<Result>(mWorker) {

@Override

protected void done() {

Message message;

Result result = null;

try {

//7.

result = get();

} catch (InterruptedException e) {

android.util.Log.w(LOG_TAG, e);

} catch (ExecutionException e) {

throw new RuntimeException("An error occured while executing doInBackground()",

e.getCause());

} catch (CancellationException e) {

message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,

new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));

message.sendToTarget();

return;

} catch (Throwable t) {

throw new RuntimeException("An error occured while executing "

+ "doInBackground()", t);

}

//8.发消息

//sHandler 是InternalHandler

message = sHandler.obtainMessage(MESSAGE_POST_RESULT,

new AsyncTaskResult<Result>(AsyncTask.this, result));AsyncTaskResult 是什么呢

message.sendToTarget();

}

};

private static class AsyncTaskResult<Data> {

final AsyncTask mTask;

final Data[] mData;

AsyncTaskResult(AsyncTask task, Data... data) {

mTask = task;

mData = data;

}

}

private static class InternalHandler extends Handler {

@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})

@Override

public void handleMessage(Message msg) {

//9.

AsyncTaskResult result = (AsyncTaskResult) msg.obj;//取出obj

switch (msg.what) {

case MESSAGE_POST_RESULT:

// There is only one result

//mTask 是AsyncTask

//10.

result.mTask.finish(result.mData[0]);

break;

case MESSAGE_POST_PROGRESS:

result.mTask.onProgressUpdate(result.mData);

break;

case MESSAGE_POST_CANCEL:

result.mTask.onCancelled();

break;

}

}

}

private void finish(Result result) {

if (isCancelled()) result = null;

//10.

onPostExecute(result);

mStatus = Status.FINISHED;

}