1、AsyncTask类的四个抽象方法
public abstract class AsyncTask<Params, Progress, Result> {
/**
Runs on the UI thread before {@link #doInBackground}.
*/
@MainThread
protected void onPreExecute() {
}
/**
* Override this method to perform a computation on a background thread.
* This method can call {@link #publishProgress} to publish updates
* on the UI thread.
**/
@WorkerThread
protected abstract Result doInBackground(Params... params);
/**
* Runs on the UI thread after {@link #publishProgress} is invoked.
* The specified values are the values passed to {@link #publishProgress}.
*/
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onProgressUpdate(Progress... values) {
}
/**
* <p>Runs on the UI thread after {@link #doInBackground}. The
* specified result is the value returned by {@link #doInBackground}.</p>
This method won't be invoked if the task was cancelled.</p>
*/
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onPostExecute(Result result) {
}
}
2、AsyncTask的执行流程
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
当我们构造一个AsyncTask的子类，并调用其execute方法时，execute方法会调用executeOnExecutor，并向其传递一个SERIAL_EXECUTOR和我们execute中的传递的参数params。
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
executeOnExecutor中，首先会调用AyncTask中的第一个回调方法onPreExecute，然后将我们的我们的参数params封装在AsyncTask构建时创建的一个FutureTask中，然后使用SERIAL_EXECUTOR去执行这个FutureTask，
private static class SerialExecutor implements Executor {
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
public synchronized void execute(final Runnable r) {
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
SERIAL_EXECUTOR是负责将所有的FutureTask加入到一个ArrayDeque的队列中进行排队，（在FutureTask加入到ArrayDeque之前会构造一个runnable，在这个runnable的run方法中首先会调用FutureTask的run方法，然后在其finally模块中调用了scheduleNext方法），如果当前没有一个任务正在运行，就调用scheduleNext从任务队列ArrayDeque中取出一个任务，使THREAD_POOL_EXECUTOR对这个任务进行执行。
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
实际上就是调用了FutureTask的run方法，在FutureTask的run方法中又会调用构造它的WorkerRunnable的call方法，在WorkerRunnable的call方法中首先会调用doInBackground方法获取任务结果，（这个doInBackground是在THREAD_POOL_EXECUTOR执行的，所以可以在里面做一些耗时操作），然后调用postResult将这个结果发送出去。
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
在postResult中，通过一个InternalHandler（InternalHandler在AsyncTask的构造方法中初始化，所以要想消息发送到主线程，AsyncTask必须在主线程中初始化）对result进行封装，然后发送出去。
private static class InternalHandler extends Handler {
public InternalHandler(Looper looper) {
super(looper);
}
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);
break;
}
}
}
在InternalHandler的handleMessage的result分支中调用了finish方法，在finish方法中，如果任务取消，就调用onCancelled方法，否则调用onPostExecute。
在doInBackground中调用publishProgress发布进度和结果的发布类似，都是通过InternalHandler构造一个message，然后将这个message发送到主线程中处理，在InternalHandler的progress分支调用onProgressUpdate。
3、AyncTask中四大原则原因
1、The AsyncTask class must be loaded on the UI thread. This is done
automatically as of android.os.Build.VERSION_CODES#JELLY_BEAN.
2、The task instance must be created on the UI thread.
在AsyncTask中会初始化InternalHandler，如果不是在主线程中，将无法获取主线程的looper，执行结果也无法发送到主线程。（新版错误）
新版初始化InternalHandler的looper是Looper.getMainLooper()，所以InternalHandler中的事件总是在主线程中处理。
正确的回答是execute方法需要在主线程中调用，进而AsyncTask实例需要在主线程中创建。（如果在子线程中创建AsyncTask实例，将无法在主进程中调用execute方法）
3、execute must be invoked on the UI thread.
AsyncTask的execute(Params…)方法必须在UI线程中被调用，原因是在execute(Params…)方法中调用了onPreExecute()方法，onPreExecute()方法是在task开始前在UI线程进行若干准备工作，例如：初始化一个进度条等，所以必须要在UI线程下执行。
4、Do not call onPreExecute()、onPostExecute、doInBackground、onProgressUpdate manually.
5、The task can be executed only once (an exception will be thrown if
a second execution is attempted.)
一个AsyncTask实例只能被执行一次，这是为了线程安全，当AsyncTask在直接调用executeOnExecutor()方法进入并行模式时，避免同一个任务同时被执行而造成混乱。
