平时经常使用AsyncTask,深入学习一下,看看它究竟是如何工作的,先看代码。
/**
* Creates a new asynchronous task. This constructor must be invoked on the UI thread.
*
* @hide
*/
public AsyncTask(@Nullable Looper callbackLooper) {
mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
? getMainHandler()
: new Handler(callbackLooper);
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;
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
@MainThread
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;
}
代码已经可以解释一切了,不过中间有些点值得深入思考一下。
1.构造函数中的Looper
在构造函数中的work里面,可以看到最终是通过 postResult(result);
来完成向主线程的回调的。
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
而getHandler里面的mHandler是构造函数中通过Looper来创建的,也就是说,这个AsyncTask的返回结果不一定非要返回到主线程。
/**
* Creates a new asynchronous task. This constructor must be invoked on the UI thread.
*
* @hide
*/
public AsyncTask(@Nullable Handler handler) {
this(handler != null ? handler.getLooper() : null);
}
public AsyncTask() {
this((Looper) null);
}
不过令人费解的是,它的构造函数全部都是hide的,也就是说一般的应用里面,只能将运行结果返回主线程。不过AsyncTask的原理很简单,大不了如果需要的话可以自己模拟写一个支持Looper的类。
2.如何结束的
上文已经讲到handler,AsyncTask中自带了一个,下面的代码中可以清楚的看到熟悉的内容: onProgressUpdate和onPostExecute。
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;
}
}
}
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
3.它是可以cancel的
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
在平时使用的时候,基本没用过这个功能,以后可以尝试一下,看了一下注释
1.结束的时候会调用onCancelled而不是onPostExecute。
2.应该在doInBackground的时候去检查是不是已经cancel,如果是的话就提前结束任务。
未完待续:线程池,FutureTask,线程的interrupt。