Android--AsyncTask源码分析
闯关关卡---第9关，闯关进度20%，继续打怪升级
一直以来都没有使用过AsyncTask，且因为有Rxjava的存在。虽然说之前有看过AsyncTask知识包括，但是时间久了也忘记了，今天重新看了一下又有新的认知
1.AsyncTask大致的原理
AsyncTask是使用线程池，FutureTask，CallBack来执行任务，默认的任务执行顺序是先进先出(FIFO)
核心线程数：Math.max(2, Math.min(CPU_COUNT - 1, 4));
最大线程数：CPU个数x2+1
存活时间：30s--非核心线程的回收时间，若allowCoreThreadTimeOut设置为turn，也作用在核心线程中
任务队列：BlockingQueue<Runnable> sPoolWorkQueue =new LinkedBlockingQueue<Runnable>(128)
2.AsyncTask核心参数及方法
public abstract class AsyncTask<Params, Progress, Result> {...}
Params：异步任务参数
Progress：任务执行进度
Result：任务执行完返回结果
onPreExecute()：任务执行前的准备工作--主线程
doInBackground(Params... params)：开启执行任务，返回执行的任务结果--子线程
onProgressUpdate(Progress... values)：任务执行的进度--主线程
onPostExecute(Result result)：任务完成的结果处理，result为doInBackground返回值
3.AsyncTask准备工作--new AsyncTask()
public AsyncTask() {
this((Looper) null);
}
public AsyncTask(@Nullable Handler handler) {
this(handler != null ? handler.getLooper() : null);
}
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);
}
}
};
}
在构造函数中，初始化Handler，实例化WorkerRunnable抽象类---实现CallBack接口，实例化FutureTask，重写done()方法，任务完成时发送Message，通过这些操作将任务装置在mFuture 中。下面是一个FutureTask简单例子
public class ThreadMain {
public static void main(String[] args) {
Callable<Integer> callable = new Callable<Integer>() {
@Override
public Integer call() throws Exception {
int i = 0;
try {
for (; i < 2; i++) {
System.out.println(Thread.currentThread().getName() + " " + i);
}
} catch (Exception e) {
e.printStackTrace();
} finally {
System.out.println("任务完成");
}
return i;
}
};
FutureTask<Integer> task = new FutureTask<Integer>(callable) {
@Override
protected void done() {
System.out.println("任务完成 done");
super.done();
}
};
task.run();
}
}
image.png
4.任务执行
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
@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;
}
在任务第一次执行，mStatus 设置为RUNNING，调用onPreExecute()在任务执行前的准备工作，exec.execute(mFuture)开启异步任务。接下来看看 sDefaultExecutor 怎么实现的
5.sDefaultExecutor 实现方式
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);
}
}
}
sDefaultExecutor--默认任务处理器，ArrayDeque是一个双向的队列(FIFO)， mTasks.offer()在最后添加一个任务元素，通过mTasks.poll()取得第一个任务元素使用THREAD_POOL_EXECUTOR线程池来执行任务。如果要并发处理调用executeOnExecutor(Executor exec,Params... params)方法，在参数1传入AsyncTask.THREAD_POOL_EXECUTOR
5.handler处理消息
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;
}
}
}
handlerMeaage主要任务处理结果还有任务进度，任务处理结果已经在构造函数实现了，但是发送任务进度消息需要调用publishProgress(Progress... values)来发送消息。
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
