1.创建AsyncTask
new AsyncTask<String,String,String>(){
// 运行在主线程中，做预备工作
onPreExecute(){
}
// 运行在子线程中，做耗时操作
String doingBackGround(String s){
}
// 运行在主线程中，耗时操作完成，更新UI
onPostExecute(String s){
}
}.execute(String);
2.AsyncTask的execute方法
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
...
mStatus = Status.RUNNING;
// 在主线程中执行准备操作
onPreExecute();
// 把params参数赋值给mWorker
mWorker.mParams = params;
// 用线程池执行mFuture
sExecutor.execute(mFuture);
// 在AsyncTask构造方法中创建了mWorker
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
...
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
...
}
};
}
// 把mWorker传递给FutureTask，callable指的就是mWorker
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
sync = new Sync(callable);
}
// 把mWorker传递给Sync，callable指的是mWorker
Sync(Callable<V> callable) {
this.callable = callable;
}
3.线程池执行FutureTask，就是执行FutureTask的run方法,代码如下：
public void run() {
// 转调
sync.innerRun();
}
void innerRun() {
if (!compareAndSetState(READY, RUNNING))
return;
runner = Thread.currentThread();
if (getState() == RUNNING) { // recheck after setting thread
V result;
try {
// 就是调用了mWorker.call方法
// 把耗时操作得到的结果赋值给result
result = callable.call();
} catch (Throwable ex) {
setException(ex);
return;
}
// 转调了sync.innerSet(v);
set(result);
} else {
releaseShared(0); // cancel
}
}
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
// 执行耗时操作 在子线程中执行
return doInBackground(mParams);
}
};
protected void set(V v) {
// 转调
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);
// 调用了FutureTask的抽象方法
done();
return;
}
}
}
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
Message message;
Result result = null;
try {
// 转调了sync.innerGet()
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);
}
// 发送了一个Message
message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(AsyncTask.this, result));
message.sendToTarget();
}
};
public V get() throws InterruptedException, ExecutionException {
// 转调
return sync.innerGet();
}
V innerGet() throws InterruptedException, ExecutionException {
acquireSharedInterruptibly(0);
if (getState() == CANCELLED)
throw new CancellationException();
if (exception != null)
throw new ExecutionException(exception);
// 把之前doinBackground方法的结果返回
return result;
}
3.在AsyncTask的成员变量，创建了InternalHandler
private static class InternalHandler extends Handler {
@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
// 结束耗时操作完成后的消息
// 调用了AsyncTask的finish方法传递的result.mData[0]就是之前
// 耗时操作返回来的结果
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 static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
// data 是返回的结果
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
private void finish(Result result) {
if (isCancelled()) result = null;
// 耗时操作完成，更新UI，执行在主线程
onPostExecute(result);
mStatus = Status.FINISHED;
}