首先看AsyncTask的execute
爲啥一個任務實例只能執行一次,如果執行第二次將會拋出異常
看下面的代碼,一共有三種狀態PENDING(尚未執行),RUNNING(執行中),FINISHED(已經執行完),只有是未執行則開始執行,否則是執行中或是已經執行完則拋異常.
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;
可以看到在execute中執行了onPreExecute()方法,說明onPreExecute也運行在主線程中
可以看到mWorker存儲了execute傳過來的參數,mWorker是WorkerRunnable類型的,而WorkerRunnable實現了Callable,
Callable類似於Runnable的接口,都是可被其它線程執行的任務,mWorker在AsyncTask的構造方法裏被初始化,可以看到call裏執行了doInBackground,所以就從主線程執行到了子線程
mWorker.mParams = params;
private final WorkerRunnable<Params, Result> mWorker;
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
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;
}
};
exec.execute(mFuture);是線程池,每execute一次會從線程池中取出一個線程執行
mFuture的聲明private final FutureTask<Result> mFuture
FutureTask可以用來封裝一個Runnable或者Callable任務,並異步執行,當用戶想要返回的結果時,只需要調用get方法獲取。
FutureTask繼承了RunnableFuture接口,它可以直接作爲一個Runnable或Callable提交到線程池執行。
在下面的初始化中new FutureTask<Result>(mWorker)可以看到保存了mWorker
也就是說exec.execute(mFuture);這句話就是要執行mFuture的run方法,run方法中result = c.call();這句話就調到了mWorker的call方法,然後就執行了doInBackground,然後將doInBackground執行結果保存在result中,run方法中還有句set(result),set中調用了done方法,這個done方法會被調用,就會執行下面的done方法.(加粗字體的是java的知識)
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);
}
}
};
然後繼續執行,最後走到result.mTask.onProgressUpdate(result.mData);因此通過handler發消息將結果返回到主線程.執行了onProgressUpdate,如果finish則執行onPostExecute
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
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;
}
值得注意的是AsyncTask三種泛型類型分別代表“啓動任務執行的輸入參數”、“後臺任務執行的進度”、“後臺計算結果的類型”。在特定場合下,並不是所有類型都被使用,如果沒有被使用,可以用java.lang.Void類型代替。