多次执行AsyncTask

Question

在我的Activity中，我使用了一个从AsyncTask扩展的类和一个参数，该参数是AsyncTask的一个实例。我打电话的时候mInstanceOfAT.execute("")一切都很好。但是当我按下再次调用AsyncTask的更新按钮时应用程序崩溃（如果网络作业不起作用）。因此出现了一个例外情况

无法执行任务：任务已经执行（任务只能执行一次）

我试过为Asyctask的实例调用cancel（true），但它也不起作用。到目前为止唯一的解决方案是创建Asyntask的新实例。这是正确的方法吗？

谢谢

Answer 1

AsyncTask 实例只能使用一次。相反，只需将您的任务称为 new MyAsyncTask().execute("");来自AsyncTask API文档：线程规则此类必须遵循一些线程规则才能正常工作：必须在UI线程上创建任务实例。必须在UI线程上调用execute（Params ...）。不要手动调用onPreExecute（），onPostExecute（Result），doInBackground（Params ...），onProgressUpdate（Progress ...）。该任务只能执行一次（如果尝试第二次执行，则会抛出异常。）

Answer 2

在Steve Prentice的回答中详细说明了ASyncTask发生故障的实例 - 但是，当您执行ASyncTask的次数受到限制时，您可以在线程运行时自由地执行您喜欢的操作。 。将可执行代码放在doInBackground（）中的循环中，并使用并发锁来触发每次执行。您可以使用publishProgress（）/ onProgressUpdate（）检索结果。例：class GetDataFromServerTask extends AsyncTask<Input, Result, Void> {&nbsp; &nbsp; private final ReentrantLock lock = new ReentrantLock();&nbsp; &nbsp; private final Condition tryAgain = lock.newCondition();&nbsp; &nbsp; private volatile boolean finished = false;&nbsp; &nbsp; @Override&nbsp; &nbsp; protected Void doInBackground(Input... params) {&nbsp; &nbsp; &nbsp; &nbsp; lock.lockInterruptibly();&nbsp; &nbsp; &nbsp; &nbsp; do {&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // This is the bulk of our task, request the data, and put in "result"&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Result result = ....&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // Return it to the activity thread using publishProgress()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; publishProgress(result);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // At the end, we acquire a lock that will delay&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // the next execution until runAgain() is called..&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; tryAgain.await();&nbsp; &nbsp; &nbsp; &nbsp; } while(!finished);&nbsp; &nbsp; &nbsp; &nbsp; lock.unlock();&nbsp; &nbsp; }&nbsp; &nbsp; @Override&nbsp; &nbsp; protected void onProgressUpdate(Result... result)&nbsp;&nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; // Treat this like onPostExecute(), do something with result&nbsp; &nbsp; &nbsp; &nbsp; // This is an example...&nbsp; &nbsp; &nbsp; &nbsp; if (result != whatWeWant && userWantsToTryAgain()) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; runAgain();&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }&nbsp; &nbsp; public void runAgain() {&nbsp; &nbsp; &nbsp; &nbsp; // Call this to request data from the server again&nbsp; &nbsp; &nbsp; &nbsp; tryAgain.signal();&nbsp; &nbsp; }&nbsp; &nbsp; public void terminateTask() {&nbsp; &nbsp; &nbsp; &nbsp; // The task will only finish when we call this method&nbsp; &nbsp; &nbsp; &nbsp; finished = true;&nbsp; &nbsp; &nbsp; &nbsp; lock.unlock();&nbsp; &nbsp; }&nbsp; &nbsp; @Override&nbsp; &nbsp; protected void onCancelled() {&nbsp; &nbsp; &nbsp; &nbsp; // Make sure we clean up if the task is killed&nbsp; &nbsp; &nbsp; &nbsp; terminateTask();&nbsp; &nbsp; }}当然，这比传统的ASyncTask使用稍微复杂一点，并且您放弃使用publishProgress（）进行实际的进度报告。但是如果你担心内存，那么这种方法将确保在运行时只有一个ASyncTask保留在堆中。

Answer 3

我遇到过同样的问题。就我而言，我有我想在做任务onCreate()和onResume(）。所以我使我的Asynctask静态，并从中获取实例。现在我们仍然有同样的问题。所以我在onPostExecute（）中做的是这样的：instance = null;请记住，我检查静态getInstance方法，我的实例不为null，否则我创建它：if (instance == null){&nbsp; &nbsp; instance = new Task();}return instance;postExecute中的方法将清空实例并重新创建它。当然这可以在课外完成。