Android 如果异步任务被取消，您要做些什么？

我正在使用AsyncTask填充SQLite数据库。我正在从某个网页下载数据并将其放入SQLite表中。问题是，我想要么下载100%的数据，要么不下载。因此，如果AsyncTask由于某种原因被中断，我希望删除迄今为止已下载的所有数据

我就是这样做的：

@Override
protected void onCancelled() {
    super.onCancelled();
    dbHandler.deleteFromDatabase(razred);
    Log.i("TAG", "AsyncTask cancelled");
}

---

我认为，如果AsyncTask以任何方式被中断，onCancelled将执行，但事实并非如此。在以任何方式取消AsyncTask时，如何擦除使用AsyncTask生成的数据？例如，活动暂停、活动破坏、互联网连接中断等。

您的做法是正确的，但在您的doInBackground中，您还需要特别致电isCancelled，检查是否取消，然后从doInBackground返回。那么你的代码就可以正常工作了

有关取消任务的信息，请参阅

以下是文档中的引用，以便于参考：

可以通过调用cancelboolean随时取消任务。调用此方法将导致对isCancelled的后续调用返回true。调用此方法后，将在doInBackgroundObject[]返回后调用onCancelledObject，而不是onPostExecuteObject。为了确保尽快取消任务，您应该始终从doInBackgroundObject[]定期检查isCancelled的返回值，如果可能的话，例如在循环中

编辑：根据请求，一些示例代码：

private class MyAsyncTask extends AsyncTask<Void,Void,Void> {

    private SQLiteDatabase db;

    @Override
    protected void onPreExecute() {
        // any kind of initialization or setup needed before the
        //     background thread kicks off. remember: this is still on
        //     on the main (UI) thread

        // since youre doing DB I/O, Ill make believe Im initializing the DB here
        db = DatabaseHelper.getInstance(MainActvity.this).getWritableDatabase();
    }

    /*
     * The background thread to do your disk and network I/O. If you need
     * to pass in any parameters, this is the first Void in the template
     */
    @Override
    protected Void doInBackground(Void... params) {

        // other stuff you need to do in the background. Since you want an
        //   all-or-nothing type thing, we will use a transaction to manually
        //   control the db
        db.beginTransaction();
        try {
            // do network I/O to retrieve what you need and then write to DB.
            ...
            ... // if theres a loop in here somewhere when reading the data, check !isCancelled() as part of the condition or as one of the first statements and then break
            ...
            db.setTransactionSuccessful(); // assuming everything works, need to set
                                         // this successful here at the end of the try
        } catch (InterruptedException ie) { // or some other exception
            cancel(true); // heres where you can call cancel() if youve been interrupted
        } catch (IOException ioe) { // if your network connection has problems
            cancel(true);
        } finally {
            db.endTransaction();
            // other cleanup, like closing the HTTP connection...
            //   no need to close the DB if you implement it properly
        }
        return null; // if there was some return value, that would go here
    }

    @Override
    protected void onCancelled(Void result) {
        // depending on how you implement doInBackground(), you may not even need this,
        //    unless you have a lot of other "state" you need to reset aside from the DB transaction
    }

    @Override
    protected void onPostExecute(Void result) {
        // any other items to do on main (UI) thread after doInBackground() finishes
        //   remember, this only gets called if cancel() is not called!
    }
}

---

希望有帮助

您走的是正确的道路，但在您的doInBackground中，您还需要专门致电isCancelled，检查它是否已取消，然后从doInBackground返回。那么你的代码就可以正常工作了

有关取消任务的信息，请参阅

以下是文档中的引用，以便于参考：

可以通过调用cancelboolean随时取消任务。调用此方法将导致对isCancelled的后续调用返回true。调用此方法后，将在doInBackgroundObject[]返回后调用onCancelledObject，而不是onPostExecuteObject。为了确保尽快取消任务，您应该始终从doInBackgroundObject[]定期检查isCancelled的返回值，如果可能的话，例如在循环中

编辑：根据请求，一些示例代码：

private class MyAsyncTask extends AsyncTask<Void,Void,Void> {

    private SQLiteDatabase db;

    @Override
    protected void onPreExecute() {
        // any kind of initialization or setup needed before the
        //     background thread kicks off. remember: this is still on
        //     on the main (UI) thread

        // since youre doing DB I/O, Ill make believe Im initializing the DB here
        db = DatabaseHelper.getInstance(MainActvity.this).getWritableDatabase();
    }

    /*
     * The background thread to do your disk and network I/O. If you need
     * to pass in any parameters, this is the first Void in the template
     */
    @Override
    protected Void doInBackground(Void... params) {

        // other stuff you need to do in the background. Since you want an
        //   all-or-nothing type thing, we will use a transaction to manually
        //   control the db
        db.beginTransaction();
        try {
            // do network I/O to retrieve what you need and then write to DB.
            ...
            ... // if theres a loop in here somewhere when reading the data, check !isCancelled() as part of the condition or as one of the first statements and then break
            ...
            db.setTransactionSuccessful(); // assuming everything works, need to set
                                         // this successful here at the end of the try
        } catch (InterruptedException ie) { // or some other exception
            cancel(true); // heres where you can call cancel() if youve been interrupted
        } catch (IOException ioe) { // if your network connection has problems
            cancel(true);
        } finally {
            db.endTransaction();
            // other cleanup, like closing the HTTP connection...
            //   no need to close the DB if you implement it properly
        }
        return null; // if there was some return value, that would go here
    }

    @Override
    protected void onCancelled(Void result) {
        // depending on how you implement doInBackground(), you may not even need this,
        //    unless you have a lot of other "state" you need to reset aside from the DB transaction
    }

    @Override
    protected void onPostExecute(Void result) {
        // any other items to do on main (UI) thread after doInBackground() finishes
        //   remember, this only gets called if cancel() is not called!
    }
}

---

希望有帮助

我认为在onpostexecute中，您可以处理任何您想处理的事情

   private class ParseDownload extends AsyncTask<Summary, Void, Boolean> {

    @Override
    protected Boolean doInBackground(Summary... urls) {


        for (Summary url : urls) {
            url.dosomething();
                    if (isCanceled();) { return false;}
        }

        return true;
    }

    @Override
    protected void onPostExecute(Boolean result) {
        if (!result) {

                            // delete * from yourtable here...
                            // and mark the download incomplete etc.
        }
    }
}

---

祝你好运

我认为在onpostexecute中，你可以处理任何你想处理的事情

   private class ParseDownload extends AsyncTask<Summary, Void, Boolean> {

    @Override
    protected Boolean doInBackground(Summary... urls) {


        for (Summary url : urls) {
            url.dosomething();
                    if (isCanceled();) { return false;}
        }

        return true;
    }

    @Override
    protected void onPostExecute(Boolean result) {
        if (!result) {

                            // delete * from yourtable here...
                            // and mark the download incomplete etc.
        }
    }
}

祝你好运

我知道这并不完全是你所要求的，但我不得不说，你使用异步任务做的都是错的

在许多情况下，异步任务将在您无法执行任何操作的情况下终止。对于像这样的关键任务，请使用服务

使用服务，您可以在系统重新启动服务之前停止，以防服务过早终止。然后，您可以继续您开始的内容，或者重新开始删除所有以前的下载…等等

对于异步任务，如果系统决定提前终止异步任务，则不会通知您，也不会重新启动异步任务。它只是在完全的沉默中死去。

我知道这并不完全是您所要求的，但我必须说，您使用AsyncTask做的都是错误的

在许多情况下，异步任务将在您无法执行任何操作的情况下终止。对于像这样的关键任务，请使用服务

使用服务，您可以在系统重新启动服务之前停止，以防服务过早终止。然后，您可以继续您开始的内容，或者重新开始删除所有以前的下载…等等

---

对于异步任务，如果系统决定提前终止异步任务，则不会通知您，也不会重新启动异步任务。它只是在完全沉默中死去。

谢谢你的回答！你有没有办法给我看一些关于如何做的示例代码？有点困惑谢谢你的回答！你有没有办法给我看一些关于如何做的示例代码？这有点令人困惑