Question

我正在学习如何取消asyncTask，因此下面的代码没有用处。

我试图调用asyncTask并执行它然后取消并执行它。

        MyAsyncTask asyncTask = new MyAsyncTask();
        Log.i("cancel","cancel 1");     
        asyncTask.execute();
        Log.i("cancel","cancel 2");
        asyncTask.onCancelled();
        Log.i("cancel","cancel 3");
        asyncTask.execute();
        Log.i("cancel","cancel done");

“取消1”和“取消2”在logcat上显示正确执行，但当“取消3”尝试执行时抛出ActivityThread.performLaunchActivity错误。（在logcat上没有显示取消3）我的asyncTask代码有什么问题吗？

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

    private ArrayList<Map<String, String>> list;
    private ProgressBar progressBar;
    @Override
    protected Void doInBackground(String... arg0) {

        progressBar = (ProgressBar)findViewById(R.id.year_expense_progressbar);
        progressBar.setVisibility(View.VISIBLE);
        getListView().setVisibility(View.GONE);
        textView.setVisibility(View.GONE);

        list = new ArrayList<Map<String, String>>();

        String time = "";
        String category = "";
        String expense = "";
        String day = "";
        String month = "";


        totalExpense = 0;       


        Cursor c = SQLLiteAdapter.fetchAllItems();

        while(c.moveToNext() != false){

            if(isCancelled()){
                Log.e("cancel","cancel inside background");
                break;
            }


            // if there is nothing is input, don't execute verifyLevel
            if(tokenizedResult.isEmpty()) break;


            category = c.getString(c.getColumnIndex(SQLLiteAdapter.col_category));
            expense = c.getString(c.getColumnIndex(SQLLiteAdapter.col_price));
            time = c.getString(c.getColumnIndex(SQLLiteAdapter.col_time));
            day = c.getString(c.getColumnIndex(SQLLiteAdapter.col_day));
            month = c.getString(c.getColumnIndex(SQLLiteAdapter.col_month));


            VerifyLevel a = new VerifyLevel(tokenizedResult,category,expense,time,day,month); 
            if(!a.isEmpty()){
                list.add(a.addToList());
            }           

            totalExpense += a.totalExpense;          
        }



        return null;
    }

    @Override
    protected void onPostExecute(Void result)
    {
        progressBar.setVisibility(View.GONE);
        getListView().setVisibility(View.VISIBLE);
        textView.setVisibility(View.VISIBLE);
        fillData(list);
        textView.setText("Total Expense is "+convertNumeric(totalExpense));         
        Log.i("yearExpense","buildList is finished");

    }

    @Override
    protected void onCancelled(){
        super.onCancelled();
        list.clear();
        progressBar.setVisibility(View.GONE);
        totalExpense = 0;
        Log.i("yearEx","Cancel asyncTask");
        Toast.makeText(getApplicationContext(), "cancelled", Toast.LENGTH_SHORT).show();

    }       
}

Answer 1

我的方法略有不同，也许有点冗长。但它一直没有任何问题。

这段代码放在doInBackground()中。如果其中包含for... loop，请使用break;语句。如果您不使用for.... loop，请将其删除。

// CHECK IF THE USER HAS CANCELLED THE TASK
if (isCancelled())  {
    cancelTask = true; // (OPTIONAL) THIS IS AN ADDITIONAL CHECK I HAVE USED. THINK OF IT AS A FAIL SAFE.
    break; // REMOVE IF NOT USED IN A FOR LOOP
}

您已宣布Asynctask：MyAsyncTask asyncTask = new MyAsyncTask();。除此之外，还创建两个boolean实例。打电话给他们，例如：

boolean blMyAsyncTask;
boolean cancelTask;

现在，在onPreExecute()中，切换blMyAsyncTask实例的状态：

blMyAsyncTask = true;

在onPostExecute()：

blMyAsyncTask = false;

并且，在同一onPostExecute()中，我还在检查cancelTask boolean的状态后执行余数函数。例如：

if (cancelTask == false)    {
    // THE NORMAL CODE YOU HAVE IN YOUR onPostExecute()
}

最后，在onDestroy()中（我使用了这个，但我怀疑onPause()也可以正常工作。从来没有在onPause()诚实地完成它），检查状态boolean blMyAsyncTask

if (blMyAsyncTask== true)   {
    asyncTask.cancel(true);
}

正如我在开始时说的那样，它很长，甚至可能很复杂，但它从未失败过。如果你愿意，我也会把它想象成一个模块化。如果我在Asycntasks添加了更多Activity，我可以在onDestroy()添加另一项检查。

Answer 2

您不应直接致电asyncTask.onCancelled();。您可以致电cancel(true)并在doInBackground()内查看isCancelled()。

Answer 3

您可以使用

asynctask.cancel(true);

或

asyncTask.wait();

如果执行此任务的线程应该被中断，则

“true”;否则，允许正在进行的任务完成

Answer 4

要取消AsyncTask，请拨打以下行。

asyncTask.cancel();

onCancelled（）是一种覆盖方法，在调用cancel时执行。

AsyncTask取消无效

4 个答案: