Question

m非常新的android，这是我在Stackoverflow上的第一个查询。这是我在使用AsyncTask时获得空指针异常。我正在使用ProgressDialog，因为我创建了一个（单独的（新））类，它扩展了AsynchTask。当我从我的主类中将参数传递给我想要显示ProgressDialog的类时，它给了我一个nullpointer异常.. 可以任何人告诉我该怎么做.. 这是代码

class Loader extends AsyncTask<Object, Object, Object> {
    ProgressDialog dialog;

    public void setDialog(ProgressDialog dialog){
        this.dialog = dialog;
    //here i get the reference of dialog from the caller class

}

提前致谢.. MAHAVEER

Answer 1

无需将对话框作为参数发送到AsyncTask，这可能会导致您的错误。您可以在ProgressDialog中创建新的AsyncTask并在onPreExecute方法中显示，因为此方法在UI线程上运行。希望这会有所帮助。

P.S。如果有错误，请尽量发布错误的logcat，这样可以让人们更容易提供帮助。祝贺你的第一篇文章！

Answer 2

使用重写的onPreExecute方法而不是setDialog

Answer 3

private class LocationControl extends AsyncTask<Context, Void, Void>
{
    private final ProgressDialog dialog = new ProgressDialog(MainActivity.this);
    protected void onPreExecute()
    {
        this.dialog.setMessage("Tap outside dialog to close");
        this.dialog.setTitle("Searching your current location...");
        //this.dialog.setTitle(R.string.hello);
        this.dialog.setCanceledOnTouchOutside(true);
        this.dialog.show();
    }

    protected Void doInBackground(Context... params)
    {
        //Wait 40 seconds to see if we can get a location from either network or GPS, otherwise stop
        Long t = Calendar.getInstance().getTimeInMillis();
        while (!hasLocation && Calendar.getInstance().getTimeInMillis() - t < 40000) {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        };
        return null;
    }

    protected void onPostExecute(final Void unused)
    {
        if(this.dialog.isShowing())
        {
            this.dialog.dismiss();
        }

        if (currentLocation != null)
        {
            GeoPoint startPoint = new GeoPoint((int)(currentLocation.getLatitude()*1E6),(int)(currentLocation.getLongitude()*1E6));
            final MapController mc = mapView.getController();
            mc.animateTo(startPoint);
            mc.setZoom(16);
        }
        else
        {
            /*
            Toast.makeText(MainActivity.this, "Location could not be found\nusing default location", Toast.LENGTH_SHORT).show();
            GeoPoint startPoint = new GeoPoint((int)(43.332628*1E6),(int)(5.353128*1E6));           
            final MapController mc = mapView.getController();
            mc.animateTo(startPoint);
            mc.setZoom(16);
            */
        }
    }
}

从onCreate()：

调用执行

locationControlTask = new LocationControl();
locationControlTask.execute(this);

如何在多个类中使用AsyncTask？它给出了空指针异常

3 个答案: