在onPreExecute()内部调用时，TextView.setText()的NullPointerException

Question

在d.android.com中为onPreExecute（）声明，它在doInBackground(Params...)之前的UI线程上运行，因此它应该很容易访问TextView，并从executed()所在的Activity执行setText()方法。

但在下面的代码中，loadingTextView是在扩展Activity的类SplashScreen中私下声明的。在onCreate()中，它与UI的TextView小部件链接。但是当AsyncTask扩展类Atom时，执行函数onPreExecute()，该函数为在其内部执行的语句loading.setText("Loading...");抛出NullPointerExcepction

这里的代码

public class SplashScreen extends Activity implements AnimationListener{
...
TextView loading=null;
...
protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
    setContentView(R.layout.splash_screen);

            try {
                a = (Atom) new Atom().execute(null,null,null);
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
                finish();
            }
...
 loading = (TextView) findViewById(R.id.textView2);
 ....
 }
 public class Atom extends AsyncTask<RSSFeed, Void, RSSFeed>{
    private RSSReader reader;
    private RSSFeed feed = null;
    private String uri = "http://website.com/feed/";
    @Override
    protected void onPreExecute() {
       super.onPreExecute();
      //------------problem----area-------------------
       loading.setText("Loading...");
      //------------problem----area-------------------  

    }
        @Override
        protected RSSFeed doInBackground(RSSFeed... arg0) {

            reader = new RSSReader();
              try {
                feed = reader.load(uri);
                Log.d("rss", feed.getTitle());

            } catch (RSSReaderException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
              return feed;
        }

        @Override
        protected void onPostExecute(RSSFeed result) {
            // TODO Auto-generated method stub
            super.onPostExecute(result);
            prg.cancel();
            t(result.getTitle().toString());
        }

        }
 }

堆栈：

03-09 10:50:12.793: W/System.err(14214): java.lang.NullPointerException
03-09 10:50:12.813: W/System.err(14214):    at in.edu.ss.er.splash.SplashScreen$Atom.onPreExecute(SplashScreen.java:158)
03-09 10:50:12.827: W/System.err(14214):    at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:586)
03-09 10:50:12.833: W/System.err(14214):    at android.os.AsyncTask.execute(AsyncTask.java:534)
03-09 10:50:12.833: W/System.err(14214):    at in.edu.ss.er.splash.SplashScreen.onCreate(SplashScreen.java:45)

Answer 1

在执行asyntask之前尝试初始化TextView。如下所示。

try {
    loading = (TextView) findViewById(R.id.textView2);
    a = (Atom) new Atom().execute(null,null,null);
} catch (Exception e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
    finish();
}

我不知道这是否正确，这是我的猜测，所以，请告诉我发生了什么。

感谢

Answer 2

只需在调用AsyncTask之前初始化文本视图。做一些类似的事情

 loading = (TextView) findViewById(R.id.textView2);
 try {
            a = (Atom) new Atom().execute(null,null,null);
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            finish();
        }

Answer 3

在调用asynctask之前，您必须初始化您的文本视图。将您的代码更改为以下内容-

protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
    setContentView(R.layout.splash_screen);
    loading = (TextView) findViewById(R.id.textView2);
            try {
                a = (Atom) new Atom().execute(null,null,null);
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
                finish();
            }
 }