當完成（）時使用ProgressDialog時，NullPointerException

Question

我有一個簡單的後臺任務來註冊，顯示用戶的進度對話框。問題是，我相信我得到NullPointerException是因爲在意圖中啓動的第二個活動以某種方式在dialogProgressHide();方法之前調用，即使該代碼在代碼中首先列出，因此不同的上下文和進度正在返回null導致錯誤。

book.saveInBackground(new SaveCallback() 
       { 

        @Override 
        public void done(ParseException e) { 
         if (e == null) 
         { 

          dialogProgressHide(); 
          Toast.makeText(getApplicationContext(), "Your book was posted correctly!", Toast.LENGTH_LONG).show(); 
          finish(); 
         } 
         else 
         { 

          Toast.makeText(getApplicationContext(), "An error has occured: " + e + "\n" + "Please try again!", Toast.LENGTH_LONG).show(); 
          dialogProgressHide(); 
         } 
        } 
       }); 

我可能是錯的，但這對我有意義，問題是我該如何解決這個問題？我只是希望對話在完成活動之前消失，以便它不會嘗試當進度不再處於上下文中時。

下面是對話的方法，如果需要：

private void dialogProgressShow() { 
      mDialog = new ProgressDialog(this); 
      mDialog.setMessage("One moment please while we post your book."); 
      mDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER); 

      mDialog.show(); 
     } 
     private void dialogProgressHide() { 
      if (mDialog.isShowing()) 
      { 
       mDialog.dismiss(); 
       mDialog = null; 
      } 
     } 
    } 

的logcat：

03-25 17:12:44.862: E/AndroidRuntime(16528): FATAL EXCEPTION: main 
03-25 17:12:44.862: E/AndroidRuntime(16528): Process: com.jameswilson.booksale, PID: 16528 
03-25 17:12:44.862: E/AndroidRuntime(16528): java.lang.NullPointerException 
03-25 17:12:44.862: E/AndroidRuntime(16528): at com.jameswilson.booksale.PostBookActivity.dialogProgressHide(PostBookActivity.java:255) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at com.jameswilson.booksale.PostBookActivity.access$1(PostBookActivity.java:254) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at com.jameswilson.booksale.PostBookActivity$2$1.done(PostBookActivity.java:144) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at com.parse.SaveCallback.internalDone(SaveCallback.java:39) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at com.parse.SaveCallback.internalDone(SaveCallback.java:27) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at com.parse.Parse$6$1.run(Parse.java:846) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at android.os.Handler.handleCallback(Handler.java:733) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at android.os.Handler.dispatchMessage(Handler.java:95) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at android.os.Looper.loop(Looper.java:136) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at android.app.ActivityThread.main(ActivityThread.java:5017) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at java.lang.reflect.Method.invokeNative(Native Method) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at java.lang.reflect.Method.invoke(Method.java:515) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
03-25 17:12:44.862: E/AndroidRuntime(16528): at dalvik.system.NativeStart.main(Native Method) 

Answer 1

檢查您的對話框參考是試圖調用`支持mDialog之前關閉它

private void dialogProgressHide() { 
    if (mDialog != null && mDialog.isShowing()) 
    { 
     mDialog.dismiss(); 
     mDialog = null; 
    } 
}