Android BottomSheetBehavior setState（）NullPointerException

Question

我試圖從Android支持設計庫實現BottomSheetBehavior。我初始化BottomSheetBehavior這樣的：

private void initBottomSheet() { 
     new AsyncTask<Void, Void, Void>() { 

      View bottomSheetFrame = rootView.findViewById(R.id.bottom_sheet); 

      } 

      @Override 
      protected Void doInBackground(Void... params) { 
       bottomSheetBehavior = BottomSheetBehavior.from(bottomSheetFrame); 

       bottomSheetBehavior.setBottomSheetCallback(new BottomSheetBehavior.BottomSheetCallback() { 
        private boolean isOnTop = false; 

        @Override 
        public void onStateChanged(@NonNull View bottomSheet, int newState) { 
         switch (newState) { 

          case BottomSheetBehavior.STATE_DRAGGING: { 
           ... 
          } 

          case BottomSheetBehavior.STATE_SETTLING: { 
           ... 
          } 

          case BottomSheetBehavior.STATE_EXPANDED: { 
           ... 
          } 

          case BottomSheetBehavior.STATE_COLLAPSED: { 
           ... 
          } 

          case BottomSheetBehavior.STATE_HIDDEN: { 
           ... 
          } 

          default: { 
           break; 
          } 
         } 
        } 

        @Override 
        public void onSlide(@NonNull View bottomSheet, float slideOffset) { 
         ... 
       }); 

       bottomSheetBehavior.setPeekHeight((int) Utils.convertDpToPixel(100f, activityContext)); 
        bottomSheetBehavior.setState(BottomSheetBehavior.STATE_HIDDEN); // NPE here 

       return null; 
      } 

      @Override 
      protected void onPostExecute(Void aVoid) { 
       super.onPostExecute(aVoid); 
      } 
     }.execute(); 
    } 

這是非常奇怪的，因爲我可以Button點擊或其他一些行動改變狀態。請幫幫我。

Answer 1

問題

NPE是因爲您在視圖之前調用BottomSheetBehavior.setState(...) - bottomSheetFrame被計劃了。此時BottomSheetBehavior在視圖上有null引用，並且無法將您的狀態應用於該視圖。

解決方案

我解決了這個問題，通過使用View.post(Runnable)方法：

View sheetView = ... ;  
BottomSheetBehavior behavior = BottomSheetBehavior.from(sheetView); 
int initState = BottomSheetBehavior.STATE_EXPANDED; 

sheetView.post(new Runnable() { 
    @Override 
    public void run() { 
     behavior.setState(initState); 
    } 
}); 

在我的情況這有助於阻止NPE-S :)