android ViewGroup刪除子視圖時應該注意的一個問題

在Activity中建立一個LinearLayout，建立一個scrollHorizon對象（scrollHorizon繼承自ViewGroup），在scrollHorizon中調用createLayout函數來載入myView（繼承自View）對象，然後調用deleteAllView函數來刪除所有的視圖。

scrollHorizon代碼：

public class scrollHorizon extends ViewGroup {private Context context;public scrollHorizon(Context context) {super(context);this.context = context;createLayout();deleteAllView();}@Overrideprotected void onLayout(boolean changed, int l, int t, int r, int b) {int childCount = this.getChildCount();int childLeft = 0;int childTop = 0;for(int i = 0; i < childCount; i++){View child = getChildAt(i);child.setVisibility(View.VISIBLE);child.measure(r - l, b - t);child.layout(childLeft, childTop, childLeft + child.getMeasuredWidth(),childTop + child.getMeasuredHeight());if(childLeft <= 320){childLeft = childLeft + child.getMeasuredWidth();}else{childLeft = 0;childTop = childTop + child.getMeasuredHeight();}}}public void createLayout(){Resources resource = this.getResources();PackageManager pmanager = context.getPackageManager();final Intent mainIntent = new Intent(Intent.ACTION_MAIN, null);                mainIntent.addCategory(Intent.CATEGORY_LAUNCHER);                        List<ResolveInfo> list = pmanager.queryIntentActivities(mainIntent, 0);                    for( int i = 0; i < list.size(); i++){                  int icon = R.drawable.contacts_button_normal;                  LinearLayout linear = new LinearLayout(context);                  linear.setLayoutParams(new LayoutParams(45, 45));                  linear.setBackgroundDrawable(this.getResources().getDrawable(R.drawable.icon));                          ImageView image2 = new ImageView(context);                  image2.setBackgroundDrawable(this.getResources().getDrawable(R.drawable.icon));                  Drawable drawable = resource.getDrawable(icon);                          ImageView image = new ImageView(context);                          image.setBackgroundDrawable(drawable);                  linear.addView(image2);                  linear.addView(image);                  this.addView(linear);               }}public void deleteAllView(){int size = this.getChildCount();for( int i = 0; i < size; i++){this.removeViewAt(i);}}}

myView代碼：

public class myView extends View {public myView(Context context) {super(context);setLayoutParams(new LayoutParams(40,40));}}

主Activity代碼：

public class scrollHorizonTest extends Activity {           public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);                LinearLayout linear = new LinearLayout(this);                scrollHorizon horizon = new scrollHorizon(this);              linear.addView(horizon);                 this.setContentView(linear);    }}

顯示結果：

如果不調用deleteAllView函數，那程式沒有問題，可以正常的進行顯示，但調用deleteAllView函數存在null 指標異常。

原因為：當你刪除掉第一個View後（假設總共有兩個View），當前ViewGroup的子視圖只有一個，而這個子視圖的位置變為0，所以當你調用removeViewAt(1)時會出現null 指標異常，正確的刪除方法應該是將deleteAllView函數中的removeViewAt(i)修改為removeViewAt(0);