Android Matrix 操作抛出 IllegalStateException

Question

我定义了一个扩展类 View 的自定义视图。

public class MyView extends View{
   public MyView(Context context, AttributeSet attrs) {
    super(context, attrs); 
       Matrix m= new Matrix();
       //this line is ok
       m.setScale(...);
   }   

   public boolean onTouchEvent(MotionEvent event){
       //this same operation throws "IllegalStateException: Matrix can not be modified" 
       m.setScale(...);
   }
}

我想知道为什么我会遇到这样的异常。我在网上搜索但根本找不到任何线索。

Answer 1

在类级别声明Matrix 对象m，如下所示...

public class MyView extends View{

   Matrix m;

   public MyView(Context context, AttributeSet attrs) {
       super(context, attrs); 

       m = new Matrix();
       m.setScale(...);
   }   

   public boolean onTouchEvent(MotionEvent event){

       m.setScale(...);

       return true;
   }
}