通过反射实现圆角ImageView

private void init(){
        paint = new Paint(Paint.ANTI_ALIAS_FLAG);        
        roundRect = new RectF(0, 0, getWidth() , getHeight());
        radius = 40;
        mPorterDuffXfermode = new PorterDuffXfermode(PorterDuff.Mode.SRC_IN) ;
    }

继承ImageView，在构造方法中调用，初始化Paint和Xfermode。

 protected void onDraw(Canvas canvas) {        
        int sc = canvas.saveLayer(0, 0, getWidth()  , getHeight(), null,
                Canvas.MATRIX_SAVE_FLAG |
                Canvas.CLIP_SAVE_FLAG |
                Canvas.HAS_ALPHA_LAYER_SAVE_FLAG |
                Canvas.FULL_COLOR_LAYER_SAVE_FLAG |
                Canvas.CLIP_TO_LAYER_SAVE_FLAG);        
        roundRect.set(0, 0, getWidth(), getHeight());
        canvas.drawRoundRect(roundRect, radius, radius, paint);        
        reflectSetXfermod();        
        super.onDraw(canvas);        
        canvas.restoreToCount(sc);
    }

重写ImageView的onDraw方法，通过xfermode实现圆角

private void reflectSetXfermod(){
        Drawable drawable =  getDrawable();
        if(drawable == null){
            return;
        }        
        
        Class bsClass = null;
        Class[] innerClasses = BitmapDrawable.class.getDeclaredClasses();
        for(Class innerClass :innerClasses)
        {
            String name = innerClass.getName();
            System.out.println("-----innerClass---"+name);
            if(name.equals("android.graphics.drawable.BitmapDrawable$BitmapState"))
            {
                bsClass = innerClass;
            }            
        }
        
        if(bsClass!= null){            
            try {
                Field mPaintField = bsClass.getDeclaredField("mPaint");
                mPaintField.setAccessible(true);
                ConstantState constantState = ((BitmapDrawable)drawable).getConstantState();
                Paint paint = (Paint)mPaintField.get(constantState);
                paint.setXfermode(mPorterDuffXfermode);
            } catch (IllegalArgumentException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (NoSuchFieldException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

通过反射的方法将xfermode设置到BitmapDrawable 里面的内部类BitmapState里的对象mPaint，用来绘制图片。