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。