一、概述
Xfermode全名transfer-mode，其作用是实现两张图叠加时的混合效果。
网上流传的关于Xfermode最出名的图来源于AndroidSDK的samples中，名叫Xfermodes.java，效果如下：
二、体验
提炼出Xfermodes.java中的核心代码，自己写了个简单粗暴的demo试试水：
1 public class ImageViewXfermode extends ImageView { 2 public ImageViewXfermode(Context context) { 3 super(context); 4 init(); 5 } 6 7 public ImageViewXfermode(Context context, AttributeSet attrs) { 8 super(context, attrs); 9 init(); 10 } 11 12 public ImageViewXfermode(Context context, AttributeSet attrs, int defStyle) { 13 super(context, attrs, defStyle); 14 init(); 15 } 16 17 private void init() { 18 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) { 19 setLayerType(View.LAYER_TYPE_SOFTWARE, null); 20 } 21 } 22 23 @Override 24 protected void onDraw(Canvas canvas) { 25 int defaultWidth = dip2px(85); //xml里view的宽度是85dp 26 int defaultdHeight = dip2px(85); //xml里view的高度是85dp 27 28 if (canvas.getHeight() == defaultWidth && canvas.getHeight() == defaultdHeight) { 29 //拿到黄色圆形的bitmap 30 Bitmap bitcircle = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888); 31 Canvas canvascicle = new Canvas(bitcircle); 32 Paint paintcicle = new Paint(Paint.ANTI_ALIAS_FLAG); 33 paintcicle.setColor(0xFFFFCC44); 34 canvascicle.drawCircle(dip2px(30), dip2px(30), dip2px(25), paintcicle); 35 36 //拿到蓝色矩形的bitmap 37 Bitmap bitrect = Bitmap.createBitmap(defaultWidth, defaultdHeight, Config.ARGB_8888); 38 Canvas canvasrect = new Canvas(bitrect); 39 Paint paintrect = new Paint(Paint.ANTI_ALIAS_FLAG); 40 paintrect.setColor(0xFF66AAFF); 41 canvasrect.drawRect(dip2px(30), dip2px(30), dip2px(80), dip2px(80), paintrect); 42 43 Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG); 44 Xfermode xfermode = new PorterDuffXfermode(Mode.LIGHTEN); 45 46 //采用saveLayer，让后续canvas的绘制在自动创建的bitmap上 47 int cnt = canvas.saveLayer(0, 0, defaultWidth, defaultdHeight, null, Canvas.ALL_SAVE_FLAG); 48 //先画圆形，圆形是dest 49 canvas.drawBitmap(bitcircle, 0, 0, paint); 50 paint.setXfermode(xfermode); 51 //后画矩形，矩形是src 52 canvas.drawBitmap(bitrect, 0, 0, paint); 53 paint.setXfermode(null); 54 canvas.restoreToCount(cnt); 55 } else { 56 super.onDraw(canvas); 57 } 58 } 59 60 private int dip2px(float dip) { 61 float scale = getResources().getDisplayMetrics().density; 62 return (int)(dip * scale + 0.5f); 63 } 64 }
效果如下：
介绍一下几个关键点：
1、关于src和dest
先绘制到canvas上的是dest，后绘制的是src
2、关于硬件加速
在sdkversion>=11时，需要关闭硬件加速（第19行），否则 Mode.CLEAR 、 Mode.DARKEN 、 Mode.LIGHTEN 三种模式下绘制效果不正常
3、saveLayer的作用
Canvas.saveLayer在Canvas.save的基础上，额外自动分配了一个bitmap，使得saveLayer之后的所有绘制都在这个新分配的bitmap上完成。
如果把saveLayer去掉呢？效果就是蓝色矩形跟黄色圆形和灰色背景都进行了 Mode.LIGHTEN 操作：
4、如果不saveLayer
有一种变通的方法，在dest对应bitmap的canvas上绘制src对应的bitmap，这样的目的与saveLayer是一致的，不在当前canvas上直接绘图：
1 @Override 2 protected void onDraw(Canvas canvas) { 3 int defaultWidth = dip2px(85); //xml里view的宽度是85dp 4 int defaultdHeight = dip2px(85); //xml里view的高度是85dp 5 6 if (canvas.getHeight() == defaultWidth && canvas.getHeight() == defaultdHeight) { 7 //拿到黄色圆形的bitmap 8 Bitmap bitcircle = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888); 9 Canvas canvascicle = new Canvas(bitcircle); 10 Paint paintcicle = new Paint(Paint.ANTI_ALIAS_FLAG); 11 paintcicle.setColor(0xFFFFCC44); 12 canvascicle.drawCircle(dip2px(30), dip2px(30), dip2px(25), paintcicle); 13 14 //拿到蓝色矩形的bitmap 15 Bitmap bitrect = Bitmap.createBitmap(defaultWidth, defaultdHeight, Config.ARGB_8888); 16 Canvas canvasrect = new Canvas(bitrect); 17 Paint paintrect = new Paint(Paint.ANTI_ALIAS_FLAG); 18 paintrect.setColor(0xFF66AAFF); 19 canvasrect.drawRect(dip2px(30), dip2px(30), dip2px(80), dip2px(80), paintrect); 20 21 Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG); 22 Xfermode xfermode = new PorterDuffXfermode(Mode.LIGHTEN); 23 24 //设置setXfermode 25 paint.setXfermode(xfermode); 26 //在圆形的canvas上画矩形，先画圆形，圆形是dest，后画矩形，矩形是src 27 canvascicle.drawBitmap(bitrect, 0, 0, paint); 28 paint.setXfermode(null); 29 //将圆形canvas上画出来的结果绘制到canvas上 30 canvas.drawBitmap(bitcircle, 0.0f, 0.0f, paint); 31 } else { 32 super.onDraw(canvas); 33 } 34 }
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三、更多玩法——圆形ImageView
利用Xfermode的特性也可以做出圆形的ImageView来。先画原图，再画圆形，采用 Mode.DST_IN 即可：
1 @Override 2 protected void onDraw(Canvas canvas) { 3 int defaultWidth = dip2px(85); //xml里view的宽度是85dp 4 int defaultdHeight = dip2px(85); //xml里view的高度是85dp 5 Drawable drawable = getDrawable(); 6 7 if (canvas.getHeight() == defaultWidth && canvas.getHeight() == defaultdHeight && drawable instanceof BitmapDrawable) { 8 //setBackgroundColor(Color.TRANSPARENT); 9 //拿到原图的bitmap 10 Bitmap bitimg = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888); 11 Canvas canvasimg = new Canvas(bitimg); 12 Matrix matrix = new Matrix(); 13 matrix.setScale(defaultWidth * 1.0f / drawable.getIntrinsicWidth(), defaultdHeight * 1.0f / drawable.getIntrinsicHeight()); 14 Paint paintimg = new Paint(Paint.ANTI_ALIAS_FLAG); 15 canvasimg.drawBitmap(((BitmapDrawable)drawable).getBitmap(), matrix, paintimg); 16 17 //拿到圆形的bitmap 18 Bitmap bitcircle = Bitmap.createBitmap(defaultWidth, defaultdHeight, Config.ARGB_8888); 19 Canvas canvascircle = new Canvas(bitcircle); 20 Paint paintcircle = new Paint(Paint.ANTI_ALIAS_FLAG); 21 paintcircle.setColor(0xFF66AAFF); 22 canvascircle.drawCircle(dip2px(85 / 2.0f), dip2px(85 / 2.0f), dip2px(85 / 2.0f), paintcircle); 23 24 Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG); 25 Xfermode xfermode = new PorterDuffXfermode(Mode.DST_IN); 26 27 //采用saveLayer，让后续canvas的绘制在自动创建的bitmap上 28 int cnt = canvas.saveLayer(0, 0, defaultWidth, defaultdHeight, null, Canvas.ALL_SAVE_FLAG); 29 //先画原图，原图是dest 30 canvas.drawBitmap(bitimg, 0, 0, paint); 31 paint.setXfermode(xfermode); 32 //后画圆形，圆形是src 33 canvas.drawBitmap(bitcircle, 0, 0, paint); 34 paint.setXfermode(null); 35 canvas.restoreToCount(cnt); 36 } else { 37 super.onDraw(canvas); 38 } 39 }
[转载请保留本文地址：http://www.cnblogs.com/goagent/p/5326438.html]