1、概述

Drawable在我们平时的开发中，基本都会用到，而且给大家非常的有用。那么什么是Drawable呢？能够在canvas上绘制的一个玩意，而且相比于View，并不需要去考虑measure、layout，仅仅只要去考虑如何draw（canavs）。当然了，对于Drawable传统的用法，大家肯定不陌生 ，今天主要给大家带来以下几个Drawable的用法：

1、自定义Drawable，相比View来说，Drawable属于轻量级的、使用也很简单。以后自定义实现一个效果的时候，可以改变View first的思想，尝试下Drawable first。

2、自定义状态，相信大家对于State Drawable都不陌生，但是有没有尝试过去自定义一个状态呢？

3、利用Drawable提升我们的UI Perfermance ， 如何利用Drawable去提升我们的UI的性能。

2、Drawable基本概念

一般情况下，除了直接使用放在Drawable下的图片，其实的Drawable的用法都和xml相关，我们可以使用shape、layer-list等标签绘制一些背景，还可以通过selector标签定义View的状态的效果等。当然了基本每个标签都对应于一个真正的实体类，关系如下：（图片来自：Cyril Mottier ：master_android_drawables）

常见的用法这里就不举例了，下面开始看本文的重点。

2、自定义Drawable

关于自定义Drawable，可以通过写一个类，然后继承自Drawable ， 类似于自定义View，当然了自定义Drawable的核心方法只有一个，那就是draw。那么自定义Drawable到底有什么实际的作用呢？能干什么呢？

相信大家对于圆角、圆形图片都不陌生，那我今天要告诉你，不需要自定义View，自定义Drawable也能实现，而且更加简单、高效、使用范围更广（你可以作为任何View的背景）。

2.1 利用 BitmapShader 

  RoundRectImageDrawable

/**
 * Created by admin on 2015/12/13.
 */
public class RoundRectImageDrawable extends Drawable {

    private Paint mPaint;
    private Bitmap mBitmap;

    private RectF rectF;

    public RoundRectImageDrawable(Bitmap bitmap) {
        mBitmap = bitmap;
        BitmapShader bitmapShader = new BitmapShader(bitmap, Shader.TileMode.CLAMP,
                Shader.TileMode.CLAMP);
        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setShader(bitmapShader);
        rectF = new RectF(0, 0, bitmap.getWidth(), bitmap.getHeight());
    }

    @Override
    public void setBounds(int left, int top, int right, int bottom) {
        super.setBounds(left, top, right, bottom);
        rectF = new RectF(left, top, right, bottom);
    }


    @Override
    public void draw(Canvas canvas) {
        canvas.drawRoundRect(rectF, 30, 30, mPaint);
    }

    @Override
    public int getIntrinsicWidth() {
        return mBitmap.getWidth();
    }

    @Override
    public int getIntrinsicHeight() {
        return mBitmap.getHeight();
    }

    @Override
    public void setAlpha(int alpha) {
        mPaint.setAlpha(alpha);
    }

    @Override
    public void setColorFilter(ColorFilter cf) {
        mPaint.setColorFilter(cf);
    }

    @Override
    public int getOpacity() {
        return PixelFormat.TRANSLUCENT;
    }

}

CircleImageDrawable

/**
 * Created by admin on 2015/12/13.
 */
public class CircleImageDrawable extends Drawable {

    private Paint mPaint;
    private int mWidth;
    private Bitmap mBitmap;
    private RectF rectF;
    public CircleImageDrawable(Bitmap bitmap) {
        mBitmap = bitmap;
        BitmapShader bitmapShader = new BitmapShader(bitmap, Shader.TileMode.CLAMP,
                Shader.TileMode.CLAMP);
        mPaint = new Paint();
        mPaint.setAntiAlias(true);


        mPaint.setShader(bitmapShader);
        rectF = new RectF(0, 0, bitmap.getWidth(), bitmap.getHeight());
        mWidth = Math.min(mBitmap.getWidth(), mBitmap.getHeight());
    }

    @Override
    public void draw(Canvas canvas) {
        canvas.drawCircle(rectF.left+mWidth / 2, rectF.top+mWidth / 2, mWidth / 2, mPaint);
    }

    @Override
    public void setBounds(int left, int top, int right, int bottom) {
        super.setBounds(left, top, right, bottom);
        rectF = new RectF(left, top, right, bottom);
        mWidth = (int)Math.min(rectF.width(), rectF.height());
    }

    @Override
    public int getIntrinsicWidth() {
        return mWidth;
    }

    @Override
    public int getIntrinsicHeight() {
        return mWidth;
    }

    @Override
    public void setAlpha(int alpha) {
        mPaint.setAlpha(alpha);
    }

    @Override
    public void setColorFilter(ColorFilter cf) {
        mPaint.setColorFilter(cf);
    }

    @Override
    public int getOpacity() {
        return PixelFormat.TRANSLUCENT;
    }

}
 2.2 利用Xfermode

   RoundRectImageDrawable
public class RoundRectImageView extends Drawable {private Canvas mCanvas;private Paint mPaint;private Bitmap mBitmap;private RectF rectF;public RoundRectImageView(Bitmap bitmap) {mBitmap = bitmap;mPaint = new Paint();mPaint.setAntiAlias(true);}@Overridepublic void draw(Canvas canvas) {Bitmap target = Bitmap.createBitmap(mBitmap.getWidth(),mBitmap.getHeight(),Config.ARGB_8888);mCanvas = new Canvas(target);mCanvas.drawRoundRect(rectF, 30, 30, mPaint);mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_IN));mCanvas.drawBitmap(mBitmap, rectF.left, rectF.top, mPaint);canvas.drawBitmap(target, 0, 0, null);}@Overridepublic void setAlpha(int alpha) {mPaint.setAlpha(alpha);}@Overridepublic void setColorFilter(ColorFilter cf) {mPaint.setColorFilter(cf);}@Overridepublic int getOpacity() {return PixelFormat.TRANSLUCENT;}    @Override    public void setBounds(int left, int top, int right, int bottom) {    super.setBounds(left, top, right, bottom);    rectF = new RectF(left, top, right, bottom);    }    @Override    public int getIntrinsicWidth() {    return mBitmap.getWidth();    }    @Override    public int getIntrinsicHeight() {    return mBitmap.getHeight();    }}
  CircleImageDrawable
public class CycleImageView extends Drawable {private Canvas mCanvas;private Paint mPaint;      private int mWidth;    private Bitmap mBitmap;     private RectF rectF;public CycleImageView(Bitmap bitmap) {mBitmap=bitmap;mPaint = new Paint();mPaint.setAntiAlias(true);mPaint.setColor(Color.BLUE); rectF = new RectF(0, 0, mBitmap.getWidth(), mBitmap.getHeight());mWidth = Math.min(mBitmap.getWidth(), mBitmap.getHeight());}@Overridepublic void draw(Canvas canvas) {Bitmap target=Bitmap.createBitmap(mWidth, mWidth, Config.ARGB_8888);mCanvas = new Canvas(target);//首先绘制一个圆形mCanvas.drawCircle(rectF.left+mWidth/2, rectF.top+mWidth/2, mWidth/2, mPaint);//设置Xfermode模式mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_IN));//绘制图片mCanvas.drawBitmap(mBitmap, 0,0, mPaint);canvas.drawBitmap(target, 0,0, null);} @Override    public void setBounds(int left, int top, int right, int bottom) {        super.setBounds(left, top, right, bottom);        rectF = new RectF(left, top, right, bottom);        mWidth = (int)Math.min(rectF.width(), rectF.height());    } @Override    public int getIntrinsicWidth() {        return mWidth;    }    @Override    public int getIntrinsicHeight() {        return mWidth;    }@Overridepublic void setAlpha(int alpha) {mPaint.setAlpha(alpha);}@Overridepublic void setColorFilter(ColorFilter cf) {mPaint.setColorFilter(cf);}@Overridepublic int getOpacity() {return PixelFormat.TRANSLUCENT;}}

 在代码中的应用：

/**
 * Created by admin on 2015/12/14.
 */
public class DrawableGeneral extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.drawablegeneral);
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(),
                R.mipmap.example);

        ( (ImageView)findViewById(R.id.pic1)).setImageDrawable(new RoundRectImageDrawable(bitmap));
        ( (ImageView)findViewById(R.id.pic2)).setImageDrawable(new CircleImageDrawable(bitmap));



    }



}