什么是Drawable ?

Android 把所有显示出来的图形都抽象为drawable ，意思就是可绘制的，包括，图片、色块、背景等。

简单的drawable都是静态的图形，当然也有其他的，比如StateListDrawable，可以根据不同的触摸来变换不同的图形。也是通常我们在drawable文件中创建的selector.xml

我们通过简单三个例子来了解一下自定义drawable

效果图如下

圆形图像的实现方式。

/**
 * Created by gesangdianzi on 2017/6/18.
 */

public class CircleDrawable extends BitmapDrawable {

    private final Paint mPaint;

    public CircleDrawable(Resources resources, Bitmap bitmap){
        super(resources,bitmap);
        BitmapShader shader=new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setShader(shader);
        mPaint.setColor(Color.RED);
    }

    @Override
    public void draw(Canvas canvas) {
        int width = getBitmap().getWidth();
        int height = getBitmap().getHeight();
        int radius = Math.min(width/2, height/2);
        int x = (width>radius+radius)?width/2:radius;
        int y = (height>radius+radius)?height/2:radius;
        canvas.drawCircle(x,y,radius,mPaint);
    }

    /**
     * 这两个方法可以获取到drawable绘制的区域，不过ImageView的源码中调用的是下面的
     * @param bounds
     */
    @Override
    public void setBounds(Rect bounds) {
        super.setBounds(bounds);
    }

    /**
     * 这两个方法可以获取到drawable绘制的区域 Imagview中会调用该方法，设置宽高
     */
    @Override
    public void setBounds(int left, int top, int right, int bottom) {
        super.setBounds(left, top, right, bottom);
    }
}

圆角图像的实现方式。

public class RoundDrawable extends BitmapDrawable {


    private final Paint paint;
    private float radisu;
    private RectF rectF;

    public RoundDrawable(Resources resources, Bitmap bitmap){
        super(resources,bitmap);
        BitmapShader shader=new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        paint = new Paint();
        paint.setAntiAlias(true);
        paint.setShader(shader);
    }

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

    public void setRadius(float radisu){

        this.radisu = radisu;
    }


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

椭圆图像的实现方式。

public class OvalDrawable extends BitmapDrawable {

    private final Paint mPaint;
    private RectF rectF;

    public OvalDrawable(Resources res, Bitmap bitmap){
        super(res,bitmap);
        BitmapShader shader=new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setDither(true);
        mPaint.setShader(shader);
    }

    @Override
    public void draw(Canvas canvas) {
        canvas.drawOval(rectF,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);
    }
}

调用

public void onClick(View v) {
        switch (v.getId()){

            case R.id.circle:
                Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.demobg);
                CircleDrawable drawable=new CircleDrawable(getResources(),bitmap);
                imageView.setImageDrawable(drawable);
                break;

            case R.id.oval:
                Bitmap ovalBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.demobg);
                OvalDrawable ovalDrawable=new OvalDrawable(getResources(),ovalBitmap);
                imageView.setImageDrawable(ovalDrawable);
                break;

            case R.id.round:
                Bitmap roundBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.demobg);
                RoundDrawable roundDrawable=new RoundDrawable(getResources(),roundBitmap);
                roundDrawable.setRadius(dp2px(30));
                imageView.setImageDrawable(roundDrawable);
                break;

        }
    }

以上三个例子非常简单，作为对自定义drawable的理解是够了