使用BitmapShader显示圆形头像

对于一些应用app特别是社交app中，通常需要用户登陆后显示一个圆形的头像作为用户的标识。普通的ImageView是不可以实现这个功能的，那么怎么才能画出一个圆形头像呢？BitmapShader可以帮我们解决这个问题。
首先来看看BitmapShader的定义，如下：

Shader used to draw a bitmap as a texture. The bitmap can be repeated or mirrored by setting the tiling mode.

着色器（shader）用于将bitmap作为纹理绘制到画布上，超出bitmap的部分根据不同的mode作出不同的绘制要求。

CLAP 渲染器超出原始的位置，将会取边缘的颜色绘制 MIRROR 以镜像的方式平铺 REPEAT 横向和纵向重复渲染，类似与平铺

生成BitmapShader的实例之后，就可以使用paint.setShader(shader)设置paint中将要画的bitmap。所以最后还是通过canvas的drawCircle来完成我们的需求的。

package com.app.motion.bitmapshadermotion.view;import android.content.Context;import android.graphics.Bitmap;import android.graphics.BitmapShader;import android.graphics.Canvas;import android.graphics.Matrix;import android.graphics.Paint;import android.graphics.Rect;import android.graphics.Shader;import android.graphics.drawable.Drawable;import android.util.AttributeSet;import android.widget.ImageView;/** * Created by joe.wang on 2016/9/12. */public class PhotoImageView extends ImageView {    private int mWdith;    private int mHeight;    private Paint mPaint;    private BitmapShader mBitmapShader;    private Bitmap mBitmap;    private Rect mDrawableRect;    private Matrix mShaderMartix;    public PhotoImageView(Context context) {        this(context, null);    }    public PhotoImageView(Context context, AttributeSet attrs) {        super(context, attrs);        init();    }    private void init() {        mPaint = new Paint();        mPaint.setAntiAlias(true);        mShaderMartix = new Matrix();    }    @Override    protected void onSizeChanged(int w, int h, int oldw, int oldh) {        super.onSizeChanged(w, h, oldw, oldh);        mWdith = w;        mHeight = h;        fillDrawableRect();        setup();    }    private void setup() {        if (mBitmap == null) {            getDrawableBitmap();        }    }    /**     * 获取当前设置的android:src属性中的图片bitmap     */    private void getDrawableBitmap() {        if (getDrawable() == null) {            return;        } else {            Drawable d = getDrawable();            Bitmap bitmap = Bitmap.createBitmap(d.getIntrinsicWidth(), d.getIntrinsicHeight(), Bitmap.Config.ARGB_8888);            Canvas canvas = new Canvas(bitmap);            d.draw(canvas);            mBitmap = bitmap;        }    }    @Override    protected void onDraw(Canvas canvas) {        if (mBitmap == null) {            super.onDraw(canvas);        } else {            getDrawableBitmap();            mBitmapShader = new BitmapShader(mBitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);            updateShaderMatrix();            mPaint.setShader(mBitmapShader);            canvas.drawCircle(mDrawableRect.centerX(), mDrawableRect.centerY(), mDrawableRect.width() / 2, mPaint);        }    }    /**     * 获取一个正方形，用于绘制圆形     */    private void fillDrawableRect() {        int avaliableWidth = mWdith - getPaddingLeft() - getPaddingRight();        int avaliableHeight = mHeight - getPaddingTop() - getPaddingBottom();        int slideLength = Math.min(avaliableWidth, avaliableHeight);        int top = getPaddingTop() + (avaliableHeight - slideLength) / 2;        int left = getPaddingLeft() + (avaliableWidth - slideLength) / 2;        mDrawableRect = new Rect(left, top, left + slideLength, top + slideLength);    }    /**     * 根据不同的比例大小设置bitmap的缩放大小和偏移量     */    private void updateShaderMatrix() {        float scale = 0;        int dx = 0;        int dy = 0;        if (mBitmap.getWidth() * mDrawableRect.height() > mBitmap.getHeight() * mDrawableRect.width()) {            scale = mDrawableRect.width() * 1.0f / mBitmap.getWidth();            dy = (int) ((mDrawableRect.height() - mBitmap.getHeight() * scale) * 0.5f);        } else {            scale = mDrawableRect.height() * 1.0f / mBitmap.getHeight();            dx = (int) ((mDrawableRect.width() - mBitmap.getWidth() * scale) * 0.5f);        }        mShaderMartix.setScale(scale, scale);        mShaderMartix.postTranslate(dx + mDrawableRect.left, dy + mDrawableRect.top);        mBitmapShader.setLocalMatrix(mShaderMartix);    }}