———继承自AppCompatImageView，没有附加属性，可像ImageView一样使用。

如需下载源码，请访问
https://github.com/fengchuanfang/CircleImageView

文章原创，转载请注明出处：
史上最简洁高效的圆形ImageView

运行效果如下：

circle_imageview.png

实现思路：

自定义CircleImageView继承自AppCompatImageView，重写onDraw方法，

通过BitmapDrawable的方法

    drawable.getBitmap()

获取ImageView所展示图片的Bitmap

通过此Bitmap创建一个着色器BitmapShader，

    BitmapShader bitmapShader = new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);

根据图片Bitmap的宽高和控件CircleImageView的宽高，通过缩放矩阵Matrix对BitmapShader进行适当缩放，使图片撑满控件

    float scale = Math.max(width / bitmap.getWidth(), height / bitmap.getHeight());
    matrix.setScale(scale, scale);//将图片宽高等比例缩放，避免拉伸
    bitmapShader.setLocalMatrix(matrix);

然后将着色器设置给画笔Paint，并通过Canvas的drawCircle方法，以控件中心为圆心画内切圆。

    paint.setShader(initBitmapShader((BitmapDrawable) drawable));//将着色器设置给画笔
    canvas.drawCircle(width / 2, height / 2, radius, paint);//使用画笔在画布上画圆

完整代码如下：

package com.edward.feng.circleimageview.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.Shader;
import android.graphics.drawable.BitmapDrawable;
import android.support.annotation.Nullable;
import android.support.v7.widget.AppCompatImageView;
import android.util.AttributeSet;

/**
 * 功能描述：一个简洁而高效的圆形ImageView
 *
 * @author (作者) edward（冯丰枫）
 * @link http://www.jianshu.com/u/f7176d6d53d2
 * 创建时间： 2018/4/17 0017
 */
public class CircleImageView extends AppCompatImageView {
    private float width;
    private float height;
    private float radius;
    private Paint paint;
    private Matrix matrix;

    public CircleImageView(Context context) {
        this(context, null);
    }

    public CircleImageView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public CircleImageView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        paint = new Paint();
        paint.setAntiAlias(true);   //设置抗锯齿
        matrix = new Matrix();      //初始化缩放矩阵
    }

    /**
     * 测量控件的宽高，并获取其内切圆的半径
     */
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        width = getMeasuredWidth();
        height = getMeasuredHeight();
        radius = Math.min(width, height) / 2;
    }

     @Override
    protected void onDraw(Canvas canvas) {
        Drawable drawable = getDrawable();
        if (drawable == null) {
            super.onDraw(canvas);
            return;
        }
        if (drawable instanceof BitmapDrawable) {
            paint.setShader(initBitmapShader((BitmapDrawable) drawable));//将着色器设置给画笔
            canvas.drawCircle(width / 2, height / 2, radius, paint);//使用画笔在画布上画圆
            return;
        }
        super.onDraw(canvas);
    }

    /**
     * 获取ImageView中资源图片的Bitmap，利用Bitmap初始化图片着色器,通过缩放矩阵将原资源图片缩放到铺满整个绘制区域，避免边界填充
     */
    private BitmapShader initBitmapShader(BitmapDrawable drawable) {
        Bitmap bitmap = drawable.getBitmap();
        BitmapShader bitmapShader = new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        float scale = Math.max(width / bitmap.getWidth(), height / bitmap.getHeight());
        matrix.setScale(scale, scale);//将图片宽高等比例缩放，避免拉伸
        bitmapShader.setLocalMatrix(matrix);
        return bitmapShader;
    }
}

在xml布局文件中，直接当imageview使用即可

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#ffeeeeff"
        android:orientation="vertical"
        android:padding="5dp"
        tools:context=".MainActivity">
    <com.edward.feng.circleimageview.view.CircleImageView
            android:id="@+id/girls01"
            android:layout_width="100dp"
            android:layout_height="100dp"
            android:src="@mipmap/natural_beauty_01"
            android:contentDescription="@string/app_name"/>
    <com.edward.feng.circleimageview.view.CircleImageView
            android:id="@+id/girls02"
            android:layout_width="130dp"
            android:layout_height="130dp"
            android:src="@mipmap/natural_beauty_02"
            android:contentDescription="@string/app_name"
            android:layout_marginTop="93dp"/>
    <com.edward.feng.circleimageview.view.CircleImageView
            android:id="@+id/girls03"
            android:layout_width="200dp"
            android:layout_height="200dp"
            android:src="@mipmap/natural_beauty_03"
            android:layout_below="@id/girls02"
            android:contentDescription="@string/app_name"/>
    <com.edward.feng.circleimageview.view.CircleImageView
            android:id="@+id/girls04"
            android:layout_width="250dp"
            android:layout_height="250dp"
            android:src="@mipmap/natural_beauty_05"
            android:layout_alignParentRight="true"
            android:layout_alignParentEnd="true"
            android:contentDescription="@string/app_name"/>
    <com.edward.feng.circleimageview.view.CircleImageView
            android:id="@+id/girls05"
            android:layout_width="170dp"
            android:layout_height="170dp"
            android:src="@mipmap/natural_beauty_04"
            android:layout_below="@id/girls04"
            android:layout_alignParentRight="true"
            android:layout_alignParentEnd="true"
            android:contentDescription="@string/app_name"/>
</RelativeLayout>