通过裁切 onDraw 里面的 canvas，让其在绘制内容之前，先裁切出一个圆形。

CircleImageView.java

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Path;
import android.util.AttributeSet;

/**
 * 通过裁切 onDraw 里面的 canvas，让其在绘制内容之前，先裁切出一个圆形出来即可！
 */
public class CircleImageView extends android.support.v7.widget.AppCompatImageView {

    private Path circlePath;

    public CircleImageView(Context context) {
        super(context);
    }

    public CircleImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public CircleImageView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }


    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int wM = MeasureSpec.getMode(widthMeasureSpec);
        int hM = MeasureSpec.getMode(heightMeasureSpec);
        int measuredHeight = getMeasuredHeight();
        int measuredWidth = getMeasuredWidth();
        if (wM != MeasureSpec.EXACTLY && hM != MeasureSpec.EXACTLY) {
            // 两个都是未指定
            int size = Math.min(measuredWidth, measuredHeight);
            setMeasuredDimension(size, size); // 变成正方形
        } else if (hM != MeasureSpec.EXACTLY) {
            // 宽度是固定的
            setMeasuredDimension(measuredWidth, measuredWidth);
        } else if (wM != MeasureSpec.EXACTLY) {
            // 高度是固定的
            setMeasuredDimension(measuredHeight, measuredHeight);
        }
    }

    @Override
    protected void onDraw(Canvas canvas) {
        if (circlePath == null) {
            circlePath = new Path();
        }
        int x = (getWidth() + getPaddingLeft() - getPaddingRight()) / 2;
        int y = (getHeight() + getPaddingTop() - getPaddingBottom()) / 2;
        int radius = Math.min((getWidth() - getPaddingLeft() - getPaddingRight()),
                (getHeight() - getPaddingTop() - getPaddingBottom())) / 2;
        circlePath.addCircle(x, y, radius, Path.Direction.CCW);
        // 只有 add 多个才能看出效果
        canvas.clipPath(circlePath);
        super.onDraw(canvas);
    }
}

used_layout.xml

...

<CircleImageView
            android:scaleType="centerCrop" // 不指定这个type的话，如果图片本身长宽不同，会在圆内留空白
            android:layout_width="wrap_content"
            android:layout_height="200dp"
            android:background="@color/colorAccent"
            app:srcCompat="@drawable/cute_cat" />

...