public class CircleView extends View{

    private static final int COLOR = Color.RED;

    private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);

    private int mWidth = 0;

    private int mHeight = 0;

    public CircleView(Context context, AttributeSet attrs) {

        super(context, attrs);

        init();

    }

    public CircleView(Context context) {

        super(context);

        init();

    }

    public CircleView(Context context, AttributeSet attrs, int defStyleAttr) {

        super(context, attrs, defStyleAttr);

        init();

    }

    private void init(){

        mPaint.setColor(COLOR);

    }

    @Override

    protected void onDraw(Canvas canvas) {

        super.onDraw(canvas);

        //获取当前View的宽/高

        mWidth = getMeasuredWidth();

        mHeight = getMeasuredHeight();

        //获取半径

        int radium = Math.min(mWidth,mHeight)/2;

        //画圆

        canvas.drawCircle(mWidth/2,mHeight/2,radium,mPaint);

    }

}

CircleView

//设定wrap_content时候的宽度

private static final int AT_WIDTH = 30;

private static final int AT_HEIGHT = 30;

//重写onMeasure()方法

@Override

    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

        //获取子View的范围

        int widthSize = MeasureSpec.getSize(widthMeasureSpec);

        int widthMode = MeasureSpec.getMode(widthMeasureSpec);

        int heightSize = MeasureSpec.getSize(heightMeasureSpec);

        int heightMode = MeasureSpec.getMode(heightMeasureSpec);

       //判断当属性为wrap_content的时候

        if (widthMode == MeasureSpec.AT_MOST && heightMode == MeasureSpec.AT_MOST){

            setMeasuredDimension(AT_WIDTH,AT_HEIGHT);

        }

        else if (widthMode == MeasureSpec.AT_MOST){

            setMeasuredDimension(AT_WIDTH,heightSize);

        }

        else if (heightMode == MeasureSpec.AT_MOST){

            setMeasuredDimension(widthSize,AT_HEIGHT);

        }

        else {

            super.onMeasure(widthMeasureSpec,heightMeasureSpec);

        }

    }    

//重写onDraw方法

 protected void onDraw(Canvas canvas) {

        super.onDraw(canvas);

        //获取padding

        int paddingLeft = getPaddingLeft();

        int paddingRight = getPaddingRight();

        int paddingTop = getPaddingTop();

        int paddingBottom = getPaddingBottom();

        //获取当前View的宽/高 减去padding

        mWidth = getMeasuredWidth() - paddingLeft - paddingRight;

        mHeight = getMeasuredHeight() - paddingTop - paddingBottom;

        //获取半径

        int radium = Math.min(mWidth,mHeight)/2;

        //画圆

        canvas.drawCircle(paddingLeft+mWidth/2,paddingTop - mHeight/2,radium,mPaint);

    }

CircleView

<resources>

    <declare-styleable name="CircleView">

        <attr name="color_circle" format="color"/>

    </declare-styleable>

</resources>

attr_circleview

<LinearLayout

    xmlns:android="http://schemas.android.com/apk/res/android"

    //这一段必须要加    app名字可以替换
    xmlns:app="http://schemas.android.com/apk/res-auto"

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    tools:context="com.maikefengchao.circleview.MainActivity">

    <com.maikefengchao.circleview.CircleView

        android:layout_width="80dp"

        android:layout_height="80dp"
        //前缀与添加的声明前缀一致
        app:color_circle="#9999"/>

</LinearLayout>

//在构造方法中使用

 public CircleView(Context context, AttributeSet attrs, int defStyleAttr) {

        super(context, attrs, defStyleAttr);

        //加载自定义属性集合CircleView

        TypedArray a = context.obtainStyledAttributes(attrs,R.styleable.CircleView);

        //解析集合中的circle_circle，设置默认颜色

        mColor = a.getColor(R.styleable.CircleView_color_circle,Color.RED);

        init();

    }