1. 继承View重写onDraw方法

public class CircleView extends View {
    private int mColor = Color.RED;
    private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);

    public CircleView(Context context, AttributeSet attrs) {
        super(context, attrs);
        setBackgroundColor(getResources().getColor(android.R.color.holo_blue_bright));
        mPaint.setColor(mColor);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        int width = getWidth();
        int height = getHeight();
        int radius = Math.min(width, height) / 2;
        canvas.drawCircle(width / 2, height / 2, radius, mPaint);
    }
}

2. 问题 padding和wrap_content无效

<qingfengmy.developmentofart._4view.CircleView
    android:layout_width="wrap_content"
    android:padding="20dp"
    android:layout_margin="20dp"
    android:layout_height="100dp" />

如上设置时，margin是有效的，因为margin在父View中处理；padding无效，因为继承自View和ViewGroup的控件，padding是需要自己处理的；wrap_content是无效的，前文分析过，在onMeasure的时候，最大模式和精确模式，测量宽高都是设置的父Veiw的剩余空间，wrap_content需要自己处理。

3. wrap_contnet的解决

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
    int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
    int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
    int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);

    int widthResult = widthSpecSize;
    int heightResult = heightSpecSize;
    if (widthSpecMode == MeasureSpec.AT_MOST) {
        widthResult = mWidth;
    }
    if (heightSpecMode == MeasureSpec.AT_MOST) {
        heightResult = mWidth;
    }
    setMeasuredDimension(widthResult, heightResult);
}

4. padding的解决

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    int paddingLeft = getPaddingLeft();
    int paddingRight = getPaddingRight();
    int paddingTop = getPaddingTop();
    int paddingBottom = getPaddingBottom();

    int width = getWidth() - paddingLeft - paddingRight;
    int height = getHeight() - paddingTop - paddingBottom;
    int radius = Math.min(width, height) / 2;
    canvas.drawCircle(paddingLeft + width / 2, paddingTop + height / 2, radius, mPaint);
}