使用CAShapeLayer 和 UIBezierPath 实现的画画功能。

1、首先根据触摸屏幕获取点信息：

- (CGPoint)pointWithTouches:(NSSet *)touches
{
    UITouch *touch = [touches anyObject];
    
    return [touch locationInView:self];
}

2、 重写响应方法：

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event

3、绘制：

在- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event 中使用CAShapeLayer 和 UIBezierPath 绘制。

CAShapeLayer：

　　 CAShapeLayer * slayer = [CAShapeLayer layer];
        slayer.path = path.CGPath;
        slayer.backgroundColor = [UIColor clearColor].CGColor;
        slayer.fillColor = [UIColor clearColor].CGColor; //颜色
        slayer.lineCap = kCALineCapRound;
        slayer.lineJoin = kCALineJoinRound;
        slayer.strokeColor = [UIColor blackColor].CGColor;
        slayer.lineWidth = path.lineWidth; // 宽度
        [self.layer addSublayer:slayer];

UIBezierPath：

    UIBezierPath * path = [[UIBezierPath alloc] init];
    path.lineWidth = width;
    path.lineCapStyle = kCGLineCapRound; //线条拐角
    path.lineJoinStyle = kCGLineCapRound; //终点处理
    [path moveToPoint:startPoint];

画画板