定义一个UIView：主要是在这个View里面加一个UIImageView,绘图都在这个UIImageView里面进行

@property(nonatomic) CGPoint prePoint;  //手指在进入move事件之前的那个点

@property(nonatomic) CGPoint oppsitePoint;  //手指在进入move事件之前的那个点

@property(nonatomic, retain) UIImageView* drawImage;

- (id)initWithFrame:(CGRect)frame

{

    self = [super initWithFrame:frame];

    if (self) {

        // Initialization code

        self.drawImage = [[UIImageView alloc] initWithImage:nil];

        self.drawImage.frame = self.frame;

        [self addSubview:_drawImage];

    }

    return self;

}

然后处理手指的事件

#pragma mark - deal touch

-(void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{

    NSLog(@"touchesBegan");

    //以下两句知道手指在屏幕上的点的信息

    UITouch* touch = [touches anyObject];

    CGPoint point = [touch locationInView:self];

    if (touch) {

        self.prePoint = point;

        _oppsitePoint = point;

        _oppsitePoint.x = 320.0f - point.x;

    }

}

-(void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{

     NSLog(@"touchesMoved");

    UITouch* touch = [touches anyObject];

    if (touch) {

        CGPoint point = [touch locationInView:self];

        UIGraphicsBeginImageContext(self.frame.size);

        [_drawImage.image drawInRect:CGRectMake(0, 0, _drawImage.frame.size.width, _drawImage.frame

                                                .size.height)];

        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);

        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 2.0f);

        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(),0.314, 0.486, 0.859, 1.0);

        CGContextBeginPath(UIGraphicsGetCurrentContext());

        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _prePoint.x, _prePoint.y);

         CGContextAddQuadCurveToPoint(UIGraphicsGetCurrentContext(), _prePoint.x, _prePoint.y, point.x, point.y);

        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _oppsitePoint.x, _oppsitePoint.y);

        CGContextAddQuadCurveToPoint(UIGraphicsGetCurrentContext(), _oppsitePoint.x, _oppsitePoint.y, 320.0f - point.x, point.y);

       // CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), point.x, point.y);

        CGContextStrokePath(UIGraphicsGetCurrentContext());

        _drawImage.image = UIGraphicsGetImageFromCurrentImageContext();

        UIGraphicsEndImageContext();

        _prePoint = point;

        _oppsitePoint = point;

        _oppsitePoint.x = 320.0f - point.x;

    }

}

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{

    NSLog(@"touchesEnded");

    //这段的作用是假设在屏幕上点击能够画出点

    UIGraphicsBeginImageContext(self.frame.size);

    [_drawImage.image drawInRect:CGRectMake(0, 0, _drawImage.frame.size.width, _drawImage.frame

                                            .size.height)];

    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);

    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 3.0f);

    CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(),0.314, 0.486, 0.859, 1.0);

    CGContextBeginPath(UIGraphicsGetCurrentContext());

    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _prePoint.x, _prePoint.y);

    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), _prePoint.x, _prePoint.y);

    CGContextStrokePath(UIGraphicsGetCurrentContext());

    _drawImage.image = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    self.isTouch = NO;

}

以下为效果图