如何使用Core Graphics在我的触摸位置绘制圆圈?

时间:2013-02-16 00:39:21

标签: ios xcode touch cgcontext

新程序员在这里。我在尝试使用Core Graphics在触摸位置周围绘制描边弧时遇到问题。我有方法绘制圆圈工作正常,我已经测试,并在我点击屏幕时注册触摸,但当我尝试调用方法绘制圆圈时,我点击,我得到错误“CG​​ContextBlahBlah:无效的上下文为0x0"

认为这是因为我没有在drawRect :()中调用该方法。

那我怎么能在触摸时调用这个方法呢?此外,如何在我的绘制方法中使用“CGPoint locationOfTouch”作为参数?

以下是我正在使用的代码块。

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint locationOfTouch = [touch locationInView:self];
    [self drawTouchCircle:(locationOfTouch)];
    [self setNeedsDisplay];
}


-(void)drawTouchCircle:(CGPoint)locationOfTouch
{
    CGContextRef ctx= UIGraphicsGetCurrentContext();

    CGContextSaveGState(ctx);

    CGContextSetLineWidth(ctx,5);
    CGContextSetRGBStrokeColor(ctx,0.8,0.8,0.8,1.0);
    CGContextAddArc(ctx,locationOfTouch.x,locationOfTouch.y,30,0.0,M_PI*2,YES);
    CGContextStrokePath(ctx);
}

提前感谢您的帮助!

1 个答案:

答案 0 :(得分:7)

是的,你是对的。问题在于,您应该实施一个drawTouchCircle方法,而不是自己调用drawRect,因此您的touches方法只需要调用setNeedsDisplaydrawRect将照顾其他人。因此,您可能希望将触摸位置保存在类属性中,然后在drawRect中检索该位置:

@interface View ()
@property (nonatomic) BOOL touched;
@property (nonatomic) CGPoint locationOfTouch;
@end

@implementation View

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesBegan:touches withEvent:event];

    self.touched = YES;
    UITouch *touch = [touches anyObject];
    self.locationOfTouch = [touch locationInView:self];
    [self setNeedsDisplay];
}

- (void)drawTouchCircle:(CGPoint)locationOfTouch
{
    CGContextRef ctx= UIGraphicsGetCurrentContext();
    CGRect bounds = [self bounds];

    CGPoint center;
    center.x = bounds.origin.x + bounds.size.width / 2.0;
    center.y = bounds.origin.y + bounds.size.height / 2.0;
    CGContextSaveGState(ctx);

    CGContextSetLineWidth(ctx,5);
    CGContextSetRGBStrokeColor(ctx,0.8,0.8,0.8,1.0);
    CGContextAddArc(ctx,locationOfTouch.x,locationOfTouch.y,30,0.0,M_PI*2,YES);
    CGContextStrokePath(ctx);
}

- (void)drawRect:(CGRect)rect
{
    if (self.touched)
        [self drawTouchCircle:self.locationOfTouch];
}

@end