如何使用Core Graphics在触摸位置绘制一个圆？ [英] How can I use Core Graphics to draw a circle at my touch location?

问题描述

这里是新程序员。我在尝试使用Core Graphics在触摸位置周围绘制笔触弧时遇到问题。我有一种方法可以很好地绘制圆，并且在点击屏幕时我已经测试并注册了触摸，但是当我尝试在点击时调用绘制圆的方法时，出现错误 CGContextBlahBlah：无效上下文0x0

New programmer here. I am having issues trying to use Core Graphics to draw a stroked arc around my touch location. I have the method to draw the circle working fine, and I have tested and am registering touches when I tap the screen, but when I try to call the method to draw the circle when I tap, I get the error "CGContextBlahBlah: invalid context 0x0"

我认为这是因为我没有在drawRect :(）中调用该方法。

I think that this is because I'm not calling the method in drawRect:().

那么我如何能够轻触调用此方法？此外，如何在我的draw方法中使用 CGPoint locationOfTouch作为参数？

So how am I able to call this method on a touch? Furthermore, how can I use the "CGPoint locationOfTouch" as a parameter in my draw method?

以下是我正在使用的代码块。

Here are the code chunks I am working with.

-(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);
}

在此先感谢您的帮助！

推荐答案

是的，您是对的。问题在于，您应该自己实现一个 drawRect 方法，而不是自己调用 drawTouchCircle ，并且因此，您的 touches 方法只需要调用 setNeedsDisplay 和 drawRect 将负责其余的工作。因此，您可能希望将触摸位置保存在class属性中，然后在 drawRect 中进行检索：

Yes, you're right. The issue is that, rather than calling drawTouchCircle yourself, you should implement a drawRect method that calls it for you, and thus your touches method only needs to call setNeedsDisplay, and drawRect will take care of the rest. You might, therefore, want to save the touch location in a class property, and then retrieve that in your drawRect:

@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

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