xcode 如何使用 Core Graphics 在我的触摸位置画一个圆圈？
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How can I use Core Graphics to draw a circle at my touch location?
提问by BendE
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"
新程序员来了。我在尝试使用 Core Graphics 在我的触摸位置周围绘制描边弧时遇到问题。我有绘制圆圈的方法工作正常，我已经测试并在点击屏幕时注册触摸，但是当我尝试在点击时调用绘制圆圈的方法时，我收到错误“CGContextBlahBlah：无效的上下文0x0"
I thinkthat this is because I'm not calling the method in drawRect:().
我认为这是因为我没有在 drawRect:() 中调用该方法。
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?
那么我怎样才能在触摸时调用这个方法呢？此外，如何在我的绘图方法中使用“CGPoint locationOfTouch”作为参数？
Here are the code chunks I am working with.
这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);
}
Thanks in advance for the help!
在此先感谢您的帮助！
回答by Rob
Yes, you're right. The issue is that, rather than calling drawTouchCircleyourself, you should implement a drawRectmethod that calls it for you, and thus your touchesmethod only needs to call setNeedsDisplay, and drawRectwill 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:
你是对的。问题是drawTouchCircle，您应该实现一个drawRect为您调用它的方法，而不是调用自己，因此您的touches方法只需要调用setNeedsDisplay，drawRect其余的会处理。因此，您可能希望将触摸位置保存在类属性中，然后在您的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
