新程序员来了。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。我有绘制圆圈的方法工作正常,并且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:无效上下文” 0x0"
I think这是因为我没有调用drawRect:()中的方法。
那么我怎样才能在触摸时调用这个方法呢?此外,如何在我的绘制方法中使用“CGPoint locationOfTouch”作为参数?
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);
}
先谢谢您的帮助!
你是对的。问题是,而不是打电话drawTouchCircle
你自己,你应该实施一个drawRect
为您调用它的方法,因此您的touches
方法只需要调用setNeedsDisplay
, and drawRect
会照顾剩下的事情。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
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