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“CGContextBlahBlah：无效的上下文为0x0\”
我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？
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,0.8,1.0);
CGContextAddArc(ctx,locationOfTouch.x,locationOfTouch.y,30,0.0,M_PI*2,YES);
CGContextStrokePath(ctx);
}
在此先感谢您的帮助！
解决方法
你是对的.问题是,你应该实现一个为你调用它的drawRect方法,而不是自己调用drawTouchCircle,因此你的touches方法只需要调用setNeedsDisplay,而drawRect将负责其余的工作.因此,您可能希望将触摸位置保存在类属性中,然后在drawRect中检索它：
@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,YES);
CGContextStrokePath(ctx);
}
- (void)drawRect:(CGRect)rect
{
if (self.touched)
[self drawTouchCircle:self.locationOfTouch];
}
@end