ios - 如何定义水平线的起点终点

Question

我想在 loc 550 处绘制一条渐变的水平线。

CGFloat colors[6] = { 
    138.0f / 255.0f, 1.0f,
    162.0f / 255.0f, 1.0f,
    206.0f / 255.0f, 1.0f};
CGFloat locations[3] = { 0.05f, 0.45f, 0.95f };
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
CGGradientRef gradient = CGGradientCreateWithColorComponents(colorSpace, colors, locations, 3);
CGColorSpaceRelease(colorSpace);

CGPoint startPoint = CGPointMake(0, 0);
CGPoint endPoint   = CGPointMake(0, 550);

 CGContextRef context = UIGraphicsGetCurrentContext(); 
 CGContextSaveGState(context);
 CGContextAddRect(context, CGRectMake(0, 550, self.view.bounds.size.width, 1));
 CGContextClip(context);

CGContextDrawLinearGradient (context, gradient, startPoint, endPoint, 0);
CGContextRestoreGState(context);           

这段代码中没有绘制线条，Xcode 给出了这个错误:

            CGContextAddRect: invalid context 0x0

Answer 1

您需要将此代码放置在 UIView 对象的 drawRect: 方法中，以确保当前上下文 存在。否则，您需要使用UIGraphicsPushContext来获取当前上下文。

来自documentation对于UIGraphicsGetCurrentContext():

The current graphics context is nil by default. Prior to calling its drawRect: method, view objects push a valid context onto the stack, making it current. If you are not using a UIView object to do your drawing, however, you must push a valid context onto the stack manually using the UIGraphicsPushContext function.

You should call this function from the main thread of your application only.