在 UIImageView 上绘制一个简单的圆圈
当视图加载到我的 iOS 应用程序中时，我动态创建一个 UIImageView 并将我的图像放置在其中。我想在 UIImageView 顶部画一个简单的圆圈，但到目前为止还无法这样做。
我编写了这段代码，并尝试在从另一个对象接收到事件时画一个圆圈......
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBStrokeColor(context, 0, 0, 255, 0.5);
CGContextFillRect(context, CGRectMake(10, 10, 10, 10));
但是什么也没有发生我收到一个错误。
<Error>: CGContextSetRGBStrokeColor: invalid context 0x0
<Error>: CGContextFillRects: invalid context 0x0
我试图超越一切，但无济于事。我做错了什么？
When the view loads in my iOS application, I dynamically create a UIImageView and place my Image inside of it. I would like to draw a simple circle on top of the UIImageView, but have been unable to do so thus far.
I wrote this code and attempted to draw a circle upon recieving an event from another object...
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBStrokeColor(context, 0, 0, 255, 0.5);
CGContextFillRect(context, CGRectMake(10, 10, 10, 10));
...but nothing happens I receive an error.
<Error>: CGContextSetRGBStrokeColor: invalid context 0x0
<Error>: CGContextFillRects: invalid context 0x0
I'm attempting to draw above everything, to no avail. What am I doing wrong?
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您的
CGContextRef是NULL，因为您调用UIGraphicsGetCurrentContext的地方没有图形上下文。当您位于视图的
drawRect:中时，已经为您设置了上下文，因此您可以进行绘制，并且可以在需要时获取当前上下文。然而，在任意方法（例如 IBAction）中，可能没有可供您绘制的图形上下文。您需要创建一个，也许使用UIGraphicsBeginImageContextWithOptions，或者（最好）在事件处理程序方法中设置一个标志，以指示下次通过drawRect 绘制一些额外的内容：，然后调用setNeedsDisplay。Your
CGContextRefisNULLbecause there is no graphics context in the place that you are callingUIGraphicsGetCurrentContext.When you're in the
drawRect:of a view, a context has already been set up for you, so you can draw and can get the current context if you need to. In an arbitrary method (such as anIBAction) however, there may not be a graphics context for you to draw into. You need to either create one, using perhapsUIGraphicsBeginImageContextWithOptions, or (preferably) just set a flag in your event handler method to indicate that something extra should be drawn the next time throughdrawRect:, and then callsetNeedsDisplay.