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1、UIKitt提供中几个图像上下文（Graphics contex）的函数,此处忽略pdf相关的处理
UIKIT_EXTERN CGContextRef UIGraphicsGetCurrentContext(void);
UIKIT_EXTERN void UIGraphicsPushContext(CGContextRef context);
UIKIT_EXTERN void UIGraphicsPopContext(void);
UIKIT_EXTERN void UIRectFillUsingBlendMode(CGRect rect, CGBlendMode blendMode);
UIKIT_EXTERN void UIRectFill(CGRect rect);
UIKIT_EXTERN void UIRectFrameUsingBlendMode(CGRect rect, CGBlendMode blendMode);
UIKIT_EXTERN void UIRectFrame(CGRect rect);
UIKIT_EXTERN void UIRectClip(CGRect rect);
// UIImage context
UIKIT_EXTERN void UIGraphicsBeginImageContext(CGSize size);
UIKIT_EXTERN void UIGraphicsBeginImageContextWithOptions(CGSize size, BOOL opaque, CGFloat scale) NS_AVAILABLE_IOS(4_0);
UIKIT_EXTERN UIImage* UIGraphicsGetImageFromCurrentImageContext(void);
UIKIT_EXTERN void UIGraphicsEndImageContext(void);
在我们对图像进行处理的时候，需要用到图像上下文CGContexRef，这个时候需要使用UIGraphicsBeginImageContext函数，用完之后UIGraphicsEndImageContext结束；当然UIKIt中，view在调用draw函数时候预先给我们准备还一个上下文，因此我们不需要显示的调用UIGraphicsEndImageContext；
2、在图像上下中绘制元素
我们可以在一个图像上下文中绘制各种各样的原始，（点，直线，UIImage,几何图形等）。
具体见连接
3、关于Graphics States
Quartz modifies the results of drawing operations according to the parameters in the current graphics state. The graphics state contains parameters that would otherwise be taken as arguments to drawing routines. Routines that draw to a graphics context consult the graphics state to determine how to render their results. For example, when you call a function to set the fill color, you are modifying a value stored in the current graphics state. Other commonly used elements of the current graphics state include the line width, the current position, and the text font size.
The graphics context contains a stack of graphics states. When Quartz creates a graphics context, the stack is empty. When you save the graphics state, Quartz pushes a copy of the current graphics state onto the stack. When you restore the graphics state, Quartz pops the graphics state off the top of the stack. The popped state becomes the current graphics state.
To save the current graphics state, use the function CGContextSaveGState to push a copy of the current graphics state onto the stack. To restore a previously saved graphics state, use the function CGContextRestoreGState to replace the current graphics state with the graphics state that’s on top of the stack.
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