關於UIKit提供幾個Graphics函數

1、UIKitt提供中幾個圖像上下文（Graphics contex）的函數,此處忽略pdf相關的處理

 

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); 

在我們對圖像進行處理的時候，需要用到圖像上下文CGContexRef，這個時候需要使用UIGraphicsBeginImageContext函數，用完之後UIGraphicsEndImageContext結束；當然UIKIt中，view在調用draw函數時候預先給我們準備還一個上下文，因此我們不需要顯示的調用UIGraphicsEndImageContext；

2、在圖像上下中繪製元素

    我們可以在一個圖像上下文中繪製各種各樣的原始，（點，直線，UIImage,幾何圖形等）。

    具體見連接

3、關於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.