Constructing Paths（建構路徑）

points -> a shape:多個點構成一個形狀

shapes -> a path:多個形狀構成一個路徑

繪制過程:

    建立路徑 －CGMutablePathRef  CGPathCreateMutable (void);  //CGMutablePathRef path = CGPathCreateMutable();

    畫線條，并将其加入路徑

          設定起點 －void  CGPathMoveToPoint (CGMutablePathRef path,const CGAffineTransform *m,CGFloat x,CGFloat y); //第二個參數:NULL

          設定終點 －void  CGPathAddLineToPoint (CGMutablePathRef path,const CGAffineTransform *m,CGFloat x,CGFloat y);

    建立上下文環境 －CGContextRef  UIGraphicsGetCurrentContext (void); //CGContextRef currentContext = UIGraphicsGetCurrentContext();

       在環境中添加路徑 － void   CGContextAddPath  (CGContextRef context,CGPathRef path); 

    設定相關屬性 - (void)setStroke  //顔色

               － void  CGContextSetLineWidth (CGContextRef c,CGFloat width); //線寬

    繪制路徑 －void  CGContextDrawPath (CGContextRef c,CGPathDrawingMode mode);

    釋放 －void  CGPathRelease (CGPathRef path);

    重要的3種繪制模式 －CGPathDrawingMode :kCGPathFill, kCGPathStroke, kCGPathFillStroke  ; 

- (void)drawRect:(CGRect)rect{

   CGMutablePathRef path = CGPathCreateMutable();

   CGRect screenBounds = [[UIScreen mainScreen] bounds];

   CGPathMoveToPoint(path, NULL, screenBounds.origin.x,screenBounds.origin.y);

   CGPathAddLineToPoint(path,NULL,screenBounds.size.width,screenBounds.size.height);

    CGPathMoveToPoint(path,NULL,screenBounds.size.width,screenBounds.origin.y);

    CGPathAddLineToPoint(path,NULL,screenBounds.origin.x,screenBounds.size.height);

   CGContextRef currentContext = UIGraphicsGetCurrentContext();

   CGContextAddPath(currentContext,path);

    [[UIColor blueColor] setStroke];

   CGContextDrawPath(currentContext,kCGPathStroke);

   CGPathRelease(path);  

}