# 矢量圖形 ????我們用Core Graphics來繪圖的一個通常原因就是只是用圖片或是圖層效果不能輕易地繪制出矢量圖形。矢量繪圖包含一下這些： * 任意多邊形（不僅僅是一個矩形） * 斜線或曲線 * 文本 * 漸變 ????舉個例子，清單13.1 展示了一個基本的畫線應用。這個應用將用戶的觸摸手勢轉換成一個`UIBezierPath`上的點，然后繪制成視圖。我們在一個`UIView`子類`DrawingView`中實現了所有的繪制邏輯，這個情況下我們沒有用上view controller。但是如果你喜歡你可以在view controller中實現觸摸事件處理。圖13.1是代碼運行結果。 清單13.1 用Core Graphics實現一個簡單的繪圖應用 ~~~ #import "DrawingView.h" @interface DrawingView () @property (nonatomic, strong) UIBezierPath *path; @end @implementation DrawingView - (void)awakeFromNib { //create a mutable path self.path = [[UIBezierPath alloc] init]; self.path.lineJoinStyle = kCGLineJoinRound; self.path.lineCapStyle = kCGLineCapRound; ? self.path.lineWidth = 5; } - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event { //get the starting point CGPoint point = [[touches anyObject] locationInView:self]; //move the path drawing cursor to the starting point [self.path moveToPoint:point]; } - (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event { //get the current point CGPoint point = [[touches anyObject] locationInView:self]; //add a new line segment to our path [self.path addLineToPoint:point]; //redraw the view [self setNeedsDisplay]; } - (void)drawRect:(CGRect)rect { //draw path [[UIColor clearColor] setFill]; [[UIColor redColor] setStroke]; [self.path stroke]; } @end ~~~  圖13.1 用Core Graphics做一個簡單的『素描』 ????這樣實現的問題在于，我們畫得越多，程序就會越慢。因為每次移動手指的時候都會重繪整個貝塞爾路徑（`UIBezierPath`），隨著路徑越來越復雜，每次重繪的工作就會增加，直接導致了幀數的下降。看來我們需要一個更好的方法了。 ????Core Animation為這些圖形類型的繪制提供了專門的類，并給他們提供硬件支持（第六章『專有圖層』有詳細提到）。`CAShapeLayer`可以繪制多邊形，直線和曲線。`CATextLayer`可以繪制文本。`CAGradientLayer`用來繪制漸變。這些總體上都比Core Graphics更快，同時他們也避免了創造一個寄宿圖。 ????如果稍微將之前的代碼變動一下，用`CAShapeLayer`替代Core Graphics，性能就會得到提高（見清單13.2）.雖然隨著路徑復雜性的增加，繪制性能依然會下降，但是只有當非常非常浮躁的繪制時才會感到明顯的幀率差異。 清單13.2 用`CAShapeLayer`重新實現繪圖應用 ~~~ #import "DrawingView.h" #import @interface DrawingView () @property (nonatomic, strong) UIBezierPath *path; @end ? @implementation DrawingView + (Class)layerClass { //this makes our view create a CAShapeLayer //instead of a CALayer for its backing layer return [CAShapeLayer class]; } - (void)awakeFromNib { //create a mutable path self.path = [[UIBezierPath alloc] init]; //configure the layer CAShapeLayer *shapeLayer = (CAShapeLayer *)self.layer; shapeLayer.strokeColor = [UIColor redColor].CGColor; shapeLayer.fillColor = [UIColor clearColor].CGColor; shapeLayer.lineJoin = kCALineJoinRound; shapeLayer.lineCap = kCALineCapRound; shapeLayer.lineWidth = 5; } - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event { //get the starting point CGPoint point = [[touches anyObject] locationInView:self]; //move the path drawing cursor to the starting point [self.path moveToPoint:point]; } - (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event { //get the current point CGPoint point = [[touches anyObject] locationInView:self]; //add a new line segment to our path [self.path addLineToPoint:point]; //update the layer with a copy of the path ((CAShapeLayer *)self.layer).path = self.path.CGPath; } @end ~~~