letter path animation

gistfile1.swift

let letters = CGPathCreateMutable()
        
        let font = CTFontCreateWithName("Helvetica-Bold", 72, nil)
        
        let attrs: [String: AnyObject] = [kCTFontAttributeName: font]
        
        let attrString = NSAttributedString(string: "~", attributes: attrs)
        
        let line = CTLineCreateWithAttributedString(attrString)
        let runArray = CTLineGetGlyphRuns(line)
        
        for index in 0 ..< CFArrayGetCount(runArray){
            let run = unsafeBitCast(CFArrayGetValueAtIndex(runArray, index), CTRunRef.self)
            let runFont = unsafeBitCast(CFDictionaryGetValue(CTRunGetAttributes(run), unsafeBitCast(kCTFontAttributeName, UnsafePointer<Void>.self)), CTFontRef.self)
            
            for runGlyphIndex in 0 ..< CTRunGetGlyphCount(run){
                let thisGlyphRange = CFRangeMake(runGlyphIndex, 1)
                
                var glyph: CGGlyph = CGGlyph()
                var position: CGPoint = CGPoint()
                CTRunGetGlyphs(run, thisGlyphRange, &glyph)
                CTRunGetPositions(run, thisGlyphRange, &position)
                
                let letter = CTFontCreatePathForGlyph(runFont, glyph, nil)
                var t = CGAffineTransformMakeTranslation(position.x, position.y)
                
                CGPathAddPath(letters, &t, letter)
            }
        }
        
        
        let shapeLayer = CAShapeLayer()
        shapeLayer.path = letters
        
        let screenSize = UIScreen.mainScreen().bounds
        shapeLayer.frame = CGRect(x: (screenSize.width-attrString.size().width)/2,
            y: (screenSize.height-attrString.size().height)/2,
            width: attrString.size().width,
            height: attrString.size().height)
        shapeLayer.strokeColor = UIColor.blackColor().CGColor
        shapeLayer.fillColor = nil
        shapeLayer.lineWidth = 3.0
        shapeLayer.geometryFlipped = true       // 上下翻转 ShapeLayer 的坐标系
        
        view.layer.addSublayer(shapeLayer)