SpriteKit 中的动画路径绘制

Question

我正在尝试使用SpriteKit 为路径上的笔画绘制动画。我已经使用SKActions 实现了一个可行的解决方案，并使用CABasicAnimations 实现了一个单独的实现。 SKAction 解决方案不是很优雅；它会在 SKAction.repeatAction(action:count:) 调用的每次迭代中创建并绘制一条新路径，每条新路径都比前一条更完整。

func start() {
    var i:Double = 1.0
    let spinAction:SKAction = SKAction.repeatAction(SKAction.sequence([
        SKAction.runBlock({
            self.drawCirclePercent(i * 0.01)

            if (++i > 100.0) {
                i = 1.0
            }
        }),
        SKAction.waitForDuration(0.01)
    ]), count: 100)
    runAction(spinAction)
}

func drawCirclePercent(percent:Double) {
    UIGraphicsBeginImageContext(self.size)

    let ctx:CGContextRef = UIGraphicsGetCurrentContext()
    CGContextSaveGState(ctx)
    CGContextSetLineWidth(ctx, lineWidth)
    CGContextSetRGBStrokeColor(ctx, 1.0, 1.0, 1.0, 1.0)
    CGContextAddArc(ctx, CGFloat(self.size.width/2.0), CGFloat(self.size.height/2.0), radius/2.0, CGFloat(M_PI * 1.5), CGFloat(M_PI * (1.5 + 2.0 * percent)), 0)
    CGContextStrokePath(ctx)

    let textureImage:UIImage = UIGraphicsGetImageFromCurrentImageContext()
    texture = SKTexture(image: textureImage)

    UIGraphicsEndImageContext()
}

虽然上面的代码有效，但它肯定不是漂亮或高效，而且很可能不是 SKActions 的预期用途。 CABasicAnimation 解决方案更加优雅和高效。

let path:CGMutablePathRef = CGPathCreateMutable()
CGPathAddArc(path, nil, 0, 0, 40.0, CGFloat(M_PI_2 * 3.0), CGFloat(M_PI_2 * 7.0), false)

let pathLayer:CAShapeLayer = CAShapeLayer()
pathLayer.frame = CGRectMake(100, 100, 80.0, 80.0)
pathLayer.path = path
pathLayer.strokeColor = SKColor.whiteColor().CGColor
pathLayer.fillColor = nil
pathLayer.lineWidth = 3.0
self.view.layer.addSublayer(pathLayer)

let pathAnimation:CABasicAnimation = CABasicAnimation(keyPath: "strokeEnd")
pathAnimation.duration = 2.0
pathAnimation.fromValue = 0.0
pathAnimation.toValue = 1.0
pathLayer.addAnimation(pathAnimation, forKey: "strokeEndAnimation")

我的问题是，我真的更希望将所有代码包含在 SKSpriteNode 的子类中（如果上述两种解决方案是我唯一的选择，我会选择第一种）。有什么方法可以改进我的SKAction 实现，使其更接近CoreAnimation 实现，而无需包含CoreAnimation？本质上，我想知道 SpriteKit 是否具有我不知道的功能，可以用来改进第一个实现。

Answer 1

您可以通过提供基于 SKShader 的 properties、v_path_distance 和 u_path_length 中的一些输出的自定义 strokeShader 来为 SKShapeNode 的路径设置动画。上面的 rickster 暗示了这一点，完整的代码如下。在着色器中，u_current_percentage 是我们添加的，它指的是我们想要向上描边的路径中的当前点。由此，场景决定了动画抚摸的节奏。另请注意，strokeShader 作为片段着色器，在每一步都输出一个 RGB，它允许沿路径进行颜色控制，例如使渐变颜色成为可能。

着色器作为文件添加到 Xcode 项目“animateStroke.fsh”：

    void main()  
{  
    if ( u_path_length == 0.0 ) {  
        gl_FragColor = vec4( 0.0, 0.0, 1.0, 1.0 );   
    } else if ( v_path_distance / u_path_length <= u_current_percentage ) {  
        gl_FragColor = vec4( 1.0, 0.0, 0.0, 1.0 );   
    } else {  
        gl_FragColor = vec4( 0.0, 0.0, 0.0, 0.0 );   
    }  
}  

以及使用它的示例 SKScene 子类：

    import SpriteKit  
    import GameplayKit  
    func shaderWithFilename( _ filename: String?, fileExtension: String?, uniforms: [SKUniform] ) -> SKShader {  
        let path = Bundle.main.path( forResource: filename, ofType: fileExtension )  
        let source = try! NSString( contentsOfFile: path!, encoding: String.Encoding.utf8.rawValue )  
        let shader = SKShader( source: source as String, uniforms: uniforms )  
        return shader  
}  
class GameScene: SKScene {  
    let strokeSizeFactor = CGFloat( 2.0 )  
    var strokeShader: SKShader!  
    var strokeLengthUniform: SKUniform!  
    var _strokeLengthFloat: Float = 0.0  
    var strokeLengthKey: String!  
    var strokeLengthFloat: Float {  
        get {  
            return _strokeLengthFloat  
        }  
        set( newStrokeLengthFloat ) {  
            _strokeLengthFloat = newStrokeLengthFloat  
            strokeLengthUniform.floatValue = newStrokeLengthFloat  
        }  
    }  

    override func didMove(to view: SKView) {  
        strokeLengthKey = "u_current_percentage"  
        strokeLengthUniform = SKUniform( name: strokeLengthKey, float: 0.0 )  
        let uniforms: [SKUniform] = [strokeLengthUniform]  
        strokeShader = shaderWithFilename( "animateStroke", fileExtension: "fsh", uniforms: uniforms )  
        strokeLengthFloat = 0.0  
        let cameraNode = SKCameraNode()  
        self.camera = cameraNode  
        let strokeHeight = CGFloat( 200 ) * strokeSizeFactor  
        let path1 = CGMutablePath()  
        path1.move( to: CGPoint.zero )  
        path1.addLine( to: CGPoint( x: 0, y: strokeHeight ) )  

        // prior to a fix in iOS 10.2, bug #27989113  "SKShader/SKShapeNode: u_path_length is not set unless shouldUseLocalStrokeBuffers() is true"  
        // add a few points to work around this bug in iOS 10-10.1 ->  
        // for i in 0...15 {  
        //    path1.addLine( to: CGPoint( x: 0, y: strokeHeight + CGFloat( 0.001 ) * CGFloat( i ) ) )  
        // }  

        path1.closeSubpath()  
        let strokeWidth = 17.0 * strokeSizeFactor  
        let path2 = CGMutablePath()  
        path2.move( to: CGPoint.zero )  
        path2.addLine( to: CGPoint( x: 0, y: strokeHeight ) )  
        path2.closeSubpath()  
        let backgroundShapeNode = SKShapeNode( path: path2 )  
        backgroundShapeNode.lineWidth = strokeWidth  
        backgroundShapeNode.zPosition = 5.0  
        backgroundShapeNode.lineCap = .round  
        backgroundShapeNode.strokeColor = SKColor.darkGray  
        addChild( backgroundShapeNode )  

        let shapeNode = SKShapeNode( path: path1 )  
        shapeNode.lineWidth = strokeWidth  
        shapeNode.lineCap = .round  
        backgroundShapeNode.addChild( shapeNode )  
        shapeNode.addChild( cameraNode )  
        shapeNode.strokeShader = strokeShader  
        backgroundShapeNode.calculateAccumulatedFrame()  

        cameraNode.position = CGPoint( x: backgroundShapeNode.frame.size.width/2.0, y: backgroundShapeNode.frame.size.height/2.0 )              
    }  

    override func update(_ currentTime: TimeInterval) {        
        // the increment chosen determines how fast the path is stroked. Note this maps to "u_current_percentage" within animateStroke.fsh  
        strokeLengthFloat += 0.01  
        if strokeLengthFloat > 1.0 {  
            strokeLengthFloat = 0.0  
        }        
    }  
}  

Answer 2

基于 Bobjt 的answer，我的着色器利用了 SKShapeNode 的光晕宽度... 所以当我使用它时，我可以使用shapeNode.glowWidth = 5.0; 等！

void main()
{
    vec4 def = SKDefaultShading();

    if ( u_path_length == 0.0 ) {
        gl_FragColor = vec4( 0xff / 255.0f, 0 / 255.0f, 0 / 255.0f, 1.0f );
    } else if ( v_path_distance / u_path_length <= u_current_percentage ) {
        gl_FragColor = vec4(
                            //0x1e / 255.0f, 0xb8 / 255.0f, 0xca / 255.0f, <-- original boring color :-P
                            0x1e / 255.0f * def.z, 0xb8 / 255.0f * def.z, 0xca / 255.0f * def.z, def.z
                            );
    } else {
        gl_FragColor = vec4( 0.0f, 0.0f, 0.0f, 0.0f );
    }
}