CAReplicatorLayer

CAReplicatorLayer

CAReplicatorLayer的目的是为了高效生成许多相似的图层。它会绘制一个或多个图层的子图层，并在每个复制体上应用不同的变换。看上去演示能够更加解释这些，我们来写个例子吧。

重复图层（Repeating Layers）

清单6.8中，我们在屏幕的中间创建了一个小白色方块图层，然后用CAReplicatorLayer生成十个图层组成一个圆圈。instanceCount属性指定了图层需要重复多少次。instanceTransform指定了一个CATransform3D3D变换（这种情况下，下一图层的位移和旋转将会移动到圆圈的下一个点）。

变换是逐步增加的，每个实例都是相对于前一实例布局。这就是为什么这些复制体最终不会出现在同意位置上，图6.8是代码运行结果。

清单6.8 用CAReplicatorLayer重复图层

 @interface ViewController ()

 @property (nonatomic, weak) IBOutlet UIView *containerView;

 @end

 @implementation ViewController
 - (void)viewDidLoad
 {
     [super viewDidLoad];
     //create a replicator layer and add it to our view
     CAReplicatorLayer *replicator = [CAReplicatorLayer layer];
     replicator.frame = self.containerView.bounds;
     [self.containerView.layer addSublayer:replicator];

     //configure the replicator
     replicator.instanceCount = ;

     //apply a transform for each instance
     CATransform3D transform = CATransform3DIdentity;
     transform = CATransform3DTranslate(transform, , , );
     transform = CATransform3DRotate(transform, M_PI / 5.0, , , );
     transform = CATransform3DTranslate(transform, , -, );
     replicator.instanceTransform = transform;

     //apply a color shift for each instance
     replicator.instanceBlueOffset = -0.1;
     replicator.instanceGreenOffset = -0.1;

     //create a sublayer and place it inside the replicator
     CALayer *layer = [CALayer layer];
     layer.frame = CGRectMake(100.0f, 100.0f, 100.0f, 100.0f);
     layer.backgroundColor = [UIColor whiteColor].CGColor;
     [replicator addSublayer:layer];
 }
 @end

Swift 3.0 代码

 //    CAReplicatorLayer
     private lazy var replicatorView: UIView! = UIView()

 func replicatorLayerOfView()  {
         self.view.addSubview(self.replicatorView)
         let replicator = CAReplicatorLayer()
         replicator.frame = CGRect(x: , y: , width: , height: )
         self.replicatorView.layer.addSublayer(replicator)

         replicator.instanceCount = 

         var transform = CATransform3DIdentity
         transform = CATransform3DTranslate(transform, , , )
         transform = CATransform3DRotate(transform, .pi / 5.0, , , )
         transform = CATransform3DTranslate(transform, , -, )
         replicator.instanceTransform = transform

         replicator.instanceBlueOffset = -0.1
         replicator.instanceGreenOffset = -0.1

         let layer = CALayer()
         layer.frame = CGRect(x: , y: , width: , height: )
         layer.backgroundColor = UIColor.white.cgColor
         replicator.addSublayer(layer)

         self.replicatorView.snp.makeConstraints { (make) in
             make.top.equalTo(self.gradientView)
             make.size.equalTo(self.gradientView)
             make.left.equalTo(self.gradientView.snp.right).inset(-)
         }

     }

图6.8 用CAReplicatorLayer创建一圈图层

注意到当图层在重复的时候，他们的颜色也在变化：这是用instanceBlueOffset和instanceGreenOffset属性实现的。通过逐步减少蓝色和绿色通道，我们逐渐将图层颜色转换成了红色。这个复制效果看起来很酷，但是CAReplicatorLayer真正应用到实际程序上的场景比如：一个游戏中导弹的轨迹云，或者粒子爆炸（尽管iOS 5已经引入了CAEmitterLayer，它更适合创建任意的粒子效果）。除此之外，还有一个实际应用是：反射。

反射

使用CAReplicatorLayer并应用一个负比例变换于一个复制图层，你就可以创建指定视图（或整个视图层次）内容的镜像图片，这样就创建了一个实时的『反射』效果。让我们来尝试实现这个创意：指定一个继承于UIView的ReflectionView，它会自动产生内容的反射效果。实现这个效果的代码很简单（见清单6.9），实际上用ReflectionView实现这个效果会更简单，我们只需要把ReflectionView的实例放置于Interface Builder（见图6.9），它就会实时生成子视图的反射，而不需要别的代码（见图6.10）.

清单6.9 用CAReplicatorLayer自动绘制反射

 #import "ReflectionView.h"
 #import 

 @implementation ReflectionView

 + (Class)layerClass
 {
     return [CAReplicatorLayer class];
 }

 - (void)setUp
 {
     //configure replicator
     CAReplicatorLayer *layer = (CAReplicatorLayer *)self.layer;
     layer.instanceCount = ;

     //move reflection instance below original and flip vertically
     CATransform3D transform = CATransform3DIdentity;
     CGFloat verticalOffset = self.bounds.size.height + ;
     transform = CATransform3DTranslate(transform, , verticalOffset, );
     transform = CATransform3DScale(transform, , -, );
     layer.instanceTransform = transform;

     //reduce alpha of reflection layer
     layer.instanceAlphaOffset = -0.6;
 }
 
 - (id)initWithFrame:(CGRect)frame
 {
     //this is called when view is created in code
     if ((self = [super initWithFrame:frame])) {
         [self setUp];
     }
     return self;
 }

 - (void)awakeFromNib
 {
     //this is called when view is created from a nib
     [self setUp];
 }
 @end