Overview

layer is the partner of its view ,a view is not redrawn frequently, its drawing is cached and the cached version is the layer.The view's graphics context is the layer's graphics context.

View & Layer

• a UIView instance has an accompanying CALayer instance, accessible as the view's layer property.
• The view is the accompanying CALayer instance's delegate.
• If you subclass UIView and you want your subclass's underlying layer to be an instance of a CALayer subclass,implement the UIView subclass's layerClass class method to return that CALayer subclass.

example

class CompassView: UiView {
    override class func layerClass() -> AnyClass {
        return CompassLayer.self
    }
}

• There's a tight integration between the two.If you set the view's backgroundColor, you are really setting the layer's backgroundColor; If you set the layer's backgroundColor directly, the view's backgroundColor is set to match and so on.

** but you must never set the delegate property of a UIView's underlying layer to break the integration between them **

• The view draws into its layer, and the layer caches that drawing; the layer can then be manipulated, changing the view's appearance ,without necessarily asking the view to redraw itself until the view is told to draw freshly(drawRect:), replacing the layer's content.

Layers & Sublayers

• A layer can have sublayers, and a layer has at most one superlayer, thus there is a tree of layers. In fact, if only take the underlying layer into consideration, the view hierarchy really is a layer hierarchy.

A hierarchy of views and the hierarchy of layers underlying it

• The layer hierarchy can go beyond the view hierarchy. Because a view has exactly one underlying layer, but a layer can have sublayers that are not the underlying layers of any view

Layers that have sublayers of their own

• Whether a layer displays regions of its sublayers that lie outside that layer's own bounds depends on the value of its masksToBounds property. And a CALayer has a hidden property, too.

Manipulating the Layer Hierarchy

Layers come with a full set of methods for reading and manipulating the layer hierarchy, parallel to the methods for reading and manipulating the view hierarchy.

• A layer has a superlayer property and a sublayers property(which is writable) with methods:
  • addSublayer:
  • insertSublayer:atIndex:
  • insertSublayer:below:
  • insertSublayer:above:
  • replaceSublayer:with:
  • removeFromSuperlayer
• A layer's sublayers has an order(back-to-front), but this is not necessarily the same as the order mentioned just before. A layer also has a zPosition(CGFloat) property and this also determines drawing order.
  All sublayers with the same zPosition are drawn in the order they are listed among their sublayers siblings, but lower zPosition siblings are drawn before higher zPosition siblings. (The default zPosition is 0.0)

Positioning a Sublayer

Layer coordinate systems and positioning are similar to those of views. A layer's own internal coordinate system is expressed by its bounds, just like a view; its size is its bounds size, and its bounds origin is the internal coordinate at its top left.

• A sublayer's position within its superlayer is defined by a combination of two properties, its position and its anchorPoint. (可以参照tuicool上的文章：http://www.tuicool.com/articles/MvI7fu3 )
  • position: A point expressed in the superlayer's coordinate system.
  • anchorPoint: where the position point is located, with respect to the layer's own bounds
• A layer's frame is a purely derived property. When you get the frame, it is calculated from the bounds size along with the position and anchorPosition. When you set the frame, you set the bounds size and position.
  a code-created layer has a frame and bounds of (0.0, 0.0, 0.0, 0.0) and will not be visible on the screen even when you add it to a superlayer that is on the screen. Be sure to give your layer a nonzero width and height before you add it to a superlayer

CAScrollLayer

If you move a CAScrollLayer's bounds origin, you are repositioning its sublayers at the same time.It is a CALayer subcalss but it provides no scrolling interface. By default, a CAScrollLayer's masksToBounds property is true.

• To move the CAScrollLayer's bounds, talk to it or to a sublayer
  • Talking to the CAScrollLayer
    • scrollToPoint: : Changes the CAScrollLayer's bounds origin to that point
    • scrollToRect: : Changes the CAScrollLayer's bounds origin minimally so that the given portion of the bounds rect is visible.
  • Talking to a sublayer
    • scrollPoint : Changes the CAScrollLayer's bounds origin so that the given point of the sublayer is at the top left of the CAScrollLayer.
    • scrollRectToVisible : Changes the CAScrollLayer's bounds origin so that the given rect of the sublayer's bounds is within the CAScrollLayer's bounds area. You can also ask the sublayer for its visibleRect, the part of this sublayer now within the CAScrollLayer's bounds.

Layout of Sublayers

There is automatic layout for layers if they are the underlying layers of views. Otherwise, there is no automatic layout for layers in iOS.

• when a layer needs layout, either because its bounds have changed or because you called setNeedsLayout, you can respond in either of two ways:
  • The layer's layoutSublayers method is called; to respond, override layoutSublayers in your CALayer subclass.
  • Alternatively, implement layoutSublayersOfLayer: in the layer's delegate. (If the layer is a view's underlying layer, the view is its delegate.)
• To do effective manual layout of sublayers, you'll probably need a way to identify or refer to the sublayers. There is no layer equivalent of viewWithTag:, so such identification and reference is entirely up to you. Key-value coding can be helpful here.
• For a view's underlying layer, layoutSublayers or layoutSublayersOfLayer: is called after the view's layoutSubviews. (未完待续...)