When setting a default placeholder and an attributedString before `_textKitComponents.textView` was created, the placeholder and the string were both appearing in the text node.
`_updateDisplayingPlaceholder` is called in `setAttributedString`, but since `_textKitComponents.textView` is nil the placeholder was not hidden. Calling `_updateDisplayingPlaceholder` after `_textKitComponents.textView` loads fixes this.
- Both ASDisplayNode and ASLayoutNode conforms to this protocol.
- ASDisplayNode can be embeded directly into layout graph.
- Eliminate ASCompositeNode.
- Fix ASStaticSizeDisplayNode not recpect min constrained size.
- Updated tests.
- Introduce ASLayoutNode and its subclasses.
- ASDisplayNode measures its ASLayoutNode and cache the result (ASLayout). Calculated size and position of each subnode can be retrieved from the calculated layout.
- Custom nodes need to override -layoutNodeThatFits:(CGSize) instead of -calculateSizeThatFits:(CGSize).
- Custom nodes do not need to layout its subnodes (in -layout:) anymore. ASDisplayNode can handle the job most of the time, by walking through its layout tree.
- ASCompositeNode is used to embed (display) subnodes to a node's layout. That way, each subnode will also be measured while the parent node is measuring. And the parent node knows where its subnodes are within its layout.
ARC doesn't play nicely with structs that contain references to
Objective-C objects, which causes breakage when using AsyncDisplayKit as
a dynamic framework (e.g., with CocoaPods 0.36+). Fixes#198.
