进入文件可以看到两个向前引用
@class ASLayout;
@class ASLayoutSpec;
因为里面定义的函数有返回相应类型，但没有必要知道其内部内容，使用向前引用以减少代码引入
第一个协议
@protocol ASLayoutElementStylability;
里面定义了一个方法，传入一个ASLayoutElementStyle来得到对应的instance对象
- (instancetype)styledWithBlock:(AS_NOESCAPE void (^)(__kindof ASLayoutElementStyle *style))styleBlock;
使用了两个陌生的指令
AS_NOESCAPE //非逃逸block __attribute__((noescape))
以及
__kindof //可接受此类型以及其子类型
第二个协议
这里只有声明，在ASTraitCollection.h中定义，具体不在这里看
@protocol ASTraitEnvironment;
定义的常量
/** A constant that indicates that the parent's size is not yet determined in a given dimension. */
AS_EXTERN CGFloat const ASLayoutElementParentDimensionUndefined;
/** A constant that indicates that the parent's size is not yet determined in either dimension. */
AS_EXTERN CGSize const ASLayoutElementParentSizeUndefined;
此处用了 AS_EXTERN 实则是 FOUNDATION_EXTERN就是extern "C"
是一个可以兼容C/C++的extern，暴露给其他文件。
定义的枚举类型
定义了两种，分别是LayoutSpec和DisplayNode
/** Type of ASLayoutElement */
typedef NS_ENUM(unsigned char, ASLayoutElementType) {
ASLayoutElementTypeLayoutSpec,
ASLayoutElementTypeDisplayNode
};
第三个协议 ASLayoutElement
布局大小的核心协议，分别遵循了ASLayoutElementExtensibility，ASTraitEnvironment，ASLayoutElementAsciiArtProtocol三个协议
/**
* The ASLayoutElement protocol declares a method for measuring the layout of an object. A layout
* is defined by an ASLayout return value, and must specify 1) the size (but not position) of the
* layoutElement object, and 2) the size and position of all of its immediate child objects. The tree
* recursion is driven by parents requesting layouts from their children in order to determine their
* size, followed by the parents setting the position of the children once the size is known
*
* The protocol also implements a "family" of LayoutElement protocols. These protocols contain layout
* options that can be used for specific layout specs. For example, ASStackLayoutSpec has options
* defining how a layoutElement should shrink or grow based upon available space.
*
* These layout options are all stored in an ASLayoutOptions class (that is defined in ASLayoutElementPrivate).
* Generally you needn't worry about the layout options class, as the layoutElement protocols allow all direct
* access to the options via convenience properties. If you are creating custom layout spec, then you can
* extend the backing layout options class to accommodate any new layout options.
*/
@protocol ASLayoutElement
包含属性
/**
* @abstract Returns type of layoutElement
*/
@property (nonatomic, readonly) ASLayoutElementType layoutElementType;
/**
* @abstract A size constraint that should apply to this ASLayoutElement.
*/
@property (nonatomic, readonly) ASLayoutElementStyle *style;
第一个方法- (nullable NSArray，获取所有子Elements，都是遵循ASLayoutElemen的id类型
/**
* @abstract Returns all children of an object which class conforms to the ASLayoutElement protocol
*/
- (nullable NSArray> *)sublayoutElements;
第二个方法- (ASLayout *)layoutThatFits:(ASSizeRange)constrainedSize;，返回一个ASLayout，不允许重写，它会通过calculateLayoutThatFits来缓存结果，调用此方法会十分昂贵如果不缓存结果
/**
* @abstract Asks the node to return a layout based on given size range.
*
* @param constrainedSize The minimum and maximum sizes the receiver should fit in.
*
* @return An ASLayout instance defining the layout of the receiver (and its children, if the box layout model is used).
*
* @discussion Though this method does not set the bounds of the view, it does have side effects--caching both the
* constraint and the result.
*
* @warning Subclasses must not override this; it caches results from -calculateLayoutThatFits:. Calling this method may
* be expensive if result is not cached.
*
* @see [ASDisplayNode(Subclassing) calculateLayoutThatFits:]
*/
- (ASLayout *)layoutThatFits:(ASSizeRange)constrainedSize;
第三个方法- (ASLayout *)layoutThatFits:(ASSizeRange)constrainedSize parentSize:(CGSize)parentSize;，同样是不允许重写的方法。在-calculateLayoutThatFits调用此方法来计算子视图的layout
/**
* Call this on children layoutElements to compute their layouts within your implementation of -calculateLayoutThatFits:.
*
* @warning You may not override this method. Override -calculateLayoutThatFits: instead.
* @warning In almost all cases, prefer the use of ASCalculateLayout in ASLayout
*
* @param constrainedSize Specifies a minimum and maximum size. The receiver must choose a size that is in this range.
* @param parentSize The parent node's size. If the parent component does not have a final size in a given dimension,
* then it should be passed as ASLayoutElementParentDimensionUndefined (for example, if the parent's width
* depends on the child's size).
*
* @discussion Though this method does not set the bounds of the view, it does have side effects--caching both the
* constraint and the result.
*
* @return An ASLayout instance defining the layout of the receiver (and its children, if the box layout model is used).
*/
- (ASLayout *)layoutThatFits:(ASSizeRange)constrainedSize parentSize:(CGSize)parentSize;
- (ASLayout *)calculateLayoutThatFits:(ASSizeRange)constrainedSize;
重写用于计算layoutElements的layout，注释说明了不应该重写-layoutThatFits:parentSize:的原因
/**
* Override this method to compute your layoutElement's layout.
*
* @discussion Why do you need to override -calculateLayoutThatFits: instead of -layoutThatFits:parentSize:?
* The base implementation of -layoutThatFits:parentSize: does the following for you:
* 1. First, it uses the parentSize parameter to resolve the nodes's size (the one assigned to the size property).
* 2. Then, it intersects the resolved size with the constrainedSize parameter. If the two don't intersect,
* constrainedSize wins. This allows a component to always override its children's sizes when computing its layout.
* (The analogy for UIView: you might return a certain size from -sizeThatFits:, but a parent view can always override
* that size and set your frame to any size.)
* 3. It caches it result for reuse
*
* @param constrainedSize A min and max size. This is computed as described in the description. The ASLayout you
* return MUST have a size between these two sizes.
*/
- (ASLayout *)calculateLayoutThatFits:(ASSizeRange)constrainedSize;
扩展方法，可以通过自身size、限制size、父视图size计算出最终的结果来得到对应的layout
/**
* In certain advanced cases, you may want to override this method. Overriding this method allows you to receive the
* layoutElement's size, parentSize, and constrained size. With these values you could calculate the final constrained size
* and call -calculateLayoutThatFits: with the result.
*
* @warning Overriding this method should be done VERY rarely.
*/
- (ASLayout *)calculateLayoutThatFits:(ASSizeRange)constrainedSize
restrictedToSize:(ASLayoutElementSize)size
relativeToParentSize:(CGSize)parentSize;
此处定义了一系列的常量，用了NSString * const可见代码的规范。
#pragma mark - ASLayoutElementStyle
AS_EXTERN NSString * const ASLayoutElementStyleWidthProperty;
AS_EXTERN NSString * const ASLayoutElementStyleMinWidthProperty;
AS_EXTERN NSString * const ASLayoutElementStyleMaxWidthProperty;
AS_EXTERN NSString * const ASLayoutElementStyleHeightProperty;
AS_EXTERN NSString * const ASLayoutElementStyleMinHeightProperty;
AS_EXTERN NSString * const ASLayoutElementStyleMaxHeightProperty;
AS_EXTERN NSString * const ASLayoutElementStyleSpacingBeforeProperty;
AS_EXTERN NSString * const ASLayoutElementStyleSpacingAfterProperty;
AS_EXTERN NSString * const ASLayoutElementStyleFlexGrowProperty;
AS_EXTERN NSString * const ASLayoutElementStyleFlexShrinkProperty;
AS_EXTERN NSString * const ASLayoutElementStyleFlexBasisProperty;
AS_EXTERN NSString * const ASLayoutElementStyleAlignSelfProperty;
AS_EXTERN NSString * const ASLayoutElementStyleAscenderProperty;
AS_EXTERN NSString * const ASLayoutElementStyleDescenderProperty;
AS_EXTERN NSString * const ASLayoutElementStyleLayoutPositionProperty;
ASLayoutElementStyleDelegate属性更换时的协议
@protocol ASLayoutElementStyleDelegate
- (void)style:(__kindof ASLayoutElementStyle *)style propertyDidChange:(NSString *)propertyName;
@end
delegate相关的操作
@interface ASLayoutElementStyle : NSObject
/**
* @abstract Initializes the layoutElement style with a specified delegate
*/
- (instancetype)initWithDelegate:(id)delegate;
/**
* @abstract The object that acts as the delegate of the style.
*
* @discussion The delegate must adopt the ASLayoutElementStyleDelegate protocol. The delegate is not retained.
*/
@property (nullable, nonatomic, weak, readonly) id delegate;
定义了一系列的参数来设定控件的大小包括
#pragma mark - Sizing
/**
* @abstract The width property specifies the width of the content area of an ASLayoutElement.
* The minWidth and maxWidth properties override width.
* Defaults to ASDimensionAuto
*/
@property (nonatomic) ASDimension width;
/**
* @abstract The height property specifies the height of the content area of an ASLayoutElement
* The minHeight and maxHeight properties override height.
* Defaults to ASDimensionAuto
*/
@property (nonatomic) ASDimension height;
/**
* @abstract The minHeight property is used to set the minimum height of a given element. It prevents the used value
* of the height property from becoming smaller than the value specified for minHeight.
* The value of minHeight overrides both maxHeight and height.
* Defaults to ASDimensionAuto
*/
@property (nonatomic) ASDimension minHeight;
/**
* @abstract The maxHeight property is used to set the maximum height of an element. It prevents the used value of the
* height property from becoming larger than the value specified for maxHeight.
* The value of maxHeight overrides height, but minHeight overrides maxHeight.
* Defaults to ASDimensionAuto
*/
@property (nonatomic) ASDimension maxHeight;
/**
* @abstract The minWidth property is used to set the minimum width of a given element. It prevents the used value of
* the width property from becoming smaller than the value specified for minWidth.
* The value of minWidth overrides both maxWidth and width.
* Defaults to ASDimensionAuto
*/
@property (nonatomic) ASDimension minWidth;
/**
* @abstract The maxWidth property is used to set the maximum width of a given element. It prevents the used value of
* the width property from becoming larger than the value specified for maxWidth.
* The value of maxWidth overrides width, but minWidth overrides maxWidth.
* Defaults to ASDimensionAuto
*/
@property (nonatomic) ASDimension maxWidth;
#pragma mark - ASLayoutElementStyleSizeHelpers
/**
* @abstract Provides a suggested size for a layout element. If the optional minSize or maxSize are provided,
* and the preferredSize exceeds these, the minSize or maxSize will be enforced. If this optional value is not
* provided, the layout element’s size will default to it’s intrinsic content size provided calculateSizeThatFits:
*
* @discussion This method is optional, but one of either preferredSize or preferredLayoutSize is required
* for nodes that either have no intrinsic content size or
* should be laid out at a different size than its intrinsic content size. For example, this property could be
* set on an ASImageNode to display at a size different from the underlying image size.
*
* @warning Calling the getter when the size's width or height are relative will cause an assert.
*/
@property (nonatomic) CGSize preferredSize;
/**
* @abstract An optional property that provides a minimum size bound for a layout element. If provided, this restriction will
* always be enforced. If a parent layout element’s minimum size is smaller than its child’s minimum size, the child’s
* minimum size will be enforced and its size will extend out of the layout spec’s.
*
* @discussion For example, if you set a preferred relative width of 50% and a minimum width of 200 points on an
* element in a full screen container, this would result in a width of 160 points on an iPhone screen. However,
* since 160 pts is lower than the minimum width of 200 pts, the minimum width would be used.
*/
@property (nonatomic) CGSize minSize;
- (CGSize)minSize UNAVAILABLE_ATTRIBUTE;
/**
* @abstract An optional property that provides a maximum size bound for a layout element. If provided, this restriction will
* always be enforced. If a child layout element’s maximum size is smaller than its parent, the child’s maximum size will
* be enforced and its size will extend out of the layout spec’s.
*
* @discussion For example, if you set a preferred relative width of 50% and a maximum width of 120 points on an
* element in a full screen container, this would result in a width of 160 points on an iPhone screen. However,
* since 160 pts is higher than the maximum width of 120 pts, the maximum width would be used.
*/
@property (nonatomic) CGSize maxSize;
- (CGSize)maxSize UNAVAILABLE_ATTRIBUTE;
/**
* @abstract Provides a suggested RELATIVE size for a layout element. An ASLayoutSize uses percentages rather
* than points to specify layout. E.g. width should be 50% of the parent’s width. If the optional minLayoutSize or
* maxLayoutSize are provided, and the preferredLayoutSize exceeds these, the minLayoutSize or maxLayoutSize
* will be enforced. If this optional value is not provided, the layout element’s size will default to its intrinsic content size
* provided calculateSizeThatFits:
*/
@property (nonatomic) ASLayoutSize preferredLayoutSize;
/**
* @abstract An optional property that provides a minimum RELATIVE size bound for a layout element. If provided, this
* restriction will always be enforced. If a parent layout element’s minimum relative size is smaller than its child’s minimum
* relative size, the child’s minimum relative size will be enforced and its size will extend out of the layout spec’s.
*/
@property (nonatomic) ASLayoutSize minLayoutSize;
/**
* @abstract An optional property that provides a maximum RELATIVE size bound for a layout element. If provided, this
* restriction will always be enforced. If a parent layout element’s maximum relative size is smaller than its child’s maximum
* relative size, the child’s maximum relative size will be enforced and its size will extend out of the layout spec’s.
*/
@property (nonatomic) ASLayoutSize maxLayoutSize;
@end