NSTextStorage

NSTextStorage 定义了 TextKit 最基本的存储机制。这个类是 NSMutableAttributedString 的半具体的子类（我特么也没看明白是啥子意思），并添加一组 NSLayoutManager 的管理行为。一个文本存储对象当字符或属性发生改变的时候会通知布局管理者，让布局管理者在需要的时候重新显示文本。

概览

NSTextSorage 对象能够在任何线程进行存取。但是你必须保证在同一时间只能在一个线程进行存取。

子类化注意事项
NSTextSorage 类实现改变管理（通过 beginEditing() 和 endEditing() ），验证属性，代理回调，布局管理通知。NSTextSorage 类是不完整的，这个类没有实现实际的属性字符串存储，子类通过重载 NSAttributedString 的两个函数进行管理：

• string
• attributes(at:effectiveRange:)

子类化也必须同时重载 NSMutableAttributedString 的两个函数：

• replaceCharacters(in:with:)
• setAttributes(_:range:)

这些函数完成改变后，然后就会调用 edited(_:range:changeInLength:)去通知父类知道 改变已经发生。

// 接收器的内容字符
/*
依附的字符串是不能通过这个属性的值进行移除。
由于性能的考虑，这个属性只会返回当前背后存储的属性字符串对象。如果你想要去维持这个你操作后返回字符串的快照，你需要对子字符串进行拷贝。

这个函数属性必须保证有效的去存储字符到属性字符串，子类必须实现这个属性。
*/
var string: String { get }

给指定索引的字符返回属性
func attributes(at location: Int, effectiveRange range: NSRangePointer?) -> [String : Any]

根据给定的返回和给定的字符串替换字符
func attributes(at location: Int, effectiveRange range: NSRangePointer?) -> [String : Any]

根据给定的范围和属性数组设置属性
func setAttributes(_ attrs: [String : Any]?, range: NSRange)

变化类型

@available(iOS 7.0, *)
public struct NSTextStorageEditActions : OptionSet {

    public init(rawValue: UInt)

    // 属性被添加， 移除，改变
    public static var editedAttributes: NSTextStorageEditActions { get }

    // 字符串被添加，移除，替换
    public static var editedCharacters: NSTextStorageEditActions { get }
}

/* Note for subclassing NSTextStorage: NSTextStorage is a semi-abstract subclass of NSMutableAttributedString.
It implements change management (beginEditing/endEditing), verification of attributes, delegate handling, and layout management notification. 
The one aspect it does not implement is the actual attributed string storage --- this is left up to the subclassers, which need to override the two NSMutableAttributedString primitives in addition to two NSAttributedString primitives:
 
 - (NSString *)string;
 - (NSDictionary *)attributesAtIndex:(NSUInteger)location effectiveRange:(NSRangePointer)range;

 - (void)replaceCharactersInRange:(NSRange)range withString:(NSString *)str;
 - (void)setAttributes:(NSDictionary *)attrs range:(NSRange)range;
 
 These primitives should perform the change then call edited:range:changeInLength: to get everything else to happen.
*/

@available(iOS 7.0, *)
open class NSTextStorage : NSMutableAttributedString {

    
    
    // NSLayoutManager objects owned by the receiver.
    /**************************** Layout manager ****************************/
    // 与存储对象相关连的 布局管理者
    open var layoutManagers: [NSLayoutManager] { get }

    
    // Adds aLayoutManager to the receiver.  Sends -[NSLayoutManager setTextStorage:] to aLayoutManager with the receiver.
    // 添加布局管理者
    open func addLayoutManager(_ aLayoutManager: NSLayoutManager)

    
    // Removes aLayoutManager from the receiver if already owned by it.  Sends -[NSLayoutManager setTextStorage:] to aLayoutManager with nil.
    // 移除布局管理者
    open func removeLayoutManager(_ aLayoutManager: NSLayoutManager)

    


    // These methods return information about the editing status. Especially useful when there are outstanding beginEditing calls or during processEditing...
    //  这些方法返回的关于编辑状态的信息特别有用（在编辑之前 和 正在编辑的过程中的编辑状态信息）
    
    // The NSTextStorageEditActions mask indicating that there are pending changes for attributes, characters, or both.
    /**************************** Pending edit info ****************************/
    open var editedMask: NSTextStorageEditActions { get }

    
    // The range for pending changes. {NSNotFound, 0} when there is no pending changes.
    open var editedRange: NSRange { get }

    
    // The length delta for the pending changes.
    open var changeInLength: Int { get }

    
    
    /**************************** Delegate ****************************/
    unowned(unsafe) open var delegate: NSTextStorageDelegate?

    
    
    // Notifies and records a recent change.  If there are no outstanding -beginEditing calls, this method calls -processEditing to trigger post-editing processes.  
    // This method has to be called by the primitives after changes are made if subclassed and overridden.  editedRange is the range in the original string (before the edit).
    /**************************** Edit management ****************************/
    open func edited(_ editedMask: NSTextStorageEditActions, range editedRange: NSRange, changeInLength delta: Int)

    
    // Sends out -textStorage:willProcessEditing, fixes the attributes, sends out -textStorage:didProcessEditing, and notifies the layout managers of change with the -processEditingForTextStorage:edited:range:changeInLength:invalidatedRange: method. 
    // Invoked from -edited:range:changeInLength: or -endEditing.
    open func processEditing()

    
    
    // Indicates if the receiver fixes invalidated attributes lazily.  The concrete UIKit subclass fixes attributes lazily by default.  
    // The abstract class (hence, all custom subclasses) is not lazy.
    /**************************** Attribute fixing ****************************/
    open var fixesAttributesLazily: Bool { get }

    
    // Notes the range of attributes that requires validation.  If the NSTextStorage is not lazy this just calls fixAttributesInRange:. 
    // If it is lazy this instead just records the range needing fixing in order to do it later.
    open func invalidateAttributes(in range: NSRange)

    
    // Ensures all attributes in range are validated and ready to be used.  An NSTextStorage that is lazy is required to call the following method before accessing any attributes.  
    // This gives the attribute fixing a chance to occur if necessary.  NSTextStorage subclasses that wish to support laziness must call it from all attribute accessors that they implement. 
    // The default concrete subclass does call this from its accessors.
    open func ensureAttributesAreFixed(in range: NSRange)
}

** 文本存储的代理方法**

/****  NSTextStorage delegate methods ****/

public protocol NSTextStorageDelegate : NSObjectProtocol {

    
    // Sent inside -processEditing right before fixing attributes.  Delegates can change the characters or attributes.
    @available(iOS 7.0, *)
    optional public func textStorage(_ textStorage: NSTextStorage, willProcessEditing editedMask: NSTextStorageEditActions, range editedRange: NSRange, changeInLength delta: Int)

    
    // Sent inside -processEditing right before notifying layout managers.  Delegates can change the attributes.
    @available(iOS 7.0, *)
    optional public func textStorage(_ textStorage: NSTextStorage, didProcessEditing editedMask: NSTextStorageEditActions, range editedRange: NSRange, changeInLength delta: Int)
}

通知
观察着不应该比代理对 textStorage 进行进一步的改变操作。 通知中会包含 编辑的步骤信息，不包含该 userInfo 信息

extension NSNotification.Name {

    
    /**** Notifications ****/
    @available(iOS 7.0, *)
    // 在调用 processEditing() 之后发送
    public static let NSTextStorageWillProcessEditing: NSNotification.Name

    @available(iOS 7.0, *)
   // 在调用 processEditing() 之前发送
    public static let NSTextStorageDidProcessEditing: NSNotification.Name
}