在oc中為了增強已有類的功能，我們經常使用分類。使用分類，我們可以在不破壞原有類的結構的前提下，對原有類進行模塊化的擴展。

但是在swift中沒有分類這種寫法了。相對應的是swift中只有擴展(Extensions)。

下面是swift中擴展(Extensions)的說明

擴展就是向一個已有的類、結構體、枚舉類型或者協議類型添加新功能（functionality）。這包括在沒有權限獲取原始源代碼的情況下擴展類型的能力（即逆向建模）。擴展和 Objective-C 中的分類（categories）類似。（不過與 Objective-C 不同的是，Swift 的擴展沒有名字。）

那么我們怎么在swift中實現oc中的分類呢？我們可以向蘋果學習。
同樣是UIView類，我們分別看看oc和swift不同的實現方式。

1.我們先看看oc中的UIView

這是 UIView中的聲明的代碼

#ifndef SDK_HIDE_TIDENS_CLASS_AVAILABLE_IOS(2_0) @interface UIView : UIResponder <NSCoding, UIAppearance, UIAppearanceContainer, UIDynamicItem, UITraitEnvironment, UICoordinateSpace, UIFocusEnvironment>#elseNS_CLASS_AVAILABLE_IOS(2_0) @interface UIView : UIResponder <NSCoding, UIAppearance, UIAppearanceContainer, UIDynamicItem, UITraitEnvironment, UICoordinateSpace>#endif+ (Class)layerClass;                        // default is [CALayer class]. Used when creating the underlying layer for the view.- (instancetype)initWithFrame:(CGRect)frame NS_DESIGNATED_INITIALIZER;- (nullable instancetype)initWithCoder:(NSCoder *)aDecoder NS_DESIGNATED_INITIALIZER;@PRoperty(nonatomic,getter=isUserInteractionEnabled) BOOL userInteractionEnabled;  // default is YES. if set to NO, user events (touch, keys) are ignored and removed from the event queue.@property(nonatomic)                                 NSInteger tag;                // default is 0@property(nonatomic,readonly,strong)                 CALayer  *layer;              // returns view's layer. Will always return a non-nil value. view is layer's delegate#ifndef SDK_HIDE_TIDE- (BOOL)canBecomeFocused NS_AVAILABLE_IOS(9_0); // NO by default@property (readonly, nonatomic, getter=isFocused) BOOL focused NS_AVAILABLE_IOS(9_0);#endif+ (UIUserInterfaceLayoutDirection)userInterfaceLayoutDirectionForSemanticContentAttribute:(UISemanticContentAttribute)attribute NS_AVAILABLE_IOS(9_0);@property (nonatomic) UISemanticContentAttribute semanticContentAttribute NS_AVAILABLE_IOS(9_0);@end

其他的屬性和方法都是使用分類的方式進行擴展的。
下面是頁面渲染相關的屬性和方法

@interface UIView(UIViewRendering)- (void)drawRect:(CGRect)rect;- (void)setNeedsDisplay;- (void)setNeedsDisplayInRect:(CGRect)rect;@property(nonatomic)                 BOOL              clipsToBounds;              // When YES, content and subviews are clipped to the bounds of the view. Default is NO.@property(nullable, nonatomic,copy)            UIColor          *backgroundColor UI_APPEARANCE_SELECTOR; // default is nil. Can be useful with the appearance proxy on custom UIView subclasses.@property(nonatomic)                 CGFloat           alpha;                      // animatable. default is 1.0@property(nonatomic,getter=isOpaque) BOOL              opaque;                     // default is YES. opaque views must fill their entire bounds or the results are undefined. the active CGContext in drawRect: will not have been cleared and may have non-zeroed pixels@property(nonatomic)                 BOOL              clearsContextBeforeDrawing; // default is YES. ignored for opaque views. for non-opaque views causes the active CGContext in drawRect: to be pre-filled with transparent pixels@property(nonatomic,getter=isHidden) BOOL              hidden;                     // default is NO. doesn't check superviews@property(nonatomic)                 UIViewContentMode contentMode;                // default is UIViewContentModeScaleToFill@property(nonatomic)                 CGRect            contentStretch NS_DEPRECATED_IOS(3_0,6_0) __TVOS_PROHIBITED; // animatable. default is unit rectangle {{0,0} {1,1}}. Now deprecated: please use -[UIImage resizableImageWithCapInsets:] to achieve the same effect.@property(nullable, nonatomic,strong)          UIView           *maskView NS_AVAILABLE_IOS(8_0);/* -tintColor always returns a color. The color returned is the first non-default value in the receiver's superview chain (starting with itself). If no non-default value is found, a system-defined color is returned. If this view's -tintAdjustmentMode returns Dimmed, then the color that is returned for -tintColor will automatically be dimmed. If your view subclass uses tintColor in its rendering, override -tintColorDidChange in order to refresh the rendering if the color changes. */@property(null_resettable, nonatomic, strong) UIColor *tintColor NS_AVAILABLE_IOS(7_0);/* -tintAdjustmentMode always returns either UIViewTintAdjustmentModeNormal or UIViewTintAdjustmentModeDimmed. The value returned is the first non-default value in the receiver's superview chain (starting with itself). If no non-default value is found, UIViewTintAdjustmentModeNormal is returned. When tintAdjustmentMode has a value of UIViewTintAdjustmentModeDimmed for a view, the color it returns from tintColor will be modified to give a dimmed appearance. When the tintAdjustmentMode of a view changes (either the view's value changing or by one of its superview's values changing), -tintColorDidChange will be called to allow the view to refresh its rendering. */@property(nonatomic) UIViewTintAdjustmentMode tintAdjustmentMode NS_AVAILABLE_IOS(7_0);/* The -tintColorDidChange message is sent to appropriate subviews of a view when its tintColor is changed by client code or to subviews in the view hierarchy of a view whose tintColor is implicitly changed when its superview or tintAdjustmentMode changes. */- (void)tintColorDidChange NS_AVAILABLE_IOS(7_0);@end

2.這是swift中的代碼

UIView類中的聲明

@available(iOS 2.0, *)    public class UIView : UIResponder, NSCoding, UIAppearance, UIAppearanceContainer, UIDynamicItem, UITraitEnvironment, UICoordinateSpace, UIFocusEnvironment {        public class func layerClass() -> AnyClass // default is [CALayer class]. Used when creating the underlying layer for the view.        public init(frame: CGRect)    public init?(coder aDecoder: NSCoder)        public var userInteractionEnabled: Bool // default is YES. if set to NO, user events (touch, keys) are ignored and removed from the event queue.    public var tag: Int // default is 0    public var layer: CALayer { get } // returns view's layer. Will always return a non-nil value. view is layer's delegate        @available(iOS 9.0, *)    public func canBecomeFocused() -> Bool // NO by default    @available(iOS 9.0, *)    public var focused: Bool { get }        @available(iOS 9.0, *)    public class func userInterfaceLayoutDirectionForSemanticContentAttribute(attribute: UISemanticContentAttribute) -> UIUserInterfaceLayoutDirection    @available(iOS 9.0, *)    public var semanticContentAttribute: UISemanticContentAttribute}

頁面渲染的代碼

extension UIView {        public func drawRect(rect: CGRect)        public func setNeedsDisplay()    public func setNeedsDisplayInRect(rect: CGRect)        public var clipsToBounds: Bool // When YES, content and subviews are clipped to the bounds of the view. Default is NO.    @NSCopying public var backgroundColor: UIColor? // default is nil. Can be useful with the appearance proxy on custom UIView subclasses.    public var alpha: CGFloat // animatable. default is 1.0    public var opaque: Bool // default is YES. opaque views must fill their entire bounds or the results are undefined. the active CGContext in drawRect: will not have been cleared and may have non-zeroed pixels    public var clearsContextBeforeDrawing: Bool // default is YES. ignored for opaque views. for non-opaque views causes the active CGContext in drawRect: to be pre-filled with transparent pixels    public var hidden: Bool // default is NO. doesn't check superviews    public var contentMode: UIViewContentMode // default is UIViewContentModeScaleToFill    // animatable. default is unit rectangle {{0,0} {1,1}}. Now deprecated: please use -[UIImage resizableImageWithCapInsets:] to achieve the same effect.        @available(iOS 8.0, *)    public var maskView: UIView?        /*     -tintColor always returns a color. The color returned is the first non-default value in the receiver's superview chain (starting with itself).     If no non-default value is found, a system-defined color is returned.     If this view's -tintAdjustmentMode returns Dimmed, then the color that is returned for -tintColor will automatically be dimmed.     If your view subclass uses tintColor in its rendering, override -tintColorDidChange in order to refresh the rendering if the color changes.     */    @available(iOS 7.0, *)    public var tintColor: UIColor!        /*     -tintAdjustmentMode always returns either UIViewTintAdjustmentModeNormal or UIViewTintAdjustmentModeDimmed. The value returned is the first non-default value in the receiver's superview chain (starting with itself).     If no non-default value is found, UIViewTintAdjustmentModeNormal is returned.     When tintAdjustmentMode has a value of UIViewTintAdjustmentModeDimmed for a view, the color it returns from tintColor will be modified to give a dimmed appearance.     When the tintAdjustmentMode of a view changes (either the view's value changing or by one of its superview's values changing), -tintColorDidChange will be called to allow the view to refresh its rendering.     */    @available(iOS 7.0, *)    public var tintAdjustmentMode: UIViewTintAdjustmentMode        /*     The -tintColorDidChange message is sent to appropriate subviews of a view when its tintColor is changed by client code or to subviews in the view hierarchy of a view whose tintColor is implicitly changed when its superview or tintAdjustmentMode changes.     */    @available(iOS 7.0, *)    public func tintColorDidChange()}

3.創(chuàng)建我們自己的extension

下面就讓我們嘗試一下，寫一個自己的swift分類
在get和set UIView的x和y屬性的時候代碼很繁瑣，如下：

//get xvar x = self.view.frame.origin.x//set xvar rect = self.view.framerect.origin.x = 100self.view.frame = rect

我們希望這里的代碼是這樣的

//get xvar x = self.view.x//set xself.view.x = 100

在oc中我們可以寫一個分類來實現，這個難度不大。懶的自己寫的可以參照這個https://github.com/findM/UIView-Positioning
在swift中我們需要寫一個extension來實現
3.1 新建swift文件

3.2 代碼實現

import Foundationimport UIKit//private var PERSON_ID_NUMBER_PROPERTY = 0extension UIView {    public var x: CGFloat{        get{            return self.frame.origin.x        }        set{            var r = self.frame            r.origin.x = newValue            self.frame = r        }    }    public var y: CGFloat{        get{            return self.frame.origin.y        }        set{            var r = self.frame            r.origin.y = newValue            self.frame = r        }    }    //其他的篇幅原因就不在這里一一實現了}

全部的實現方法請參考這里https://github.com/findM/UIView-Positioning-Swift