在oc中为了增强已有类的功能，我们经常使用分类。使用分类，我们可以在不破坏原有类的结构的前提下，对原有类进行模块化的扩展。

但是在swift中没有分类这种写法了。相对应的是swift中只有扩展(Extensions)。

下面是swift中扩展(Extensions)的说明

扩展就是向一个已有的类、结构体、枚举类型或者协议类型添加新功能（functionality）。这包括在没有权限获取原始源代码的情况下扩展类型的能力（即逆向建模）。扩展和 Objective-C 中的分类（categories）类似。（不过与 Objective-C 不同的是，Swift 的扩展没有名字。）

那么我们怎么在swift中实现oc中的分类呢？我们可以向苹果学习。
同样是UIView类，我们分别看看oc和swift不同的实现方式。

1.我们先看看oc中的UIView

这是 UIView中的声明的代码

#ifndef SDK_HIDE_TIDE
NS_CLASS_AVAILABLE_IOS(2_0) @interface UIView : UIResponder <NSCoding, UIAppearance, UIAppearanceContainer, UIDynamicItem, UITraitEnvironment, UICoordinateSpace, UIFocusEnvironment>
#else
NS_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

其他的属性和方法都是使用分类的方式进行扩展的。
下面是页面渲染相关的属性和方法

@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.这是swift中的代码

UIView类中的声明

@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
}

页面渲染的代码

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.创建我们自己的extension

下面就让我们尝试一下，写一个自己的swift分类
在get和set UIView的x和y属性的时候代码很繁琐，如下：

//get x
var x = self.view.frame.origin.x

//set x
var rect = self.view.frame
rect.origin.x = 100
self.view.frame = rect

我们希望这里的代码是这样的

//get x
var x = self.view.x
//set x
self.view.x = 100

在oc中我们可以写一个分类来实现，这个难度不大。懒的自己写的可以参照这个https://github.com/findM/UIView-Positioning
在swift中我们需要写一个extension来实现
3.1 新建swift文件

3.2 代码实现

import Foundation
import UIKit

//private var PERSON_ID_NUMBER_PROPERTY = 0

extension 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这里一一实现了
}

全部的实现方法请参考这里https://github.com/findM/UIView-Positioning-Swift