扩展（extension）
扩展是向一个已有的类、结构体或枚举类型添加新的功能。在swift中扩展是没有名字的，但在Objective-C中Category是有名字的，而且只能扩展类（类别）

例如在OC中我们想直接拿到某个view的frame并修改,只需要新建一个UIView+Frame的Category

//UIView+Frame.h
#import <UIKit/UIKit.h>

@interface UIView (Frame)

@property (nonatomic ,assign) CGFloat width;
@property (nonatomic ,assign) CGFloat height;

@property (nonatomic ,assign) CGFloat x;
@property (nonatomic ,assign) CGFloat y;

@property (nonatomic ,assign) CGFloat centerX;
@property (nonatomic ,assign) CGFloat centerY;

//UIView+Frame.m
#import "UIView+Frame.h"
@implementation UIView (Frame)

- (void)setCenterX:(CGFloat)centerX{
    CGPoint center = self.center;
    center.x = centerX;
    self.center = center;
}

- (void)setCenterY:(CGFloat)centerY{
    CGPoint center = self.center;
    center.y = centerY;
    self.center = center;
}
- (CGFloat)centerX{
    return self.center.x;
}

- (CGFloat)centerY{
    return self.center.y;
}

- (void)setWidth:(CGFloat)width{
    CGRect frame = self.frame; 
    frame.size.width = width;   
    self.frame = frame;    
}

- (void)setHeight:(CGFloat)height{
    CGRect frame = self.frame;
    frame.size.height = height;
    self.frame = frame;
}

- (void)setX:(CGFloat)x{
    CGRect frame = self.frame;
    frame.origin.x = x;
    self.frame = frame;
}

- (void)setY:(CGFloat)y{
    CGRect frame = self.frame;
    frame.origin.y = y;
    self.frame = frame;
}

- (CGFloat)width{
    return self.bounds.size.width;
}

- (CGFloat)height{
    return self.bounds.size.height;
}

- (CGFloat)x{
    return self.frame.origin.x;
}

- (CGFloat)y{
    return self.frame.origin.y;
}

就可以在项目中丝滑的使用

//MyProjectViewController.m
    NSUInteger index = self.scrollView.contentOffset.x / self.scrollView.width;
    UIViewController *childVC = self.childViewControllers[index];
    childVC.view.y = 0;
    childVC.view.x = self.scrollView.contentOffset.x;
    childVC.view.width = self.scrollView.width;
    childVC.view.height = self.scrollView.height;

而在swift中,如果不进行"扩展",我们只能这么写:

//MyProjectViewController.swift
    let index = Int(self.scrollView.contentOffset.x / self.scrollView.frame.width)
    var childVC = self.childViewControllers[index]
    childVC.view.frame = CGRect(x: self.scrollView.contentOffset.x, y: 0, width: self.scrollView.frame.width, height: self.scrollView.frame.height)

    self.scrollView.addSubview(childVC.view)

要添加UIView的分类,只需要新建一个swift文件

//UIView+Frame.swift
import UIKit

extension UIView {
    var width : CGFloat {
        get{
            return self.bounds.size.width
        }
        set(width) {
            //var frame = self.frame
            //frame.size.width = width
            //self.frame = frame
            self.frame.size = CGSize(width: width, height: self.frame.height)
        }
    }
}

即可以在项目中使用了:

//MyProjectViewController.swift
    let index = Int(self.scrollView.contentOffset.x / self.scrollView.frame.width)
    var childVC = self.childViewControllers[index]
    childVC.view.x......
    childVC.view.y......
    childVC.view.width = self.scrollView.width
    childVC.view.height......

    self.scrollView.addSubview(childVC.view)