OC和Swift中实现Method Swizzling

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曾经在我刚工作的时候产品提出一个需求,进行页面和事件追踪,当时用的友盟,于是傻乎乎的在每个ViewControlelr的viewWillAppear和viewDidAppear中都调用友盟的接口。后来接触到Method Swizzling,才了解到这个基于Runtime的强大的方式。Method Swizzling是一个非常有用的技巧,可以全局hook,进行行为统计。

Method Swizzling原理

每个类都有一个方法列表,存放着selector的名字和方法实现的映射关系。IMP有点类似函数指针,指向具体的Method实现。
在Objective-C中调用一个方法,其实是向一个对象发送消息,查找消息的唯一依据是selector的名字。利用Objective-C的动态特性,可以实现在运行时偷换selector对应的方法实现,达到给方法挂钩的目的。

在实现Method Swizzling时,核心代码主要就是一个runtime的C语言API:

OBJC_EXPORT void method_exchangeImplementations(Method m1, Method m2) 
 __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);

原来的Dispatch Table


Screen Shot 2018-04-14 at 4.36.47 PM.png

然后调用method_exchangeImplementations方法交换后变成的原来的Dispatch Table


Screen Shot 2018-04-14 at 4.38.47 PM.png

Method Swizzling实战

OC中使用

#import "UIViewController+swizzling.h"
#import <objc/runtime.h>
 
@implementation UIViewController (swizzling)
 
+ (void)load


{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        SEL origSel = @selector(viewDidAppear:);
        SEL swizSel = @selector(swiz_viewDidAppear:);
        [UIViewController swizzleMethods:[self class] originalSelector:origSel swizzledSelector:swizSel];
    }
    
}
 
//exchange implementation of two methods
+ (void)swizzleMethods:(Class)class originalSelector:(SEL)origSel swizzledSelector:(SEL)swizSel
{
    Method origMethod = class_getInstanceMethod(class, origSel);
    Method swizMethod = class_getInstanceMethod(class, swizSel);
     
    //class_addMethod will fail if original method already exists
    BOOL didAddMethod = class_addMethod(class, origSel, method_getImplementation(swizMethod), method_getTypeEncoding(swizMethod));
    if (didAddMethod) {
        class_replaceMethod(class, swizSel, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
    } else {
        //origMethod and swizMethod already exist
        method_exchangeImplementations(origMethod, swizMethod);
    }
}
 
- (void)swiz_viewDidAppear:(BOOL)animated
{
    NSLog(@"I am in - [swiz_viewDidAppear:]");
    //handle viewController transistion counting here, before ViewController instance calls its -[viewDidAppear:] method
    //需要注入的代码写在此处
    [self swiz_viewDidAppear:animated];
}
 
@end

Swift中是如何使用的呢?

要在 Swift 自定义类中使用 Method Swizzling 有两个必要条件:

  1. 包含 swizzle 方法的类需要继承自 NSObject
  2. 需要 swizzle 的方法必须有动态属性(dynamic attribute)

Swift3中重写initialize方法

public override class func initialize() {


}

Swift4后initialize失效了,可以按照下面这种方式实现

//
//  ViewController+swizzling.swift
//  RuntimeDemo
//
//  Created by roy.cao on 14/04/2018.
//  Copyright © 2018 roy.cao. All rights reserved.
//

import Foundation
import UIKit



private let onceToken = "Method Swizzling"
extension UIViewController {
    
    public class func initializeMethod() {
        // Make sure This isn't a subclass of UIViewController, So that It applies to all UIViewController childs
        
        if self != UIViewController.self {
            return
        }
        
        /**
         let _:() = {
         
         }()
        */
        //DispatchQueue函数保证代码只被执行一次,防止又被交换回去导致得不到想要的效果
        DispatchQueue.once(token: onceToken) {
            
            let originalSelector = #selector(UIViewController.viewWillAppear(_:))
            let swizzledSelector = #selector(UIViewController.swizzled_viewWillAppear(animated:))
            
            let originalMethod = class_getInstanceMethod(self, originalSelector)
            let swizzledMethod = class_getInstanceMethod(self, swizzledSelector)
            //在进行 Swizzling 的时候,需要用 class_addMethod 先进行判断一下原有类中是否有要替换方法的实现
            let didAddMethod: Bool = class_addMethod(self, originalSelector, method_getImplementation(swizzledMethod!), method_getTypeEncoding(swizzledMethod!))
            //如果 class_addMethod 返回 yes,说明当前类中没有要替换方法的实现,所以需要在父类中查找,这时候就用到 method_getImplemetation 去获取 class_getInstanceMethod 里面的方法实现,然后再进行 class_replaceMethod 来实现 Swizzing
            
            if didAddMethod {
                class_replaceMethod(self, swizzledSelector, method_getImplementation(originalMethod!), method_getTypeEncoding(originalMethod!))
            } else {
                method_exchangeImplementations(originalMethod!, swizzledMethod!)
            }
            
        }
        
        
    }
    
    
    @objc func swizzled_viewWillAppear(animated: Bool) {
        //需要注入的代码写在此处
        self.swizzled_viewWillAppear(animated: animated)
    }
    
}

didFinishLaunchingWithOptions中调用initializeMethod,因此swizzling applies and stays.

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        
        UIViewController.initializeMethod()

        return true
 }

注意

在Swift中尽量少使用Method Swizzling,毕竟Swift 关于方法派发是使用静态方法,只有当你的问题不能用 Swift 的方式解决,也不能用子类、协议或扩展解决时才使用。不过听说苹果又想在Swift5中增加Runtime,其实私心是支持的,Runtime很强大。
个人建议其实不光在Swift中尽量少使用Method Swizzling,在OC中也尽量少使用,只有当没有其他更好的方式解决你的需要时才用。

参考文章
Swift 4 Method Swizzling (Part 2/2)
如何优雅地在Swift4中实现Method Swizzling
Effective Method Swizzling in Swift
Objective-C的hook方案(一): Method Swizzling

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