iOS Method Swizzling

前言

有个奇怪的需求：每个UIViewController在viewDidAppear:和viewWillAppear:方法中都需要执行一些操作。这里介绍一种iOS Method Swizzling方法:让viewDidAppear:和customViewDidAppear:方法调换实现方式；让viewWillAppear:和customViewWillAppear:方法调换实现方式。

实现过程

1. 新建UIViewController的一个category - Swizzle
   1. 在Swizzle中实现customViewDidAppear:方法和customViewWillAppear:方法。
   2. 并且重写load方法，在load方法中将指向viewDidAppear:实现的指针和指向customViewDidAppear:实现的指针对调；将指向viewWillAppear:实现的指针和指向customViewWillAppear:实现的指针对调
2. 函数指针对调实现
3. 在AppDelegate.m中import这个Swizzle category。那么在所有的UIViewController执行load方法时都会执行Swizzle中重写的load方法，以实现方法的对调；所以之后正常调用viewDidAppear:或是viewWillAppear:时，其实是调用的customViewDidAppear:和customViewWillAppear:方法

具体代码实现

1. Swizzle category

        @implementation UIViewController (Swizzle)
   
        + (void)load {
            swizzleAllViewController();
        }
   
        - (void)customViewDidAppear:(BOOL)animated {
            // do something here
            [self customViewDidAppear:animated]; // 其实是调用的viewDidAppear:
        }
   
        - (void)customviewWillAppear:(BOOL)animated {
            // do something here
            [self customviewWillAppear:animated]; // 其实是调用的viewWillAppear:
        }
   
        @end
   
        // 函数指针对调
        void swizzleAllViewController() {
            Swizzle([UIViewController class], @selector(viewDidAppear:), @selector(customViewDidAppear:));
            //Swizzle([UIViewController class], @selector(viewWillDisappear:), @selector(customViewWillDisappear:));
            Swizzle([UIViewController class], @selector(viewWillAppear:), @selector(customviewWillAppear:));
        }
   

2. 函数指针对调的实现

        void Swizzle(Class c, SEL origSEL, SEL newSEL) {
            Method origMethod = class_getInstanceMethod(c, origSEL);
            Method newMethod = nil;
            if (!origMethod) {
                origMethod = class_getClassMethod(c, origSEL);
                if (!origMethod) {
                    return;
                }
                newMethod = class_getClassMethod(c, newSEL);
                if (!newMethod) {
                    return;
                }
            } else {
                newMethod = class_getInstanceMethod(c, newSEL);
                if (!newMethod) {
                    return;
                }
            }
   
            if (class_addMethod(c, origSEL, method_getImplementation(newMethod), method_getTypeEncoding(newMethod))) {
                NSLog(@"vanney code log... errorhappened");
                class_replaceMethod(c, newSEL, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
            } else {
                method_exchangeImplementations(origMethod, newMethod);
            }
        }
   

P.S

以上代码都参考自Coding-iOS

参考

• Coding-iOS
• Objective-C的hook方案（一）: Method Swizzling
• NSObject +load and +initialize - What do they do?

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