IOS之方法混写(swizzling.)

OC中的混写(swizzling)是指透明地把一个方法换成另外一个。简明的说就是在运行时替换方法。利用方法混写可以改变那些没有源代码的对象(包括系统对象)的行为。

方法混写的代码看起来相对比较直观的，举个例子说明一下，之前做本地化翻译的时候就有用到 swizzling 方法。直接去swizze方法 awakeFromNib 然后替换成自己的方法实现以实现本地化翻译。详细可以看这篇文章：IOS本地化应用程序。

主要用到的代码也就这两句：

+(void)load  
{  
    // Autoload : swizzle -awakeFromNib with -localizeNibObject as soon as the app (and thus this class) is loaded  
    Method localizeNibObject = class_getInstanceMethod([NSObject class], @selector(localizeNibObject));  
    Method awakeFromNib = class_getInstanceMethod([NSObject class], @selector(awakeFromNib));  
    method_exchangeImplementations(awakeFromNib, localizeNibObject);  
} 

-(void)localizeNibObject  
{   
     //本地化操作....
    // Call the original awakeFromNib method  
    [self localizeNibObject]; // this actually calls the original awakeFromNib (and not localizeNibObject) because we did some method swizzling  
} 

如 NSobject + AGi18n 分类代码：

#import "NSObject+AGi18n.h"
#import <objc/runtime.h>

@implementation NSObject (AGi18n)

//By default do nothing when localizing
- (void)localizeFromNib {
}


#pragma mark - Method swizzling

+ (void)load {
    Method awakeFromNibOriginal = class_getInstanceMethod(self, @selector(awakeFromNib));
    Method awakeFromNibCustom = class_getInstanceMethod(self, @selector(awakeFromNibCustom));
        
    //Swizzle methods
    method_exchangeImplementations(awakeFromNibOriginal, awakeFromNibCustom);
}

- (void)awakeFromNibCustom {
    //Call standard methods
    [self awakeFromNibCustom];
    
    //Localize
    [self localizeFromNib];
}

@end

#import "UIButton+AGi18n.h"

@implementation UIButton (AGi18n)

- (void)localizeFromNib {

    //Replace text with localizable version
    NSArray *states = @[@(UIControlStateNormal), @(UIControlStateHighlighted), @(UIControlStateDisabled), @(UIControlStateSelected), @(UIControlStateApplication)];
    for (NSNumber *state in states) {
        NSString *title = [self titleForState:state.integerValue];
        if (title.length > 0) {
            [self setTitle:[[NSBundle mainBundle] localizedStringForKey:title value:@"" table:nil] forState:state.integerValue];
        }
    }
}

@end

《IOS 7 Programming:Pushing the Limits》一书中对方法混写举了一个在NSNotificationCenter中添加观察者时打印日志的例子。通过上下两个例子的对比，AGi18n在 swizzling 时还是欠缺部分考虑的。

@interface NSObject(RNSwizzle)
+(IMP)swizzleSelector:(SEL)origSelector withIMP:(IMP)newIMP;
@end

@implementation NSObject(RNSwizzle)
+(IMP)swizzleSelector:(SEL)origSelector withIMP:(IMP)newIMP{
    Class class = [self class];
    Method origMethod = class_getInstanceMethod(class, origSelector);
    IMP origIMP = method_getImplementation(origMethod);
    if (!class_addMethod(self, origSelector, newIMP, method_getTypeEncoding(origMethod))) {
        method_setImplementation(origMethod, newIMP);
    }
    
    return origIMP;
}

@end

如果方法根本没有实现，或者是某个父类实现的，那就需要用 class_addMethod 添加方法，这跟通常的覆盖方法是一样的。如果 class_addMethod 失败了，我们就知道了是此类直接实现了正在混写的方法，那么就转而用 method_setImplementation 来把旧实现替换为 新实现。

好了之后，返回原来的 IMP，调用者怎么用返回值是它自己的事。

参考：《IOS 7 Programming:Pushing the Limits》

IOS之方法混写(swizzling.)

原文：http://blog.csdn.net/chaoyuan899/article/details/44323145