iOS消息转发

1.在说消息转发前需要先理解objc_magsent， objc_msgSend函数会依据接收者与选择子（SEL）的类型来调整适当的方法。为了完成此操作，该方法需要在接收者所属的类中搜寻其“方法列表”如果能找到与选择子名称相符的方法，就调整至其代码，如果找不到，就沿着继承体系继续向上查找，等找到合适的方法之后在跳转，如果还是找不到相符合的方法那就执行消息转发

2.上面是消息转发的条件消息转发包括一下：

1.动态方法解析（+ (BOOL)resolveInstanceMethod:(SEL)selector）他的作用是 类是否新增实力方法处理选择子有返回yes没有No

2.备用接收者 (- (id)forwardingTargetForSelector:(SEL)aSelector) 如果1返回类NO就到2，2的作用是把消息转给其他接收者来处理消息 没有就返回nil

3.完整消息转发 （-(void)forwardInvocation:(NSInvocation *)anInvocation） 如果2返回nil 创建NSInvocation 对象 封装全部细节 消息派发系统把消息派给目标对象

举个例子 模拟下字典 为属性添加set get 方法

@interface EOCAutoDictionary : NSObject
@property(nonatomic,strong) NSString *string;
@property(nonatomic,strong) NSNumber *number;
@property(nonatomic,strong) NSDate *date;
@property(nonatomic,strong) id opaqueObject;
@end

#import "EOCAutoDictionary.h"
#import <objc/runtime.h>
@interface EOCAutoDictionary()
@property(nonatomic,strong) NSMutableDictionary *backingStore;
@end
@implementation EOCAutoDictionary
@dynamic string,number,date,opaqueObject;//这个是让用户自己实现get set 方法 
- (id)init{
    if (self = [super init]) {
        _backingStore = [NSMutableDictionary new];
    }
    return self;
}
+ (BOOL)resolveInstanceMethod:(SEL)selector{
    NSString *selectorString = NSStringFromSelector(selector);
    if ([selectorString hasPrefix:@"set"]) {
        class_addMethod(self,
                        selector,
                        (IMP)autoDictionarySetter,
                        "v@:@");
    }else{
        class_addMethod(self,
                        selector,
                        (IMP)autoDictionaryGetter,
                        "@@:");
    }
    
    return YES;
    
}

id autoDictionaryGetter(id self ,SEL _cmd){
    EOCAutoDictionary *typedSelf = (EOCAutoDictionary *)self;
    NSMutableDictionary *backingStore = typedSelf.backingStore;
    NSString *key = NSStringFromSelector(_cmd);
    return [backingStore objectForKey:key];
}
void autoDictionarySetter(id self ,SEL _cmd,id value){
     EOCAutoDictionary *typedSelf = (EOCAutoDictionary *)self;
     NSMutableDictionary *backingStore = typedSelf.backingStore;
    NSString *selectorString = NSStringFromSelector(_cmd);
    NSMutableString *key = [selectorString mutableCopy];
    [key deleteCharactersInRange:NSMakeRange(key.length-1, 1)];
    [key deleteCharactersInRange:NSMakeRange(0, 3)];
    NSString *lowercaseFirstChar = [[key substringToIndex:1] lowercaseString];
    [key replaceCharactersInRange:NSMakeRange(0, 1) withString:lowercaseFirstChar];
    if (value) {
        [backingStore setObject:value forKey:key];
    }else{
        [backingStore removeObjectForKey:key];
    }
    
}

外面调用

 EOCAutoDictionary *dict = [EOCAutoDictionary new];
    dict.date = [NSDate dateWithTimeIntervalSince1970:475372800];
    NSLog(@"date = %@",dict.date);