iOS多继承的实现及区别

// 编程技能
@protocol Program <NSObject>
- (void)program;
@end
// 绘画技能
@protocol Draw <NSObject>
- (void)draw;
@end
// 歌唱技能
@protocol Sing <NSObject>
- (void)sing;
@end

// 原本一个什么也不会的程序员
// 学会了多个技能
@interface Programmer : NSObject <Draw, Sing>
// 继承的协议方法自动公有，无须声明接口
@end

@interface Programmer () <Program>
// 继承的协议方法自动私有，无须声明接口
@end

// 需要自行实现协议方法
@implementation Programmer
- (void)program {
    NSLog(@"I'm writing bugs!");
}
- (void)draw {
    NSLog(@"I can draw");
}
- (void)sing {
    NSLog(@"Lalalallalalala");
}
@end

/*      分类头文件       */
#import "Programmer.h"

@interface Programmer (Program)
// 声明属性
@property (nonatomic, assign) NSString *motto;
// 声明公有方法
- (void)draw;
- (void)sing;
@end


/*      分类实现文件       */
#import <objc/runtime.h>

@implementation Programmer (Program)
// 为Catagory添加属性
static const char kMottoKey;
- (void)setMotto:(NSString *)motto {
    objc_setAssociatedObject(self, &kMottoKey, motto, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
- (NSString *)motto {
    return objc_getAssociatedObject(self, &kMottoKey);
}
// 私有方法
- (void)program {
    NSLog(@"I'm writing bugs!");
}
// 实现公有方法
- (void)draw {
    NSLog(@"I can draw");
}
- (void)sing {
    NSLog(@"Lalalalallalala");
}
@end

/*  Programmer实现文件  */
@implementation Programmer

// 通过消息转发实现多继承
- (id)forwardingTargetForSelector:(SEL)aSelector {
    Singer *singer = [[Singer alloc] init];
    Artist *artist = [[Artist alloc] init];
    
    if ([singer respondsToSelector:aSelector]) {
        return singer;
    }
    else if ([artist respondsToSelector:aSelector]) {
        return artist;
    }
    return nil;
}
@end

/*  Artist代码  */
@interface Artist : NSObject
// 画家可以绘画
- (void)draw;
@end

@implementation Artist
- (void)draw {
    NSLog(@"I can draw");
}
@end

/*  Singer代码  */
@interface Singer : NSObject
// 歌手会唱歌
- (void)sing;
@end

@implementation Singer
- (void)sing {
    NSLog(@"Lalalalalala");
}
@end

Programmer *programmer = [[Programmer alloc] init];

// 在performSelector中使用NSSelectorFromString会造成警告
// 可以通过设置不检测performSelector内存泄露关闭警告
[programmer performSelector:NSSelectorFromString(@"sing") withObject:nil];

// 或者通过类型强转来实现，无警告
[(Artist *)programmer draw];

@interface Programmer ()
{
    // 由于要频繁用到，我们可以创建成员实例
    Singer *_singer;
    Artist *_artist;
}
@end

@implementation Programmer

- (instancetype)init {
    if (self = [super init]) {
        _singer = [[Singer alloc] init];
        _artist = [[Artist alloc] init];
    }
    return self;
}

// 在转发消息前先对方法重新签名
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    // 尝试自行实现方法签名
    NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
    
    // 若无法实现，尝试通过多继承得到的方法实现
    if (signature == nil) {
        // 判断该方法是哪个父类的，并通过其创建方法签名
        if ([_singer respondsToSelector:aSelector]) {
            signature = [_singer methodSignatureForSelector:aSelector];
        }
        else if ([_artist respondsToSelector:aSelector]) {
            signature = [_artist methodSignatureForSelector:aSelector];
        }
    }
    
    return signature;
}

// 为方法签名后，转发消息
- (void)forwardInvocation:(NSInvocation *)anInvocation {
    SEL sel = [anInvocation selector];
    
    // 判断哪个类实现了该方法
    if ([_singer respondsToSelector:sel]) {
        [anInvocation invokeWithTarget:_singer];
    }
    else if ([_artist respondsToSelector:sel]) {
        [anInvocation invokeWithTarget:_artist];
    }
}

@end

/*  Singer和Artist的实现同上述代码  */