16 runtime之面试题(class+super)

面试题

一 下面代码的执行结果

• Student继承自Person类，下面代码执行结果

#import "Student.h"

@implementation Student
- (instancetype)init {
    if (self = [super init]) {
        NSLog(@"[self class] = %@", [self class]); // Student
        NSLog(@"[self superclass] = %@", [self superclass]); // Person

        NSLog(@"--------------------------------");

        // objc_msgSendSuper({self, [Merson class]}, @selector(class));
        NSLog(@"[super class] = %@", [super class]); // Student
        NSLog(@"[super superclass] = %@", [super superclass]); // Person
    }
    return self;
}
@end

运行结果

1653926-22eae5166916f4d4.png

结论如下

• [super message]的底层实现
  • 1.消息接收者仍然是子类对象
  • 2.从父类开始查找方法的实现
  • 3.super调用的receiver仍然是MJStudent对象

struct objc_super {
    __unsafe_unretained _Nonnull id receiver; // 消息接收者
    __unsafe_unretained _Nonnull Class super_class; // 消息接收者的父类
};

• NSObject类实现猜测如下

@implementation NSObject
- (Class)class
{
    return object_getClass(self);
}

- (Class)superclass
{
    return class_getSuperclass(object_getClass(self));
}
@end

所以[super class]的打印结果是Student，因为调用class方法最终都是调用NSObject的Class方法，又因为调用方法最终是走objc_msgSend方法，该方法取决于谁是接受者，通过底层代码可知，[super message]的消息接受者人仍然是子类对象，所以打印Student。

二 利用消息转发做什么事情

• 可以拦截方法找不到导致的奔溃

• Person类文件

@interface Person : NSObject
- (void)run;
- (void)test1;
- (void)test2;
@end

@implementation Person
- (void)run {
    NSLog(@"%s",__func__);
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    // 本来能调用的方法
    if ([self respondsToSelector:aSelector]) {
        return [super methodSignatureForSelector:aSelector];
    }

    // 找不到的方法
    return [NSMethodSignature signatureWithObjCTypes:"v@:"];
}

// 找不到的方法，都会来到这里
- (void)forwardInvocation:(NSInvocation *)anInvocation {
    NSLog(@"找不到%@方法", NSStringFromSelector(anInvocation.selector));
}
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Person *per = [[Person alloc] init];
        [per run];
        [per test1];
        [per test2];
    }
    return 0;
}

运行结果

1653926-8359076e7b926782.png

三 isKindOfClass 和 isMemberOfClass的理解

// 这句代码的方法调用者不管是哪个类（只要是NSObject体系下的），都返回YES
NSLog(@"%d", [[NSObject class] isKindOfClass:[NSObject class]]);
NSLog(@"%d", [[NSObject class] isMemberOfClass:[NSObject class]]);
NSLog(@"%d", [[Person class] isKindOfClass:[Person class]]);
NSLog(@"%d", [[Person class] isMemberOfClass:[Person class]]);

打印结果

1653926-c17aae722026d49e.png

NSObject底层方法的实现

判断对象类型

• -(BOOL) isKindOfClass: classObj判断是否是这个类或者这个类的子类的实例
• -(BOOL) isMemberOfClass: classObj 判断是否是这个类的实例

@implementation NSObject

- (BOOL)isMemberOfClass:(Class)cls {
    return [self class] == cls;
}

- (BOOL)isKindOfClass:(Class)cls {
    for (Class tcls = [self class]; tcls; tcls = tcls->superclass) {
        if (tcls == cls) return YES;
    }
    return NO;
}

+ (BOOL)isMemberOfClass:(Class)cls {
    return object_getClass((id)self) == cls;
}

+ (BOOL)isKindOfClass:(Class)cls {
    for (Class tcls = object_getClass((id)self); tcls; tcls = tcls->superclass) {
        if (tcls == cls) return YES;
    }
    return NO;
}
@end

• 下面代码执行结果如何

// 实例对象
id person = [[Person alloc] init];
NSLog(@"%d", [person isMemberOfClass:[Person class]]);
NSLog(@"%d", [person isMemberOfClass:[NSObject class]]);
NSLog(@"%d", [person isKindOfClass:[Person class]]);
NSLog(@"%d", [person isKindOfClass:[NSObject class]]);

NSLog(@"------------类对象-------------");
// 类对象
NSLog(@"%d", [Person isKindOfClass:[NSObject class]]);
NSLog(@"%d", [Person isMemberOfClass:[NSObject class]]);
NSLog(@"%d", [Person isMemberOfClass:object_getClass([Person class])]);
NSLog(@"%d", [Person isKindOfClass:object_getClass([NSObject class])]);

执行结果

1653926-c69614fd777906a4.png

下面代码执行结果如何

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSLog(@"%d", [[NSObject class] isKindOfClass:[NSObject class]]);
        NSLog(@"%d", [[NSObject class] isMemberOfClass:[NSObject class]]);
        NSLog(@"%d", [[MJPerson class] isKindOfClass:[MJPerson class]]);
        NSLog(@"%d", [[MJPerson class] isMemberOfClass:[MJPerson class]]);

        NSLog(@"-----------");
        // 这句代码的方法调用者不管是哪个类（只要是NSObject体系下的），都返回YES
        NSLog(@"%d", [NSObject isKindOfClass:[NSObject class]]); // 1
        NSLog(@"%d", [NSObject isMemberOfClass:[NSObject class]]); // 0
        NSLog(@"%d", [MJPerson isKindOfClass:[MJPerson class]]); // 0
        NSLog(@"%d", [MJPerson isMemberOfClass:[MJPerson class]]); // 0
        NSLog(@"-----------");

        id person = [[MJPerson alloc] init];

        NSLog(@"%d", [person isMemberOfClass:[MJPerson class]]);
        NSLog(@"%d", [person isMemberOfClass:[NSObject class]]);
        NSLog(@"%d", [person isKindOfClass:[MJPerson class]]);
        NSLog(@"%d", [person isKindOfClass:[NSObject class]]);
        NSLog(@"-----------");

        NSLog(@"%d", [MJPerson isMemberOfClass:object_getClass([MJPerson class])]);
        NSLog(@"%d", [MJPerson isKindOfClass:object_getClass([NSObject class])]);
        NSLog(@"%d", [MJPerson isKindOfClass:[NSObject class]]);
        NSLog(@"%d", [MJPerson isMemberOfClass:[NSObject class]]);
    }
    return 0;
}

打印结果

1653926-60371a1365d92614.png

1. -(BOOL) isKindOfClass: classObj判断是否是这个类或者这个类的子类的实例
对象是类的实例，类是元类的实例
2. -(BOOL) isMemberOfClass: classObj 判断是否是这个类的实例
对象是类的实例，类是元类的实例

面试题四 下面代码执行结果

@interface Person : NSObject
@property (copy, nonatomic) NSString *name;
- (void)print;
@end

@implementation Person
- (void)print {
    NSLog(@"my name is %@", self.name);
}
@end

- (void)viewDidLoad {
    [super viewDidLoad];

    id cls = [Person class];

    void *obj = &cls;

    [(__bridge id)obj print];
}

1653926-8c9016a577eb4ae9.png

- (void)viewDidLoad {
    [super viewDidLoad];
    NSString *test = @"123";

    id cls = [Person class];

    void *obj = &cls;

    [(__bridge id)obj print];
}

1653926-719580be033fc5a6.png

分析如下

• print为什么能够调用成功

1653926-2580deefa6ad1434.png

• 为什么self.name变成了ViewController等其他内容

1653926-60665602d1f18dda.png

1653926-ab16cbfb4b8d70f4.png

局部变量在栈空间，栈空间地址分配是由高到底分配，所以先声明的局部变量在更高的地址上

一 super的本质

• super调用，底层会转换为objc_msgSendSuper2函数的调用，接收2个参数
• struct objc_super2
• SEL

struct objc_super2 {
  id receiver;
  Class current_class;
};

• receiver是消息接收者
• current_class是receiver的Class对象

打印当前栈内存信息

1653926-6d557ac5124be3a5.png

1653926-b28831b7f01c0f33.png

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本文参考:
路飞_Luck (https://www.jianshu.com/p/07f7b96bb03f)
以及借鉴MJ的教程视频
非常感谢.

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项目连接地址 - runtime-super
项目连接地址 - runtime-super2