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
image.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
消息转发.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
image.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
image.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
image.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];
}
image.png
- (void)viewDidLoad {
[super viewDidLoad];
NSString *test = @"123";
id cls = [Person class];
void *obj = &cls;
[(__bridge id)obj print];
}
image.png
分析如下
- print为什么能够调用成功
image.png
- 为什么self.name变成了ViewController等其他内容
image.png
image.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
image.png
image.png
本文主要参考MJ老师的教案，非常感谢MJ老师。
项目连接地址 - runtime-super
项目连接地址 - runtime-super2
关于runtime更多文章请看如下链接
iOS-runtime-API详解+使用
iOS Runtime原理及使用
iOS - runtime如何通过selector找到对应的 IMP地址(分别考虑类方法和实例方法)
iOS - Runtime之面试题详解一
iOS-runtime之面试题详解二
iOS runtime的使用场景-实战篇
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