objective-c - 为什么resolveInstanceMethod : called twice sometimes

最近，我正在研究Objective-C中的运行时。

我创建了一个名为TO的类:

@interface TO : NSObject
@end

#import "TO.h"

@implementation TO

- (id)forwardingTargetForSelector:(SEL)aSelector {
    NSLog(@"%@ sel: %@", NSStringFromSelector(_cmd), NSStringFromSelector(aSelector));
    return nil;
}

- (BOOL)respondsToSelector:(SEL)aSelector {
    NSLog(@"%@ sel: %@", NSStringFromSelector(_cmd), NSStringFromSelector(aSelector));
    return NO;
}

+ (BOOL)resolveClassMethod:(SEL)sel {
    NSLog(@"%@ sel: %@", NSStringFromSelector(_cmd), NSStringFromSelector(sel));
    return NO;
}

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    NSLog(@"%@ sel: %@", NSStringFromSelector(_cmd), NSStringFromSelector(sel));
    return NO;
}

+ (IMP)instanceMethodForSelector:(SEL)aSelector {
    NSLog(@"%@ sel: %@", NSStringFromSelector(_cmd), NSStringFromSelector(aSelector));
    return nil;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation {
    NSLog(@"%@", NSStringFromSelector(_cmd));
}

@end

然后，我在某个地方调用一个无法识别的选择器:

TO *to = [TO new];
id res = [(NSString *)to uppercaseString];

随后，我得到以下输出:

2015-12-22 22:27:04.319 OCDemo[81920:7728539] resolveInstanceMethod: sel: uppercaseString
2015-12-22 22:27:04.320 OCDemo[81920:7728539] forwardingTargetForSelector: sel: uppercaseString
2015-12-22 22:27:04.320 OCDemo[81920:7728539] resolveInstanceMethod: sel: uppercaseString
2015-12-22 22:27:04.320 OCDemo[81920:7728539] -[TO uppercaseString]: unrecognized selector sent to instance 0x7fdd3ad120a0
2015-12-22 22:27:04.322 OCDemo[81920:7728539] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[TO uppercaseString]: unrecognized selector sent to instance 0x7fdd3ad120a0'

如我们所见，resolveInstanceMethod:被调用了两次。

但是，如果我先调用-[description]:

TO *to = [TO new];
[to description];
id res = [(NSString *)to uppercaseString];

然后，输出将是:

2015-12-22 22:58:50.458 OCDemo[82137:7813436] resolveInstanceMethod: sel: uppercaseString
2015-12-22 22:58:50.459 OCDemo[82137:7813436] forwardingTargetForSelector: sel: uppercaseString
2015-12-22 22:58:50.459 OCDemo[82137:7813436] -[TO uppercaseString]: unrecognized selector sent to instance 0x7f9bcad59960
2015-12-22 22:58:50.461 OCDemo[82137:7813436] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[TO uppercaseString]: unrecognized selector sent to instance 0x7f9bcad59960'

这次resolveInstanceMethod:仅被调用了一次。

有人可以解释一下吗？

最佳答案

看起来对给定类的第一个方法调用(在您的示例中为[to description])缓存了类选择器。实际上，对于无法识别的选择器，resolveInstanceMethod:仅被调用一次。

使用函数刷新给定类的方法缓存时，您会看到此信息

#import <objc/runtime.h>
void _objc_flush_caches(Class cls)

修改示例:

TO *to = [TO new];
[to description];
_objc_flush_caches([to class]);
id res = [(NSString *)to uppercaseString];

给出以下输出:

2017-04-15 21:48:21.575527+0200 pro objective c games[29820:1116842] resolveInstanceMethod: sel: uppercaseString
2017-04-15 21:48:21.575760+0200 pro objective c games[29820:1116842] forwardingTargetForSelector: sel: uppercaseString
2017-04-15 21:48:21.575808+0200 pro objective c games[29820:1116842] resolveInstanceMethod: sel: uppercaseString
2017-04-15 21:48:21.575833+0200 pro objective c games[29820:1116842] -[TO uppercaseString]: unrecognized selector sent to instance 0x100303600
2017-04-15 21:48:21.579989+0200 pro objective c games[29820:1116842] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[TO uppercaseString]: unrecognized selector sent to instance 0x100303600'

如您所见，即使resolveInstanceMethod:之前被调用过，[to description]也被调用了两次。当您注释掉\\_objc_flush_caches([to class]);时，resolveInstanceMethod:将被调用一次。就像在第二个例子中一样。这表明您注意到的行为与方法缓存有关。