如何通过NSInvocation调用类方法？

Question

我们可以通过调用NSObject的+ (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector方法来获取NSMethodSignature。然后使用+ (NSInvocation *)invocationWithMethodSignature:(NSMethodSignature *)signature构建NSInvocation。但问题在于，我们只能获取实例方法的方法签名，那么类方法怎么办呢？

- Fang Yang

Answer 1

您可以通过以下步骤获取类方法：

NSMethodSignature *pMS = [[YourObject class] methodSignatureForSelector: (SEL)aSelector];

[YourObject class]片段返回一个类对象（单例），您可以使用该对象来获取类方法。

补充：以下是评论中的新信息。您可以直接执行以下操作：

NSMethodSignature *pMS = [YourObject methodSignatureForSelector: (SEL)aSelector];

- Kent

---

你甚至不需要 class 部分。你可以直接将它发送到 YourObject，它仍然可以工作，因为类名也是一等对象，同时也是 typedefs。 - Richard J. Ross III

谢谢！但是这只适用于声明为实例方法的方法，该方法由“-”启动。我无法获取声明为类方法的methodSignature，该方法由“+”启动。 - Fang Yang

不正确。有两种不同的方法：instanceMethodSignatureForSelector:和methodSignatureForSelector:。一个用于实例方法，另一个用于类方法。 - Kent

ADD 部分不正确，请参考 @Ben Flynn 的测试代码。 - xi.lin

Answer 2

这里提供有关通过NSInvocation调用类（和实例）方法的额外细节，展示了methodForSignature：和instanceMethodForSignature：

@interface SomeObject : NSObject
+ (void)instanceAndClass;
- (void)instanceAndClass;
+ (void)classOnly;
- (void)instanceOnly;
@end

@implementation SomeObject
+ (void)instanceAndClass { NSLog(@"Class version"); }
- (void)instanceAndClass { NSLog(@"Instance version"); }    
+ (void)classOnly { NSLog(@"Class only"); }
- (void)instanceOnly { NSLog(@"Instance only"); }
@end

@implementation Foo

- (void)invoke:(id)receiver signature:(NSMethodSignature *)signature selector:(SEL)selector {
    if (!signature) {
        NSLog(@"method signature is nil");
        return;
    }        
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
    invocation.selector = selector;
    invocation.target = receiver;
    [invocation invoke];
}

- (void)foo {
    NSMethodSignature *signature;
    SomeObject *someObject = [SomeObject new];

    signature = [someObject methodSignatureForSelector:@selector(instanceAndClass)];
    [self invoke:someObject signature:signature selector:@selector(instanceAndClass)];
    // 2015-07-10 11:01:10.227 FooBar[65312:3097866] Instance version
    [self invoke:[someObject class] signature:signature selector:@selector(instanceAndClass)];
    // 2015-07-10 11:01:10.227 FooBar[65312:3097866] Class version

    signature = [someObject methodSignatureForSelector:@selector(classOnly)];
    [self invoke:someObject signature:signature selector:@selector(classOnly)];
    // 2015-07-10 11:01:10.227 FooBar[65312:3097866] method signature is nil

    signature = [[someObject class] methodSignatureForSelector:@selector(classOnly)];
    [self invoke:[someObject class] signature:signature selector:@selector(classOnly)];
    // 2015-07-10 11:01:10.228 FooBar[65312:3097866] Class only

    signature = [someObject methodSignatureForSelector:@selector(instanceOnly)];
    [self invoke:someObject signature:signature selector:@selector(instanceOnly)];
    // 2015-07-10 11:01:10.228 FooBar[65312:3097866] Instance only

    signature = [[someObject class] methodSignatureForSelector:@selector(instanceOnly)];
    [self invoke:[someObject class] signature:signature selector:@selector(instanceOnly)];
    2015-07-10 11:01:10.228 FooBar[65312:3097866] method signature is nil

    signature = [[someObject class] instanceMethodSignatureForSelector:@selector(instanceOnly)];
    [self invoke:someObject signature:signature selector:@selector(instanceOnly)];
    // 2015-07-10 11:01:10.228 FooBar[65312:3097866] Instance only
}

- Ben Flynn