为什么 class object 可以调用“methodSignatureForSelector:”实例方法

Question

What make me doubt is that the class object can invoke instance method.

The method of " methodSignatureForSelector is instance method but when I invoke it by instance object, it go wrong.

NSString *classStr = @"NSInvocationObject";

// 获取class
Class objClass = NSClassFromString(classStr);

// 获取函数
SEL selector = NSSelectorFromString(@"classTest");

// 获取对象
Class stclass = [objClass class];
// - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE(""); here is a instance method ,but be invoke by objClass 
NSMethodSignature *singature = [objClass methodSignatureForSelector:selector];

NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:singature];

invocation.target = objClass;
invocation.selector = selector;
[invocation invoke];ere

Answer 1

Selector

• Selector identifies method by name, not method implementations.
• The hallo method in one class has the same selector as hallo method in another class.
• A class method and an instance method with the same name have assigned the same selector.

Selector just identifies method by name and it's irrelevant if it's a class or an instance.

• -(void)hallo; -> selector = hallo
• +(void)hallo; -> selector = hallo

NSMethodSignature

Method signature describes type information of the return value and parameters. It doesn't say anything about method name, if it's an instance method, a class method, ... Just types .

• -(void)hallo; - signature/encoding = v@:
  • v - void, return type
  • @ - object
  • : - selector
• +(void)hallo; - signature/encoding = v@:

Type Encodings for more info.

To make it more obvious, imagine you have following methods:

- (void)foo {
}

+ (NSUInteger)foo {
    return 2;
}

Both of them have foo as a selector, but they do differ in encoding:

• -(void)foo - signature/encoding = v@:
• +(NSUInteger)foo - signature/encoding = Q@:

Usage

Here's an example. If your class method & instance method equals (name & return type & parameter types), you can use the same selector and method signature on the class and on the instance. invokeWithTarget: says - send a message (identifier by selector = method name & signature = return value & parameter types) - target can be anything that can receive this message.

Simplified a bit, but should explain what's going on here.

Sample class:

@interface MyClass: NSObject

+(void)hallo; // Class method
-(void)hallo; // Instance method

@end

@implementation MyClass

+(void)hallo {
    NSLog(@"classHallo");
}

-(void)hallo {
    NSLog(@"instanceHallo");
}

@end

Invocations:

Class c = [MyClass class];   // Class of MyClass
MyClass *i = [MyClass new];  // Instance of MyClass

// Selector identifies method by name
// It's irrelevant if it's a class or an instance method
SEL selector = @selector(hallo);

// Signature just contains types of return value & parameters
// It's irrelevant if it's a class or an instance method
// Get a signature of the +hallo method
NSMethodSignature *signature = [c methodSignatureForSelector:selector];

// Create invocation
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
invocation.selector = selector;  // Set the selector
[invocation invokeWithTarget:c]; // Invokes +hallo (class)
[invocation invokeWithTarget:i]; // Invokes -hallo (instance)

// Get a signature of the -hallo method
signature = [i methodSignatureForSelector:selector];

// Create invocation
invocation = [NSInvocation invocationWithMethodSignature:signature];
invocation.selector = selector;  // Set the selector
[invocation invokeWithTarget:c]; // Invokes +hallo (class)
[invocation invokeWithTarget:i]; // Invokes -hallo (instance)

Documentation

Archived, but still good to learn about:

• Type Encodings
• Selectors

Comments

MyClass only have a instance method: -(void)hallo; but when I execute the NSMethodSignature *signature = [c methodSignatureForSelector:@selector(hallo)] , I can't get a signature of the method, signature is nil , if the selector just identifies method by name and it's irrelevant if it's a class or an instance, I should get a signature of method.

No. Check the methodSignatureForSelector: method aSelector argument documentation:

A selector that identifies the method for which to return the implementation address. When the receiver is an instance, aSelector should identify an instance method; when the receiver is a class, it should identify a class method.

What selector identifies? Method by name and it's hallo in our case - NOT +(void)hallo and NOT -(void)hallo , just hallo - this is what I meant with irrelevant comment.

When the receiver is an instance, aSelector should identify an instance method;

i is the receiver, i is an instance, so the following code asks for the -(void)hallo method signature, not +(void)hallo .

MyClass *i = [MyClass new];   // Instance of MyClass
SEL selector = @selector(hallo);
NSMethodSignature *signature = [i methodSignatureForSelector:selector];

when the receiver is a class, it should identify a class method.

c is the receiver, c is a class, so the following code asks for the +(void)hallo method signature, not -(void)hallo .

Class c = [MyClass class];   // Class of MyClass
SEL selector = @selector(hallo);
NSMethodSignature *signature = [c methodSignatureForSelector:selector];

---

There's also + (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector which you can invoke on a class to get an instance method selector:

Class c = [MyClass class];   // Class of MyClass
SEL selector = @selector(hallo);
NSMethodSignature *signature = [c instanceMethodSignatureForSelector:selector];

---

i think i had understanded why the class object can inkove instance method, the class object's isa point to a meta-class that the meat-class is still a NSObject type (same as meta-class: NSObject); so the class object is same to a isntance object in the sense, this mean that class'object and instance'object can inkove instance method

I either don't understand what do you mean with this or you're misusing the instance method term.

Objective-C Classes Are also Objects :

In Objective-C, a class is itself an object with an opaque type called Class. Classes can't have properties defined using the declaration syntax shown earlier for instances, but they can receive messages .

Objects Send and Receive Messages

Sending an Objective-C message is conceptually very much like calling a C function.

[i hallo] actually means send a hallo message to i & [c hallo] actually means send a hallo message to c where i is an instance and c is a Class .

It seems to me that you're just using wrong term. If I understood your comment correctly, yes, you can send a message to either an instance or a class.

Here're other resources related to this topic:

• Objective C message dispatch mechanism
• Objective-C Direct Methods (new stuff)