NSInvocation类上的setSelector方法的目的是什么？

Question

我不明白为什么我们必须setSelector在NSInvocation对象上调用该方法,因为该信息已经通过invocationWithMethodSignature.

要创建NSInvocation对象,我们执行以下操作:

SEL someSelector;
NSMethodSignature *signature;
NSInvocation *invocation;

someSelector = @selector(sayHelloWithString:);

//Here we use the selector to create the signature
signature = [SomeObject instanceMethodSignatureForSelector:someSelector];
invocation = [NSInvocation invocationWithMethodSignature:signature];

//Here, we again set the same selector
[invocation setSelector:someSelector];
[invocation setTarget:someObjectInstance];
[invocation setArgument:@"Loving C" atIndex:2];

请注意,我们将选择器传递给[SomeObject instanceMethodSignatureForSelector: someSelector];并再次传递给[invocation setSelector:someSelector];.

有什么我想念的吗？

Answer 1

签名不是选择器.选择器是消息的名称.签名定义参数和返回值.您可以拥有许多具有相同签名的选择器,反之亦然.如果你看NSMethodSignature,你会注意到没有-selector方法; 签名不带有特定的选择器.

考虑以下

- (void)setLocation:(CGFloat)aLocation;
- (void)setLocation:(MyLocation*)aLocation;

它们具有相同的选择器@selector(setLocation:),但具有不同的签名.

- (void)setX:(CGFloat)x;
- (void)setY:(CGFloat)y;

它们具有相同的签名,但选择器不同.

ObjC编程语言中的选择器可能是理解这一点的有用参考.