如何从NSInvocation获取NSString结果？

Question

以下代码按预期工作:

NSLog(@"%@", [NSString stringWithString:@"test"]; // Logs "test"

但当我用一个替换它时NSInvocation,我得到一个完全不同的结果:

Class class = [NSString class];
SEL selector = @selector(stringWithString:);

NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:
                          [class methodSignatureForSelector:selector]];
[invocation setTarget:class];
[invocation setSelector:selector];
[invocation setArgument:@"test" atIndex:2];
[invocation invoke];

id returnValue = nil;
[invocation getReturnValue:&returnValue];
NSLog(@"%@", returnValue); // Logs "NSCFString"

我搜索过高低,但无法弄清楚这一点.有帮助吗？谢谢!

Answer 1

从NSInvocation类引用:

当参数值是对象时,将指针传递给应从中复制对象的变量(或内存):

NSArray *anArray;    
[invocation setArgument:&anArray atIndex:3];

因为@"test"实际上构建了NSString的一个实例,所以你应该使用

NSString *testString = @"test";
[invocation setArgument:&testString atIndex:2];