NSInvocation nil参数

Question

我如何(或者甚至可以)将nil参数传递给NSInvocation对象？

我试着这样做:

NSMethodSignature* signature = [AClass instanceMethodSignatureForSelector:@selector(aMethod:theOtherArg:)];
NSInvocation* invocation = [NSInvocation invocationWithMethodSignature: signature];

[invocation setTarget: aTargetObj];
[invocation setSelector: @selector(aMethod:theOtherArg:)];

/* I've tried both this way */
AnObj* arg1 = nil;
AnotherObj* arg2 = nil;
[invocation setArgument: &arg1 atIndex:2];
[invocation setArgument: &arg2 atIndex:3];

/* and this way */
//[invocation setArgument: nil atIndex:2];
//[invocation setArgument: nil atIndex:3];

NSInvocationOperation* operation = [[NSInvocationOperation alloc] initWithInvocation:invocation];
//opQueue is an NSOperationQueue object
[opQueue addOperation:operation];

第一种方法会因此消息而崩溃:

Thread 0 Crashed:
0   libSystem.B.dylib              0x927c1f10 strlen + 16
1   com.apple.CoreFoundation       0x92dd1654 __NSI3 + 500
2   com.apple.CoreFoundation       0x92dd1994 -[NSInvocation retainArguments] + 132
3   com.apple.Foundation           0x96a50c5e -[NSInvocationOperation initWithInvocation:] + 78

第二种方法会因此消息而崩溃:

Error: Exiting due to caught object *** -[NSInvocation setArgument:atIndex:]: NULL address argument

在此先感谢您的帮助!

Answer 1

是的,你可以传递零参数.更确切地说,您可以传递内容为nil的有效指针.

我无法重现您的特定问题.这是我用来测试它的代码.你的程序中可能还有其他东西导致错误.

#import <Foundation/Foundation.h>

@interface Person : NSObject {
    NSString *name;
    NSUInteger age;
}
- (void)setName:(NSString *)personName age:(NSNumber *)personAge;
@end

@implementation Person
- (void)setName:(NSString *)personName age:(NSNumber *)personAge {
    NSLog(@"setName:%@ age:%@", personName, personAge);
    name = [personName copy];
    age = [personAge unsignedIntegerValue];
}
@end

int main() {
    NSAutoreleasePool *pool = [NSAutoreleasePool new];            
    Person *person = [Person new];

    SEL sel = @selector(setName:age:);

    NSMethodSignature *sig = [Person instanceMethodSignatureForSelector:sel];
    NSInvocation *inv = [NSInvocation invocationWithMethodSignature:sig];

    [inv setTarget:person];
    [inv setSelector:sel];

    NSString *name = nil;
    NSNumber *age = nil;

    [inv setArgument:&name atIndex:2];
    [inv setArgument:&age atIndex:3];

    // [inv retainArguments];
    // [inv invoke];

    NSInvocationOperation *op = [[[NSInvocationOperation alloc] initWithInvocation:inv] autorelease];
    NSOperationQueue *queue = [NSOperationQueue new];
    [queue addOperation:op];

    [pool release];
    return 0;
}

我还测试了它,没有使用NSInvocationOperation并直接发送-retainArguments,因为这似乎是触发你的错误.

对于记录,您的第二种方法将无法工作,因为 - [NSInvocation setArgument:atIndex:]需要一个指向将被复制的缓冲区的有效内存地址.对于对象参数,缓冲区必须包含对象的地址.对象的地址可以为零,但缓冲区的地址必须有效.