更加灵活的“ performSelector

I want to invoke a selector of a method that has the usual NSError** argument:

-(int) getItemsSince:(NSDate *)when dataSelector:(SEL)getDataSelector error:(NSError**)outError  {
    NSArray *data = nil;
    if([service respondsToSelector:getDataSelector]) {
        data = [service performSelector:getDataSelector withObject:whenwithObject:outError];
        // etc.

... which the compiler doesn't like:

warning: passing argument 3 of 'performSelector:withObject:withObject:' fromincompatible pointer type

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签出 NSInvocation，这使您可以更灵活的方式的"performSelector"。

if ([service respondsToSelector:getDataSelector]) {
    NSArray *data;
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:
        [service methodSignatureForSelector:getDataSelector]];
    [invocation setTarget:delegate];
    [invocation setSelector:getDataSelector];
    // Note: Indexes 0 and 1 correspond to the implicit arguments self and _cmd, 
    // which are set using setTarget and setSelector.
    [invocation setArgument:when atIndex:2]; 
    [invocation setArgument:outError atIndex:3];
    [invocation invoke];
    [invocation getReturnValue:&data];
}

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perform selector调用超过两个以上参数的方法

也可以把所有的参数放到字典里, 像NSNotification或者NSTimer的userinfo