动态调用方法时会用到，例子
-(NSString *)myMethod:(NSString *)param1 withParam2:(NSNumber *)param2
{
NSString *result = @"objc";
NSLog(@"par = %@",param1);
NSLog(@"par 2 = %@",param2);
return result;
}
-(void)invokeMyMethodDynamically
{
SEL selector = @selector(myMethod:withParam2:);
//获得类和方法的签名
NSMethodSignature *methodSignature = [[self class] instanceMethodSignatureForSelector:selector];
//从签名获得调用对象
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];
//设置target
[invocation setTarget:self];
//设置selector
[invocation setSelector:selector];
NSString *returnValue = nil;
NSString *argument1 = @"fist";
NSNumber *argument2 = [NSNumber numberWithInt:102];
//设置参数，第一个参数index为2
[invocation setArgument:&argument1 atIndex:2];
[invocation setArgument:&argument2 atIndex:3];
//retain一遍参数
[invocation retainArguments];
//调用
[invocation invoke];
//得到返回值，此时不会再调用，只是返回值
[invocation getReturnValue:&returnValue];
NSLog(@"return value = %@",returnValue);
}
另外一个例子:
SEL selector = @selector(myMethod:setValue2:);
NSMethodSignature *signature = [MyObject instanceMethodSignatureF orSelector:selector];
NSInvocation *invocation = [NSInvocation invocationWithMethodSign ature:signature];
[invocation setSelector:selector];
NSString *str1 = @"someString";
NSString *str2 = @"someOtherString";
//The invocation object must retain its arguments
[str1 retain];
[str2 retain];
//Set the arguments
[invocation setTarget:targetInstance];
[invocation setArgument:&str1 atIndex:2];
[invocation setArgument:&str2 atIndex:3];
[NSTimer scheduledTimerWithTimeIn terval:0.1 invocation:invocation repeats:YES]