NSInvocation直接调用某个对象的消息

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在 iOS中可以直接调用某个对象的消息方式有两种：

一种是performSelector:withObject；

再一种就是NSInvocation。

第一种方式比较简单，能完成简单的调用。但是对于>2个的参数或者有返回值的处理，那就需要做些额外工作才能搞定。那么在这种情况下，我们就可以使用NSInvocation来进行这些相对复杂的操作。

main.h

1#import <Foundation/Foundation.h>2#import"MyClass.h"34intmain (int argc,constchar * argv[])5{67NSAutoreleasePool* pool=[[NSAutoreleasePoolalloc]init];89MyClass*myClass=[[MyClassalloc]init];10NSString*myString=@"My string";1112//普通调用13NSString*normalInvokeString=[myClassappendMyString:myString];14NSLog(@"The normal invoke string is: %@", normalInvokeString);1516//NSInvocation调用17SEL mySelector=@selector(appendMyString:); //

SEL selector =NSSelectorFromString(@"setStatusBarOrientation:"); 多种方式

18NSMethodSignature* sig=[[myClassclass]19instanceMethodSignatureForSelector: mySelector];2021NSInvocation* myInvocation=[NSInvocationinvocationWithMethodSignature: sig];22[myInvocationsetTarget: myClass];23[myInvocationsetSelector: mySelector];2425[myInvocationsetArgument:&myStringatIndex: 2];2627NSString* result=nil;28[myInvocationretainArguments];29[myInvocationinvoke];30[myInvocationgetReturnValue:&result];31NSLog(@"The NSInvocation invoke string is: %@", result);3233 35 36return0;37}

MyClass.h

1#import <Foundation/Foundation.h>234@interfaceMyClass: NSObject { 5}67- (NSString*)appendMyString:(NSString*)string;89@end

MyClass.m

1#import"MyClass.h"234@implementationMyClass56 1516- (NSString*)appendMyString:(NSString*)string17{18NSString*mString=[NSStringstringWithFormat:@"%@ after append method",string];1920return mString;21}22
2728@end

这里说明一下[myInvocation setArgument: &myString atIndex: 2];为什么index从2开始，原因为：0 1 两个参数已经被target 和selector占用。

在附上一个实例,解决了ipad 横屏时,进入app 右侧黑屏的问题

//解决横屏启动时,app右侧是黑的问题    if ([[UIApplication sharedApplication] respondsToSelector:@selector(setStatusBarOrientation:)]){        SEL selector = NSSelectorFromString(@"setStatusBarOrientation:");        NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:[UIApplication instanceMethodSignatureForSelector:selector]];        UIDeviceOrientation orentation = UIDeviceOrientationPortrait;        [invocation setSelector:selector];        [invocation setTarget:[UIApplication sharedApplication]];        [invocation setArgument:&orentation atIndex:2];        [invocation invoke];    }

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