NSInvocation的使用

在 iOS中可以直接调用 某个对象的消息 方式有2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

NSInvocation可以处理参数、返回值。会java的人都知道反射操作，其实NSInvocation就相当于反射操作。

下面这个例子描述了如何使用NSInvocation，以下例子中如果要正常运行，需要把不存在的类进行正确填写。

//方法签名类，需要被调用消息所属的类AsynInvoke ,被调用的消息invokeMethod:NSMethodSignature *sig= [[AsynInvoke class] instanceMethodSignatureForSelector:@selector(invokeMethod:)];//根据方法签名创建一个NSInvocationNSInvocation *invocation=[NSInvocation invocationWithMethodSignature:sig];//设置调用者也就是AsynInvoked的实例对象，在这里我用self替代[invocation setTarget:self];//设置被调用的消息[invocation setSelector:@selector(invokeMethod:)];//如果此消息有参数需要传入，那么就需要按照如下方法进行参数设置，需要注意的是，atIndex的下标必须从2开始。原因为：0 1 两个参数已经被target 和selector占用NSInteger num=10;[invocation setArgument:&num atIndex:2];//retain 所有参数，防止参数被释放dealloc[invocation retainArguments];//消息调用[invocation invoke];//如果调用的消息有返回值，那么可进行以下处理//获得返回值类型const char *returnType = sig.methodReturnType;//声明返回值变量id returnValue;//如果没有返回值，也就是消息声明为void，那么returnValue=nilif( !strcmp(returnType, @encode(void)) ){returnValue =  nil;}//如果返回值为对象，那么为变量赋值else if( !strcmp(returnType, @encode(id)) ){[invocation getReturnValue:&returnValue];}else{//如果返回值为普通类型NSInteger  BOOL//返回值长度NSUInteger length = [sig methodReturnLength];//根据长度申请内存void *buffer = (void *)malloc(length);//为变量赋值[invocation getReturnValue:buffer];if( !strcmp(returnType, @encode(BOOL)) ) {returnValue = [NSNumber numberWithBool:*((BOOL*)buffer)];}else if( !strcmp(returnType, @encode(NSInteger)) ){returnValue = [NSNumber numberWithInteger:*((NSInteger*)buffer)];}returnValue = [NSValue valueWithBytes:buffer objCType:returnType];}

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NSInvocation 调用示例

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CurrentDate.h

#import <Foundation/Foundation.h>@interface CurrentDate : NSObject {}- (NSString *) stringForDate: (NSDate *)date               usingFormatter: (NSDateFormatter *)formatter;@end

CurrentDate.m

#import "CurrentDate.h"@implementation CurrentDate- (NSString *) stringForDate: (NSDate *)date               usingFormatter: (NSDateFormatter *)formatter{    return [formatter stringFromDate: date];}@end

main.m

#import <Foundation/Foundation.h>#import "CurrentDate.h"//参考：http://theocacao.com/document.page/264int main (int argc, constchar* argv[]){    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];    //原始调用    NSDateFormatter * dateFormat = [[NSDateFormatter alloc]                                     initWithDateFormat:@"%b %d %Y"                                     allowNaturalLanguage: NO];    CurrentDate * currentDateClassObject = [[CurrentDate alloc] init];    NSString * currentDate = [currentDateClassObject                               stringForDate: [NSDate date]                               usingFormatter: dateFormat];    NSLog(@"currentDate: %@", currentDate);    //NSInvocation调用    SEL mySelector = @selector(stringForDate:usingFormatter:);    NSMethodSignature * sig = [[currentDateClassObject class]                                instanceMethodSignatureForSelector: mySelector];    NSInvocation * myInvocation = [NSInvocation invocationWithMethodSignature: sig];    [myInvocation setTarget: currentDateClassObject];    [myInvocation setSelector: mySelector];    NSDate * myDate = [NSDate date];    [myInvocation setArgument: &myDate atIndex: 2];    NSDateFormatter * dateFormatter = [[NSDateFormatter alloc] init];    [dateFormatter setDateStyle: NSDateFormatterMediumStyle];        [myInvocation setArgument: &dateFormatter atIndex: 3];    NSString * result = nil;        [myInvocation retainArguments];        [myInvocation invoke];    [myInvocation getReturnValue: &result];    NSLog(@"The result is: %@", result);    [pool drain];    return0;}

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动态调用方法的时候会用到

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示例一：

-(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:);     NSMethodSignature *methodSignature = [[self class] instanceMethodSignatureForSelector:selector];//获得类和方法的签名     NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];     //从签名获得调用对象     [invocation setTarget:self];     //设置target     [invocation setSelector:selector];//设置selector     NSString *returnValue = nil;     NSString *argument1 = @"fist";     NSNumber *argument2 = [NSNumber numberWithInt:102];     [invocation setArgument:&argument1 atIndex:2];//设置参数，第一个参数index为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。）    [invocation setArgument:&argument2 atIndex:3];     [invocation retainArguments];//retain一遍参数     [invocation invoke];//调用     [invocation getReturnValue:&returnValue];//得到返回值，此时不会再调用，只是返回值     NSLog(@"return value = %@",returnValue); } 

示例二：

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]