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NSInvocation

NSInvocation *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=nil

if( !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];

}

NSInvocation调用

CurrentDate.h

#import <Foundation/Foundation.h>

@interface CurrentDate : NSObject {

}

- (NSString *) stringForDate: (NSDate *)date

usingFormatter: (NSDateFormatter *)formatter;

@end
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CurrentDate.m

#import "CurrentDate.h"

@implementation CurrentDate

- (NSString *) stringForDate: (NSDate *)date

usingFormatter: (NSDateFormatter *)formatter

{

return [formatter stringFromDate: date];

}

@end
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main.m

#import <Foundation/Foundation.h>

#import "CurrentDate.h"

//参考：http://theocacao.com/document.page/264

int 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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NSMethodSignature和NSInvocation的使用

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:); 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 [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); } 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]