iOS NSInvocation用法简介

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。

//test performSelector      注意：此种方法只使用于最多两个参数，多个参数无能为力

- (void)testMutiParams {
    //test perform
    NSString *strRe = [self performSelector:@selector(appendString:String2:) withObject:@"hello" withObject:@"world"];
    NSLog(@"strRe :%@",strRe);
   
    //question ?
    NSString *strRe1 = [self performSelector:@selector(appendString:String2:String3:) withObject:@"hello" withObject:@"world"];
    NSLog(@"strRe :%@",strRe1);

    NSString *strMy1 = @"My string1";
    NSString *strMy2 = @"My string2";
    NSString *strMy3 = @"My string3";
   
    SEL mySelector = @selector(appendString:String2:String3:);
    NSMethodSignature *sig = [[self class]instanceMethodSignatureForSelector:mySelector];
   
    NSInvocation *myInvocation = [NSInvocation invocationWithMethodSignature:sig];
    [myInvocation setTarget:self];
    [myInvocation setSelector:mySelector];
    [myInvocation setArgument:&strMy1 atIndex:2];
    [myInvocation setArgument:&strMy2 atIndex:3];
    [myInvocation setArgument:&strMy3 atIndex:4];
   
    NSString *strResult = nil;
    [myInvocation retainArguments];
    [myInvocation invoke];
    [myInvocation getReturnValue:&strResult];
    NSLog(@"The NSInvocation invoke string is :%@",strResult);

}

- (NSString *)appendString:(NSString *)str1 String2:(NSString *)str2 String3:(NSString *)str3 {
    return [NSString stringWithFormat:@"%@,%@,%@",str1,str2,str3];

}

- (void)testInvocation {

    MyClass *myClass = [[MyClass alloc] init];
    NSString *strMy = @"My string";
   
    //common selector
    NSString *normalInvokeString = [myClass appendMyString:strMy];
    NSLog(@"The normal invoke string is :%@",normalInvokeString);
   
    //NSInvocation selector
    SEL mySelector = @selector(appendMyString:);
    NSMethodSignature *sig = [[myClass class]instanceMethodSignatureForSelector:mySelector];
   
    NSInvocation *myInvocation = [NSInvocation invocationWithMethodSignature:sig];
    [myInvocation setTarget:myClass];
    [myInvocation setSelector:mySelector];
    [myInvocation setArgument:&strMy atIndex:2];
   
    NSString *strResult = nil;
    [myInvocation retainArguments];
    [myInvocation invoke];
    [myInvocation getReturnValue:&strResult];
    NSLog(@"The NSInvocation invoke string is :%@",strResult);
 
}

类实现：

#import <Foundation/Foundation.h>

@interface MyClass : NSObject

- (NSString *)appendMyString:(NSString *)string;

@end

#import "MyClass.h"

@implementation MyClass

- (NSString *)appendMyString:(NSString *)string {
    NSString *mString = [NSString stringWithFormat:@"%@ after append method", string];
    return mString;
}

@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。