iOS 反射函数: performSelector, NSInvocation, objc_msgSend

当咱们有方法名和参数列表，想要动态地给对象发送消息，可用经过反射函数机制来实现，有两种经常使用的作法：

 

1、performSelector

1 - (id)performSelector:(SEL)aSelector;
2 - (id)performSelector:(SEL)aSelector withObject:(id)object;
3 - (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;

经常使用的方法有这三个，其中aSelector能够经过 NSSelectorFromString 方法拿到

SEL aSelector = NSSelectorFromString(selString);

可是 performSelector 的缺点是最多只支持传递两个参数

http://my.oschina.net/ososchina/blog/644117

这篇文章给出的方法二说是能够支持多参数，可是我尝试了下未成功

 

2、NSInvocation

很少说，直接上栗子

// 测试反射函数
- (void)printWithString:(NSString *)string withNum:(NSNumber *)number withArray:(NSArray *)array {
    NSLog(@"%@, %@, %@", string, number, array[1]);
}

- (void)test {
    NSString *str = @"哈哈哈";
    NSNumber *num = @20;
    NSArray *arr = @[@"ABC", @"DEF"];
//    [self printWithString:str withNum:num withArray:arr];
    SEL sel = NSSelectorFromString(@"printWithString:withNum:withArray:");
    NSArray *objs = [NSArray arrayWithObjects:str, num, arr, nil];
    [self performSelector:sel withObjects:objs];
}


- (id)performSelector:(SEL)selector withObjects:(NSArray *)objects
{
    // 方法签名(方法的描述)
    NSMethodSignature *signature = [[self class] instanceMethodSignatureForSelector:selector];
    if (signature == nil) {
        
        //能够抛出异常也能够不操做。
    }
    
    // NSInvocation : 利用一个NSInvocation对象包装一次方法调用（方法调用者、方法名、方法参数、方法返回值）
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
    invocation.target = self;
    invocation.selector = selector;
    
    // 设置参数
    NSInteger paramsCount = signature.numberOfArguments - 2; // 除self、_cmd之外的参数个数
    paramsCount = MIN(paramsCount, objects.count);
    for (NSInteger i = 0; i < paramsCount; i++) {
        id object = objects[i];
        if ([object isKindOfClass:[NSNull class]]) continue;
        [invocation setArgument:&object atIndex:i + 2];
    }
    
    // 调用方法
    [invocation invoke];
    
    // 获取返回值
    id returnValue = nil;
    if (signature.methodReturnLength) { // 有返回值类型，才去得到返回值
        [invocation getReturnValue:&returnValue];
    }
    
    return returnValue;
}

 

3、objc_msgSend

objc_msgSend的写法要复杂一点，具体能够参看这篇文章，讲的很清楚

http://www.jianshu.com/p/efeb33712445

可是有个缺点是，须要指定好传递参数的类型，是否是能够直接都用id呢？

经测试id可用

// objc_msgSend
SEL sel = NSSelectorFromString(@"printWithString:withNum:withArray:");
((void (*) (id, SEL, NSString *, NSNumber *, NSArray *)) objc_msgSend) (self, sel, str, num, arr);