iOS 反射函數: performSelector, NSInvocation, objc_msgSend

當咱們有方法名和參數列表，想要動態地給對象發送消息，可用經過反射函數機制來實現，有兩種經常使用的作法：

 

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;

經常使用的方法有這三個，其中aSelector能夠經過 NSSelectorFromString 方法拿到

SEL aSelector = NSSelectorFromString(selString);

可是 performSelector 的缺點是最多隻支持傳遞兩個參數

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

這篇文章給出的方法二說是能夠支持多參數，可是我嘗試了下未成功

 

2、NSInvocation

很少說，直接上栗子

// 測試反射函數
- (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
{
    // 方法簽名(方法的描述)
    NSMethodSignature *signature = [[self class] instanceMethodSignatureForSelector:selector];
    if (signature == nil) {
        
        //能夠拋出異常也能夠不操做。
    }
    
    // NSInvocation : 利用一個NSInvocation對象包裝一次方法調用（方法調用者、方法名、方法參數、方法返回值）
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
    invocation.target = self;
    invocation.selector = selector;
    
    // 設置參數
    NSInteger paramsCount = signature.numberOfArguments - 2; // 除self、_cmd之外的參數個數
    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];
    }
    
    // 調用方法
    [invocation invoke];
    
    // 獲取返回值
    id returnValue = nil;
    if (signature.methodReturnLength) { // 有返回值類型，纔去得到返回值
        [invocation getReturnValue:&returnValue];
    }
    
    return returnValue;
}

 

3、objc_msgSend

objc_msgSend的寫法要複雜一點，具體能夠參看這篇文章，講的很清楚

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

可是有個缺點是，須要指定好傳遞參數的類型，是否是能夠直接都用id呢？

經測試id可用

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