把可变参装入数组

+ (NSMutableArray *)arrayWithObjects:(id)firstObj, ... NS_REQUIRES_NIL_TERMINATION {
    NSMutableArray *array = [NSMutableArray array];
    va_list argList;
    if (firstObj) {
        [array addObject:firstObj];
        va_start(argList, firstObj);
        id temp;
        while ((temp = va_arg(argList, id))) {
            [array addObject:temp];
        }
    }
    va_end(argList);
    
    return array;
}

rac_liftSelector:withSignals:....
我们仿写一个：

//
//  RACSignal.h
//  Created by liuweizhen on 2017/6/15.
//  Copyright © 2017年 刘威振. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface RACSignal : NSObject

@property (nonatomic, copy) NSString *name;
@end

#import "RACSignal.h"

@implementation RACSignal

@end

//
//  NSObject+RACLifting.h
//  Created by liuweizhen on 2017/6/15.
//  Copyright © 2017年 刘威振. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "RACSignal.h"

@interface NSObject (RACLifting)

- (void)rac_liftSelector:(SEL)selector withSignals:(RACSignal *)firstSignal, ... NS_REQUIRES_NIL_TERMINATION;
- (void)rac_liftSelector:(SEL)selector withSignalsFromArray:(NSArray *)signals;

@end

//
//  NSObject+RACLifting.m
//  VA_LIST
//
//  Created by liuweizhen on 2017/6/15.
//  Copyright © 2017年 刘威振. All rights reserved.
//

#import "NSObject+RACLifting.h"

@implementation NSObject (RACLifting)

- (void)rac_liftSelector:(SEL)selector withSignals:(RACSignal *)firstSignal, ... {
    NSCParameterAssert(firstSignal != nil);
    NSMutableArray *signals = [NSMutableArray array];
    
    va_list args;
    va_start(args, firstSignal); // 指向第一个可变参
    for (id currentSignal = firstSignal; currentSignal != nil; currentSignal = va_arg(args, id)) {
        NSCAssert([currentSignal isKindOfClass:RACSignal.class], @"Argument %@ is not a RACSignal", currentSignal);
        [signals addObject:currentSignal];
    }
    va_end(args);
    [self rac_liftSelector:selector withSignalsFromArray:signals];
}

- (void)rac_liftSelector:(SEL)selector withSignalsFromArray:(NSArray *)signals {
    NSCParameterAssert(signals != nil);
    NSCParameterAssert(signals.count > 0);
    
    // 方法签名
    NSMethodSignature *methodSignature = [self methodSignatureForSelector:selector];
    NSCAssert(methodSignature != nil, @"%@ does not respond to %@", self, NSStringFromSelector(selector));
    NSUInteger numberOfArguments __attribute__((unused)) = methodSignature.numberOfArguments - 2; // From Apple: There are always at least two arguments, because an NSMethodSignature object includes the implicit arguments self and _cmd, which are the first two arguments passed to every method implementation.
    NSCAssert(numberOfArguments == signals.count, @"Wrong number of signals for %@ (expected %lu, got %lu)", NSStringFromSelector(selector), (unsigned long)numberOfArguments, (unsigned long)signals.count);
    
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];
    invocation.selector = selector;
    
    for (NSInteger i = 0; i < signals.count; i++) {
        NSInteger j = i + 2;
        RACSignal *signal = signals[i];
        [invocation setArgument:&signal atIndex:j];
    }
//    
//    RACSignal *signal0 = signals[0];
//    RACSignal *signal1 = signals[1];
//    RACSignal *signal2 = signals[2];
//    // 前两个被selector和target占用, 所以从2开始
//    [invocation setArgument:&signal0 atIndex:2];
//    [invocation setArgument:&signal1 atIndex:3];
//    [invocation setArgument:&signal2 atIndex:4];
// invocation.target = self;
    [invocation invokeWithTarget:self];
}

@end