iOS开发 「RAC」RAC事件监听方法（rac_signalF

以UIButton的点击事件为例，可以代替add target/action for particular event事件

    // 只要按钮产生这个事件,就会产生一个信号，相当于Button 的add target/action for particular event
    //- (void)addTarget:(nullable id)target action:(SEL)action forControlEvents:(UIControlEvents)controlEvents;
    [[_btn rac_signalForControlEvents:UIControlEventTouchUpInside] subscribeNext:^(id x) {
        //x返回的就是被监听的button对象
        NSLog(@"按钮被点击%@",x);
    }];
    _btn.rac_command = [[RACCommand alloc] initWithSignalBlock:^RACSignal *(id input) {
        NSLog(@"按钮点击");
        return [RACSignal empty];
    }];