RACCommand中的信号

示例：

 RACSignal* textSignal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        [subscriber sendNext:@(1)];
        [subscriber sendNext:@(2)];
        [subscriber sendError:[NSError new]];
        return nil;
    }];

    RACCommand* textCommad = [[RACCommand alloc]initWithSignalBlock:^RACSignal *(id input) {
        return textSignal;
    }];

    self.createButton.rac_command = textCommad;

    [textCommad.executing subscribeNext:^(id x) {
        NSLog(@"executing%@",x);
    }];

    [textCommad.executionSignals subscribeNext:^(id x) {
        NSLog(@"executionSignals%@",x);
    }];

    [[textCommad.executionSignals switchToLatest]subscribeNext:^(id x) {
        NSLog(@"executionSignals switchLatest%@",x);
    }];

    [textCommad.errors subscribeNext:^(id x) {
        NSLog(@"errors");
    }];

输出：

2015-08-09 22:17:27.610 ReactiveCocoaDemo[7181:141024] executing0

2015-08-09 22:17:30.325 ReactiveCocoaDemo[7181:141024] executing1

2015-08-09 22:17:30.325 ReactiveCocoaDemo[7181:141024] executionSignals<RACDynamicSignal: 0x7f89b3c6ca20> name: 

2015-08-09 22:17:30.326 ReactiveCocoaDemo[7181:141024] executionSignals switchLatest1

2015-08-09 22:17:30.326 ReactiveCocoaDemo[7181:141024] executionSignals switchLatest2

2015-08-09 22:17:30.327 ReactiveCocoaDemo[7181:141024] errors

2015-08-09 22:17:30.327 ReactiveCocoaDemo[7181:141024] executing0

结论：

1.executing信号一绑定就会sendNext:@(NO)；如果想忽略第一次的Next，使用[executing skip:1]

2.按钮点击事件发生时，首先exectuing会sendNext:@(YES)；然后executionSignals会sendNext一个RACSignal对象，该对象就是RACCommand创建时传入的block的返回值。

3.注意executionSignals是信号的信号，即它的值类型为RACSignal,而我们一般希望捕获的是RACSignal所携带的值，因此可以使用switchToLatest或flatten的方法来做到。

4.errors包含了RACComand执行过程产生的所有错误。

5.等到RACCommand中的Signal都完毕了（complete或error）,exectuting会sendNext:@(NO).