观察每一个项目在RACSequence，更新观测时序列具有新项目

问题描述：

这是an earlier question that asked about observing every item in a RACSequence类似的问题 - 正确答案是这样的：观察每一个项目在RACSequence，更新观测时序列具有新项目

RACSignal *valid = [[RACSignal combineLatest: 
        [self.viewModels map:^id(ViewModel *viewModel) { 
         return RACObserve(viewModel, state); 
        }] 
        ] 
        map:^(RACTuple *states) { 
         return @([states.rac_sequence all:^BOOL(NSNumber *state) { 
         return state.unsignedIntegerValue == Completed; 
         }]); 
        } 
        ];

我对这个变化是，我想还处理ViewModel实例在序列中添加/删除的情况。我可以通过使存储在实例变量或属性中的RACDisposable失效来实现这一点，但如果不添加任何额外的状态，这样做会很好。什么是正确的方法来做到这一点？

答

我发现了一个旧后答案通过@贾斯汀 - 斯帕尔 - 夏天：https://stackoverflow.com/a/19711002/63580

下面是具体针对我的问题上面为后人版本：

@weakify(self); 

RACSignal *enabled = [[RACObserve(self, viewModels) 
    // Map _each_ array of view models to a signal determining whether the command 
    // should be enabled. 
    map:^(NSArray *viewModels) { 
     RACSequence *selectionSignals = [[viewModels.rac_sequence 
      map:^(ViewModel *viewModel) { 
       // RACObserve() implicitly retains `self`, so we need to avoid 
       // a retain cycle. 
       @strongify(self); 

       // Observe each view model's `state` property for changes. 
       return RACObserve(viewModel, state); 
      }] 
      // Ensure we always have one YES for the -and below. 
      startWith:[RACSignal return:@YES]]; 

     // Sends YES whenever all of the view models are selected, NO otherwise. 
     return [[RACSignal 
      combineLatest:selectionSignals] 
      and]; 
    }] 
    // Then, ensure that we only subscribe to the _latest_ signal returned from 
    // the block above (i.e., the observations from the latest `viewModels`). 
    switchToLatest];

观察每一个项目在RACSequence，更新观测时序列具有新项目

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