我如何在没有嵌套订阅的情况下订阅命令执行信号的完成? [英] How can I subscribe to the completion of a command's execution signals without a nested subscription?

问题描述

我尝试了以下操作，但没有成功.使用-subscribeNext:的等效项可以正常工作.

I tried the following without success. The equivalent using -subscribeNext: works as expected.

// A
[[_viewModel.loginCommand.executionSignals flatten] subscribeCompleted:^{
    NSLog(@"A");
}];

我唯一可行的实现如下:

My only working implementation is as follows:

// B
[_viewModel.loginCommand.executionSignals subscribeNext:^(RACSignal *loginSignal) {
    [loginSignal subscribeCompleted:^{
        NSLog(@"B");
    }];
}];

为什么-flatten在"A"中不起作用，如何重写"B"以不使用嵌套订阅?

Why doesn't -flatten work in "A", and how I can I rewrite "B" to not use a nested subscription?

推荐答案

-flatten运算符返回仅在所有内部信号都已完成时才完成的信号，这也需要外部信号也要完成. -concat也是如此.因此，一旦您应用了任何一个运算符，结果信号就不会表示单个完成，而只能表示最终的汇总完成.

The -flatten operator returns a signal that completes only when all of the inner signals have completed, which requires the outer signal to complete as well. The same is true of -concat. Because of this, once you apply either operator, the resulting signal has no representation of individual completion, only the final aggregate completion.

作为嵌套订阅的替代，您可以转换内部信号，以便它们发送表示完成的值.一种方法是使用-materialize:

Alternative to nested subscriptions, you could transform the inner signals so that they send a value that signifies completion. One way of doing this is with -materialize:

[[[_viewModel.loginCommand.executionSignals
    map:^(RACSignal *loginSignal) {
        // Using -ignoreValues ensures only the completion event is sent.
        return [[loginSignal ignoreValues] materialize];
    }]
    concat]
    subscribeNext:^(RACEvent *event) {
        NSLog(@"Completed: %@", event);
    }];

请注意，我使用了-concat而不是-flatten，因为它与RACCommand的默认串行执行的语义相匹配.在这种情况下，它们最终会执行相同的操作，因为命令一次只执行一个信号，所以-flatten会退化为-concat的行为.

Note that I used -concat instead of -flatten, since it matches the semantics of RACCommand's default serial execution. They ultimately do the same in this case, -flatten degenerates to the behavior of -concat because the command only executes signals one at a time.

使用-materialize并不是唯一的方法，它只是发送一个表示完成的值，但这可能是您认为对用例有意义的任何值.

Using -materialize isn't the only way to do this, it just happens to send a value that represents completion, but that could be any value that you find appropriately significant for your use case.

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