Fix Flow leak when has .one() step and is only Flow emits rxDismissed #trivial

[asiliuk]

Description

This leak can happen in following situation:

• Coordinator leaves longer then presented Flow, for example as let on not yet migrated ViewModel who is presenting already migrated to Flow components
• You present modally flow whose root is UINavigationController
• It returns .one() with child Presentable and puts its ViewController inside root.viewControllers
• User swipes to dismiss whole modally presented UINavigationViewController

What is happening in that case:

1. Both UINavigationController (Flow implementing class) and child Presentable receive didDisappear

2. Because of implementation of rx.dismissed on UIViewController event on child presentable is filtered

.filter { [weak base] _ in base?.isBeingDismissed ?? true }

And isBeingDismissed of a child is false for some reason

3. In func coordinate(...) of FlowCoordinator there's this code

.flatMap { [weak self] in
    self?.steps(from: $0, within: flow, allowStepWhenDismissed: allowStepWhenDismissed) ?? Signal.empty()
}

Since rxDimiss of child Presentable is filtered out this code is equivalent of

.flatMap { [weak self] _ in
    .never()
}

4. So first part of pipe in func coordinate() is destroyed because Flow rxDismissed emitted event
5. Bottom part leaves forever and is disposed only when FlowCoordinator with its disposeBag is destroyed
6. And because FlowCoordinator in some cases can leave longer that Flow that it presents it can cause unexpected leaks

Basically fix is to destroy whole pipe when presented Flow is dimissed

Checklist

• this PR is based on develop or a 'develop related' branch
• the commits inside this PR have explicit commit messages
• the Jazzy documentation has been generated (if needed -> Jazzy RxFlow)

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[asiliuk]

@twittemb Hi! Do you have any thoughts about this?

[twittemb]

Sorry guys, I’ll review it ASAP I promise (before the end of the week).

[twittemb]

Thanks a lot