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transitions-reversal.md

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Reversing transitions

A killer feature of the framework is that transitions can be automatically reversed. You don't need to do anything special! A reversal happens whenever this happens:

  1. There's a configuration change from any X state to any Y state of the RoutingSource
  2. The transition animation is playing and hasn't settled yet
  3. There's a configuration change from Y state back to X state.

Note hat states here don't reference individual configurations, but the whole state of the RoutingSource!

For example, in the case of the back stack, the whole content of the back stack is considered its state, so these sequences would trigger transition & reversal:

T0: [A, B, C]       // element on screen: C
push(D)             // e.g. from business logic
T1: [A, B, C, D]    // transition plays: C exiting, D entering
pop()               // easily triggered by back press
T2: [A, B, C]       // transition reverses to starting point: C is restored, D exits

T0: [A, B, C]       // element on screen: C
replace(D)          // e.g. user selects a menu item to change current screen
T1: [A, B, D]       // transition plays: C exiting, D entering
replace(C)          // e.g. user selects the previous menu item
T2: [A, B, C]       // transition reverses to starting point: C is restored, D exits

But this would NOT work:

T0: [A, B, C]       // element on screen: C
push(D)             // e.g. from business logic
T1: [A, B, C, D]    // transition plays: C exiting, D entering
newRoot(C)
T2: [C]             // no reversal, even though C is the same as in T0 - the whole of the state is different

Transition reversal is automatic

The coolest thing is probably that back press has no special role here at all! Transition reversal automatically works regardless of:

  • what kind of routing source you have (back stack or not)
  • what triggered the configuration change starting the initial transition
  • what triggered the configuration change that would lead back to the original state

As long as there's a transition on the fly running between any X→Y states, any incoming transition of Y→X would reverse it automatically. At its core this is triggered by the simplest tool possible, an object equality check:

fun isReverseOf(other: TransitionDescriptor) =
    from == other.to && to == other.from