Pass prevState as argument to onLeave

[albertogasparin]

This is a little enhancement I'm asking for. Currently, both onEnter and onChange have router state passed as an argument. That allows us doing stuff with router location and parameters. However, it is not the case for onLeave. Is there any particular reason?

Use case example: I'd like to handle socket.io room joining/leaving on the router.

<Route path="/" component={App}>
  <Route 
    path="/chat/:id" 
    component={Chat} 
    onEnter={(nextState) => { 
      joinChat(nextState.params.id); 
    }}
    onChange={(prevState, nextState) => { 
      leaveChat(prevState.params.id); 
      joinChat(nextState.params.id); 
    }}
    onLeave={(prevState) => { 
      leaveChat(prevState.params.id); 
    }}
  />
</Route>

I know this could be handled by the component itself, but I'm questioning why not passing the previous state to the onLeave callback, as it might be helpful in some situations.

[timdorr]

I know this could be handled by the component itself

That's the reason right there. Make use of lifecycle methods as much as possible.

In your example, you can move the onChange up to your top level route instead.

[taion]

Is it weird that the API is inconsistent here? I do think it would feel more consistent if onLeave received prevState? It'd be a pretty easy change.

[taion]

I'm inclined to add prevState there just for the sake of consistency. It's just really weird that onLeave doesn't have the signature you'd expect there.

One reason you may not want to just use lifecycle hooks there is (as with onChange) that you might be using named components.

[timdorr]

Sure, I'll whip up a PR now.