React-router: Support function for Prompt when prop

[julienbachmann]

Now the Prompt component only support a boolean that enable or not the navigation. This means that we need to know at at render time that navigation is allowed or not. Sometimes we want to check a condition just at navigation time and in this case we should be able to pass a function instead of a boolean (history lib already support a function in the block function so it should be trivial to implement this feature).
I would like to be able to write something like that:

<Prompt when={(location, action) => {
  // The location and action arguments indicate the location
  // we're transitioning to and how we're getting there.

  // A common use case is to prevent the user from leaving the
  // page if there's a form they haven't submitted yet.
  if (input.value !== '')
    return 'Are you sure you want to leave this page?'
}}

and when we use a function for the when prop then we do not provide the message prop as the function already return the message to display.

[timdorr]

Seems like a good idea. This would be best as a PR and should be pretty simple. If someone wants to put one together (with tests!), that would be awesome.

[ryanflorence]

Ah, so this would be handy when you are rendering a prompt in a deep view pretty far away from a route component?

[taurose]

As far as I can tell, this should already be possible by passing a function to message and setting when to true (which is the default, so it can be omitted).

<Prompt message={(location, action) => {
  // The location and action arguments indicate the location
  // we're transitioning to and how we're getting there.

  // A common use case is to prevent the user from leaving the
  // page if there's a form they haven't submitted yet.
  if (input.value !== '')
    return 'Are you sure you want to leave this page?'
}} />

docs

[ryanflorence]

Good call, thanks.

[julienbachmann]

:+1:

[rubencodes]

This is definitely possible using the above code, but seems like a hacky workaround. Would y'all be open to a PR that allows a function for the when parameter?