how to stop subscription?

[诺铁]

hi,

in the guide http://guide.elm-lang.org/architecture/effects/time.html

there is an exercise:

Exercises:

• Add a button to pause the clock, turning the Time subscription off.

but I search the api doc, and didn't find out how to do this? any hints?

[Max Goldstein]

What's the type of the thing passed to Html.App.program that controls subscriptions?

[Gage Peterson]

I'm not sure it's possible at this point. One way would be to simply ignore them based on some state in the model. Like a boolean called "trackTime" that would only react to the time if it was true. 

[诺铁]

I add a "shouldStop: Bool" flag to the model. and check it in subscription definition:

subscriptions : Model -> Sub Msg
subscriptions model =
  if model.shouldStop then
    Sub.none
  else
    Time.every second Tick

It works!  but this confuse me. 

doesn't this mean,whenever model is changed, the subscription is reevaluated?

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[James Wilson]

Correct: Every time the global update function in your app runs for any reason, the global subscriptions object is reevaluated as well, and effect managers receive the new list of current subscriptions.

[Gage Peterson]

Wow! I didn't know that. 

On Fri, May 27, 2016 at 2:37 AM James Wilson <jam...@gmail.com> wrote:

Correct: Every time the global update function in your app runs for any reason, the global subscriptions object is reevaluated as well, and effect managers receive the new list of current subscriptions.

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[jonathan de montalembert]

I believe that every functions that receive a model are reevaluated when the model changes.