Avoid capturing ComponentDef in a closure for a long time #83
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gasi
approved these changes
Jul 17, 2024
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| - Fixed a bug that allowed `ComponentDef` to be captured in closures for a long | ||
| time, which could lead to using stale values in complex scenarios where | ||
| `ComponentDef` is not constant, but depends on arguments. See [#83](https://github.com/collegevine/purescript-elmish/pull/83). |
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Nit: Within the same repo, this syntax will autolink AFAIK 👍
Suggested change
| `ComponentDef` is not constant, but depends on arguments. See [#83](https://github.com/collegevine/purescript-elmish/pull/83). | |
| `ComponentDef` is not constant, but depends on arguments. See #83. |
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No, it doesn't. I tried this first.
mcordova47
approved these changes
Jul 17, 2024
| @@ -61,6 +63,35 @@ spec = describe "Elmish.Component.wrapWithLocalState" do | |||
| findAll ".t--counter" <#> length >>= shouldEqual 2 | |||
| findAll ".t--counter" >>= forEach (nearestEnclosingReactComponentName >>= shouldEqual "Elmish_Counter") | |||
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| it "calls the correct closure of `update` when dispatching events" $ | |||
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The
ComponentDefis captured in closures defined underwhereinbindComponent. In most cases this is fine, becauseComponentDefis constant anyway. And even if not,bindComponentis called on every render, which means a new, fresh value ofComponentDefis re-captured on every render.But what if such closure could survive several renders and several changes of
ComponentDef? If the closure is adispatchfunction, the next message that is issued through it will call a stale version ofupdate. This is particularly important now that we added subscriptions, which keep the initial value ofdispatchpretty much indefinitely.See the new test I added in this PR to see how this could happen. If the changes to
bindComponentare undone, that test fails in exactly the way the comment in it predicts.To fix this, I am now keeping
ComponentDefas a prop on the component, and always retrieving it from the component props when needed, rather than capturing it in a closure. This way, whenever theComponentDefis required, it is always the one from the latest render.There will be another fix in
purescript-hooksto take advantage of this fix to fix the next level of the same problem.