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docs: add a note about <select> and nested <option>s in Ivy compat guide (#42338) 

An internal change in Ivy has surfaced issues in previosly broken code. This change adds a note to the
Ivy compatibility guide as well as the TrackByFunction api docs.

Fixes #35896

PR Close #42338
This commit is contained in:
Igor Minar 2021-05-25 17:24:41 -07:00 committed by Andrew Kushnir
parent 306f7ac543
commit e73fef4d0b
2 changed files with 11 additions and 4 deletions
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13
aio/content/guide/ivy-compatibility.md
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@ -85,7 +85,12 @@ For more information about the updated style precedence in Ivy, refer to the [st
* If selecting the native `<option>` element in a `<select>` where the `<option>`s are created using `*ngFor`, use the `[selected]` property of an `<option>` instead of binding to the `[value]` property of the `<select>` element (previously, you could bind to either.) [details](guide/ivy-compatibility-examples#select-value-binding)
* Embedded views (such as ones created by `*ngFor`) are now inserted in front of anchor DOM comment node (for example, `<!--ng-for-of-->`) rather than behind it as was the case previously.
In most cases this does not have any impact on rendered DOM.
In some cases (such as animations delaying the removal of an embedded view) any new embedded views will be inserted after the embedded view being animated away.
This difference only last while the animation is active, and might alter the visual appearance of the animation.
Once the animation is finished the resulting rendered DOM is identical to that rendered with View Engine.
In most cases this has no impact on rendered DOM.
When animations delay the removal of an embedded view, any new embedded views will be inserted after the embedded view that will be removed once the animation completes.
This difference only lasts while the animation is active, and might alter the visual appearance of the animation.
When the animation is finished, the resulting rendered DOM is identical to that rendered with View Engine.
One additional exception is the `<select>` element with `<option>` elements dynamically rendered using `NgForOf`. If a [`trackBy`](api/common/NgForOf#ngForTrackBy) function is not provided, the selected `<option>` will not be preserved when the iterable used by the `NgForOf` changes.
With View Engine, this programming error was obscured, and often not visible.
To avoid this problem, provide the `trackBy` function to correctly associate the model with the rendered DOM elements.

2
packages/core/src/change_detection/differs/iterable_differs.ts
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@ -126,6 +126,8 @@ export interface IterableChangeRecord<V> {
* - preserve any DOM-specific UI state (like cursor position, focus, text selection) when the
* iterable is modified
* - enable animation of item addition, removal, and iterable reordering
* - preserve the value of the `<select>` element when nested `<option>` elements are dynamically
* populated using `NgForOf` and the bound iterable is updated
*
* A common use for custom `trackBy` functions is when the model that `NgForOf` iterates over
* contains a property with a unique identifier. For example, given a model: