fix(forms): improve select performance #61949
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We defer the update until after rendering is complete for two reasons: first, to avoid repeatedly calling `writeValue` on every option element until we find the selected one (could be the very last element). Second, to ensure that we perform the write after the DOM elements have been created (this doesn't happen until the end of change detection when animations are enabled). This is needed to efficiently set the select value when adding/removing options. The previous approach resulted in exponentially more `_compareValue` calls than the number of option elements (issue angular#41330). Finally, this PR fixes an issue with delayed element removal when using the animations module (in all browsers). Previously when a selected option was removed (so no option matched the ngModel anymore), Angular changed the select element value before actually removing the option from the DOM. Then when the option was finally removed from the DOM, the browser would change the select value to that of the first option, even though it didn't match the ngModel (issue angular#18430). Note that this is still somewhat of an application problem when using `ngModel`. The model value still needs to be updated to a valid value when the selected value is deleted or it will be out of sync with the DOM. Fixes angular#41330, fixes angular#18430.
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We defer the update until after rendering
is complete for two reasons: first, to avoid repeatedly calling
writeValueon every option element until we find the selected one (could be the very last element). Second, to ensure that we perform the write after the DOM elements have been created (this doesn't happen until the end of change detection when animations are enabled).This is needed to efficiently set the select value when adding/removing options. The previous approach resulted in exponentially more
_compareValuecalls than the number of option elements (issue #41330).Finally, this PR fixes an issue with delayed element removal when using the animations module (in all browsers). Previously when a selected option was removed (so no option matched the ngModel anymore), Angular changed the select element value before actually removing the option from the DOM. Then when the option was finally removed from the DOM, the browser would change the select value to that of the first option, even though it didn't match the ngModel (issue #18430). Note that this is still somewhat of an application problem when using
ngModel. The model value still needs to be updated to a valid value when the selected value is deleted or it will be out of sync with the DOM.Fixes #41330, fixes #18430.
TGP
requires an update to the screenshots in one test, with minor shifting in the placement of the text in the select control (see cl/767187673)
Though there is a green TGP, this changes the timing of values being written to the DOM, which still caries some risk. We should merge to
mainonly to give more time for things to be discovered internally prior to external release.