[Table] Rows are not provided correct $implicit with trackBy

[quanterion]

Bug:

MatTable doesn't refresh row columns if [trackBy] function used. For example if I replace rows array with a new one having the same ids, but completely different data MatTable won't update. I suspect the same applies to CdkTable.
It looks like it correctly updates table when removing / adding rows but it fails to trigger ChangeDetection on components if no rows added or removed

What is the expected behavior?

When used with trackBy function Table should trigger change detection on all existing rows, just like ngFor with trackBy do.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-tbrfqj?file=app%2Fapp.component.html

[willshowell]

Wouldn't this be what you want?

trackBy is used to uniquely identify rows when you want to use some identifier other than the Object Reference. By using the same id for the swapped row, you're purposefully tricking the table into thinking that nothing has changed.

[quanterion]

I suppose using trackBy is intended to prevent recreating of components and should not stop change detection to happen. I updated stackblitz template to show that ngFor+trackBy updates correctly.

[andrewseguin]

Thanks for reporting this issue. The intention of trackBy is to make sure we are being smart about when to build/teardown rows, which is a separate concern than change detection. In fact, since cell templates are written by the user and not the table, the table has no control over when change detection occurs for those cells.

It turns out that when trackBy is used, a row can be re-used for a different data object, but the table never updates row/cell $implicit data (the exported variable for cells). ngFor does have this built in by checking for identity changes in the data, but it didn't get brought over to the table's logic.

Thanks again for the issue, this was a major one that I'm surprised got this far.

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