bug(mat-tree): How to maintain tree expansion state if TreeControl is deprecated?

[danielsharvey]

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

pre v18.2.0

Description

Previously, it was possible to maintain the expansion state independent of the MatTree (rendered) component by using a TreeControl.

With TreeControl deprecated, this does not appear possible any longer.

I rely on this feature so would appreciate consideration of a path forward.

Of note, the selection model is maintained privately in MatTree:

components/src/cdk/tree/tree.ts

Lines 366 to 372 in b961966

	_getExpansionModel() {
	if (!this.treeControl) {
	this._expansionModel ??= new SelectionModel<K>(true);
	return this._expansionModel;
	}
	return this.treeControl.expansionModel;
	}

Note that this is not a duplicate of #29856, which looks to relate to accessing some of the methods of TreeControl, which have been made available on MatTree itself.

Reproduction

I can produce a reproduction but I believe the regression has been clearly explained above and a reproduction does not add value.

Expected Behavior

There is a mechanism for maintaining expansion state independent of the rendered component.

Actual Behavior

Expansion state is lost when the component is destroyed.

Environment

Angular CLI: 18.0.3
Node: 18.20.5
Package Manager: npm 10.8.2
OS: darwin arm64

Angular: 18.0.2
... animations, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... router

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.1800.3
@angular-devkit/build-angular     18.0.3
@angular-devkit/core              18.0.3
@angular-devkit/schematics        18.0.3
@angular/cdk                      18.2.14
@angular/cli                      18.0.3
@angular/flex-layout              14.0.0-beta.41
@angular/material                 18.2.14
@angular/material-luxon-adapter   18.2.14
@schematics/angular               18.0.3
ng-packagr                        18.0.0
rxjs                              7.8.1
typescript                        5.4.5
zone.js                           0.14.4

[Sebi11]

I agree!
#29856 (comment) with ViewChild might be a solution, but we must wait for the MatTree component to be rendered before calling methods on it.

[WwwHhhYran]

I agree! #29856 (comment) with ViewChild might be a solution, but we must wait for the MatTree component to be rendered before calling methods on it.

That's true 👍, but I feel that deprecating TreeControl has made the control of trees even more complex.

[WwwHhhYran]

Execuse, any one can address this issue ? @crisbeto @andrewseguin @@BobobUnicorn

I think the original TreeControl was able to ensure the isolation of the view from the model, which made it easier for us to manage the data and the state of the tree. But nowadays it's only possible to manage it through tree instances which is obviously not convenient. Let me give you an example:

In the old version (before v 18.2.0), I would pass only some of the data to be rendered to the tree view, and all the data to the Tree Control, and I could manage the nodes through the tree control's expansionModel. if I need to expand some other nodes, I can do it through the tree control's expand method and combine node isExpanded status to pass the certain node data to the tree.

But now, if I use levelAccessor property, I have to pass all the data to the tree, and then use style.display combined with the shouldRender method to hide some nodes that shouldn't be shown, like the official demo. If I don't pass all the data to the tree, it may cause methods like expandAll, expandDescendants, etc. to be incorrect. I dont think this is an elegant approach, and this approach may lead to inconvenient use。

[tutkli]

Before I used TreeControl to mix virtual scrolling with a tree. Similar to this example, but with the deprecation of these classes it would be impossible in the future. Rendering huge trees will be very expensive performance wise.