Make CollapsingHeader truncateable

[higumachan]

I am currently using a patch that allows the CollapsingHeader to truncate the header. I am considering creating a PR if this patch would be useful to others. What do you think? The API I created for truncation looks like this.

CollapsingHeader::new("collapsing_header_label")
    .header_truncate()
    .body(|ui| { ... })

[YgorSouza]

You can make a PR and see if it gets accepted, but know that the similar PR #4045 was rejected. It is relatively easy to make a collapsing header with any UI you want, as shown in this example from the Misc Demos section in the demo app:

egui/crates/egui_demo_lib/src/demo/misc_demo_window.rs

Lines 439 to 450 in 5cf99c6

	ui.label("Example of a collapsing header with custom header:");
	let id = ui.make_persistent_id("my_collapsing_header");
	egui::collapsing_header::CollapsingState::load_with_default_open(ui.ctx(), id, true)
	.show_header(ui, |ui| {
	ui.toggle_value(&mut self.selected, "Click to select/unselect");
	ui.radio_value(&mut self.radio_value, false, "");
	ui.radio_value(&mut self.radio_value, true, "");
	})
	.body(|ui| {
	ui.label("The body is always custom");
	});

So arbitrarily adding some of these options to the default header seems redundant.

[higumachan]

Thank you for response.

I know that with CollapsingState, I can create an arbitrary widget. However, the problem in this case is that the body does not toggle when the user clicks on the arbitrary widget.

Is there a simple solution to this problem?

Or would it make more sense to create a mode in CollapsingState that toggles from a click on the header? (I have tried to implement that as a PoC as well.)

[YgorSouza]

I don't know if there is an easy way to do that currently, because the header response does not sense clicks, and the UI you put inside the header doesn't have to return a Response, so the header can't use it. And also the show_header method consumes the CollapsingState, so you can't toggle it manually afterward. So I guess you would have to store the state yourself using ui.data, toggle it if your custom UI's response was clicked, and then force it manually using set_open (i.e., duplicate some of the CollapsingState's inner logic).

I think this could be implemented in egui with the following code after this part:

egui/crates/egui/src/containers/collapsing_header.rs

Lines 147 to 153 in 5cf99c6

	let header_response = ui.horizontal(|ui| {
	let prev_item_spacing = ui.spacing_mut().item_spacing;
	ui.spacing_mut().item_spacing.x = 0.0; // the toggler button uses the full indent width
	let collapser = self.show_default_button_indented(ui);
	ui.spacing_mut().item_spacing = prev_item_spacing;
	(collapser, add_header(ui))
	});

let clicked = ui
    .allocate_rect(header_response.response.rect, Sense::click())
    .clicked();
if clicked {
    self.toggle(ui);
}

But it has to be opt-in, since it makes the custom UI you put in the header non-clickable. So there would have to be a method like toggle_if_clicked_on_header or something like that, which makes this code run.

So maybe you could try pitching that as a feature request issue. I think people are more likely to notice those than the discussions.