Conditional item gets re-instantiated even if the result is the same

[bjorn]

Platform: Linux / winit / FemtoVG / X11, Language: Rust

Consider the following code:

if (Facade.portfolio.file-name != ""): MainContent {}

When I call facade.set_portfolio, the MainContent appears to be always re-instantiated (judging by the loss of its state), even if the result of the condition stays true. This is undesirable.

I'm working it around in my application by instead doing:

MainContent {
    visible: Facade.portfolio.file-name != "";
}

But this of course keeps the instance around, even when it is not needed.

[ogoffart]

The thing is that we have to reset the whole thing if the model is dirty in

slint/internal/core/model.rs

Lines 871 to 872 in c695869

	if model.is_dirty() {
	*self.data().inner.borrow_mut() = RepeaterInner::default();

But we could actually check if the model has changed before doing that.

Now the problem is how to compare two model. Because the generated expression in rust is sp::ModelRc::new(#model as bool), so it's always a different model. So we need to find a trick to make it work. Maybe instead of using a ModelRc for bool model, we could use something else.

[bjorn]

So we need to find a trick to make it work. Maybe instead of using a ModelRc for bool model, we could use something else.

Or maybe it is possible to do a special case check for comparing bool models? Using as_any / downcast_ref, maybe?

[tp971]

I just came across this issue after updating an older private project; this seems to be a recession from 1.2.2 to 1.3.0. In my case, I had a conditional button in a video-player-like application, which is almost impossible to click because the player state is updated 60 times a second:

if (!state.playing): Button {
    icon: @image-url("icons/play-fill.svg");
    enabled: state.ready && !state.fastforward;
    clicked => {
        play();
        focus.focus();
    }
}
if (state.playing): Button {
    icon: @image-url("icons/pause-fill.svg");
    enabled: !state.fastforward;
    clicked => {
        pause();
        focus.focus();
    }
}

I could workaround this issue by "pulling the ifs inside":

Button {
    icon: !state.playing ? @image-url("icons/play-fill.svg") : @image-url("icons/pause-fill.svg");
    enabled: state.ready && !state.fastforward;
    clicked => {
        if (!state.playing) {
            play();
            focus.focus();
        } else {
            pause();
            focus.focus();
        }
    }
}