How to create a generic over generated globals and components?

[john-basilio]

EDIT: This project is in Rust.

I am currently developing a cross-platoform (Linux+Android) application that pretty much uses the same core logic with major difference being UI design and platform-specific compatibility features.

I have two root components mainly, DesktopWindow and MainWindow (Linux/Android), which share a global named ProfilesList.

In one of my modules, I have:

pub fn start_backend<T>
(
    db_path: &PathBuf, 
    window: &T, 
) 
where 
    T: slint::ComponentHandle,
{
    // ...snip
    let profile_list = ProfilesList::get(&window);
}

I would then get a compiler error:

error[E0277]: the trait bound `ProfilesList<'_>: Global<'_, _>` is not satisfied
  --> src/libcore/mod.rs:20:24
   |
20 | ...list = ProfilesList::get(&win...
   |           ^^^^^^^^^^^^ unsatisfied trait bound
   |
help: the trait `slint::Global<'_, &T>` is not implemented for `ProfilesList<'_>`
  --> /home/inuxiuz/Projects/mini_ledger/target/debug/build/mini_ledger/2e0a6d3b97085295/out/main_window.rs:34:25
   |
34 | ...)] pub struct r#ProfilesList < 'a > (:...
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: `ProfilesList<'_>` implements trait `Global<'a, Component>`
  --> /home/inuxiuz/Projects/mini_ledger/target/debug/build/mini_ledger/2e0a6d3b97085295/out/main_window.rs:47:6
   |
47 |      impl < 'a >...List < 'a > {
   |      ^^^^^^^^^^^...^^^^^^^^^^^ `Global<'_, DesktopWindow>`
...
55 |      impl < 'a >...List < 'a > {
   |      ^^^^^^^^^^^...^^^^^^^^^^^ `Global<'_, MainWindow>`

I wonder if there's a way to resolve this.

My last choice would be to have the project support only one platform then either fork or create an entirely new one for another platform.

[john-basilio]

One more thing to add:

Since the Android block is in lib.rs and the Linux's is in main.rs I did try the approach to have used the ProfilesList on each of the files directly, which worked. Just wondering if this can be genericized to make the code less repetitive.

[ogoffart]

You need to add more bounds:

pub fn start_backend<T>
(
    db_path: &PathBuf, 
    window: &'a T, 
) 
where 
    T: slint::ComponentHandle,
    ProfileList: slint::Global<'a, T>
{
    // ...snip
    let profile_list = ProfilesList::get(window);
}

something like that, but i haven't tried.

[tk1475]

ogoffart's bound is the right idea. There are two small things to get it to compile:

1. The trait is Global<'a, Component> with fn get(component: &'a Component) -> Self (in internal/core/api.rs), so the lifetime has to be declared on the function and tie the borrow of window to ProfilesList<'a>.
2. window is already a &T, so ProfilesList::get(&window) asks for Global<'_, &T>. That's the &T in your error message. Pass window directly.

use slint::Global; // for ProfilesList::get

pub fn start_backend<'a, T>(db_path: &PathBuf, window: &'a T)
where
    T: slint::ComponentHandle,
    ProfilesList<'a>: slint::Global<'a, T>,
{
    let profile_list = ProfilesList::get(window);
    // or: let profile_list = window.global::<ProfilesList>();

    // ...snip, profile_list.set_xxx(...) / on_xxx(...) work as usual
}

Since your error shows both Global<'_, DesktopWindow> and Global<'_, MainWindow> implemented on the same ProfilesList type, the bound is satisfied for both, and you can call start_backend(&db, &desktop_window) from main.rs and start_backend(&db, &main_window) from lib.rs. Inside the function ProfilesList is still the concrete generated type, so all its generated setters and callbacks are available without any extra trait bounds. Only getting it from the component needs the bound.

If you later need to keep the component around inside callbacks, take window.as_weak() (that gives a slint::Weak<T>) and upgrade it inside the closure as usual.

[hasanerman]

I tried the bound suggested above and it works: one generic function can serve both root components.

fn start_backend<'a, T>(window: &'a T)
where
    T: slint::ComponentHandle,
    ProfilesList<'a>: slint::Global<'a, T>,
{
    let profiles = ProfilesList::get(window); // or window.global::<ProfilesList>()
    profiles.on_add(|name| println!("add {name}"));
}

Tested with Slint 1.18.1 on Linux (Kali, rustc 1.99.0) and Windows (rustc 1.98.1), using i-slint-backend-testing: I created both windows, bound them through generic functions (both the ProfilesList::get(window) and window.global::<ProfilesList>() variants), invoked the callback and checked it fires for both.

One detail: the E0277 in the question comes from the missing Global bound. Without it I get exactly that error (rustc even suggests the where clause). Passing &window where window: &T still compiles (deref coercion), so the extra reference is not the cause.