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Clarify that init callback can't be used from backend code and why (#5645)

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Danut Enachioiu 2024-07-18 21:55:01 +02:00 • committed by GitHub
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docs/reference/src/language/builtins/callbacks.md
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@ -31,18 +31,33 @@ export component MyWindow inherits Window {
``` ```
Don't use this callback to initialize properties, because this violates the declarative principle. Don't use this callback to initialize properties, because this violates the declarative principle.
Avoid using this callback, unless you need it, for example, in order to notify some native code:
Even though the `init` callback exists on all components, it cannot be set from application code,
i.e. an `on_init` function does not exist in the generated code. This is because the callback is invoked during the creation of the component, before you could call `on_init` to actually set it.
While the `init` callback can invoke other callbacks, e.g. one defined in a `global` section, and
you _can_ bind these in the backend, this doesn't work for statically-created components, including
the window itself, because you need an instance to set the globals binding. But it is possible
to use this for dynamically created components (for example ones behind an `if`):
```slint,no-preview ```slint,no-preview
global SystemService { export global SystemService {
// This callback can be implemented in native code using the Slint API // This callback can be implemented in native code using the Slint API
callback ensure_service_running(); callback ensure_service_running();
} }
export component MySystemButton inherits Rectangle { component MySystemButton inherits Rectangle {
init => { init => {
SystemService.ensure_service_running(); SystemService.ensure_service_running();
} }
// ... // ...
} }
export component AppWindow inherits Window {
in property <bool> show-button: false;
// MySystemButton isn't initialized at first, only when show-button is set to true.
// At that point, its init callback will call ensure_service_running()
if show-button : MySystemButton {}
}
``` ```