adwaita.docc/Advanced/CreatingWidgets.md

Creating widgets

Widgets are special views that do not provide a collection of other views as a content, but have functions that are called when creating or updating the view. Normally, a widget manages a GTK or Libadwaita widget using the C API.

Recreate the Text widget

In this tutorial, we will recreate the Text widget. A widget conforms to the Widget protocol:

struct CustomText: Widget { }

You can add properties to the widget:

struct CustomText: Widget {

    var text: String

}

This widget can be called in a view body using CustomText(text: "Hello, world!"). Now, add the two functions required by the protocol:

import CAdw

struct CustomText: Widget {

    var text: String

    public func container(modifiers: [(View) -> View]) -> ViewStorage { }
    public func update(_ storage: ViewStorage, modifiers: [(View) -> View], updateProperties: Bool) { }

}

Import CAdw which exposes the whole C Libadwaita and Gtk API to Swift.

The container(modifiers:) function

This function initializes the widget when the widget appears for the first time. It expects a ViewStorage as the return type. In our case, this function is very simple:

func container(modifiers: [(View) -> View]) -> ViewStorage {
    .init(gtk_label_new(text)?.opaque())
}

The update(_:modifiers:updateProperties:) function

Whenever a state of the app changes, the Widget/update(_:modifiers:updateProperties:) function of the widget gets called. You get the view storage that you have previously initialized as a parameter. Update the storage to reflect the current state of the widget:

func update(_ storage: ViewStorage, modifiers: [(View) -> View], updateProperties: Bool) {
    if updateProperties {
        gtk_label_set_label(storage.pointer, text)
    }
}

Containers

Some widgets act as containers that accept other widgets as children. In that case, use the ViewStorage's content property for storing their view storages. In the Widget/update(_:modifiers:updateProperties:) function, update the children's storages. Take a look at the code of the container widgets in this library as a reference.