SpinButton (JavaScript)

A SpinButton is not related to a Spinner. It's a text entry field which only accepts numbers, and which has plus and minus buttons to let you change the value without having to type anything in.

It's best used when it's obvious that only a number can be entered into it. In this example, two SpinButtons are used for the number of kittens and number of cans of tuna to give them.

Libraries to import

These are the libraries we need to import for this application to run. Remember that the line which tells GNOME that we're using Gjs always needs to go at the start.

Creating the application window

All the code for this sample goes in the SpinButtonExample class. The above code creates a Gtk.Application for our widgets and window to go in.

The _buildUI function is where we put all the code to create the application's user interface. The first step is creating a new Gtk.ApplicationWindow to put all our widgets into.

Creating the SpinButtons

We can use the new_with_range function to create a new SpinButton quickly. Its first parameter is the SpinButton's starting value, its second is the maximum value, and its third is how much to increment it when the plus or minus buttons are pressed.

After we create the first SpinButton, we connect its value-changed signal to a function which handles what happens when the number inside either SpinButton changes.

If you want more fine-grained control over a SpinButton, or want to create a bunch of SpinButtons which all use the same parameters, you can create an object called an Adjustment. Then you can use that object as a new SpinButton's adjustment property, and it sets all the values at once. Afterwards, you can change all the SpinButtons which use that adjustment by changing the Adjustment object's properties.

The commented-out lines here show things you can do to customize your SpinButton. You can set the number of digits which go after the decimal point, for example, or tell it to wrap the number around if it goes past the upper or lower bounds that you set.

For the kitties' sake, please don't use set_digits to allow a decimal number of kittens.

Creating the rest of the UI

We create each Label individually, and then string them together along with the SpinButtons. The last label needs to show the number of cans of tuna per kitten, so it has a variable in the middle, which corresponds to an equation that uses the SpinButtons' get_value functions to find out what they're set at. JavaScript's Math function's floor method is used to round the number of cans of tuna per kitten down to the nearest whole number.

Here we use Grid widgets to keep everything organized. One Grid holds the labels and SpinButtons in order, while the next puts that Grid on top and the final Label on the bottom.

There isn't a wrong way to organize things in Grids, so long as you like how it turns out. In this case, the top Grid has a margin on the bottom to keep it evenly spaced from the bottom Label, and the bottom Label is inside a separate Grid so it will be centered relative to the Labels and SpinButtons on the top.

Finally, we add the larger Grid to the window, then tell the window to show itself and all the widgets inside of it.

Function which handles the SpinButtons' numerical values being adjusted

Here we update the perKitten variable based on the SpinButtons' new values, and use the set_label property to refresh what _lastLabel shows. Since both SpinButtons have their value-changed signal connected to this function, every time either of the numbers changes this function will update the Label.

Finally, we create a new instance of the finished SpinButtonExample class, and set the application running.

Complete code sample

#!/usr/bin/gjs

imports.gi.versions.Gtk = '3.0';

const Gio = imports.gi.Gio;
const Gtk = imports.gi.Gtk;

class SpinButtonExample {

    // Create the application itself
    constructor() {
        this.application = new Gtk.Application({
            application_id: 'org.example.jsspinbutton'
        });

        // Connect 'activate' and 'startup' signals to the callback functions
        this.application.connect('activate', this._onActivate.bind(this));
        this.application.connect('startup', this._onStartup.bind(this));
    }

    // Callback function for 'activate' signal presents window when active
    _onActivate() {
        this._window.present();
    }

    // Callback function for 'startup' signal builds the UI
    _onStartup() {
        this._buildUI();
    }

    // Build the application's UI
    _buildUI() {

        // Create the application window
        this._window = new Gtk.ApplicationWindow({
            application: this.application,
            window_position: Gtk.WindowPosition.CENTER,
            border_width: 20,
            title: "Kitten Feeder"});

        // Create the first spinbutton using a function
        this._kittens = Gtk.SpinButton.new_with_range (1, 9001, 1);
        this._kittens.connect ("value-changed", this._newValue.bind(this));

        // Create an adjustment to use for the second spinbutton
        this._adjustment = new Gtk.Adjustment ({
            value: 1,
            lower: 0,
            upper: 9001,
            step_increment: 1,
            page_increment: 10 });

        // Create the second spinbutton
        this._tuna = new Gtk.SpinButton ({ adjustment: this._adjustment });
        this._tuna.connect ("value-changed", this._newValue.bind(this));

        // this._tuna.set_digits (1);
        // this._tuna.set_wrap (true);

        // Create the text labels to go with the spinbuttons
        this._startLabel = new Gtk.Label ({ label: "There are " });
        this._kittenLabel = new Gtk.Label ({ label: " kitten(s), and "});
        this._tunaLabel = new Gtk.Label ({ label: " can(s) of tuna."});
        this.perKitten = Math.floor((this._tuna.get_value() / this._kittens.get_value()));
        this._lastLabel = new Gtk.Label ({
            label: "That's " + this.perKitten + " can(s) of tuna per kitten." });

        // Create a grid to put the spinbuttons and their labels in
        this._spinGrid = new Gtk.Grid ({
            halign: Gtk.Align.CENTER,
            valign: Gtk.Align.CENTER,
            margin_bottom: 20 });

        // Attach everything to the grid
        this._spinGrid.attach (this._startLabel, 0, 0, 1, 1);
        this._spinGrid.attach (this._kittens, 1, 0, 1, 1);
        this._spinGrid.attach (this._kittenLabel, 2, 0, 1, 1);
        this._spinGrid.attach (this._tuna, 3, 0, 1, 1);
        this._spinGrid.attach (this._tunaLabel, 4, 0, 1, 1);

        // Create a main grid to hold it and the last label
        this._mainGrid = new Gtk.Grid ({
            halign: Gtk.Align.CENTER,
            valign: Gtk.Align.CENTER });

        // Attach the smaller grid and the last label to the main grid
        this._mainGrid.attach (this._spinGrid, 0, 0, 1, 1);
        this._mainGrid.attach (this._lastLabel, 0, 1, 1, 1);

        // Add the main grid to the window
        this._window.add (this._mainGrid);

        // Show the window and all child widgets
        this._window.show_all();
    }

    _newValue() {
        // Update the label which shows how many cans there are per kitten
        this.perKitten = Math.floor((this._tuna.get_value() / this._kittens.get_value()))
        this._lastLabel.set_label ("That's " + this.perKitten + " can(s) of tuna per kitten.");
    }
};

// Run the application
let app = new SpinButtonExample ();
app.application.run (ARGV);

In-depth documentation

Gtk.Adjustment

Gtk.Application

Gtk.ApplicationWindow

Gtk.Grid

Gtk.Label

Gtk.SpinButton