Scale (JavaScript)

A Scale is a horizontal or vertical slider, that represents a value inside a numerical range. When you create a new Scale, you set what its default position is, what the numbers at the top and bottom of the range are, and things like how much it moves up or down when you click on the Scale to either side of the knob. To keep from having to type all that in every time you create a new Scale, you can create an object called an Adjustment which keeps track of all that, then tell each new Scale to use that Adjustment.

This scale is a simple widget that lets you adjust the size of an iceberg that penguins live on. The number of penguins on the iceberg is the product of the values of the two sliders. Try playing with them and seeing what happens.

Zu importierende Bibliotheken


#!/usr/bin/gjs

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

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.

Entwurf des Anwendungsfensters


const ScaleExample = new Lang.Class({
    Name: 'Scale Example',

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

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

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

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

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


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

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

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 scales


        // Create the horizontal scale
        this._hScale = Gtk.Scale.new_with_range (Gtk.Orientation.HORIZONTAL, 0.0, 100.0, 5.0);
        this._hScale.set_valign (Gtk.Align.START);
        this._hScale.set_value (50);
        this._hScale.set_digits (0);
        // this._hScale.set_draw_value (false);

The new_with_range method is one way to create a new Scale widget. The parameters it takes are a Gtk.Orientation, the minimum value, the maximum value, and the increment for a single step. After that we use the Scale's methods to set its starting value, and how many decimal places it runs to. We also set its vertical alignment in this case, to control where it appears in the window.

We can use the set_draw_value method to tell it whether or not to show the number next to the sliding scale. It's commented out in this example.


        // Create a master adjustment to use for the vertical (or any other) scale
        this._adjustment = new Gtk.Adjustment ({
            value: 95,
            lower: 0,
            upper: 100,
            step_increment: 5,
            page_increment: 10 });

An Adjustment is an object we can use to simplify things when creating a new Scale. The Adjustment's "value" property is what the Scale's default value is, while "upper" and "lower" make the high and low ends of the numerical range. Meanwhile, the increment values show how much the slider moves when you do things like click on it.


        // Create a vertical scale using the adjustment we just made
        this._vScale = new Gtk.Scale ({
            orientation: Gtk.Orientation.VERTICAL,
            adjustment: this._adjustment,
            digits: 0,
            // draw_value: false,
            margin_left: 10 });

Here we create a new Scale object using _adjustment as its "adjustment" property. This is a great shortcut. We still have to tell it to round off the decimal place, though. Note that the draw_value property is commented out; this is how you tell it not to show the number next to the Scale when you're creating one this way.


        // Create the label that shows the product of the two values
        this._product = (this._hScale.get_value() * this._vScale.get_value());
        this._label = new Gtk.Label ({
            label: (String(this._product) + " penguins on the iceberg."),
            height_request: 200,
            width_request: 200,
            wrap: true});

        // Connect the two scales to functions which recalculate the label
        this._hScale.connect ("value-changed", Lang.bind (this, this._recalc));
        this._vScale.connect ("value-changed", Lang.bind (this, this._recalc));

We can use the get_value method to find out the numerical value a Scale is set at. We can then do whatever we want with it, including multiply the two Scales' values together and have a Label show us the product. We set the label's text to wrap around, because we're having it display a silly message too.

After we create the Label, we connect the two Scales' "value-changed" signals to _recalc, a function that will recalculate the number of penguins on the iceberg and come up with a new message.


        // Create a grid to arrange things in
        this._UIGrid = new Gtk.Grid ({
            halign: Gtk.Align.CENTER,
            valign: Gtk.Align.CENTER,
            margin_top: 20,
            margin_left: 20});

        // Attach everything to the grid
        this._UIGrid.attach (this._label, 0, 0, 1, 1);
        this._UIGrid.attach (this._hScale, 0, 1, 1, 1);
        this._UIGrid.attach (this._vScale, 1, 0, 1, 1);

Here we create a Grid to put everything in, then attach all our widgets to it. Note that here and on some of the widgets themselves we're using margins to keep things neatly spaced.


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

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

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

Function which handles the scales' values changing


    _recalc: function() {

        // Figure out what the product of the two scales' values is
        var product = (this._hScale.get_value() * this._vScale.get_value());

        // Create a blank comment line in case there isn't a silly comment to make
        var comment = "";

        // Make a silly comment based on the number of penguins
        if (product > 9000) {
            comment = "It's over 9000!";
        }
        else if (product < 1000 && product > 0) {
            comment = "They're getting lonely.";
        }
        else if (product == 0) {
            comment = "They're all gone ...";
        }
        else comment = "";

        // Set ._label's new text
        this._label.set_label (String (product) + " penguins on the iceberg. " + comment);

    }

});

Remember, we can get a Scale's value using its get_value method. Here we simply recalculate what the product of the two values is after one of the Scales is moved, add in a silly message depending on how many penguins are left, and change the wording on _label to show the new number and message.


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

Finally, we create a new instance of the finished ScaleExample 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 ScaleExample {

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

        // 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: "Birds on a Floe"});

        // Create the horizontal scale
        this._hScale = Gtk.Scale.new_with_range (Gtk.Orientation.HORIZONTAL, 0.0, 100.0, 5.0);
        this._hScale.set_valign (Gtk.Align.START);
        this._hScale.set_value (50);
        this._hScale.set_digits (0);
        // this._hScale.set_draw_value (false);

        // Create a master adjustment to use for the vertical (or any other) scale
        this._adjustment = new Gtk.Adjustment ({
            value: 95,
            lower: 0,
            upper: 100,
            step_increment: 5,
            page_increment: 10 });

        // Create a vertical scale using the adjustment we just made
        this._vScale = new Gtk.Scale ({
            orientation: Gtk.Orientation.VERTICAL,
            adjustment: this._adjustment,
            digits: 0,
            // draw_value: false,
            margin_left: 10 });

        // Create the label that shows the product of the two values
        this._product = (this._hScale.get_value() * this._vScale.get_value());
        this._label = new Gtk.Label ({
            label: (String(this._product) + " penguins on the iceberg."),
            height_request: 200,
            width_request: 200,
            wrap: true});

        // Connect the two scales to functions which recalculate the label
        this._hScale.connect ("value-changed", this._recalc.bind(this));
        this._vScale.connect ("value-changed", this._recalc.bind(this));

        // Create a grid to arrange things in
        this._UIGrid = new Gtk.Grid ({
            halign: Gtk.Align.CENTER,
            valign: Gtk.Align.CENTER,
            margin_top: 20,
            margin_left: 20});

        // Attach everything to the grid
        this._UIGrid.attach (this._label, 0, 0, 1, 1);
        this._UIGrid.attach (this._hScale, 0, 1, 1, 1);
        this._UIGrid.attach (this._vScale, 1, 0, 1, 1);

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

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

    _recalc() {

        // Figure out what the product of the two scales' values is
        var product = (this._hScale.get_value() * this._vScale.get_value());

        // Create a blank comment line in case there isn't a silly comment to make
        var comment = "";

        // Make a silly comment based on the number of penguins
        if (product > 9000) {
            comment = "It's over 9000!";
        }
        else if (product < 1000 && product > 0) {
            comment = "They're getting lonely.";
        }
        else if (product == 0) {
            comment = "They're all gone ...";
        }
        else comment = "";

        // Set ._label's new text
        this._label.set_label (String (product) + " penguins on the iceberg. " + comment);
    }
};

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

Weiterführende Dokumentation

Gtk.Adjustment

Gtk.Application

Gtk.ApplicationWindow

Gtk.Grid

Gtk.Label

Gtk.Scale