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TIP: Animate Line Under Button

Sometimes a UI isn't what it seems
TIP: Animate Line Under Button

TIP: Animate Line Under Button

I got a question the other day about a UI design for an upcoming Codename One app. In this UI the title floated on top of the content as the content scrolled past and tabs were a part of the title area in the style of material design.
Our tabs component predated material design by many years and wasn’t designed for this sort of UI. When you scroll out of a tab it scrolls out. Using it for this sort of UI doesn’t make much sense. The solution was to use a custom title area with two toggle radio buttons representing each tab with a line below to indicate the selected button.

That’s pretty easy to do in Codename One, we can just style a toggle button with the right colors or border but the question is: "How do we animate the line motion".

Well…​ That’s actually really easy to accomplish once we decide the line isn’t a part of the button…​

This is a quick demo I made using maps and a floating toolbar and here is the annotated code:

public void init(Object context) {
    theme = UIManager.initFirstTheme("/theme");

    // Enable Toolbar on all Forms by default (1)

public void start() {
    if(current != null){;
    Form m = new Form("Map", new BorderLayout());
    Toolbar tb = new Toolbar(true); (2)
    m.add(BorderLayout.CENTER, new MapContainer("... PLACE A KEY HERE ...")); (3)

    Label title = new Label("Map", "Title");

    ButtonGroup bg = new ButtonGroup();
    RadioButton modeA = RadioButton.createToggle("Mode A", bg); (4)
    RadioButton modeB = RadioButton.createToggle("Mode B", bg);
    RadioButton modeC = RadioButton.createToggle("Mode C", bg);
    Container radioGrid = GridLayout.encloseIn(3, modeA, modeB, modeC);

    Label whiteLine = new Label(); (5)
    whiteLine.getUnselectedStyle().setPadding(1, 0, 1, 1);
    Container lineGrid = GridLayout.encloseIn(3, whiteLine, new Label(), new Label()); (6)
    bg.addActionListener(e -> { (7)
        int offset = radioGrid.getComponentIndex(e.getComponent());
        lineGrid.addComponent(offset, whiteLine);

    Container titleArea = BoxLayout.encloseY(title, radioGrid, lineGrid); (8)
1 I commented out the toolbar as I will use an overlay toolbar on top of the map
2 When creating a toolbar instance with true, it "floats" on top of the UI notice the transparency to the underlying map
3 The API key is required if you want to use the JavaScript version
4 The radio toggle buttons are pretty standard, I styled them to look the same in all modes
5 I could have used the style to do the "white line" but chose to do it in code. It’s just a white background padding
6 Both the button and the white line are in 1x3 grids which means the elements will inherently have the same size
7 I listen on the button group and when a radio is selected I just move the white line to the selected radio offset. Then animate the layout
8 Box Y gives components the same width so the grids both get the same width and will thus match in size

Check out the full code on github here:

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It’s small things like that and attention to detail that can transform your app into a loved app. If you See similar effects or don’t fully understand why the code above works just ask in the comments!

We’re here to help you make great looking apps.

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Posted by Shai Almog

Shai is the co-founder of Codename One. He's been a professional programmer for over 25 years. During that time he has worked with dozens of companies including Sun Microsystems.
For more follow Shai on Twitter & github.