Calculator: make_widget and grid layout


The previous tutorial built a custom widget with derive(Widget), used layout(column) and used a message for event handling. This tutorial follows on from there, introducing the make_widget! macro, using layout(grid), and some more message-based event handling.

We use this to build a simple but fully functional calculator, including the expected keyboard bindings, in under 200 lines of code.

The make_widget macro

If you thought in the last tutorial that having to derive a whole custom widget just for simple layout and event handling was a bit much, you're not alone. The make_widget! macro makes this much easier, but does have some rough edges.

Looking back at our Counter widget, there are a few things which are either "boring details of a widget" or redundant:

  • #[derive(Debug, Widget)]: all widgets must have these derives
  • the #[widget_core] and #[layout_data] fields
  • declaring the field types, then constructing in a separate function
  • having to write the type when this could be deduced
  • having to name a field we never access

The make_widget! macro addresses all of the above. But be warned, Rust does not support type-inference on fields so the macro has to emulate this with generics, and that has its rough edges (including atrocious error messages).

Enough talk, lets see it in action.


A calculator needs a lot of buttons, and we'll use the grid layout with make_widget:

fn main() {
let buttons = make_widget! {
    #[handler(msg = Key)]
    struct {
        #[widget(col = 0, row = 0)]
        _ = TextButton::new_msg("&clear", Key::Clear).with_keys(&[VK::Delete]),
        #[widget(col = 1, row = 0)]
        _ = TextButton::new_msg("&÷", Key::Divide).with_keys(&[VK::Slash]),
        #[widget(col = 2, row = 0)]
        _ = TextButton::new_msg("&×", Key::Multiply).with_keys(&[VK::Asterisk]),
        #[widget(col = 3, row = 0)]
        _ = TextButton::new_msg("&−", Key::Subtract),
        #[widget(col = 0, row = 1)]
        _ = TextButton::new_msg("&7", Key::Char('7')),
        #[widget(col = 1, row = 1)]
        _ = TextButton::new_msg("&8", Key::Char('8')),
        #[widget(col = 2, row = 1)]
        _ = TextButton::new_msg("&9", Key::Char('9')),
        #[widget(col = 3, row = 1, rspan = 2)]
        _ = TextButton::new_msg("&+", Key::Add),
        #[widget(col = 0, row = 2)]
        _ = TextButton::new_msg("&4", Key::Char('4')),
        #[widget(col = 1, row = 2)]
        _ = TextButton::new_msg("&5", Key::Char('5')),
        #[widget(col = 2, row = 2)]
        _ = TextButton::new_msg("&6", Key::Char('6')),
        #[widget(col = 0, row = 3)]
        _ = TextButton::new_msg("&1", Key::Char('1')),
        #[widget(col = 1, row = 3)]
        _ = TextButton::new_msg("&2", Key::Char('2')),
        #[widget(col = 2, row = 3)]
        _ = TextButton::new_msg("&3", Key::Char('3')),
        #[widget(col = 3, row = 3, rspan = 2)]
        _ = TextButton::new_msg("&=", Key::Equals).with_keys(&[VK::Return, VK::NumpadEnter]),
        #[widget(col = 0, row = 4, cspan = 2)]
        _ = TextButton::new_msg("&0", Key::Char('0')),
        #[widget(col = 2, row = 4)]
        _ = TextButton::new_msg("&.", Key::Char('.')),
    impl kas::WidgetConfig {
        fn configure(&mut self, mgr: &mut Manager) {
            // Enable key bindings without Alt held:

Here, you see, we didn't need to use #[derive(Debug, Widget)] since the make_widget macro does it for us. We also didn't need to mention the core or layout-data fields. We didn't bother naming those buttons or even explicitly typing them (naming and typing are optional). Also notice that the struct type is unnamed, and similarly the following impl block doesn't name type.

Grid layout

This is pretty simple to use: #[layout(grid)], and specify col and row in the #[widget] attribute. The number of rows and columns is auto-detected. Both row and col are optional, with the default value 0.

In a few cases we see spans: e.g. cspan = 2 implies that two columns are spanned, starting from the column selected by col. Both rspan and cspan default to 1 if not specified.

Key bindings

Like several other GUI toolkits, KAS allows shortcut keys to be specified through the label, e.g. &clear binds the C key. (Usually this method of binding is only seen in menus, but in KAS it can be used on most "activatable" controls.) TextButton allows additional keys can be specified through TextButton::with_keys.

Both the above types of bindings are usually only accessible while the Alt key is held — the difference being that those specified through a label also underline the letter/symbol following the &.

In a calculator, it is desirable that these keys are accessible without Alt held. Manager::enable_alt_bypass enables this (for the whole window). We could potentially call this method elsewhere, e.g. in an event handler, but we do so in the WidgetConfig::configure method. (Notice how we used #[widget(config=noauto)] to opt out of deriving WidgetConfig).

The Key message

One last thing to notice from the above snippet is that each button returns some variant of the Key enum as its message when pressed. We should go ahead and define this:

fn main() {
#[derive(Clone, Debug, VoidMsg)]
enum Key {

The only thing of note here is that Key uses derive(VoidMsg): this is a macro which implements From<VoidMsg> for the type. All message types should do this.

Building our calculator

Now, lets put our buttons in a calculator:

fn main() -> Result<(), kas_wgpu::Error> {

    let buttons = /* snip */;
    let content = make_widget! {
        #[handler(msg = VoidMsg)]
        struct {
            #[widget] display: impl HasString = EditBox::new("0").editable(false).multi_line(true),
            #[widget(handler = handle_button)] buttons -> Key = buttons,
            calc: Calculator = Calculator::new(),
        impl {
            fn handle_button(&mut self, mgr: &mut Manager, msg: Key) -> Response<VoidMsg> {
                if self.calc.handle(msg) {
                    *mgr |= self.display.set_string(self.calc.display());
    let window = Window::new("Calculator", content);

    let theme = kas_theme::ShadedTheme::new().with_font_size(16.0);

By now, most of this code should be clear enough, but a few things are worth mentioning:

  • We use an EditBox instead of a Label for our display
  • Instead of giving display an explicit type, we write display: impl HasString. This mirrors Rust's "argument position impl trait" (APIT) syntax, and just means that display has some unnamed type which implements HasString.
  • buttons -> Key is a different kind of type-restriction, only usable on widgets, which says that the type is some widget with message type Key

The calculator logic itself is pushed into the Calculator struct, with interaction limited to new, handle(msg) and display methods. Implementing this is left as an exercise to the user (you have 106 lines left to fit within our arbitrary goal of "under 200 lines").


The full code for our example can be found here. Run it with:

cargo run --example calculator

We introduced the grid layout, the make_widget macro and implemented a significantly more complex app than the previous tutorial's counter.

This tutorial series has now introduced all three of KAS's macros, but without refering to an API reference. If you've been using Rust (or other programming languages) for a while you probably know why that is: there isn't a standard way to document macros. What there is is the kas::macros module documentation, but, like most macro documentation, it is more "by example" than a true reference.