Callbacks

Components may declare callbacks, that communicate changes of state to the outside. Callbacks are invoked by “calling” them like you would call a function.

You react to callback invocation by declaring a handler using the => arrow syntax. The built-in TouchArea element declares a clicked callback, that’s invoked when the user touches the rectangular area covered by the element, or clicks into it with the mouse. In the example below, the invocation of that callback is forwarded to another custom callback (hello) by declaring a handler and invoking our custom callback:

export component Example inherits Rectangle {
    // declare a callback
    callback hello;

    area := TouchArea {
        // sets a handler with `=>`
        clicked => {
            // emit the callback
            root.hello()
        }
    }
}

It’s possible to add parameters to a callback:

export component Example inherits Rectangle {
    // declares a callback
    callback hello(int, string);
    hello(aa, bb) => { /* ... */ }
}

Callbacks may also return a value:

export component Example inherits Rectangle {
    // declares a callback with a return value
    callback hello(int, int) -> int;
    hello(aa, bb) => { aa + bb }
}

Aliases

It’s possible to declare callback aliases in a similar way to two-way bindings:

export component Example inherits Rectangle {
    callback clicked <=> area.clicked;
    area := TouchArea {}
}