Callbacks

A callback is a member that carries an invocation from one place to another. It’s declared with the callback keyword, invoked by calling it, and reacted to by a handler set with the => arrow.[sls.callback.def]

export component Example inherits Window {
    callback hello;
    TouchArea {
        clicked => { root.hello(); }
    }
}

slint

Declaration

A callback declaration adds a callback to the element in whose body it appears. It consists of the keyword callback, a name, and ;.[sls.callback.decl.form]

A callback shall only be declared on the root element, so that every callback of a component is one the application implements.[sls.callback.decl.root-only]

A callback shall have neither parameters nor a return value, so its declaration carries neither a parameter list nor a -> return type.[sls.callback.decl.no-params]

A callback shall not be declared pure, and shall not be declared as an alias.[sls.callback.decl.plain]

A callback name shall not collide with another member of the same element: a declaration whose name matches an existing property, function, or callback is an error.[sls.callback.decl.unique]

Calling

A callback is invoked with call syntax: the callback, followed by parentheses holding the arguments.[sls.callback.call.form]

Invoking a callback runs its handler. A callback without a handler does nothing when invoked.[sls.callback.call.handler]

export component Example inherits Window {
    callback acknowledged;
    TouchArea {
        clicked => { root.acknowledged(); }
    }
}

slint

Handlers

A handler reacts to a callback’s invocation. It’s written as the callback’s name, =>, and either a single expression or a code block.[sls.callback.handler.form]

A callback has at most one handler; setting a second handler for the same callback is an error.[sls.callback.handler.one]

A handler may be set on a callback of a child element, such as TouchArea’s clicked.[sls.callback.handler.target]

A handler body shall consist of callback invocations and nothing else.[sls.callback.handler.body]

A handler shall not be set on a callback declared on the root element: the application implements those.[sls.callback.handler.not-root]

Each invocation runs the handler of the callback, so a cycle of handlers, each invoking the next, is an error.[sls.callback.handler.no-cycle]

export component Example inherits Window {
    callback first;
    callback second;
    TouchArea {
        clicked => {
            root.first();
            root.second();
        }
    }
}

slint