How do I focus an input with Cycle.js and RxJS?

How do I focus an input with Cycle? Do I need to reach inside the DOM and call .focus() either with or without jQuery, or is there some other way with Cycle/RxJS?

Yes, you do need to reach inside the DOM and call .focus() either with or without jQuery. However this is a side-effect and it is Cycle.js convention to move these kinds of side effects to a so-called driver.

The two questions the driver needs to know are:

• which element do you want to focus?
• when do you want to focus the element?

The answer to both questions can be provided by a single stream of DOM elements.

Create the driver

First make your driver. Let's call it SetFocus. We'll make it a so-called read-only driver. It will read from the app's sinks but it will not provide a source to the app. Because it is reading, the driver's function will need to accept a formal parameter that will be a stream, call it elem$:

function makeSetFocusDriver() {
  function SetFocusDriver(elem$) {
    elem$.subscribe(elem => {
      elem.focus();
    });
  }
  return SetFocusDriver;
}

This driver takes whatever DOM element arrives in the stream and calls .focus() on it.

Use the Driver

Add it to the list of drivers provided to the Cycle.run function:

Cycle.run(main, {
  DOM: makeDOMDriver('#app'),
  SetFocus: makeSetFocusDriver() // add a driver
});

Then in your main function:

function main({DOM}) {

  // setup some code to produce the elem$ stream
  // that will be read by the driver ...
  // [1]: say _when_ we want to focus, perhaps we need to focus when
  //      the user clicked somewhere, or maybe when some model value
  //      has changed
  // [2]: say _what_ we want to focus
  //      provide the textbox dom element as actual value to the stream
  //      the result is:
  //      |----o-----o-----o--->
  //      where each o indicates we want to focus the textfield
  //      with the class 'field'
  const textbox$ = DOM.select('.field').observable.flatMap(x => x); // [2]
  const focusNeeded = [
    clickingSomewhere$,    // [1]
    someKindofStateChange$ // [1]
  ];
  const focus$ = Observable.merge(...focusNeeded)
    .withLatestFrom(textbox$, (_, textbox) => textbox); // [2]

  // ...

  // [*]: Add driver to sinks, the driver reads from sinks.
  //      Cycle.js will call your driver function with the parameter
  //      `elem$` being supplied with the argument of `focus$`
  return {
    DOM: vtree$,
    SetFocus: focus$, // [*]
  };
}

You can then configure focusNeeded to say when you want .field to be focused.