Handling User Input

In order to allow users to interact with your app, you will want to capture and handle user input. If you are developing a TV based app this will often be key input via a remote control.

Blits offers an intuitive and straightforward interface to handle key input in Components.

Focus

Before diving into the specifics of key handling, it is important to understand the basic concept of focus.

In a Blits app, there is always one Component that has the focus. By default, this will be the root Application component.

The component that has focus, is the one that is responsible for handling the user input at that moment.

For example, when a user clicks the right or left button while a Poster Component has focus, it is that instance of the Poster Component that will receive the first key press event.

Configuring Input Handling

Within the Component configuration object, the input key is used to define how the component should react to specific key presses when it has focus. The input key should be an object literal of functions for each input event that the component wants to handle.

Each function corresponds to a key press name, such as up, down, enter, space, back, 1, 2, a etc and each function defined in the input object, receives the full InputEvent object as its first argument.

export default Blits.Component('MyComponent', {
  // ...
  input: {
    up(e) {
      // Logic to execute when users press up
    },
    down(e) {
      // Logic to execute when users press down
    },
    enter(e) {
      // Logic to execute when users press enter
    },
  }
}

Catch-All Handling

To allow a focused component to respond to any key and act as a catch-all, you can add an any() function to the input object. As it receives the InputEvent object as the first argument, you can abstract the key press in there and handle (or ignore) it as you wish.

{
  input: {
    any(e) {
      // Logic to execute for any key press
    },
  }
}

Event Handling Chain

If the currently focused component does not handle a key press, Blits will traverse up the component hierarchy, checking for any parent component that does have a function defined for that key press in the input-key. This input event handling chain continues until it reaches the root Application component.

When a component handles a key press by having a corresponding function specified, said component receives focus, and the event handling chain stops by default. However, if you want the input event to propagate up the hierarchy further, you can move the focus to the parent element and pass the InputEvent object on in that function call.

{
  input: {
    enter() {
      // Give focus to the parent
      this.parent.focus();
    },
    back(e) {
      // Give focus to the parent and let the user input event bubble
      this.parent.focus(e);
    },
  }
}

Key-up handling

The functions specified in the input configuration are invoked when a key is pressed down (i.e. the keydown event listener). But sometimes you may also want to execute some logic when a key is released (i.e. the keyup event listener).

Instead of introducing a separate key on the Component configuration object for key release callbacks, Blits relies on the concept that a keyup event is always preceeded by a keydown event.

Following this logic, whenever you return a function in an input (key down) handler, this function will be executed upon release (i.e. the keyup event) of that key .

When an input key is being a hold down, it will execute the key down handler multipe times. Upon key release, only the last returned key up callback function will be executed.

Blits.Component('MyComponemnt', {
  //
  input: {
    enter() {
      // execute logic on key down
      this.pressedEnter = true
      return () => {
        // execute logic on key up
        this.pressedEnter = false
      }
    },
    space(e) {
      // not logic on key down
      return () => {
        // only execute logic on key up
        console.log('Space key released')
      }
    },
    left() {
      // some logic on key down here ..
      this.leftHold = true
      // return a reference to a Component method
      // (instead of creating a new function on the fly)
      return this.leftKeyUp
    }
  },
  methods: {
    leftKeyUp() {
      console.log('Left key up')
      this.leftHold = false
    }
  }
}

Custom Keycode mapping

Blits comes with a default keycode mapping. This mapping is a sensible default that works in your desktop browser and with most RDK based devices.

But it's possible that the keycodes and mapping of your target device are slightly or even completely different.

In Blits, you can easily configure the key mapping to match your needs. In the src/index.js file where we instantiate the App via the Blits.Launch function, we can add an extra key, called keymap, to the settings object.

The keymap should contain an object literal, where you map a key or keyCode (from the KeyboardEvent) to an event name that you can use in your Components.

You can use a site like keyjs.dev to find the appropriate key and keyCode for your device

// src/index.js
Blits.Launch(App, 'app', {
  w: 1920,
  h: 1080,
  //...
  keymap: {
    // switch left and right using the key
    ArrowLeft: 'right',
    ArrowRight: 'left',
    // switch up and down using the keyCode
    38: 'down',
    40: 'up',
    // register new handlers
    '.': 'dot', // dot() can now be used in the input object
    // key code for letter 's'
    83: 'search' // search() can now be used in the input object
  }
})

The custom keymap object will be merged with the default key mapping, that looks like this:

const defaultKeyMap = {
  ArrowLeft: 'left',
  ArrowRight: 'right',
  ArrowUp: 'up',
  ArrowDown: 'down',
  Enter: 'enter',
  ' ': 'space',
  Backspace: 'back',
  Escape: 'escape',
  37: 'left',
  39: 'right',
  38: 'up',
  40: 'down',
  13: 'enter',
  32: 'space',
  8: 'back',
  27: 'escape',
}