Input compatibility

On ChromeOS devices, many users interact with apps using a keyboard, mouse, trackpad, stylus, or gamepad. While these input devices are also used on Android phones, they are not as common and are often overlooked by developers.

Developers who want their app to work well with input on ChromeOS, and other large screen Android capable devices, should look at the following optimizations:

• Add and test basic keyboard support, such as keyboard navigation via arrow and tab keys, Enter key for confirming text entry, and Space for play/pause in media apps.
• Add standard keyboard shortcuts where applicable, for example ctrl+z for undo, ctrl+s for save.
• Test basic mouse interactions in the manner of right-click for context menu, icon changes on hover, and mouse wheel/trackpad scroll events on custom views.
• Test app-specific input devices such as stylus for drawing apps, game controllers for games, and MIDI controllers for music apps.
• Consider advanced input support that could make the app stand-out in desktop environments: touchpad as a cross-fader for DJ apps, mouse capture for games, and extensive keyboard shortcuts for power users.

Keyboard

The way your app responds to keyboard input contributes to a good desktop experience. There are three kinds of keyboard input: Navigation, Keystrokes, and Shortcuts.

Navigation

Keyboard navigation is rarely implemented in touch-centric apps, but users expect it when they are using an app and have their hands on a keyboard. It can also be essential for users with accessibility needs on both phones and desktop devices.

For many apps, simple arrow key and tab navigation is all that is needed and is mostly handled automatically by the Android framework. For example, a view of a Button is focusable by default, and keyboard navigation should generally work without any additional code. In order to enable keyboard navigation for views that are not focusable by default, developers should mark them as focusable. This can be done programmatically or in XML, as shown below. See the Focus Handling⁠⁠ documentation for more information.

yourView.isFocusable = true

Alternatively you can set the focusable attribute in your layout file:

android:focusable="true"

Once focus is enabled, the Android framework will create a navigational mapping for all focusable views based on their position. This usually works as expected and no further work is needed. When the default mapping is not correct for an app’s needs, it can be overridden as follows:

// Arrow keys
yourView.nextFocusLeftId = R.id.view_to_left
yourView.nextFocusRightId = R.id.view_to_right
yourView.nextFocusTopId = R.id.view_above
yourView.nextFocusBottomId = R.id.view_below

// Tab key
yourView.nextFocusForwardId = R.id.next_view

It is good practice to try to access every piece of your app’s functionality before each release using the keyboard only. It should be easy to access the most common actions without mouse of touch input.

Keystrokes

For text input that would be handled by an on-screen virtual keyboard (IME⁠⁠) such as an EditText, apps should behave as expected on ChromeOS with no additional work from the developer. For keystrokes that cannot be anticipated by the framework, apps will need to handle the behavior themselves. This is especially true for apps with custom views.

Some examples are chat apps that use the enter key to send a message, media apps that start/stop playback with the space key, and games that control movement with the w,a,s, and d keys.

Most apps override the onKeyUp⁠⁠ event and add the expected behavior for each received keycode, as shown below.

override fun onKeyUp(keyCode: Int, event: KeyEvent): Boolean {
    return when (keyCode) {
        KeyEvent.KEYCODE_ENTER -> {
            sendChatMessage()
            true
        }
        KeyEvent.KEYCODE_SPACE -> {
            playOrPauseMedia()
            true
        }
        else -> super.onKeyUp(keyCode, event)
    }
}

Using onKeyUp prevents apps from receiving multiple events if a key is held down or released slowly. Games and apps that expect users to hold down keyboard keys can look for the onKeyDown⁠⁠ event.

Depending on an app’s needs, overriding onKeyUp for the entire Activity usually provides the needed behavior. If desired, an onKeyListener⁠⁠ can be added to a specific view instead. For example an app may only listen for the Enter key in specific EditText, and not the Activity, in order to implement send functionality only when the user is typing in a chat box.

When you add keyboard support, follow the Android Keyboard handling documentation⁠⁠.

Shortcuts

Common ctrl, alt, and shift-based shortcuts are expected in desktop environments. If an app does not implement them, the experience can feel frustrating and broken to users. Advanced users also appreciate shortcuts for frequently used app-specific tasks. Shortcuts make an app easier to use and differentiate it from apps that don’t have shortcuts.

Some common shortcuts include save (ctrl+s), undo (ctrl+z), and redo (ctrl+shift+z). For an example of some more advanced shortcuts, see the list of VLC Media Player shortcut keys⁠⁠.

Shortcuts can be implemented using dispatchKeyShortcutEvent⁠⁠. This intercepts all meta-key combinations (alt, ctrl, and shift) for a given keycode. To check for a specific meta-key, use KeyEvent.isCtrlPressed()⁠⁠, KeyEvent.isShiftPressed()⁠⁠, KeyEvent.isAltPressed()⁠⁠, or KeyEvent.hasModifiers()⁠⁠.

Separating shortcut code from other keystroke handling (such as onKeyUp or onKeyDown) can make code maintenance easier and maintains the default acceptance of meta-keys without having to manually implement meta-key checks in every case. Allowing all meta-key combinations can also be more convenient for users who are accustomed to different keyboard layouts and operating systems.

override fun dispatchKeyShortcutEvent(event: KeyEvent): Boolean {
  return when (event.keyCode) {
    KeyEvent.KEYCODE_O -> {
      openFile() // Ctrl+O, Shift+O, Alt+O
      true
    }
    KeyEvent.KEYCODE_Z-> {
      if (event.isCtrlPressed) {
        if (event.isShiftPressed) {
          redoLastAction() // Ctrl+Shift+Z pressed
          true
        } else {
          undoLastAction() // Ctrl+Z pressed
          true
        }
      }
    }
    else -> {
      return super.dispatchKeyShortcutEvent(event)
    }
  }
}

You can also implement shortcuts in onKeyUp by checking for KeyEvent.isCtrlPressed()⁠⁠, KeyEvent.isShiftPressed()⁠⁠, or KeyEvent.isAltPressed()⁠⁠ in the same manner as above. This can be easier to maintain if the meta-behavior is more of a modification to an app behavior than a shortcut. For example, when w means “walk forward” and shift+w means “run forward”.

override fun onKeyUp(keyCode: Int, event: KeyEvent): Boolean {
  return when(keyCode) {
    KeyEvent.KEYCODE_W-> {
      if (event.isShiftPressed) {
        if (event.isCtrlPressed) {
          flyForward() // Ctrl+Shift+W pressed
          true
        } else {
          runForward() // Shift+W pressed
          true
        }
      } else {
        walkForward() // W pressed
      }
    }
    else -> super.onKeyUp(keyCode, event)
  }
}

Mouse and touchpad support

ChromeOS automatically handles most mouse and trackpad events so that they act like touch events on an Android phone. This includes two-finger touchpad/mouse wheel scrolling. Most apps generally only need to handle three desktop-centric events: Right-click, Hover, and Drag-and-Drop.

Right-click

Any actions that cause an app to show a context menu, like a long-press on a list item, should also react to right-click events. To handle right-click events, apps should register a View.OnContextClickListener⁠⁠. For details on constructing a context menu, see the Android context menu documentation⁠⁠

yourView.setOnContextClickListener {
  showContextMenu()
  true
}

Hover

Developers can make their app layouts feel polished and easier to use by handling hover events. This is especially true for custom views. The two most common examples of this are:

• Indicating to users if an element has interactive behavior, such as being clickable or editable, by changing the mouse pointer icon
• Adding visual feedback to items in a large list or grid when the pointer is hovering over them

// Change the icon to a "hand" pointer on hover,
// Highlight the view by changing the background.
yourView.setOnHoverListener { view, _ ->
  addVisualHighlighting(true)
  view.pointerIcon =
    PointerIcon.getSystemIcon(applicationContext,
    PointerIcon.TYPE_HAND)
  false // listener did not consume the event.
}

Drag-and-Drop

In a multi-window environment, users expect to be able to drag-and-drop items between apps. This is true for ChromeOS devices as well as tablets, phones, and foldables in split-screen mode.

Developers should consider whether users are likely to drag items into their app. Some common examples include: photo editors should expect to receive photos, audio players should expect to receive audio files, and drawing programs should expect to receive photos.

To add drag-and-drop support, follow the Android Drag-and-Drop documentation⁠⁠ and take a look at this ChromeOS blog post⁠⁠.

Special considerations for ChromeOS

• To handle files from the ChromeOS Files app, look for the MIME type application/x-arc-uri-list
• Remember to request permission via requestDragAndDropPermissions to access items dragged in from outside the app
• An item must have the View.DRAG_FLAG_GLOBAL flag in order to be dragged out to other applications

Multi-select support

If your app contains lists or grids, consider whether your users would benefit from support for multi-select. A high-quality multi-select experience with mouse and trackpad often includes features such as band selection. Implementing this on your own can be challenging, but you can leverage the Recyclerview Selection library⁠⁠.

Advanced pointer support

Apps that do advanced handling of mouse and touchpad input should follow the Android documentation for View.onGenericMotionEvent()⁠⁠ and use MotionEvent.getSource()⁠⁠ to distinguish between SOURCE_MOUSE⁠⁠ and SOURCE_TOUCHSCREEN⁠⁠.

Examine the MotionEvent to implement the required behavior:

• Movement generates ACTION_HOVER_MOVE events
• Buttons generate ACTION_BUTTON_PRESS and ACTION_BUTTON_RELEASE events. You can also check the current state of all mouse/trackpad buttons using getButtonState().
• Mouse wheel scrolling generates ACTION_SCROLL events

Stylus

Many Chromebooks come with a stylus, and Android apps handle that as touchscreen input. Some devices might also have a USB or bluetooth drawing table, like the Wacom Intuos⁠⁠. Android apps can receive bluetooth input, but won’t work with USB input.

A stylus event is reported as a touchscreen event via View.onTouchEvent()⁠⁠ or View.onGenericMotionEvent()⁠⁠, and contains a MotionEvent.getSource()⁠⁠ of type SOURCE_STYLUS⁠⁠. The MotionEvent will also contain additional data:

• MotionEvent.getToolType()⁠⁠ will return TOOL_TYPE_FINGER⁠⁠, TOOL_TYPE_STYLUS⁠⁠, or TOOL_TYPE_ERASER⁠⁠ depending on the tool that made contact with the surface
• MotionEvent.getPressure()⁠⁠ will report the physical pressure applied to the stylus pen, if supported
• MotionEvent.getAxisValue()⁠⁠ with MotionEvent.AXIS_TILT⁠⁠ and MotionEvent.AXIS_ORIENTATION⁠⁠ which can be used to read the physical tilt and orientation of the stylus, if supported

Historical points

Android batches input events and delivers them once per frame. A stylus pen can report events at much higher frequencies than the display. When creating drawing apps, it is important to check for events that may be in the recent past by using the getHistorical APIs:

• MotionEvent.getHistoricalX()
• MotionEvent.getHistoricalY()
• MotionEvent.getHistoricalPressure()
• MotionEvent.getHistoricalAxisValue()

Palm rejection

ChromeOS attempts to recognize when a user’s palm is resting on the touchscreen. However, this is not always possible. Sometimes a touch event might be reported to the app before the OS recognizes it as a palm. In that case, touches will be cancelled by reporting an ACTION_CANCEL event.

This event tells the app that certain touches are invalid and it should undo all interactions caused by those touches. For example, a drawing app might temporarily draw new lines as soon as they are received to provide the lowest latency, but only commit them permanently to the canvas once the touch series is finished cleanly. If touch events are cancelled in the meantime, the temporary lines can be easily erased.

Note-taking apps

ChromeOS has a special intent that surfaces registered note-taking apps to users. To register an app as a note-taking app, add the following to the Android manifest:

<intent-filter>
    <action android:name="org.chromium.arc.intent.action.CREATE_NOTE" />
    <category android:name="android.intent.category.DEFAULT" />
  </intent-filter>

When an app is registered, the user can select it as the default note-taking app. When a new note is requested, the app should create an empty note ready for stylus input. When the user wishes to annotate an image (such as a screenshot or downloaded image), the app launches with ClipData containing one or more items with content:// URIs. The app should create a note that uses the first attached image as a background image and enter a mode where the user can draw on the with a stylus.

Testing note-taking intents without a stylus

To test if an app responds correctly to note-taking intents without an active stylus, use the following method to display the note-taking options:

1. Switch to dev mode and make the device writable⁠⁠
2. Press ctrl+alt+f2 to open a terminal
3. Run the command sudo vi /etc/chrome_dev.conf
4. Press i to edit and add --ash-enable-palette to a new line at the end of the file
5. Save by pressing Esc and then typing :, w, q and pressing Enter
6. Press ctrl+alt+f1 to return to the regular ChromeOS UI
7. Log out and back in

There should now be a stylus menu in the shelf:

• Tap the stylus button in the shelf and choose New note. This should open a blank drawing note
• Take a screenshot. From shelf select stylus button > Capture screen or download an image. There should be the option to “Annotate image” in the notification. This should launch the app with the image ready to be annotated.

Game Controllers

Chromebooks support up to four game controllers. Developers should use the standard Android Game Controller APIs⁠⁠ to handle them.

Buttons are mapped to common values following a common mapping⁠⁠. Unfortunately, not all game controller manufacturers follow the same mapping conventions. You can provide a much better experience if you allow users to select different popular controller mappings.

Input translation mode

ChromeOS enables an input translation mode by default. For most Android apps this mode helps apps work as expected in a desktop environment. Some examples include automatically enabling two-finger scrolling on the touchpad, mouse wheel scrolling, and mapping raw display coordinates to window coordinates. Generally, app developers do not need to implement any of these behaviors themselves.

If an app implements custom input behavior, for example defining a custom two-finger touchpad pinch action, or these input translations do not provide the input events expected by the app, you can disable the input translation mode by adding the following tag to the Android manifest:

<uses-feature android:name="android.hardware.type.pc" 
              android:required="false" />