android – Custom ViewGroup focus handling-ThrowExceptions

Exception or error:

Let’s say I have a custom ViewGroup which is focusable and has some child views which are focusable as well (A custom vertical menu for Android set-top-boxes which should react on remote controller).

I need to pass a focus to some of the child views whenever the custom ViewGroup gains the focus.

I set descendantFocusability to beforeDescendants and set OnFocusChangeListener to the custom ViewGroup but OnFocusChangeListener.onFocusChanged()is never called. It looks like beforeDescendants does not work as I expected. Actually setting the beforeDescendants works the same as setting the afterDescendants – The focus is taken by the nearest child view and the custom ViewGroup does not have an opportunity to decide which child view should take the focus.

How can I achieve the desired behavior? What are the best practices to handling focuses in ViewGroups?

How to solve:

After digging in Android sources I understand how to dispatch focus to ViewGroup‘s children. For example I have custom ViewGroup with few EditText fields inside. When the user clicks on ViewGroup (outside of each EditText) I want to dispatch focus to one of fields.

To do this we need to set descendantFocusability to FOCUS_AFTER_DESCENDANTS. It means that we can handle focus event before our custom view will try to focus itself:

setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);

Next step is to override onRequestFocusInDescendants method, it will be invoked before custom view’s requestFocus if flag FOCUS_AFTER_DESCENDANTS was set. In this method we can request child focus:

@Override
protected boolean onRequestFocusInDescendants(final int dir, final Rect rect) {
    return getChildAt(this.target).requestFocus();
}

One important thing with this method: if we return true here, our custom view will not be requested to focus (only with FOCUS_AFTER_DESCENDANTS).

In such a manner we can change target field to change which child will be focused when ViewGroup is clicked.


For better understanding you can find requestFocus method in ViewGroup in Android sources.

###

I have the similar problem. In our android TV application we wanna select the last focused position of our custom menu (it is custom vertical LinearLayout).

Your should override method addFocusables(views: ArrayList<View>, direction: Int, focusableMode: Int) . This method is called when ViewRootImpl searches all available focusable views. Flags FOCUS_AFTER_DESCENDANTS and FOCUS_BEFORE_DESCENDANTS are only determine position of insertion parent view into focusable views. In case of FOCUS_BEFORE_DESCENDANTS your custom view will be added before your child. In case of FOCUS_AFTER_DESCENDANTS your custom view will be added after your child.

The logic of overriding is to add only your custom view in the list, when your didn’t have focused child before.

override fun addFocusables(views: ArrayList<View>, direction: Int, focusableMode: Int) {
    // if we did have focused child before, we must add only parent view
    // otherwise our child already has focus, and we don't wont to change focus behavior
    if (focusedChild == null) {
        views.add(this)
    } else {
        super.addFocusables(views, direction, focusableMode)
    }
}

You also must override requestFocus(direction: Int, previouslyFocusedRect: Rect) because you mast pass focus into you child or decide that your cannot do it.

override fun requestFocus(direction: Int, previouslyFocusedRect: Rect): Boolean = 
        getViewToFocus()?.requestFocus() ?: false

In this example getViewToFocus() returns the child which I want to be focused or null if we don’t have any child

private fun getViewToFocus() =
        when {
            lastSelectedPos in 0..childCount -> getChildAt(lastSelectedPos)
            isNotEmpty() -> getChildAt(0)
            else -> null
        }

I use Kotlin in examples, but your can use java as well. Enjoy!

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