Feature request: ViewTreeObserver

[jonasfa]

Add support for ViewTreeObserver listeners:

• onScroll
• onLayout
• onDraw
• etc

See: https://developer.android.com/reference/android/view/ViewTreeObserver.html

[JakeWharton]

What do you envision this looking like?

[jonasfa]

@OnScroll(R.id.scroll_view)
void syncFloatingView() {
}

instead of

scrollView.getViewTreeObserver().addOnScrollChangedListener(new ViewTreeObserver.OnScrollChangedListener() {
    public void onScrollChanged() {
    }
});

[jonasfa]

@OnLayout(R.id.bottom_button)
void updateGoogleMapBottomPadding() {
}

instead of

bottomButton.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
    public void onGlobalLayout() {
    }
});

[SimonVT]

The ids on the annotations are unnecessary, it's the same observer for the entire view tree. Not sure this is a good idea tho. E.g. for a scrollview you generally wouldn't want to be notified of any scroll. ViewTreeObserver is not that widely used, and adding annotations could encourage people to use it for the wrong reasons.

[jonasfa]

@SimonVT I agree it's not ideal to get all scrolling notifications. The problem is there isn't any other option as View/ScrollView doesn't provide a scroll listener. On the other hand, Layout notifications would be great to set up padding for layouts with floating views. Ie: to set a MapView bottomPadding to match a floating button's height. See: https://developers.google.com/maps/documentation/android/map#map_padding

[SimonVT]

It's better to subclass ScrollView and create your own lister than to use ViewTreeObserver. If you want to get notified of a View being laid out, there's View#OnLayoutChangeListener. There's not really a lot of reasons to use ViewTreeObserver in your average app.

[jonasfa]

Good point, but ButterKnife doesn't have an annotation for View.OnLayoutChangeListener yet. Should we open an issue to discuss it?

Also, View.OnLayoutChangeListener is unfortunately API 11+ only. I know Gingerbread is almost dead, but it's not dead yet. It's probably dead enough to keep using ViewTreeObserver manually for a litle longer though?