Also allow user to override layoutConstants in
BaseMessageCollectionViewCell and allow user to specify
VerticalAlignment of avatar: Top, Bottom or Center
-This commit exposes messageViewModel, createBubbleView() and init
functions of BaseMessageCollectionViewCell class to other modules by
changing their access modifiers to ‘public’.
-This also requires to change the access level of createBubbleView()
function of TextMessageCollectionViewCell and
PhotoMessageCollectionViewCell to be made ‘public’
Zhao Wang
Barış Çamur
Diego Sanchez