Carousel

Carousel

Carousel is a motion helper object to easily build carousels views.

Concept

Create a motion layout with 3 states (make sure to give them Ids)

• previous
• start
• next

In the layout, have a collection of views to represent your carousel items.

If the start state corresponds to that base layout, the previous state should be done in such a way that the carousel items will be shifted by one -- meaning that if we have 3 views A, B, C in the start state, in the previous state B should be put where A was, C where B was, and A can be pushed away. In the next state, we do the opposite, with A being set where B was, B being set where C was, and C being pushed aside.

Transitions

With those three constraintsets defined, we need to create two transitions -- forward and backward -- between the start and next, and start and previous. Typically here, for Carousel we would add an OnSwipe handler to trigger the transitions via a gesture.

Adding the Carousel

Once this basic motion scene is created, we only need to add a Carousel helper to the layout and references those views (in the same order we implemented our previous/next animation).

The Carousel helper also need a couple of attributes to be set up:

• app:carousel_firstView : the view that will represent the first element of the carousel, in our example, B
• app:carousel_previousState : the constraintset id of the previous state
• app:carousel_nextState : the constraintset id of the next state
• app:carousel_backwardTransition : the transition id applied between start -> previous
• app:carousel_forwardTransition : the transition id applied between start -> next

Finally, we also need to set up a Carousel adapter in code:

    carousel.setAdapter(new Carousel.Adapter() {
        @Override
        public int count() {
            // need to return the number of items we have in the carousel
        }

        @Override
        public void populate(View view, int index) {
            // need to implement this to populate the view at the given index
        }

        @Override
        public void onNewItem(int index) {
             // called when an item is set
        }
    });

Additional notes

Depending on the current item "selected" in the Carousel, the views representing the items before or after may need to be hidden in order to correctly account for the Carousel start and end. The Carousel helper will handle this automatically for you, by default marking those views as View.INVISIBLE in those situations (so the overall layout doesn't change). An alternative mode is available to that the Carousel helper instead marks those views as View.GONE.

    app:carousel_emptyViewsBehavior="gone"