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Carousel with MotionLayout

Carousel is a motion helper object to easily build custom "carousel" views – showing a list of elements that a user can skim through. Compared to other solutions to implement such views, this helper lets you quickly create complex motion and dimension changes for your carousel by taking advantage of MotionLayout.

The Carousel widget supports lists with a start and end as well as circular wrap-around lists.

Let's imagine that we want to build a simple horizontal carousel view, with a centered view enlarged:

Our basic layout contains several views, representing our carousel items:

Create a MotionLayout with three states (make sure to give them Ids):

  • previous
  • start
  • next

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.

For example let's imagine we have 5 views: A, B, C, D, E in the start state, with B, C, D visible and A and E outside of the screen. We want to set up the previous state such that the positions of A, B, C, D now are where B, C, D, E were, with the views moving from left to right. In the next state, the opposite needs to happen, with B, C, D, E moving to where A, B, C, D were, and the views moving from right to left.

What is critical is that the views end up exactly where the original views started; the way the carousel give the illusion of an infinite collection of elements is by moving the actual views back to where they were, but reinitializing them with the new matching content. The following diagram shows this mechanism (pay attention to the "item #" values):

Transitions

With those three ConstraintSets defined in your motion scene file, create two transitions – forward and backward – between the start and next, and start and previous. Let's add an OnSwipe handler to trigger the transitions via a gesture. For example:

    <Transition
        motion:constraintSetStart="@id/start"
        motion:constraintSetEnd="@+id/next"
        motion:duration="1000"
        android:id="@+id/forward">
        <OnSwipe
            motion:dragDirection="dragLeft"
            motion:touchAnchorSide="left" />
    </Transition>

    <Transition
        motion:constraintSetStart="@+id/start"
        motion:constraintSetEnd="@+id/previous"
        android:id="@+id/backward">
        <OnSwipe
            motion:dragDirection="dragRight"
            motion:touchAnchorSide="right" />
    </Transition>

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, C
  • 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

For example, you'd have something like this in your layout XML file:

    <androidx.constraintlayout.motion.widget.MotionLayout ... >

        <ImageView  android:id="@+id/imageView0" .. />
        <ImageView  android:id="@+id/imageView1" .. />
        <ImageView  android:id="@+id/imageView2" .. />
        <ImageView  android:id="@+id/imageView3" .. />
        <ImageView  android:id="@+id/imageView4" .. />

        <androidx.constraintlayout.helper.widget.Carousel
            android:id="@+id/carousel"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:carousel_forwardTransition="@+id/forward"
            app:carousel_backwardTransition="@+id/backward"
            app:carousel_previousState="@+id/previous"
            app:carousel_nextState="@+id/next"
            app:carousel_infinite="true"
            app:carousel_firstView="@+id/imageView2"
            app:constraint_referenced_ids="imageView0,imageView1,imageView2,imageView3,imageView4" />

    </androidx.constraintlayout.motion.widget.MotionLayout>

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

Kotlin

carousel.setAdapter(object : Carousel.Adapter {
            override fun count(): Int {
              // need to return the number of items we have in the carousel
            }

            override fun populate(view: View, index: Int) {
                // need to implement this to populate the view at the given index
            }

            override fun onNewItem(index: Int) {
                // called when an item is set
            }
        })

Java

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 in which the Carousel helper instead marks those views as View.GONE. This mode can be set using the following property:

app:carousel_emptyViewsBehavior="gone"

Examples

For more examples using the Carousel helper, see the example projects on GitHub.