I tried to set an onSwipe handler on a NestedScrollView to drive the animation in a MotionLayout. However the first time the animation doesn't work. But if I start the animation d, then the onSwipe handler begins to work as expected. Here is the relevant code for the NestedScrollView:
<androidx.core.widget.NestedScrollView
android:id="@+id/nsv_desc"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="160dp"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@id/slider_view"
android:clipToPadding="true"
android:background="?colorSurface">
...
<Constraint android:id="@+id/nsv_desc">
<Layout
motion:layout_constraintEnd_toEndOf="parent"
android:layout_width="0dp"
android:layout_height="0dp"
motion:layout_constraintTop_toBottomOf="@id/slider_view"
motion:layout_constraintStart_toStartOf="parent"
android:layout_marginTop="@dimen/margin_all_large"
motion:layout_constraintBottom_toBottomOf="parent"
android:layout_marginBottom="@dimen/margin_all_normal" />
<CustomAttribute
motion:attributeName="elevation"
motion:customDimension="0dp" />
</Constraint>
...
<OnSwipe
motion:touchAnchorId="@id/nsv_desc"
motion:dragDirection="dragUp"
motion:touchAnchorSide="top"/>
...
<KeyAttribute
motion:motionTarget="@id/nsv_desc"
motion:framePosition="25"
android:elevation="4dp"
android:translationY="40dp" />
EDIT: SOLUTION I fixed the bug. It turned out that it was very specific to my app. For anyone interested: suppose you have multiple ConstraintSets and transitions between them, like A -> B -> C -> D ecc. If you transition (with the onSwipe handler or programmatically) from A to B and reach the state B, then you have to call motionLayout.setTransition to specify that your motion layout must now apply the transition from B to C when you swipe on the widgets. If you call the transition from B to C programmatically it automatically applies that transition (that's the reason of the behavior described in the question).
