How to animate shared element in transition by moving the element in source activity to dest position?

Question

I have a shared element in the transition from activity A to activity B. The shared element in A, however, uses a TransitionDrawable, and sometimes I want to fire the transition of the drawable, using startTransition, as part of the transition. (The shared element in B looks like the end state of the transition.)

The shared element transition documentation states that the implementation moves the shared element in B to the screen position of the shared element in A. Then it will run the shared element enter transition.

Is it possible to move the element in A to the B element's position (letting me run startTransition via shared element callback) instead of the other way? P.S.: I know about shared element exit transitions, but I don't think that helps me, because that always runs before the activity transition starts, and I don't see a way to change that, in the docs.

Answer 1

I leveraged the shared element snapshots via SharedElementCallback. While it's fairly complicated overall, here's the portion of my code that handles taking the snapshot and recreating a TransitionDrawable using the snapshot as the bottom layer.

override fun onSharedElementEnd(sharedElementNames: MutableList<String>,
                                sharedElements: MutableList<View>,
                                sharedElementSnapshots: MutableList<View>?) {
    if (sharedElements.size < 1)
        return
    val shElem = sharedElements[0]
    if (shElem !is ImageView)
        return
    mainPhoto = shElem
    if (mainPhoto!!.drawable !is TransitionDrawable) {
        val layers = arrayOf(sharedElementSnapshots!![0].background, mainPhoto!!.drawable)
        if (exiting)
            layers.reverse()
        val transDrwbl = android.graphics.drawable.TransitionDrawable(layers)
        mainPhoto!!.setImageDrawable(transDrwbl)
    } else
        needToUseReverse = true
}

/**
 * I'm copying these from v21 SharedElementCallback.java because, while the
 * new functionality in v24 (v23?) seems nice, I don't want something unexpected
 * to happen on newer versions
 */
override fun onCaptureSharedElementSnapshot(sharedElement: View, viewToGlobalMatrix: Matrix,
                                            screenBounds: RectF): Parcelable {
    val mTempMatrix = Matrix(viewToGlobalMatrix)
    val transitionUtils = Class.forName("android.transition.TransitionUtils")
    val createViewBitmap = transitionUtils.getDeclaredMethod("createViewBitmap", View::class.java, Matrix::class.java, RectF::class.java)
    val pcbl = createViewBitmap.invoke(null, sharedElement, mTempMatrix, screenBounds) as Parcelable
    return pcbl
}

override fun onMapSharedElements(names: MutableList<String>, sharedElements: MutableMap<String, View>) {
    for (key in sharedElements.keys.toList())
        if (key != currentSharedElementName)
            sharedElements.remove(key)
}

override fun onCreateSnapshotView(context: Context, snapshot: Parcelable): View? {
    var view: View? = null
    if (snapshot is Bitmap) {
        view = View(context)
        val resources = context.resources
        view.background = BitmapDrawable(resources, snapshot)
    }
    return view
}