In these video tutorials I explain what shape keys in Blender are and how to use them. Then I bring them to Unity and Unreal Engine 4. This is the first video in that I use shape keys in Blender 2.79 to create a transformation from a cube into a kind of low poly tree.
I export the object as fbx, so that I can import it to any game engine that supports shape keys.
The first game engine I use for this is Unity 2017. You can import shape keys with the fbx importer, the feature is called Blend Shapes in Unity:
I want to fade in the transformation when the game starts so I write a little C# script in which a timer logic is scripted to increase the value of the blend shape for the tree.
In Unreal Engine 4 the support for shape keys is called morph targets. The concept is quite similar, you have to import the fbx as skeletal mesh, no matter if you use bones or not, and also check the option Import with Morph targets:
Instead of writing a script I used an Actor Blueprint with a timeline node to control the morph targets on startup.
What you can also do to control the value of shape keys is to use drivers in Blender. As interface you can use for example a bone, that you move up and down or to the left or right to change the value of the shape key.
In this video I explained how to setup such a driver with Blender 2.79.