Make a animated transition like this in material nodes

Question

I want to make a transition effect like this

Made with windows movie maker

excuse the poor choice of images as an example.

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I've tried gradient texture but can only slide it on one side, So far I have this

Answer 1

That can be something like this:

Get the X component of generated coordinates (or UV), recenter it between -1 and 1, then absolute to have symmetrical value.

Then animate (keyframe) the "greater than" math node and plug it as factor in a mix color node (using colors, or images as you like).