Currently, I'm using Photoshop to design a mobile UI and I use PNG Express to generate assets for various size and devices. I want to know if there is any such supporting tool for Illustrator to generate assets if we design in it.
you can use this famous extension made for Illustrator.
PS: I recommend you to use a more appropriate tool to make UI. I suggest you Sketch App or keep using Photoshop. Exporting assets from illustrator is a bit harder. With Sketch you can export your assets easily and you can work with vectors and symbols.