Sketch + Origami
Origami lets you create links between layers in your prototype and layers in your Sketch file. Once they're linked up, hitting a keyboard shortcut will instantly update your prototype with all of the latest assets without interrupting it. You can even update assets with one hand while interacting with them in Origami Live with the other.
If you're a Photoshop user, we've got you covered too. Origami layers can also be linked to PSD files.