If you're just looking for a rectangular clipping mask, you can do that in the beta today. Just create a custom component, double-click to edit it, and then uncheck Modify > Auto Size Component Bounds. You'll see an indicator in the upper-right of the artboard that says "Clipping On." Now everything will clip to the bounds you set for the component. (We're working on making this easier to find!)
You also can fake fancier masks within the component by placing translucent gradients around the inside of the border, and ordering them higher in the Layers panel than the content that is going to move around and clip.
Hope that helps!
Hi awesome quick reply thank you very much!
Is there a way to adjust the manual clipping mask so that you can animate the image within the mask?
In other words, If I want to have a transition that only reveals the masked portion of the image (say, scrolling onto the page) how can I do that?
Currently, if I set the imported artwork as a custom componenet and adjust the clipping mask, when I get back to the primary artboard and apply a move transition it will animate the whole component including the mask. Is there a way to force the mask to stay stationary so I can create the desired animation?
If you go back into the custom component and apply the Move effect to content inside there, you'll get the behavior you want -- the content moves, and the mask (defined by the component's boundaries) stays put. If placing the Move effects inside the component is problematic for some reason, please let us know a bit more about the app you're creating and we can try to help more.
Thanks for the reply, I'm still working with the implementation but am hopeful it should provide just enough to achieve the effect. The only downside is being unable to animate the mask itself as that would open a world of possibilities.