Add read operator.
Add read operator.

Summary: This is a convenience operator for fetching a value from a stream instantaneously. We use this operator to do things like pull a value when a gesture recognizer begins (e.g. the translated operator).

Reviewers: O4 Material Apple platform reviewers, #material_motion, O2 Material Motion, appsforartists

Reviewed By: #material_motion, O2 Material Motion, appsforartists

Subscribers: appsforartists

Tags: #material_motion

Differential Revision: http://codereview.cc/D2529

material-motion/material-motion-swift,material-motion/material-motion-swift