Observer

The Observer is a redstone-related block that will be added in Update 0.15.0.

Usage
The Observer block is a BUD (Block Update Detector) block that will send a short redstone pulse out the back of the block if the block it's facing updates (e.g. Air -> Cobblestone, untouced cake -> half cake, etc).

Trivia

 * BUD is a pre-existing concept in Minecraft, and Pistons, as well as other redstone blocks, will get activated if a block near them is updated.
 * For example, if a block nearby is set on Fire, the Piston will get activated.
 * The Observer Block will be added to Minecraft Pocket Edition  before PC.