The Observer Block is currently in development and will be introduced to Minecraft Pocket Edition in the 0.15.0 update. This is an entirely new feature in Minecraft. And there is no word on when it will be available for other platforms of Minecraft. So far, the Observer is Minecraft PE exclusive.
The Observer block is a Block Update Detector or BUD block that will be used to activate Pistons if a block near it is changed.
For example, if a block near the Observer is set on Fire, the Piston will get activated.
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