What happens if play "Imperial decree" then i move my units to a battlefield with enemy units, then i play "Cannon Barrage". will the enemy units die before units deal damage to each other causing my units to die too from the imperial decree?

Yes — the enemy units die during the Showdown, before combat damage is ever assigned, so your own units never get damaged and your Imperial Decree won't kill them.

Cannon Barrage is a Reaction, so you resolve it while the Combat Showdown is still open. Its 2 damage makes the enemy units "take damage," and Imperial Decree's trigger fires immediately and goes on the chain (FAQ #9789). Once those kill triggers resolve, all damaged defending units are gone. The Combat Damage Step only happens if both attacking and defending units remain at the battlefield (Rule 465.1), so with no defenders left no damage is dealt to your attackers — nothing damages them, so nothing triggers Imperial Decree against them.

Two caveats: the kill triggers sit on the chain, so your opponent can respond before they resolve. And if any enemy unit's damage is fully prevented (Counter Strike, Zhonya's Hourglass, etc.), that unit survives, combat damage is then dealt simultaneously, and any unit that takes damage — including your attacker — dies to Imperial Decree.