An ability that reads “Whenever [a creature] attacks, . . .” triggers if that creature is declared as an attacker. Similarly, “Whenever [a creature] attacks [a player, planeswalker, or battle], . . .” triggers if that creature is declared as an attacker attacking that player or permanent. Such abilities won’t trigger if a creature is put onto the battlefield attacking.
May 3, 2019 · War of the Spark
An ability that reads “Whenever [a creature] attacks, . . .” triggers if that creature is declared as an attacker. Similarly, “Whenever [a creature] attacks [a player or planeswalker], . . .” triggers if that creature is declared as an attacker attacking that player or planeswalker. Such abilities won’t trigger if a creature is put onto the battlefield attacking.
January 25, 2019 · Ravnica Allegiance
An ability that reads "Whenever [a creature] attacks, . . ." triggers if that creature is declared as an attacker. Similarly, "Whenever [a creature] attacks [a player or planeswalker], . . ." triggers if that creature is declared as an attacker attacking that player or planeswalker. Such abilities won't trigger if a creature is put onto the battlefield attacking.
August 25, 2017 · Commander 2017
An ability that reads “Whenever [a creature] attacks, . . .” triggers if that creature is declared as an attacker. Similarly, “Whenever [a creature] attacks [a player or planeswalker], . . .” triggers if that creature is declared as an attacker attacking that player or planeswalker. Such abilities won’t trigger if a creature is put onto the battlefield attacking.
September 30, 2016 · Kaladesh
If the effect that puts a creature onto the battlefield attacking specifies it’s attacking a certain player, and that player is no longer in the game when the effect resolves, the creature is put onto the battlefield but is never considered an attacking creature. The same is true if the effect specifies a creature is put onto the battlefield attacking a planeswalker and that planeswalker is no longer on the battlefield or is no longer a planeswalker when the effect resolves.