Auto casting is a property of an ability that allows it to be casted automatically without the unit's controller needing to activate it.
Enabling Auto Casting
To enable auto cast on an Ability simply ensure that the Auto Cast flag is checked under Stats - Flags. To have auto casting On by default, make sure that the Auto Cast On flag is checked as well.
In addition to filtering that already exists for your ability, you can add more filtering for auto casting so that the ability is not casted needlessly. There are a few properties that aid in this:
Acquire level is something that is important to pay attention to. There are 4 different acquire levels:
This will specify the minimum level the unit must be in for the ability to be auto casted. Most units default to the offensive state when not doing anything.
Filters specify what kinds of (properties of) units the auto cast will target. For instance, you can exclude invulnerable units if your auto casted ability only does damage since casting the ability will not damage the unit.
The range at which the unit will attempt to cast the Ability. This can be longer than your abilities' range.
You can validate the state of the caster or target. You can use this to ensure that no more important actions are already taking place for the unit.