Attribute Modifier (Data Type)

An Object that describes how one number is changed. Attribute powers (apoli:attribute) and resource operations (apoli:modify_resource and its apoli:change_resource alias) both use it, as one data type for both.

One type for attributes and resources

Apace's Apoli has AttributeModifier for the apoli:attribute power and a separate Modifier for apoli:modify_resource, which differ only in whether the attribute field is required. Here they are one type with every field optional, and each power or action enforces the fields it needs.

Fields

Field	Type	Default	Description
`operation`	Attribute Modifier Operation	required	The operation this modifier performs.
`value`	Float or Expression	`0`	The modifier value. `amount` is accepted as an alias. An expression is evaluated each time the modifier is applied.
`attribute`	Identifier	optional	The vanilla attribute ID this modifier targets. apoli:attribute requires it, and apoli:modify_resource ignores it, since that targets a resource and not an attribute.
`name`	String	derived from the power ID	The vanilla AttributeModifier name, used to seed the modifier's UUID.
`resource`	Identifier	optional	When set, the value of this resource (or cooldown) power is used as the modifier value instead of `value`.
`position`	Integer or Expression	optional	Which slot of `resource` to read, when it names a table apoli:resource. Ignored without `resource`. `index` and `slot` are accepted as aliases.
`modifier`	Attribute Modifier	optional	A nested modifier applied to this modifier's own value before it acts, for example to scale a value that comes from a resource.

Where the input value comes from

When the modifier is applied to a current base or total, the value fed into the operation is worked out like this:

If resource is set, it starts from that resource's current value, or the value in slot position when one is given.
Otherwise, it evaluates value (a number or an expression).
If a nested modifier is set, it runs that modifier on the value from step 1 or 2 and uses the result.

This one rule covers both of Apace's separate forms, a resource as the value and a modifier of a modifier.

Inside a value expression, the value variable is the unmodified input the modifier chain is applied to: the current resource value in apoli:modify_resource, the attribute's base value in apoli:attribute, or the incoming amount in modify_* powers. So "value": "value * 0.5" with "operation": "set_total" halves whatever comes in. Every other expression variable (health, xp_level, resource IDs and so on) is available too.

Examples

Add 9 to a base value, inside apoli:modify_resource:

"modifier": { "operation": "add_base_early", "value": 9 }

Multiply the current value by 2 additively, in the Apace style:

"modifier": { "operation": "multiply_base_additive", "value": 2 }

Use another resource's value as the modifier value, so the input is the value of example:resource:

"modifier": {
    "operation": "add_base_early",
    "resource": "example:resource"
}

A resource's value, scaled down to 0.001× with a nested modifier, in the style of Apace's nested modifier example:

"modifier": {
    "operation": "add_base_early",
    "resource": "example:resource",
    "modifier": { "operation": "multiply_total_multiplicative", "value": -0.999 }
}

A vanilla attribute modifier, inside apoli:attribute:

"modifier": {
    "attribute": "minecraft:generic.attack_damage",
    "operation": "addition",
    "value": 9
}

An expression value, for a max health bonus that grows with the player's current XP level:

"modifier": {
    "attribute": "minecraft:generic.max_health",
    "operation": "addition",
    "value": "xp_level / 2"
}