Trait awarded on advancement

[boru]

I created a custom trait called Swamp Savvy. I want to apply it to goblins when they advance from level 0 to 1.

Here is the trait WML. It works when I apply it to units directly.

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#define TRAIT_SWAMPSAVVY
    # Units with trait Swamp Savvy move quicker and have better defense in swamp terrain.
    [trait]
        id=swamp_savvy
        availability="any"
        male_name= _ "swamp savvy"
        female_name= _ "female^swamp savvy"
        description= _ "Better defense and quicker in swamp"
        [effect]
            apply_to=movement_costs
            replace=true
            [movement_costs]
				swamp=2
            [/movement_costs]
        [/effect]
        [effect]
            apply_to=defense
            replace=true
            [defense]
				swamp=50
            [/defense]
        [/effect]
    [/trait]
#enddef

The part I'm tripping over is, how to apply this trait to every unit that levels up. I tried an event but it doesn't work.

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    [event]
        name=post advance
        first_time_only=no
        [filter]
            type=Goblin Spearman
        [/filter]
        {MODIFY_UNIT () trait (id=swamp_savvy)}
    [/event]

I tried a few permutations of this before concluding the proper way of doing really could be miles away from what I'm trying. Thanks in advance for any help.

[zookeeper]

This seems to work for adding healthy to all advancing units, so just change the trait and add whatever event filter you want:

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    [event]
        name=advance
        first_time_only=no

        [set_variables]
            name=existing_traits.trait
            to_variable=unit.modifications.trait
        [/set_variables]

        [set_variables]
            name=new_traits

            [value]
                {TRAIT_HEALTHY}
            [/value]
        [/set_variables]

        [set_variables]
            name=unit.modifications.trait

            [insert_tag]
                name=value
                variable=existing_traits.trait
            [/insert_tag]

            [insert_tag]
                name=value
                variable=new_traits.trait
            [/insert_tag]
        [/set_variables]

        [unstore_unit]
            variable=unit
            find_vacant=no
            advance=no
        [/unstore_unit]

        {CLEAR_VARIABLE existing_traits,new_traits}
    [/event]

You'll want to use an advance event instead of post advance to ensure that the unit stats update correctly during leveling. If you used post advance here, the healthy trait would show up on the unit just fine but it wouldn't actually have any effect, since direct variable manipulation like that doesn't cause the units max hitpoints to actually be re-calculated.

[boru]

Thanks, that did it!

I had post advance in there for several reasons, the main one was that I had tried originally with advance and knew that wasn't working, also because I was testing by advancing units with the debugger and wasn't certain that the combination of :unit advances=1 and [event] name=advance might be at odds. But I'm unfamiliar with the [insert_tag] method and it never would have occurred to me as a way to award a trait. Thanks very much!

EDIT: Just wanted to mention, if anyone else wants to use this code, don't forget to add a filter for the intended unit, or you'll be awarding the trait to every unit that advances.

[boru]

Okay, I'm further on in my campaign and now I want my leader to recruit some level 1 units, instead of zeroes all the time. These units ought to gain the swamp savvy trait on recruitment.

I thought I could use the same basic code as above, with a minor variation, but what happens is, the name of the trait is inserted in the sidebar description, but the trait itself does not work.

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#define SS_TRAIT_ON_RECRUITMENT TYPE
# swamp savvy trait given to level 1 units:

    [event]
        name=recruit
        first_time_only=no
 
		[filter]
			type={TYPE}
		[/filter]

        [set_variables]
            name=existing_traits.trait
            to_variable=unit.modifications.trait
        [/set_variables]

        [set_variables]
            name=new_traits

            [value]
                {TRAIT_SWAMPSAVVY}
            [/value]
        [/set_variables]

        [set_variables]
            name=unit.modifications.trait

            [insert_tag]
                name=value
                variable=existing_traits.trait
            [/insert_tag]

            [insert_tag]
                name=value
                variable=new_traits.trait
            [/insert_tag]
        [/set_variables]

        [unstore_unit]
            variable=unit
            find_vacant=no
            advance=no
        [/unstore_unit]

        {CLEAR_VARIABLE existing_traits,new_traits}
    [/event]
#enddef

I added the following line before the [/event] tag, which fixes it:

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{ADVANCE_UNIT x,y=$x1,$y1 {TYPE} }

however, the visual effect of a unit being recruited, then fading to white and having the XP bar advance is really weird. Is there a simpler, cleaner way to do this?

[zookeeper]

Okay, I'm further on in my campaign and now I want my leader to recruit some level 1 units, instead of zeroes all the time. These units ought to gain the swamp savvy trait on recruitment.

I thought I could use the same basic code as above, with a minor variation, but what happens is, the name of the trait is inserted in the sidebar description, but the trait itself does not work.

Try adding {CLEAR_VARIABLE unit.defense,unit.movement_costs} right before the [unstore_unit].

[boru]

You have emerged victorious!

Again, thanks.

[ravenyear]

I found another way to add a trait to a unit when it advances by triggering on the post_advance event.

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#define ADD_TRAIT_TO_UNIT_ON_EVENT TRAIT TRAIT_ID UNIT_TYPE EVENT_NAME
    [event]
        name={EVENT_NAME}
        first_time_only=no

        [filter]
            type={UNIT_TYPE}
        [/filter]

        [modify_unit]
            [filter]
                id=$unit.id
                # prevents adding trait again in case of AMLA where UNIT_TYPE advances to itself
                [not]
                    [filter_wml]
                        [modifications]
                            [trait]
                                id={TRAIT_ID}
                            [/trait]
                        [/modifications]
                    [/filter_wml]
                [/not]
            [/filter]

            # adds new trait to unit
            {TRAIT}
        [/modify_unit]
    [/event]
#enddef

Following the example that boru gave, here's how we can add the TRAIT_SWAMPSAVVY trait (assuming its trait id is swampsavvy) to a Goblin Impaler unit whenever:
1) a unit advances into a Goblin Impaler

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    {ADD_TRAIT_TO_UNIT_ON_EVENT {TRAIT_SWAMPSAVVY} swampsavvy (Goblin Impaler) post_advance}

2) a Golbin Impaler unit is recruited

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    {ADD_TRAIT_TO_UNIT_ON_EVENT {TRAIT_SWAMPSAVVY} swampsavvy (Goblin Impaler) recruit}

The reason snippet 1) triggers on the post_advance event instead of advance event is because when specifying [event]name=advance and [modify_unit] together, the advancement-choice dialog gets stuck in a loop and keeps on showing up. See issue: https://github.com/wesnoth/wesnoth/issues/5298