Building Cost System

The building cost system checks construction requirements and consumes resources after a building is confirmed.

The plugin does not restrict resource types. Inventory items, currency, character attributes, and other resources can all be integrated through a custom Building Cost Evaluator.

Workflow

Read the Cost Set
→ Check costs
→ Generate UI information
→ Revalidate on the server
→ Consume resources
→ Create the building

Client-side evaluation is used only for UI display and local pre-checks. Final validation and resource consumption should be performed by the server.

Cost Set

A Building Cost Set is the cost configuration asset used by a building. It can contain multiple Cost Entry entries.

For example:

Cost	Amount
Wood	20
Stone	10
Gold	100

After creating it, assign it to the Cost Set property of the corresponding Building Data Asset.

When no Cost Set is configured, the plugin does not perform cost validation. This is suitable for Creative Mode, administrator building, or projects that handle costs through their own systems.

Cost Entry

Each Cost Entry represents one cost requirement.

Field	Description
Evaluator Class	The Evaluator responsible for processing the current cost.
Target Data	The resource, item, or other target data.
Amount	The quantity to validate and consume.

The same Evaluator can use Target Data to process multiple targets, such as Wood, Stone, and Iron. A separate class is not required for every resource type.

Evaluator

A Building Cost Evaluator integrates the plugin with the project's resource systems. Its primary responsibilities include:

Operation	Description
Can Afford	Checks whether the current resources satisfy the requirement.
Consume	Consumes resources after the building is confirmed.
Evaluate for UI	Generates UI display information and failure reasons.

Projects can create separate Evaluators for inventories, currencies, character attributes, or storage systems.

Default Evaluators

Default Evaluators can be registered under:

Edit → Project Settings → Building System → Cost

Each rule contains:

Field	Description
Cost Type Tag	The cost type.
Evaluator Class	The default Evaluator associated with that type.

For example:

Cost Type Tag	Evaluator Class
Building.Cost.Resource	BP_ResourceCostEvaluator
Building.Cost.Currency	BP_CurrencyCostEvaluator
Building.Cost.Item	BP_ItemCostEvaluator

When a Cost Entry does not directly specify an Evaluator, the system can find the default implementation based on its cost type.

For other settings, see Plugin Settings.

Cost Context

When a cost check is performed, the Evaluator receives the current building context, such as:

• Player Controller
• Current world
• Building Data
• Build Component

The Evaluator uses this information to locate the player's inventory, currency, or other resource data.

Note

The plugin does not require resource components to be stored on the Pawn, Player State, or any other specific object.

The Evaluator determines how the required resource data is obtained.

UI Evaluation

Evaluate for UI can generate:

• Current amount
• Required amount
• Cost display text
• Whether the requirement is satisfied
• Failure reason

For example:

Wood 12 / 20
Stone 30 / 10

UI evaluation must not consume resources.

Multiplayer

When a building is confirmed, the server should process the request in the following order:

Recheck costs
→ Validate placement
→ Consume resources
→ Create the building

The client cannot make the final decision about whether the player has sufficient resources and cannot perform final resource consumption.

Resources should not be consumed if building creation fails.

Custom Evaluators

A custom Evaluator usually needs to:

1. Obtain the player from the Cost Context.
2. Read the target and amount from the Cost Entry.
3. Query the project's resource system.
4. Return the validation result and failure reason.
5. Generate display information for the UI.
6. Consume resources during the server confirmation stage.

After implementation, the Evaluator can be assigned directly to a Cost Entry or registered as a default Evaluator.

Example

The following example uses a custom Player State to validate and deduct gold.

Can Afford

Consume

Evaluate for UI

Troubleshooting

Issue	What to Check
Resources are always reported as insufficient	Check whether the Evaluator correctly obtains the player and resource data.
Evaluator cannot be found	Check Evaluator Class and the default Evaluator rules.
Costs are not displayed in the UI	Check Evaluate for UI.
Resources are not consumed after construction	Check whether the server executes Consume.
Resources are consumed more than once	Ensure that Consume is executed only once during server confirmation.
Resources are consumed even though construction fails	Check the execution order of validation, consumption, and building creation.