1.4 KiB
Dynamic Constants Provider
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Usage Scenario
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A system has a large number of constants, but only a minor set will be used by any script.
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The system constants are expensive to load.
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The system constants set is too massive to push into a custom [
Scope]. -
The values of system constants are volatile and call-dependent.
Key Concepts
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Use a [variable resolver] to intercept variable access.
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Only load a variable when it is being used.
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Perform a lookup based on variable name, and provide the correct data value.
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May even perform back-end network access or look up the latest value from a database.
Implementation
let mut engine = Engine::new();
// Create shared data provider.
// Assume that SystemValuesProvider::get(&str) -> Option<value> gets a value.
let provider = Arc::new(SystemValuesProvider::new());
// Clone the shared provider
let db = provider.clone();
// Register a variable resolver.
// Move the shared provider into the closure.
engine.on_var(move |name, _, _, _| Ok(db.get(name).map(Dynamic::from)));
Values are Constants
All values provided by a [variable resolver] are constants due to their dynamic nature. They cannot be assigned to.
In order to change values in an external system, register a dedicated API for that purpose.