56 lines
1.4 KiB
Markdown
56 lines
1.4 KiB
Markdown
Dynamic Constants Provider
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==========================
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{{#include ../links.md}}
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```admonish info "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`].
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* The values of system constants are volatile and call-dependent.
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```
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```admonish abstract "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.
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```
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Implementation
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--------------
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```rust
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let mut engine = Engine::new();
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// Create shared data provider.
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// Assume that SystemValuesProvider::get(&str) -> Option<value> gets a value.
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let provider = Arc::new(SystemValuesProvider::new());
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// Clone the shared provider
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let db = provider.clone();
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// Register a variable resolver.
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// Move the shared provider into the closure.
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engine.on_var(move |name, _, _| Ok(db.get(name).map(Dynamic::from)));
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```
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```admonish note.small "Values are constants"
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All values provided by a [variable resolver] are _[constants]_ due to their dynamic nature.
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They cannot be assigned to.
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In order to change values in an external system, register a dedicated API for that purpose.
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```
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