Create mechanism for providing lazily evaluated context data

Sources/Stencil/Context.swift

@@ -11,19 +11,21 @@ public class Context {
   /// The context's environment, such as registered extensions, classes, …
   public let environment: Environment
 
+  init(dictionaries: [[String: Any?]], environment: Environment) {
+    self.dictionaries = dictionaries
+    self.environment = environment
+  }
+
   /// Create a context from a dictionary (and an env.)
   ///
   /// - Parameters:
   ///   - dictionary: The context's data
   ///   - environment: Environment such as extensions, …
-  public init(dictionary: [String: Any] = [:], environment: Environment? = nil) {
-    if !dictionary.isEmpty {
-      dictionaries = [dictionary]
-    } else {
-      dictionaries = []
-    }
-
-    self.environment = environment ?? Environment()
+  public convenience init(dictionary: [String: Any] = [:], environment: Environment? = nil) {
+    self.init(
+      dictionaries: dictionary.isEmpty ? [] : [dictionary],
+      environment: environment ?? Environment()
+    )
   }
 
   /// Access variables in this context by name

Sources/Stencil/LazyValueWrapper.swift

@@ -0,0 +1,69 @@
+//
+// Stencil
+// Copyright © 2022 Stencil
+// MIT Licence
+//
+
+/// Used to lazily set context data. Useful for example if you have some data that requires heavy calculations, and may
+/// not be used in every render possiblity.
+public final class LazyValueWrapper {
+  private let closure: (Context) throws -> Any
+  private let context: Context?
+  private var cachedValue: Any?
+
+  /// Create a wrapper that'll use a **reference** to the current context.
+  /// This means when the closure is evaluated, it'll use the **active** context at that moment.
+  ///
+  /// - Parameters:
+  ///   - closure: The closure to lazily evaluate
+  public init(closure: @escaping (Context) throws -> Any) {
+    self.context = nil
+    self.closure = closure
+  }
+
+  /// Create a wrapper that'll create a **copy** of the current context.
+  /// This means when the closure is evaluated, it'll use the context **as it was** when this wrapper was created.
+  ///
+  /// - Parameters:
+  ///   - context: The context to use during evaluation
+  ///   - closure: The closure to lazily evaluate
+  /// - Note: This will use more memory than the other `init` as it needs to keep a copy of the full context around.
+  public init(copying context: Context, closure: @escaping (Context) throws -> Any) {
+    self.context = Context(dictionaries: context.dictionaries, environment: context.environment)
+    self.closure = closure
+  }
+
+  /// Shortcut for creating a lazy wrapper when you don't need access to the Stencil context.
+  ///
+  /// - Parameters:
+  ///   - closure: The closure to lazily evaluate
+  public init(_ closure: @autoclosure @escaping () throws -> Any) {
+    self.context = nil
+    self.closure = { _ in try closure() }
+  }
+}
+
+extension LazyValueWrapper {
+  func value(context: Context) throws -> Any {
+    if let value = cachedValue {
+      return value
+    } else {
+      let value = try closure(self.context ?? context)
+      cachedValue = value
+      return value
+    }
+  }
+}
+
+extension LazyValueWrapper: Resolvable {
+  public func resolve(_ context: Context) throws -> Any? {
+    let value = try self.value(context: context)
+    return try (value as? Resolvable)?.resolve(context) ?? value
+  }
+}
+
+extension LazyValueWrapper: Normalizable {
+  public func normalize() -> Any? {
+    (cachedValue as? Normalizable)?.normalize() ?? cachedValue
+  }
+}

Sources/Stencil/Variable.swift

@@ -78,6 +78,8 @@ public struct Variable: Equatable, Resolvable {
 
       if current == nil {
         return nil
+      } else if let lazyCurrent = current as? LazyValueWrapper {
+        current = try lazyCurrent.value(context: context)
       }
     }