Add backing storage and caching for @Attribute (#997)

* Add backing storage and caching for `@Attribute`

* Remove print

Author: carson-katri

Sources/LiveViewNative/Property Wrappers/Attribute.swift

@@ -47,40 +47,66 @@ public struct Attribute<T>: DynamicProperty {
     private let name: AttributeName
     private let defaultValue: T?
     private let transform: (LiveViewNativeCore.Attribute?) throws -> T
+    
+    @StateObject private var storage: Storage = .init()
 
     /// Create an `Attribute` with an ``AttributeDecodable`` type.
     public init(wrappedValue: T? = nil, _ name: AttributeName) where T: AttributeDecodable {
-        self.name = name
-        self.defaultValue = wrappedValue
-        self.transform = { try T(from: $0) }
+        self.init(wrappedValue: wrappedValue, name, transform: { try T(from: $0) }, element: nil)
     }
 
     /// Create an `Attribute` with a `transform` function that converts the attribute into the desired type.
     public init(wrappedValue: T? = nil, _ name: AttributeName, transform: @escaping (LiveViewNativeCore.Attribute?) throws -> T) {
-        self.name = name
-        self.defaultValue = wrappedValue
-        self.transform = transform
+        self.init(wrappedValue: wrappedValue, name, transform: transform, element: nil)
     }
 
     init(wrappedValue: T? = nil, _ name: AttributeName, element: ElementNode) where T: AttributeDecodable {
-        self.name = name
-        self.defaultValue = wrappedValue
-        self.transform = { try T(from: $0) }
-        self._element = .init(element: element)
+        self.init(wrappedValue: wrappedValue, name, transform: { try T(from: $0) }, element: element)
     }
 
-    init(wrappedValue: T? = nil, _ name: AttributeName, transform: @escaping (LiveViewNativeCore.Attribute?) throws -> T, element: ElementNode) {
+    init(wrappedValue: T? = nil, _ name: AttributeName, transform: @escaping (LiveViewNativeCore.Attribute?) throws -> T, element: ElementNode?) {
         self.name = name
         self.defaultValue = wrappedValue
         self.transform = transform
-        self._element = .init(element: element)
+        if let element {
+            self._element = .init(element: element)
+        } else {
+            self._element = .init()
+        }
     }
 
     /// Gets the decoded value of the attribute.
     ///
     /// If attribute decoding fails, this will return the default value the property wrapper was constructed with.
     /// If no default value was provided, the app will crash.
     public var wrappedValue: T {
+        // Use the cached value if possible. Otherwise, decode on demand.
+        guard let value = storage.value else {
+            return decode()
+        }
+        return value
+    }
+    
+    /// Container that holds the cached decoded attribute value, and the previous attribute for comparison.
+    private final class Storage: ObservableObject {
+        var value: T?
+        var previousValue: LiveViewNativeCore.Attribute?
+    }
+    
+    /// Before the View's body is produced, check if the attribute needs to be recomputed.
+    public mutating func update() {
+        let attribute = element.attribute(named: name)
+        
+        // Only recompute the attribute if the attribute is different.
+        guard storage.previousValue != attribute
+        else { return }
+        
+        self.storage.value = decode()
+        self.storage.previousValue = attribute
+    }
+    
+    /// Decodes the attribute if possible.
+    private func decode() -> T {
         do {
             return try transform(element.attribute(named: name))
         } catch {

Sources/LiveViewNative/ViewTree.swift

@@ -201,19 +201,18 @@ private struct ModifierObserver<Parent: View, R: RootRegistry>: View {
     let parent: Parent
     @ObservedElement private var element
     @LiveContext<R> private var context
+    @Attribute("modifiers", transform: { attribute in
+        guard let encoded = attribute?.value else { return [] }
+        
+        let decoder = makeJSONDecoder()
+        
+        guard let decoded = try? decoder.decode([ModifierContainer<R>].self, from: Data(encoded.utf8))
+        else { return [] }
+        
+        return decoded
+    }) private var modifiers: [ModifierContainer<R>]
 
     var body: some View {
-        let modifiers: [ModifierContainer<R>]
-        if let encoded = element.attributeValue(for: "modifiers") {
-            let decoder = makeJSONDecoder()
-            if let decoded = try? decoder.decode([ModifierContainer<R>].self, from: Data(encoded.utf8)) {
-                modifiers = decoded
-            } else {
-                modifiers = []
-            }
-        } else {
-            modifiers = []
-        }
         return parent
             .applyModifiers(modifiers[...], element: element, context: context.storage)
     }