WIP: SwiftUI utils

Sources/ObjectReader.swift

@@ -0,0 +1,100 @@
+//
+//  ListState.swift
+//  CoreStore
+//
+//  Created by John Rommel Estropia on 2020/12/26.
+//  Copyright © 2020 John Rommel Estropia. All rights reserved.
+//
+
+#if canImport(Combine) && canImport(SwiftUI)
+
+import Combine
+import SwiftUI
+
+
+// MARK: - ObjectReader
+
+@available(iOS 13.0, tvOS 13.0, watchOS 6.0, macOS 10.15, *)
+public struct ObjectReader<Object: DynamicObject, Content: View, Placeholder: View>: View {
+    
+    // MARK: Internal
+    
+    public init(
+        _ objectPublisher: ObjectPublisher<Object>?,
+        @ViewBuilder content: @escaping (ObjectSnapshot<Object>) -> Content,
+        @ViewBuilder placeholder: @escaping () -> Placeholder
+    ) {
+        
+        self.objectPublisher = .init(
+            objectPublisher.flatMap {
+                
+                guard let dataStack = $0.cs_dataStack() else {
+                    
+                    return nil
+                }
+                return $0.asPublisher(in: dataStack)
+            }
+        )
+        self.content = content
+        self.placeholder = placeholder
+    }
+    
+    public init(
+        _ objectPublisher: ObjectPublisher<Object>?,
+        @ViewBuilder content: @escaping (ObjectSnapshot<Object>) -> Content
+    ) where Placeholder == EmptyView {
+        
+        self.init(
+            objectPublisher,
+            content: content,
+            placeholder: EmptyView.init
+        )
+    }
+    
+    
+    // MARK: View
+    
+    public var body: some View {
+        
+        if let snapshot = self.objectPublisher.wrappedValue?.snapshot {
+            
+            self.content(snapshot)
+        }
+        else {
+            
+            self.placeholder()
+        }
+    }
+    
+    
+    // MARK: Private
+    
+    @ObservedObject
+    private var objectPublisher: OptionalObservedObject<ObjectPublisher<Object>>
+    
+    private let content: (ObjectSnapshot<Object>) -> Content
+    private let placeholder: () -> Placeholder
+    
+    
+    // MARK: - OptionalObservedObject
+    
+    fileprivate final class OptionalObservedObject<T: ObservableObject>: ObservableObject where ObservableObjectPublisher == T.ObjectWillChangePublisher {
+        
+        // MARK: Internal
+        
+        let wrappedValue: T?
+        
+        init(_ wrappedValue: T?) {
+            
+            self.wrappedValue = wrappedValue
+            self.objectWillChange = wrappedValue.map(\.objectWillChange) ?? .init()
+        }
+        
+        // MARK: ObservableObject
+        
+        let objectWillChange: ObservableObjectPublisher
+    }
+}
+
+#endif
+