From ad6aea5414743b2e7b39c37cd5ec262441855454 Mon Sep 17 00:00:00 2001
From: Red Davis <me@red.to>
Date: Wed, 2 Mar 2022 06:43:50 +0000
Subject: [PATCH] Readonly view model value binding (#16)

* Expose state so people can easily build their own bindings

* Add a readonly binding
---
 RedUx/Source/ViewModel.swift          | 31 ++++++++++++++++++++++++++-
 RedUxTests/Tests/ViewModelTests.swift | 26 ++++++++++++++++++++++
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/RedUx/Source/ViewModel.swift b/RedUx/Source/ViewModel.swift
index 2497e2d..3734ec6 100644
--- a/RedUx/Source/ViewModel.swift
+++ b/RedUx/Source/ViewModel.swift
@@ -42,7 +42,7 @@ import SwiftUI
 public final class ViewModel<State: Equatable, Event>: ObservableObject {
     
     /// The state of the store.
-    @Published var state: State
+    @Published public var state: State
     
     // Private
     private var stateTask: Task<Void, Never>?
@@ -172,4 +172,33 @@ extension ViewModel {
             event: { _ in event }
         )
     }
+    
+    /// Creates a readonly `Binding`.
+    ///
+    /// This makes working with SwiftUI components easier.
+    ///
+    /// For example:
+    ///
+    /// ```swift
+    /// .fullScreenCover(
+    ///     isPresented: self.viewModel.binding(
+    ///         value: { !$0.isLoggedIn }
+    ///     ),
+    ///     onDismiss: nil,
+    ///     content: {
+    ///         Text("Logged out")
+    ///     }
+    /// )
+    /// ```
+    /// - Parameters:
+    ///   - value: A function to extract the value from the state.
+    /// - Returns: A readonly binding.
+    public func binding<ScopedState>(
+        value: @escaping (State) -> ScopedState
+    ) -> Binding<ScopedState> {
+        Binding(
+            get: { value(self.state) },
+            set: { _, _ in }
+        )
+    }
 }
diff --git a/RedUxTests/Tests/ViewModelTests.swift b/RedUxTests/Tests/ViewModelTests.swift
index ff53458..1227540 100644
--- a/RedUxTests/Tests/ViewModelTests.swift
+++ b/RedUxTests/Tests/ViewModelTests.swift
@@ -99,6 +99,8 @@ final class ViewModelTests: XCTestCase {
         )
     }
     
+    // MARK: Bindings
+    
     func testBindingWithValue() {
         let binding = self.viewModel.binding(
             value: \.value,
@@ -190,4 +192,28 @@ final class ViewModelTests: XCTestCase {
             [.setValue(self.value)]
         )
     }
+    
+    func testReadonlyBindingReceivesValueChange() {
+        let binding = self.viewModel.binding(
+            value: \.value
+        )
+        
+        XCTAssertNil(binding.wrappedValue)
+        self.viewModel.send(.setValue(self.value))
+        XCTAssertEventuallyEqual(
+            binding.wrappedValue,
+            self.value
+        )
+    }
+    
+    func testReadonlyBindingDoesNotEmitsEventOnWrappedValueChange() {
+        let binding = self.viewModel.binding(
+            value: \.value
+        )
+        binding.wrappedValue = "whatever"
+        
+        XCTAssertNil(binding.wrappedValue)
+        XCTAssertNil(self.store.state.value)
+        XCTAssertNil(self.viewModel.state.value)
+    }
 }