Support modifying parameters in multiple previews (#40)

Resolves #14

This allows for consumers to override parameters in previews by using `Exhibit.preview(parameters: [String: Any]) -> some View`

This allows for multiple previews to be had for easing development.

In order to support the passing of parameters in this way, some refactoring was required:

`Exhibit` is now a strict model, not a view itself, allowing it to be generic and more easily understood.

`AnyExhibit` is added which is a type-erased version for use in arrays, and then `AnyExhibitView` displays the actual exhibit with support for context observing.

For some reason, a compiler error unfortunately occurs in the preview definition if `layout` is left as a closure on `Exhibit`. The only solution I could find which left types intact is to move the layout override to a separate function in the `Exhibit` protocol.

Example/CustomButton.swift

@@ -19,5 +19,13 @@ struct CustomButton_Previews: ExhibitProvider, PreviewProvider {
             action: context.parameter(name: "action")
         )
     }
+
+    static var previews: some View {
+        exhibit.preview()
+            .previewLayout(.sizeThatFits)
+
+        exhibit.preview(parameters: ["title": "Other"])
+            .previewLayout(.sizeThatFits)
+    }
 }
 

Example/CustomToggle.swift

@@ -16,8 +16,9 @@ struct CustomToggle_Previews: ExhibitProvider, PreviewProvider {
             title: context.parameter(name: "title", defaultValue: "Title"),
             isOn: context.parameter(name: "isOn")
         )
-    } layout: { exhibit in
-        exhibit
-            .padding()
+    }
+
+    static func exhibitLayout(_ content: CustomToggle) -> some View {
+        content.padding()
     }
 }

Example/Exhibition.generated.swift

@@ -8,9 +8,9 @@ public struct Exhibition: View {
         NavigationView {
             ExhibitListView(
                 exhibits: [
-                    CustomButton_Previews.exhibit,
-                    CustomDatePicker_Previews.exhibit,
-                    CustomToggle_Previews.exhibit,
+                    CustomButton_Previews.anyExhibit,
+                    CustomDatePicker_Previews.anyExhibit,
+                    CustomToggle_Previews.anyExhibit,
                 ]
             )
         }

Exhibition.swifttemplate

@@ -9,7 +9,7 @@ public struct Exhibition: View {
         NavigationView {
             ExhibitListView(
                 exhibits: [<% for type in types.types where type.inheritedTypes.contains("ExhibitProvider") { %>
-                    <%= type.name %>.exhibit,<% } %>
+                    <%= type.name %>.anyExhibit,<% } %>
                 ]
             )
         }

Sources/Exhibition/Exhibit.Context.swift → Sources/Exhibition/Context.swift

@@ -1,47 +1,49 @@
 import SwiftUI
 
-extension Exhibit {
-    public class Context: ObservableObject {
-        @Published var parameters: [String: Any] = [:]
-        @Published var log: [String] = []
-
-        public func parameter<T>(name: String, defaultValue: T) -> T {
-            guard let binding = parameters[name] else {
-                parameters[name] = defaultValue
-                return defaultValue
-            }
-
-            return binding as! T
-        }
-
-        public func parameter<T>(name: String) -> T where T: Defaultable {
-            parameter(name: name, defaultValue: T.defaultValue)
-        }
-
-        public func parameter<T>(name: String, defaultValue: T) -> Binding<T> {
-            return Binding(
-                get: { [unowned self] in
-                    self.parameter(name: name, defaultValue: defaultValue)
-                },
-                set: { [unowned self] newValue in
-                    parameters[name] = newValue
-                }
-            )
-        }
-
-        public func parameter<T>(name: String) -> Binding<T> where T: Defaultable {
-            parameter(name: name, defaultValue: T.defaultValue)
+public class Context: ObservableObject {
+    @Published var parameters: [String: Any]
+    @Published var log: [String] = []
+
+    init(parameters: [String: Any] = [:]) {
+        self.parameters = parameters
+    }
+
+    public func parameter<T>(name: String, defaultValue: T) -> T {
+        guard let binding = parameters[name] else {
+            parameters[name] = defaultValue
+            return defaultValue
         }
 
-        public func log(_ text: String) {
-            log.append(text)
-        }
+        return binding as! T
+    }
+
+    public func parameter<T>(name: String) -> T where T: Defaultable {
+        parameter(name: name, defaultValue: T.defaultValue)
+    }
+
+    public func parameter<T>(name: String, defaultValue: T) -> Binding<T> {
+        return Binding(
+            get: { [unowned self] in
+                self.parameter(name: name, defaultValue: defaultValue)
+            },
+            set: { [unowned self] newValue in
+                parameters[name] = newValue
+            }
+        )
+    }
+
+    public func parameter<T>(name: String) -> Binding<T> where T: Defaultable {
+        parameter(name: name, defaultValue: T.defaultValue)
+    }
+
+    public func log(_ text: String) {
+        log.append(text)
     }
 }
 
 // MARK: - Closure Parameters
 
-extension Exhibit.Context {
+extension Context {
     /// A closure parameter with no arguments
     ///
     /// EG: `action: () -> Void`

Sources/Exhibition/DebugView.swift

@@ -2,7 +2,7 @@ import SwiftUI
 
 /// A presented view used to modify accessibility and exhibit parameters.
 struct DebugView: View {
-    @ObservedObject var context: Exhibit.Context
+    @ObservedObject var context: Context
 
     @Binding var preferredColorScheme: ColorScheme
     @Binding var layoutDirection: LayoutDirection

Sources/Exhibition/Exhibit.swift

@@ -1,48 +1,65 @@
 import SwiftUI
 
-public struct Exhibit: View {
+public struct Exhibit<Content: View> {
 
     let name: String
     let section: String
-    let view: (Context) -> AnyView
-    let layout: (Self) -> AnyView
-
-    @ObservedObject var context = Context()
+    let content: (Context) -> Content
 
-    public init<T: View, S: View>(
+    public init(
         name: String,
         section: String = "",
-        @ViewBuilder builder: @escaping (Context) -> T,
-        @ViewBuilder layout: @escaping (Self) -> S
+        @ViewBuilder builder: @escaping (Context) -> Content
     ) {
         self.name = name
         self.section = section
-        view = { context in AnyView(builder(context)) }
-        self.layout = { exhibit in AnyView(layout(exhibit)) }
+        self.content = builder
+    }
+}
+
+extension Exhibit {
+    @ViewBuilder public func preview(parameters: [String: Any] = [:]) -> some View {
+        ExhibitView(
+            exhibit: self,
+            context: Context(parameters: parameters)
+        )
+    }
+}
+
+struct ExhibitView<Content: View>: View {
+    let exhibit: Exhibit<Content>
+    @ObservedObject var context: Context
+
+    var body: some View {
+        exhibit.content(context)
     }
+}
+
+public struct AnyExhibit {
+    let name: String
+    let section: String
+    let content: (Context) -> AnyView
 
-    public var body: some View {
-        view(context)
-            .navigationTitle(name)
+    init<Content: View, Layout: View>(_ exhibit: Exhibit<Content>, layout: @escaping (Content) -> Layout) {
+        self.name = exhibit.name
+        self.section = exhibit.section
+        self.content = { context in
+            AnyView(layout(exhibit.content(context)))
+        }
     }
 }
 
-extension Exhibit: Identifiable {
+extension AnyExhibit: Identifiable {
     public var id: String {
         name
     }
 }
 
-extension Exhibit {
-    public init<T: View>(
-        name: String,
-        section: String = "",
-        @ViewBuilder builder: @escaping (Context) -> T
-    ) {
-        self.init(
-            name: name,
-            section: section,
-            builder: builder
-        ) { AnyView($0) }
+struct AnyExhibitView: View {
+    let exhibit: AnyExhibit
+    @ObservedObject var context: Context
+
+    var body: some View {
+        exhibit.content(context)
     }
 }

Sources/Exhibition/ExhibitListView.swift

@@ -2,7 +2,7 @@ import SwiftUI
 
 public struct ExhibitListView: View {
 
-    let exhibits: [Exhibit]
+    let exhibits: [AnyExhibit]
 
     @State var displayed: AnyHashable?
     @State var rootDebugViewPresented: Bool = false
@@ -13,7 +13,7 @@ public struct ExhibitListView: View {
     @State var preferredColorScheme: ColorScheme = .light
     @State var layoutDirection: LayoutDirection = .leftToRight
 
-    private var sections: [String: [Exhibit]] {
+    private var sections: [String: [AnyExhibit]] {
         Dictionary(grouping: searchResults, by: \.section)
     }
 
@@ -30,7 +30,7 @@ public struct ExhibitListView: View {
         )
     }
 
-    public init(exhibits: [Exhibit]) {
+    public init(exhibits: [AnyExhibit]) {
         self.exhibits = exhibits
     }
 
@@ -78,8 +78,9 @@ public struct ExhibitListView: View {
         .environment(\.layoutDirection, layoutDirection)
     }
 
-    private func debuggable(_ exhibit: Exhibit) -> some View {
-        exhibit.layout(exhibit)
+    private func debuggable(_ exhibit: AnyExhibit) -> some View {
+        let context = Context()
+        return AnyExhibitView(exhibit: exhibit, context: context)
             .toolbar {
                 ToolbarItem {
                     Button {
@@ -91,7 +92,7 @@ public struct ExhibitListView: View {
             }
             .sheet(isPresented: $exhibitDebugViewPresented) {
                 DebugView(
-                    context: exhibit.context,
+                    context: context,
                     preferredColorScheme: $preferredColorScheme,
                     layoutDirection: $layoutDirection
                 )
@@ -103,7 +104,7 @@ public struct ExhibitListView: View {
             .parameterView(ClosureParameterView.self)
     }
 
-    private var searchResults: [Exhibit] {
+    private var searchResults: [AnyExhibit] {
         if searchText.isEmpty {
             return exhibits
         } else {
@@ -116,15 +117,23 @@ public struct ExhibitListView: View {
 }
 
 struct ExhibitListView_Previews: PreviewProvider {
+    struct First: ExhibitProvider {
+        static let exhibit = Exhibit(name: "Text", section: "Section 1") { context in
+            Text(context.parameter(name: "Content", defaultValue: "Text"))
+        }
+    }
+
+    struct Second: ExhibitProvider {
+        static let exhibit = Exhibit(name: "Text", section: "Section 1") { context in
+            Text(context.parameter(name: "Content", defaultValue: "Text"))
+        }
+    }
+
     static var previews: some View {
         ExhibitListView(
             exhibits: [
-                .init(name: "Text", section: "Section 1") { context in
-                    Text(context.parameter(name: "Content", defaultValue: "Text"))
-                },
-                .init(name: "Text2", section: "Section 2") { context in
-                    Text(context.parameter(name: "Content", defaultValue: "Text"))
-                }
+                First.anyExhibit,
+                Second.anyExhibit,
             ]
         )
     }

Sources/Exhibition/ExhibitProvider.swift

@@ -1,12 +1,28 @@
 import SwiftUI
 
 public protocol ExhibitProvider {
-    static var exhibit: Exhibit { get }
+    associatedtype Content: View
+    associatedtype Layout: View
+
+    static var exhibit: Exhibit<Content> { get }
+
+    static func exhibitLayout(_ content: Content) -> Layout
 }
 
 public extension ExhibitProvider {
     static var previews: some View {
         exhibit
+            .preview()
             .previewLayout(.sizeThatFits)
     }
+
+    static var anyExhibit: AnyExhibit {
+        AnyExhibit(exhibit, layout: exhibitLayout)
+    }
+}
+
+public extension ExhibitProvider where Content == Layout {
+    static func exhibitLayout(_ content: Content) -> Layout {
+        content
+    }
 }

Sources/Exhibition/ParameterView.swift

@@ -25,7 +25,7 @@ extension View {
 // MARK: - Internal
 
 /// Type erased parameter view for storage in an array.
-typealias AnyParameterView = (String, Any, Exhibit.Context) -> AnyView?
+typealias AnyParameterView = (String, Any, Context) -> AnyView?
 func erase<P: ParameterView>(_ parameterView: P.Type) -> AnyParameterView {
     return { name, value, parameters in
         guard let value = value as? P.Value else {