Add `PreviewProvider` protocol (#328)

* Add `PreviewProvider` protocol

No functionality behind, just makes it easier to integrate with existing SwiftUI projects.

* Update PreviewProvider.swift

* Add missing preview-related types and modifiers

Sources/TokamakCore/PreviewProvider.swift

@@ -0,0 +1,64 @@
+// Copyright 2020 Tokamak contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/// This protocol has no functionality currently, and is only provided for compatibility purposes.
+public protocol PreviewProvider {
+  associatedtype Previews: View
+
+  @ViewBuilder static var previews: Previews { get }
+}
+
+public struct PreviewDevice: RawRepresentable, ExpressibleByStringLiteral {
+  public let rawValue: String
+  public init(rawValue: String) {
+    self.rawValue = rawValue
+  }
+
+  public init(stringLiteral: String) {
+    rawValue = stringLiteral
+  }
+}
+
+public protocol PreviewContextKey {
+  associatedtype Value
+  static var defaultValue: Self.Value { get }
+}
+
+public protocol PreviewContext {
+  subscript<Key>(key: Key.Type) -> Key.Value where Key: PreviewContextKey { get }
+}
+
+public enum PreviewLayout {
+  case device
+  case sizeThatFits
+  case fixed(width: CGFloat, height: CGFloat)
+}
+
+public extension View {
+  func previewDevice(_ value: PreviewDevice?) -> some View {
+    self
+  }
+
+  func previewLayout(_ value: PreviewLayout) -> some View {
+    self
+  }
+
+  func previewDisplayName(_ value: String?) -> some View {
+    self
+  }
+
+  func previewContext<C>(_ value: C) -> some View where C: PreviewContext {
+    self
+  }
+}

Sources/TokamakDOM/Core.swift

@@ -139,3 +139,5 @@ public extension Text {
     _concatenating(lhs: lhs, rhs: rhs)
   }
 }
+
+public typealias PreviewProvider = TokamakCore.PreviewProvider

Sources/TokamakDemo/ToggleDemo.swift

@@ -27,3 +27,9 @@ public struct ToggleDemo: View {
     }
   }
 }
+
+struct ToggleDemo_Previews: PreviewProvider {
+  static var previews: some View {
+    ToggleDemo()
+  }
+}

Sources/TokamakStaticHTML/Core.swift

@@ -97,3 +97,5 @@ public extension Text {
     _concatenating(lhs: lhs, rhs: rhs)
   }
 }
+
+public typealias PreviewProvider = TokamakCore.PreviewProvider