StatefulPreview

README.md

StatefulPreview.swift

What's it?

StatefulPreview is a simple struct wrapped in a PreviewProvider extension that enables stateful previews for SwiftUI views.

Why?

Fields wrapped in @Binding annotation, are not easy to preview as stateful value. It's possible to wrap a value using .constant utilty but it will not allow to modify the value.

Usage

This example shows how to use StatefulPreview with a simple value type.

struct CountView: View {
  @Binding var count: Int

  var body: some View {
    HStack {
          Text("The current count is: ")
        + Text("\(count)")
            .bold()

      Button { count += 1 } label: { Text("Add + 1") }
          .padding(6)
          .background(Color.blue)
          .foregroundColor(.white)
          .cornerRadius(12)
    }
  }
}

struct CountView_Previews: PreviewProvider {
    static var previews: some View {
        Stateful(value: 1) { count in
            CountView(count: count)
        }
    }
}

License

MIT

StatefulPreview.swift

/*
  StatefulPreview.swift
  
  MIT License
  Copyright (c) 2020 Jan Cassio
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:
  The above copyright notice and this permission notice shall be included in all
  copies or substantial portions of the Software.
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.

*/
import Foundation
import SwiftUI

extension PreviewProvider {
    
    /// Assign a value as binding. The state is internally preserved, enabling to use stateful values into the preview.
    /// - Parameters:
    ///   - value: A value type.
    ///   - content: A block that return a `View`. The block will provide a `Binding<Value>` as parameter.
    /// - Returns: A stateful view that can be used in preview.
    static func Stateful<Value, Content: View>(value: Value, @ViewBuilder content: @escaping(Binding<Value>) -> Content) -> some View {
        StatefulPreviewWrapper(value, content: content)
    }
}

fileprivate struct StatefulPreviewWrapper<Value, Content: View>: View {
    @State var value: Value
    var content: (Binding<Value>) -> Content
    
    var body: some View {
        content($value)
    }
    
    init(_ value: Value, content: @escaping (Binding<Value>) -> Content) {
        self._value = State(wrappedValue: value)
        self.content = content
    }
}