A SwiftUI View to display SVGs using WKWebView

SVGWebView.swift

// Created by Helge Heß on 06.04.21.
// Also available as a package: https://github.com/ZeeZide/SVGWebView

import SwiftUI
import WebKit

/**
 * Display an SVG using a `WKWebView`.
 *
 * Used by [SVG Shaper for SwiftUI](https://zeezide.de/en/products/svgshaper/)
 * to display the SVG preview in the sidebar.
 *
 * This patches the XML of the SVG to fit the WebView contents.
 *
 * IMPORTANT: On macOS `WKWebView` requires the "outgoing internet connection"
 *            entitlement to operate, otherwise it'll show up blank.
 * Xcode Previews do not work quite right with the iOS variant, best to test in
 * a real simulator.
 */
public struct SVGWebView: View {
  
  private let svg: String
  
  public init(svg: String) { self.svg = svg }
  
  public var body: some View {
    WebView(html:
      "<div style=\"width: 100%; height: 100%;\">\(rewriteSVGSize(svg))</div>"
    )
  }
  
  /// A hacky way to patch the size in the SVG root tag.
  private func rewriteSVGSize(_ string: String) -> String {
    guard let startRange = string.range(of: "<svg") else { return string }
    let remainder = startRange.upperBound..<string.endIndex
    guard let endRange = string.range(of: ">", range: remainder) else {
      return string
    }
    
    let tagRange = startRange.lowerBound..<endRange.upperBound
    let oldTag   = string[tagRange]
    
    var attrs : [ String : String ] = {
      final class Handler: NSObject, XMLParserDelegate {
        var attrs : [ String : String ]?
        
        func parser(_ parser: XMLParser, didStartElement: String,
                    namespaceURI: String?, qualifiedName: String?,
                    attributes: [ String : String ])
        {
          self.attrs = attributes
        }
      }
      let parser  = XMLParser(data: Data((string[tagRange] + "</svg>").utf8))
      let handler = Handler()
      parser.delegate = handler
        
      guard parser.parse() else { return [:] }
      return handler.attrs ?? [:]
    }()
    
    if attrs["viewBox"] == nil &&
      (attrs["width"] != nil || attrs["height"] != nil)
    { // convert to viewBox
      let w = attrs.removeValue(forKey: "width")  ?? "100%"
      let h = attrs.removeValue(forKey: "height") ?? "100%"
      let x = attrs.removeValue(forKey: "x")      ?? "0"
      let y = attrs.removeValue(forKey: "y")      ?? "0"
      attrs["viewBox"] = "\(x) \(y) \(w) \(h)"
    }
    attrs.removeValue(forKey: "x")
    attrs.removeValue(forKey: "y")
    attrs["width"]  = "100%"
    attrs["height"] = "100%"
    
    func renderTag(_ tag: String, attributes: [ String : String ]) -> String {
      var ms = "<\(tag)"
      for ( key, value ) in attributes {
        ms += " \(key)=\""
        ms += value
          .replacingOccurrences(of: "&",  with: "&amp;")
          .replacingOccurrences(of: "<",  with: "&lt;")
          .replacingOccurrences(of: ">",  with: "&gt;")
          .replacingOccurrences(of: "'",  with: "&apos;")
          .replacingOccurrences(of: "\"", with: "&quot;")
        ms += "\""
      }
      ms += ">"
      return ms
    }
    
    let newTag = renderTag("svg", attributes: attrs)
    return newTag == oldTag
         ? string
         : string.replacingCharacters(in: tagRange, with: newTag)
  }

  #if os(macOS)
    typealias UXViewRepresentable = NSViewRepresentable
  #else
    typealias UXViewRepresentable = UIViewRepresentable
  #endif
  
  private struct WebView : UXViewRepresentable {
    
    let html : String
    
    private func makeWebView() -> WKWebView {
      let prefs = WKPreferences()
      #if os(macOS)
        if #available(macOS 10.5, *) {} else { prefs.javaEnabled = false }
      #endif
      if #available(macOS 11, *) {} else { prefs.javaScriptEnabled = false }
      prefs.javaScriptCanOpenWindowsAutomatically = false
      
      let config = WKWebViewConfiguration()
      config.preferences = prefs
      config.allowsAirPlayForMediaPlayback = false
      
      if #available(macOS 10.5, *) {
        let pagePrefs : WKWebpagePreferences = {
          let prefs = WKWebpagePreferences()
          prefs.preferredContentMode = .desktop
          if #available(macOS 11, *) {
            prefs.allowsContentJavaScript = false
          }
          return prefs
        }()
        config.defaultWebpagePreferences = pagePrefs
      }
      
      let webView = WKWebView(frame: .zero, configuration: config)
      #if !os(macOS)
        webView.scrollView.isScrollEnabled = false
      #endif
      
      webView.loadHTMLString(html, baseURL: nil)
      
      // Sometimes necessary to make things show up initially. No idea why.
      DispatchQueue.main.async {
        let old = webView.frame
        webView.frame = .zero
        webView.frame = old
      }
      
      return webView
    }
    private func updateWebView(_ webView: WKWebView, context: Context) {
      webView.loadHTMLString(html, baseURL: nil)
    }

    #if os(macOS)
      func makeNSView(context: Context) -> WKWebView {
        return makeWebView()
      }
      func updateNSView(_ webView: WKWebView, context: Context) {
        updateWebView(webView, context: context)
      }
    #else // iOS etc
      func makeUIView(context: Context) -> WKWebView {
        return makeWebView()
      }
      func updateUIView(_ webView: WKWebView, context: Context) {
        updateWebView(webView, context: context)
      }
    #endif
  }
}

struct SVGWebView_Previews : PreviewProvider {

  static var previews: some View {
    SVGWebView(svg:
      """
      <svg viewBox="0 0 100 100">
        <rect x="10" y="10" width="80" height="80"
              fill="gold" stroke="blue" stroke-width="4" />
      </svg>
      """
    )
    .frame(width: 300, height: 200)

    SVGWebView(svg:
      """
      <svg width="120" height="120" version="1.1" xmlns="http://www.w3.org/2000/svg">
        <defs>
            <linearGradient id="Gradient1">
              <stop offset="0%"   stop-color="red"/>
              <stop offset="50%"  stop-color="black" stop-opacity="0"/>
              <stop offset="100%" stop-color="blue"/>
            </linearGradient>
        </defs>
        <rect x="10" y="10" rx="15" ry="15" width="100" height="100"
              fill="url(#Gradient1)" />
      </svg>
      """)
      .frame(width: 200, height: 200)
  }
}

On macOS, make sure to enable the "Outgoing Internet Connection" (Client)" entitlement in the sandbox settings, WKWebView requires this even if no outgoing Internet access is done.

Example usage in SVG Shaper for SwiftUI (it is the View displaying the SVG in the upper left):

Note: SVG Shaper is for converting SVGs to SwiftUI source code (which then gets compiled). SVGWebView is for displaying SVG resources (e.g. loaded from a bundle or the web) at runtime. They serve different purposes.

This is intended as a no-dependency option, to avoid linking a 3rd party SVG framework. If that's no problem, there are nice options for the latter:

• https://github.com/IconJar/IJSVG (used that initially, very nice, not too large)
• https://github.com/SVGKit/SVGKit/ (didn't try this, but supposed to be good)

Finally !!!! been searching for a mac solution for a week