[DownView] Add parameter so bundle is writable

Source/Views/DownView.swift

@@ -28,10 +28,12 @@ open class DownView: WKWebView {
     ///   - configuration: Optional custom web view configuration.
     ///   - options: `DownOptions` to modify parsing or rendering, defaulting to `.default`
     ///   - didLoadSuccessfully: Optional callback for when the web content has loaded successfully
+    ///   - writableBundle: Whether or not the bundle folder is writable.
     /// - Throws: `DownErrors` depending on the scenario
-    public init(frame: CGRect, markdownString: String, openLinksInBrowser: Bool = true, templateBundle: Bundle? = nil, configuration: WKWebViewConfiguration? = nil, options: DownOptions = .default, didLoadSuccessfully: DownViewClosure? = nil) throws {
+    public init(frame: CGRect, markdownString: String, openLinksInBrowser: Bool = true, templateBundle: Bundle? = nil, writableBundle: Bool = false, configuration: WKWebViewConfiguration? = nil, options: DownOptions = .default, didLoadSuccessfully: DownViewClosure? = nil) throws {
         self.options = options
         self.didLoadSuccessfully = didLoadSuccessfully
+        self.writableBundle = writableBundle
 
         if let templateBundle = templateBundle {
             self.bundle = templateBundle
@@ -86,6 +88,7 @@ open class DownView: WKWebView {
     // MARK: - Private Properties
 
     let bundle: Bundle
+    let writableBundle: Bool
     var options: DownOptions
 
     private lazy var baseURL: URL = {
@@ -113,7 +116,12 @@ private extension DownView {
         let pageHTMLString = try htmlFromTemplate(htmlString)
 
         #if os(iOS)
-            loadHTMLString(pageHTMLString, baseURL: baseURL)
+            if writableBundle {
+                let newIndexUrl = try writeTempIndexFile(pageHTMLString: pageHTMLString)
+                loadFileURL(newIndexUrl, allowingReadAccessTo: newIndexUrl.deletingLastPathComponent())
+            } else {
+                loadHTMLString(pageHTMLString, baseURL: baseURL)
+            }
         #elseif os(macOS)
             let indexURL = try createTemporaryBundle(pageHTMLString: pageHTMLString)
             loadFileURL(indexURL, allowingReadAccessTo: indexURL.deletingLastPathComponent())
@@ -125,6 +133,14 @@ private extension DownView {
         return template.replacingOccurrences(of: "DOWN_HTML", with: htmlString)
     }
 
+    #if os(iOS)
+    func writeTempIndexFile(pageHTMLString: String) throws -> URL {
+        let newIndexUrl = bundle.resourceURL!.appendingPathComponent("tmp_index.html")
+        try pageHTMLString.write(to: newIndexUrl, atomically: true, encoding: .utf8)
+        return newIndexUrl
+    }
+    #endif
+
     #if os(macOS)
     func createTemporaryBundle(pageHTMLString: String) throws -> URL {
         guard let bundleResourceURL = bundle.resourceURL

Tests/DownViewTests.swift

@@ -97,6 +97,40 @@ class DownViewTests: XCTestCase {
         }
     }
 
+    func testInstantiationWithCustomWritableTemplateBundle() {
+        let expect1 = expectation(description: "DownView accepts and loads custom bundle files from a user writable location")
+
+        guard
+            let bundle = Bundle(for: type(of: self)).url(forResource: "TestDownView", withExtension: "bundle"),
+            let templateBundle = Bundle(url: bundle)
+        else {
+            XCTFail("Test template bundle not found in test target!")
+            return
+        }
+
+        let markdownString = """
+```swift
+let x = 1
+```
+"""
+        var downView: DownView?
+        downView = try? DownView(frame: .zero, markdownString: markdownString, templateBundle: templateBundle, writableBundle: true, didLoadSuccessfully: {
+            self._pageContents(for: downView!) { htmlString in
+                XCTAssertTrue(htmlString!.contains("css/down.min.css"))
+                XCTAssertTrue(htmlString!.contains("hljs-keyword"))
+                XCTAssertTrue(htmlString!.contains("But also, custom HTML!"))
+
+                expect1.fulfill()
+            }
+        })
+
+        waitForExpectations(timeout: 10) { (error: Error?) in
+            if let error = error {
+                XCTFail("waitForExpectationsWithTimeout errored: \(error)")
+            }
+        }
+    }
+
 	func testDownOptions() {
         let markdownString = "## [Down](https://github.com/iwasrobbed/Down)\n\n<strong>I'm strong!</strong>"
         let renderedHTML = "<strong>I'm strong!</strong>"