What's the difference between UIWebView and WKWebView when loading local resources

A couple points:

1. Apple recommends that you use WKWebview for iOS 8 and later. I would avoid writing new code with UIWebView.

In apps that run in iOS 8 and later, use the WKWebView class instead of using UIWebView. Additionally, consider setting the WKPreferences property javaScriptEnabled to false if you render files that are not supposed to run JavaScript.

2. Apple has been trying to move away from path and instead wants to use URI even for local files. They recommend that you NOT use /path/to/file.png and use file:///path/to/file.png instead.

As to why one URL works and the other does not, let's make a minimal example:

let realPath = "/path/to/file.png"let url = URL(string: realPath)               // /path/to/file.pnglet fileUrl = URL(fileURLWithPath: realPath)  // file:///path/to/file.png

• url does not provide the scheme (a.k.a protocol). It should only be used in conjunction with another URL to give the absolute address of the resource you are trying to reach. UIWebView supports it for backwards-compatibility reasons but Apple decided to start clean with WKWebView.
• fileURL has a scheme (file://) that tells the resource is located on the local file system. Other common schemes are http, https, ftp, etc. It's a complete address to a resource so both views know how to resolve it.

This might be for security reasons, or just how the WKWebView API was implemented.

WKWebView has a specific instance method for loading local resources called loadFileURL(_:allowingReadAccessTo:). This was introduced in iOS 9.

Note

If you are targeting iOS 8.0 or newer, you should be using WKWebView instead of UIWebView. See: https://developer.apple.com/reference/webkit/wkwebview