WKWebView fails to load images and CSS using loadHTMLString(_, baseURL:)

Ios Problem Overview

> In apps that run in iOS 8 and later, use the WKWebView class instead of using UIWebView.

Thus, I have replaced my good old UIWebView with a shiny new WKWebView. But what I thought to be an easy exercise (simply swapping the classes and replacing the delegate methods) turned out to be a real mess.

The Problem

When I load an HTML string using

loadHTMLString(String, baseURL: URL?)

the web view loads and renders the pure HTML but it doesn't load any images or CSS files referenced inside the htmlString.

This happens only on a real device!
In Simultor all referenced resources are loaded correctly.

Example

I have defined a simple htmlString in my view controller class:

let imageName = "image.png"

let libraryURL: URL // The default Library URL

var htmlString: String {
    return "<html> ... <img src=\"\(imageName)\" /> ... </html>"
    // "..." represents more valid HTML code incl. header and body tags
}

The image is stored in the root Library folder so its URL is:

let imageURL = libraryURL.appendingPathComponent(imageName)

Now I load the htmlString into the web view:

webView.loadHTMLString(htmlString, baseURL: libraryURL)

and it doesn't load the image even though the baseURL is set correctly.

Ideas for a Solution

Maybe WKWebView has a problem with resolving relative paths so my first idea was to use absolute paths inside the HTML string instead.
→ ❌ Doesn't work.
Two answers to another SO post suggested that using
loadFileURL(URL, allowingReadAccessTo: URL)
instead of loadHTMLString(...) works in iOS 9+.
→ ✅ That works.

However, I cannot use solution 2 because my HTML files are encrypted and the decrypted files must not be stored on the disk.

Question

Is there any way to load local resources like images and styles using the WKWebView's

loadHTMLString(String, baseURL: URL?)

function? Or is still a bug in iOS 9+?

(I just cannot believe that Apple provides and recommends using a web view that cannot load any local web content from inside an HTML string?!)

Ios Solutions

Solution 1 - Ios

Without taking a look at your actual project it's difficult to give some hundreed percent sure advices.

However:

class ViewController: UIViewController {

    var webView = WKWebView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        webView.translatesAutoresizingMaskIntoConstraints = false
        let views = [
            "webView" : webView
        ]
        view.addSubview(webView)
        var constraints = NSLayoutConstraint.constraintsWithVisualFormat("H:|[webView]|", options: [.AlignAllLeading, .AlignAllTrailing], metrics: nil, views: views)
        constraints.appendContentsOf(NSLayoutConstraint.constraintsWithVisualFormat("V:|[webView]|", options: [.AlignAllTop, .AlignAllBottom], metrics: nil, views: views))
        NSLayoutConstraint.activateConstraints(constraints)
        
        let path = NSBundle.mainBundle().pathForResource("ios - WKWebView fails to load images and CSS using loadHTMLString(_, baseURL_) - Stack Overflow", ofType: "htm")
        let url = NSURL(fileURLWithPath: path!)
        webView.loadHTMLString(try! String(contentsOfURL: url), baseURL: url.URLByDeletingLastPathComponent)

        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

I think the key point here is baseUrl parameter, you should setup it correctly. In my case i've used html's url without last path component - e.g. containing folder. This works fine on both device & simulator - check device snapshot. I've uploaded sample project to https://github.com/soxjke/WKWebViewTest so you can take a look (i've removed codesigning info from git)

So, to recap - method is working, functionality is working, just you do something wrong with it. To help you get what's wrong with your solutions, i'll add some suggestions:

Remember, that simulator filesystem is case-insensitive, device filesystem is case-sensitive. So if you have your filenames in html in lowercase - this won't work on device. 8fFsD.png != 8ffsd.png
Remember, that when copying resources, XCode ignores your folder structure. So if your html has <img src="./img/1.png"> and your XCOde project has folder structure like

test.htm

img/
```
1.png 
2.png
```

After build it will be flattened, so test.htm and 1.png and 2.png will reside on same level

test.htm
1.png 
2.png

I'm almost sure, after you verify these two assumptions, you'll get this method working.

Solution 2 - Ios

I had this problem today, I've found the solution and potentially the cause:

loadHTMLString(String, baseURL: URL?)

This function doesn't allow the rendered HTML to access local media, as far as I'm aware, this is because it would be an injection risk, this could allow rendered HTML to access and manipulate your local file system. With a html string, that could come from anywhere or anyone.

loadFileURL(URL, allowingReadAccessTo: URL)

With this function, you point the WKWebview to the html file in your FileManager, and to the containing folder with 'allowingReadAccessTo'. Because the html is stored within the FileManager, it will allow the rendered HTML to access locally stored media.

If you don't have the html file stored locally for some reason(I assume you do), You could write the html sting into a .html file, then point to the URL of that file. However, this is just subverting Apple's protection, so do it at your own peril (don't do it).

This is just the solution that worked for me and my understanding of why we're having the problem to begin with.

Edit #1: Typo.

Edit #2: I've since found another nuance, When stating the 'allowingReadAccessTo:' URL, if the HTML itself needs to access things in parent folders (ie: .css, .js files), you need to specify the parent folder, not necessarily the location of the HTML itself, this will then implicitly allow access to the child folders as required also. For me, this problem was only apparent on a physical device, this didn't seem to have an effect whilst running in simulator, likely another discrepancy between how permissions work on simulator and a physical device.

Solution 3 - Ios

Well you should be able to use local images and CSS files (and JavaScript files for that matter) with WKWebViews with the function that you have already found. My guess is that the problem is with your baseURL variable.

Update 7.5.2017:

I have completely updated the code from another SO answer of mine that used to be linked to my answer here. I have a working project for loadHTMLString() and .loadFileURL()

Solution 4 - Ios

Personally, I had to switch to using XWebView as the out-of-the-box behavior of WKWebView does not allow loading of local files. XWebView tricks it by loading up a local web server in the background and directing local traffic thru it. (XWebView is based on top of WKWebView)

Seems a bit overkill, but that is what I ended up having to do.

Solution 5 - Ios

I've been experimenting with this as well, with similar restrictions, and the problem appears to be that paths aren't resolved unless baseURL references the application bundle. It doesn't work if you, for example, have something in the application's documents.

Edit: I have filed a radar for this rdar://29130863

Solution 6 - Ios

Try to create baseURL using:

let baseURL = URL(fileURLWithPath: "#path#")

instead of:

let baseURL = URL(string: "#path#")

The main difference is that the first method adds file:// prefix before the path.

Solution 7 - Ios

You can base64 encode the images... I know that works. Not sure if it will be appropriate for your use case though.

Kind of funny, I just ran into this problem while doing the opposite - moving from base64 encoded to image files.

Solution 8 - Ios

When I used UIWebview, I used baseURL as,

let baseUrl = NSURL(string: Bundle.main.path(forResource: "cms", ofType: "html")!)! as URL

webView.loadHTMLString(bodyPage, baseURL: baseUrl)

But for the WKWebView, I used baseURL as

let baseUrl = Bundle.main.bundleURL

webView.loadHTMLString(bodyPage, baseURL: baseUrl)

This works for me.

Solution 9 - Ios

I know this is quite old already, but I ran into the exact same problem and it took me hours of trials and even to find this thread with the same problem (Xamarin Forms App)

My issue was: parsing remote HTML content into a string and also adding locally saved images (also downloaded dynamically, no resource of the app). On the simulator all works well, but on acutal device the local images are not showing (also no ? or anything indicating an error, just a blank frame). The Xamarin webview also offers the "BaseURL" option which didn't help, also not to use the BaseURL on the custom iOS wkWebView.

The only working solution as pointed out by Scott above, is to write the HTML into a file and then use the "LoadFileUrl" function and allow read access to the base directory. This also works with absolute file paths for images in the HTML (not only relative to the basedir, but of course somewhere within the basedir).

My custom webview renderer to load web and local content looks like this now:

protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e) {
    base.OnElementPropertyChanged(sender, e);
    NSUrl baseURL = new NSUrl(App.dirNews, true);
    string viewFile = Path.Combine(App.dirNews, "view.html");
    NSUrl fileURL = new NSUrl(viewFile, false);

    switch (e.PropertyName) {
        case "Url":
            System.Console.WriteLine("--- Loading Web page ---");
            System.Console.WriteLine("--- " + Element.Url + " ---");
            NSUrlRequest myRequest = new NSUrlRequest(new NSUrl(Element.Url), NSUrlRequestCachePolicy.ReloadIgnoringLocalAndRemoteCacheData, 120);
            Control.LoadRequest(myRequest);
            break;

        case "HTML":
            System.Console.WriteLine("--- Showing HTTP content ---");
            File.WriteAllText(viewFile, Element.HTML, System.Text.Encoding.UTF8);
            Control.LoadFileUrl(fileURL, baseURL);
            break;
    }
}

Solution 10 - Ios

I was able to reproduce a similar issue. WKWebView loads my images specially if they are located remotely, apart from my app server.

For servers that are not SSL-secured (http instead of https), you can set your info.plist as per below:

App Transport Security Settings

 - Allow Arbitrary Loads in Web Content (Set to YES)
 - Allow Arbitrary Loads (Set to YES)

The problem was actually in the server. The server application was either:

Changing the image src from "http://IP-or-domain/uploads/file.jpg" to "../../uploads/file.jpg"

- OR -

The image src was "http://localhost/uploads/file.jpg" or "http://127.0.0.1/uploads/file.jpg" instead of "http://YOUR-SERVER-IP-ADDRESS/uploads/file.jpg"

In these cases, the actual device wont be able to locate the image. This only works with iOS Simulator because the virtual device is the same as the server and development machine. It can read LOCALHOST and 127.0.0.1.

In my server, I was using a Rich Text Editor (TinyMCE) and it automatically removes the IP address after it detects that it's the same source.

Solution 11 - Ios

WKWebView can load image or css file from NSTemporaryDirectory, so you can copy your files to NSTemporaryDirectory, and then load it. It works for me on iOS 14! see this issue. ios-wkwebview-loadhtmlstring-baseurl-fails-to-load-images-and-read-css

Content Type	Original Author	Original Content on Stackoverflow
Question	Mischa	View Question on Stackoverflow
Solution 1 - Ios	Petro Korienev	View Answer on Stackoverflow
Solution 2 - Ios	Scott Browne	View Answer on Stackoverflow
Solution 3 - Ios	tech4242	View Answer on Stackoverflow
Solution 4 - Ios	Chris Brandsma	View Answer on Stackoverflow
Solution 5 - Ios	ianyh	View Answer on Stackoverflow
Solution 6 - Ios	kowal	View Answer on Stackoverflow
Solution 7 - Ios	moliveira	View Answer on Stackoverflow
Solution 8 - Ios	lbear9090	View Answer on Stackoverflow
Solution 9 - Ios	Kerry	View Answer on Stackoverflow
Solution 10 - Ios	Keith Manilla	View Answer on Stackoverflow
Solution 11 - Ios	Phil	View Answer on Stackoverflow