Understanding iOS Web ViewCSS Styling Limitations

As an aspiring iOS developer, you’ve encountered a common challenge when trying to customize the appearance of websites displayed in your app’s UIWebView or WKWebView. The question on everyone’s mind is: “Can I change the CSS of an external site to make it more mobile-friendly?”

Understanding Web Views

Before diving into the CSS styling limitations, let’s take a brief look at what UIWebView and WKWebView are. Both are designed to display web content within your iOS app.

• UIWebView: The first generation of web view on iOS. It is now deprecated in favor of WKWebView.
• WKWebView: Apple introduced this new generation of web views with the release of Xcode 9.0, as part of iOS 10. It provides a more secure and efficient way to display web content.

CSS Styling Limitations

Unfortunately, changing the CSS of an external site is not as straightforward as you might expect. The main reason lies in how web views handle CSS styling:

• No Access to Page Source: When using UIWebView or WKWebView, you don’t have direct access to the HTML source code of the webpage.
• CSS Rendering: Web views use a custom renderer, which is responsible for rendering the webpage. The renderer’s behavior and capabilities are controlled by Apple and can limit customization options.

Given these constraints, it might seem like changing CSS isn’t an option. However, we’ll explore alternative approaches to achieve your goal.

Using Proxies

One possible solution involves creating a proxy server that modifies the CSS of external sites before forwarding the request to the original server. This approach has its advantages and disadvantages:

Advantages:

• Flexibility: With a proxy server, you can customize the CSS styles as needed.
• Security: By controlling the requests and responses, you can protect against malicious content.

Disadvantages:

• Complexity: Setting up a proxy server requires more development effort and expertise.
• Overhead: Creating an additional server to handle HTTP requests introduces latency and may impact performance.

Alternative Approaches

While creating a proxy server is one viable solution, there are other options you can explore:

1. CSS Spritesheets:
   • If the website uses a fixed-width sprite sheet for its graphics, you might be able to manipulate this asset directly in your app.
2. JavaScript and CSS Injection:
   • With some JavaScript expertise, you could inject custom styles into the webpage using JavaScript injection or similar techniques. However, this is more complex and may not work across all websites due to security measures.

iOS SDK and HTML5 APIs

In recent years, Apple has introduced various APIs and tools that make it easier to manipulate web content in your app:

• WKWebView’s evaluateJavaScript method:
  • Allows you to execute custom JavaScript code on the webpage.
• WKWebView’s accessFrameName method:
  • Enables access to specific frame elements within a webpage, which can be useful for injecting styles.

While these APIs provide more flexibility than before, they still don’t allow direct access to CSS styling. You’ll need to use JavaScript to manipulate the DOM and apply your custom styles.

Example: Injecting Custom Styles Using evaluateJavaScript

Here’s an example of how you could inject custom styles into a webpage using evaluateJavaScript:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let script = "document.head.insertAdjacentHTML('beforeend', '<style>body { background-color: #f2f2f2; }</style>');"
        evaluateJavaScript(script) { (result, error) in
            if let error = error {
                print("Error injecting styles: \(error)")
            }
        }
    }

}

In this example, we’re using evaluateJavaScript to execute a JavaScript script that injects custom styles into the webpage.

Conclusion

Changing the CSS of an external site is not as straightforward as you might expect. While creating a proxy server provides flexibility and security benefits, it introduces complexity and overhead. Alternative approaches like CSS spritesheets, JavaScript injection, and iOS SDK APIs can help you achieve your goal, but require more expertise and development effort.

When working with web views on iOS, keep in mind the limitations of CSS styling and explore alternative solutions to find the best approach for your project.

Last modified on 2023-06-18