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Introduction

WebView is an essential component in Android for displaying web content in mobile apps. It allows developers to display web pages, HTML content, and other web-based resources inside their apps. However, it also provides a way for developers to intercept web requests and load local files, which can be useful for a variety of purposes.

Usecase

One common use case for intercepting web requests is to handle authentication. For example, if a user is required to log in to view certain web content, the app can intercept the request and present a login screen to the user. Once the user enters their credentials, the app can then modify the request and include the necessary authentication information before sending it to the web server.

Another use case is to load local resources when a specific web request is made. For example, if the user has selected a particular theme for the app, the app can intercept web requests for CSS files and load the local CSS file that matches the theme.

To intercept web requests in an Android WebView and load local files, you can use the WebViewClient.shouldInterceptRequest() method and the AssetManager class. The shouldInterceptRequest method is called for each resource requested by WebView, and it allows the app to handle the request. The AssetManager class can be used to access local files in the app’s assets folder.

Examples

Here is an example of how to use the shouldInterceptRequest() method and the AssetManager class to load a local CSS file when a specific web request is made:

WebView webView = findViewById(R.id.web_view);
webView.setWebViewClient(new WebViewClient() {
    @Override
    public WebResourceResponse shouldInterceptRequest(WebView view, WebResourceRequest request) {
        // Check if the request is for a specific resource
        if (request.getUrl().getPath().equals("/css/styles.css")) {
            try {
                // Load the local CSS file from the assets folder
                AssetManager assetManager = getAssets();
                InputStream inputStream = assetManager.open("styles.css");
                return new WebResourceResponse("text/css", "UTF-8", inputStream);
            } catch (IOException e) {
                e.printStackTrace();
                return null;
            }
        }
        return super.shouldInterceptRequest(view, request);
    }
});
webView.loadUrl("https://example.com");

In this example, the app is checking if the request is for the path /css/styles.css and if it is, it loads the local CSS file from the assets folder, otherwise, it is returning the default response.

Loading a local image file:
if (request.getUrl().getPath().endsWith(".jpg")) {
    try {
        // Load the local image file from the assets folder
        AssetManager assetManager = getAssets();
        InputStream inputStream = assetManager.open("image.jpg");
        return new WebResourceResponse("image/jpeg", "UTF-8", inputStream);
    } catch (IOException e) {
        e.printStackTrace();
        return null;
    }
}
Loading a local JavaScript file:
if (request.getUrl().getPath().endsWith(".js")) {
    try {
        // Load the local JavaScript file from the assets folder
        AssetManager assetManager = getAssets();
        InputStream inputStream = assetManager.open("script.js");
        return new WebResourceResponse("application/javascript", "UTF-8", inputStream);
    } catch (IOException e) {
        e.printStackTrace();
        return null;
    }
}
Loading a local HTML file:
if (request.getUrl().getPath().endsWith("/about")) {
    try {
        // Load the local HTML file from the assets folder
        AssetManager assetManager = getAssets();
        InputStream inputStream = assetManager.open("about.html");
        return new WebResourceResponse("text/html", "UTF-8", inputStream);
    } catch (IOException e) {
        e.printStackTrace();
        return null;
    }
}

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You can see in the above examples, the app checks the URL path of the request to determine if it should load a local file. Then it uses the AssetManager class to open the local file from the assets folder and returns it as a WebResourceResponse.

It’s also worth noting that you should use the shouldInterceptRequest() method judiciously, as it can slow down the performance of your webview if you are intercepting too many requests or if the files you’re loading are large.

Intercepting web requests and loading local files can be a powerful tool for customizing the behaviour of a WebView, but it should be used with care. It’s essential to understand the implications of handling requests and to test the app thoroughly to ensure that it behaves as expected.

Conclusion

In conclusion, intercepting web requests and loading local files using Android WebView provides a great way to handle authentication, load resources based on user preferences, and many other use cases. With the help of the shouldInterceptRequesta() method and the AssetManager class, developers can achieve this easily. However, it is important to be mindful of the implications of handling requests and to test the app thoroughly to ensure that it behaves as expected.

This article was previously published on proandroiddev.com

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