Is there a way to modify HTML content in a UIWebView using JavaScript and then take a screenshot of the updated WebView? Perhaps with the help of CGD?

Question

Is there a way to modify HTML content in a UIWebView using JavaScript and then take a screenshot of the updated WebView? Perhaps with the help of CGD?

I'm currently using a UIWebView to display a webpage and I'm trying to hide certain elements on the page, then take a screenshot of it. Once I have the image, I will unhide the elements to return the webView back to normal.

Here's my progress so far:

NSString * result1 = [webView stringByEvaluatingJavaScriptFromString:@"(function(){document.getElementById(\"bottom-bar\").style.display = 'none';return document.getElementById(\"bottom-bar\").style.display;})();"];
UIGraphicsBeginImageContextWithOptions(webView.scrollView.contentSize.width, NO, 0.0);
        {
            CGPoint savedContentOffset = webView.scrollView.contentOffset;
            CGRect savedFrame = webView.scrollView.frame;

            webView.scrollView.contentOffset = CGPointZero;
            webView.scrollView.frame = CGRectMake(0, 0, webView.scrollView.contentSize.width, webView.scrollView.contentSize.height);

            [webView.scrollView.layer renderInContext: UIGraphicsGetCurrentContext()];
            image = UIGraphicsGetImageFromCurrentImageContext();

            webView.scrollView.contentOffset = savedContentOffset;
            webView.scrollView.frame = savedFrame;
        }
        UIGraphicsEndImageContext();
NSString * result2 = [webView stringByEvaluatingJavaScriptFromString:@"document.getElementById(\"bottom-bar\").style.display = 'block';"];

Unfortunately, I'm not able to obtain the desired image.

Instead, I only get an image of the original webView without hiding the element.

When checking the console, I see the correct results "none" and "block", and the element "bottom-bar" briefly disappears from the screen.

I suspect there may be background threads or queues that affect the execution of

stringByEvaluatingJavaScriptFromString

or renderInContext. Is there a way for me to get accurate results?

UPDATED: Could GCD and Blocks help me capture the image? I understand that the webView runs JavaScript on a separate webThread and updates the UI on the main thread. Can I somehow wait until the webView finishes updating its UI before taking the screenshot? Is there a way to receive notification from the webThread when it completes a task?

javascript ios multithreading uiwebview grand-central-dispatch

Answer 1

Answer №1

From my understanding, the work involving internal UIWebView operations is carried out on a separate thread known as the WebThread, not on the main app thread. There are a couple of ways you could potentially tackle this issue:

One option is to delay the screen capture by utilizing something like dispatch_async, ensuring that it is called after the HTML element is hidden.
Alternatively, you can initiate a callback from JavaScript to native code once the element has been concealed. You can achieve this by adding a simulated location change at the conclusion of the initial JavaScript call.

window.location.href='app://captureScreenShot';

Subsequently, intercept this request in the

webView:shouldStartLoadWithRequest:navigationType

delegate method, handle your screen capture tasks, and then terminate the request.

- (BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)request navigationType:(UIWebViewNavigationType)navigationType {
NSURL *url = request.URL;
if ([url.scheme isEqualToString:@"app"] && [url.host isEqualToString:@"captureScreenShot"]) {
    // Perform screenshot capture here
    return NO;
}
return YES;

}

By adopting this approach, you can ensure that the screen capture functionality always executes following the completion of the JavaScript operations.

Answer 2

From my understanding, the work involving internal UIWebView operations is carried out on a separate thread known as the WebThread, not on the main app thread. There are a couple of ways you could potentially tackle this issue:

One option is to delay the screen capture by utilizing something like dispatch_async, ensuring that it is called after the HTML element is hidden.
Alternatively, you can initiate a callback from JavaScript to native code once the element has been concealed. You can achieve this by adding a simulated location change at the conclusion of the initial JavaScript call.

window.location.href='app://captureScreenShot';

Subsequently, intercept this request in the

webView:shouldStartLoadWithRequest:navigationType

delegate method, handle your screen capture tasks, and then terminate the request.

- (BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)request navigationType:(UIWebViewNavigationType)navigationType {
NSURL *url = request.URL;
if ([url.scheme isEqualToString:@"app"] && [url.host isEqualToString:@"captureScreenShot"]) {
    // Perform screenshot capture here
    return NO;
}
return YES;

}

By adopting this approach, you can ensure that the screen capture functionality always executes following the completion of the JavaScript operations.

Answer 3

Answer №2

To integrate a javascript function into the webview, you can write it as an NSString in Objective-C.

Here is an example:

int zoom = 10;
NSString *zoomString = [NSString stringWithFormat:@"document.getElementsByTagName('body')[0].style.webkitTextSizeAdjust= '%d%%'", zoom];
[self.webView stringByEvaluatingJavaScriptFromString:zoomString];

This code snippet is particularly useful for capturing screenshots.

Answer 4

To integrate a javascript function into the webview, you can write it as an NSString in Objective-C.

Here is an example:

int zoom = 10;
NSString *zoomString = [NSString stringWithFormat:@"document.getElementsByTagName('body')[0].style.webkitTextSizeAdjust= '%d%%'", zoom];
[self.webView stringByEvaluatingJavaScriptFromString:zoomString];

This code snippet is particularly useful for capturing screenshots.

Is there a way to modify HTML content in a UIWebView using JavaScript and then take a screenshot of the updated WebView? Perhaps with the help of CGD?

Answer №1

Answer №2

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