Set the height of a UIWebView to the height of it’s HTML content

Sometimes you need to know the height of a html document that is loaded into a UIWebView. For example if you want to set the height of the UIWebView to the height of its content.

The logical way would be this:

1. add a Javascript function to the HTML that returns the height of the document.
2. add the call to this Javascript function to the UIWebViewDelegate’s method webViewDidFinishLoad:

This sounds easy, but if you look at the results you’ll realize that the values for the document height are not correct and pretty random.

The problem is: webViewDidFinishLoad: get’s called when the HTML is fully loaded BUT it still has to be fully rendered before you can determine its height!

So the call comes too early. You could delay the call but that’s not the way to go here as this is still unreliable and you want the height as soon as possible.

The solution is to revert the process. Instead of Objective-C asking the Javascript for the height, have the Javascript call Objective-C as soon as it knows the height of the document.

Here’s how to do it:

1. Add a Javascript function to the HTML document

Add this function either to the head or the body of your HTML

This function gets called as soon as the HTML document is fully rendered. It puts the height into an URL and sends a request with this URL.
If you are asking yourself why I use “ready://” instead of “http://”: I do this because sending a request is the only way, how the Javascript inside a UIWebView can send messages to the UIWebViewDelegate. So this is not a “real” HTTP Request. Instead you can use the URL scheme to make things easier on the Objective-C part (as you will see in step 2).

2. Intercept the request in the UIWebView’s delegate

A UIWebViewDelegate has a method, that get’s called everytime the HTML inside the UIWebView sends a request. Here’s what to do in this method:

- (BOOL)webView:(UIWebView*)webView shouldStartLoadWithRequest:(NSURLRequest*)request navigationType:(UIWebViewNavigationType)navigationType { 
  NSURL *url = [request URL]; 
  if (navigationType == UIWebViewNavigationTypeOther) { 
    if ([[url scheme] isEqualToString:@"ready"]) { 
      float contentHeight = [[url host] floatValue]; 
      CGRect fr = webview.frame; 
      fr.size = CGSizeMake(webview.frame.size.width, contentHeight); 
      webview.frame = fr; 
      return NO; 
    } 
  }
  return YES; 
}

Here you can see why I used the custom URL scheme “ready”. It makes it easy to identify my request. The URL that the javascript requested has the format “ready://1200” meaning that the HTML content is 1200px high. So I use the scheme “ready” to identify my request and the “host” part of the URL to sent the actual height. Then it’s easy to set the height of the UIWebView to the height of the HTML document’s content.

Return NO to stop the UIWebView from trying to load the request. Don’t forget to return YES for all other requests or the HTML content won’t even get loaded into the UIWebView in the first place.