Nov 09

UIWebView has very few instance methods. One of them is stringByEvaluatingJavaScriptFromString, which very powerful and unfortunately poorly documented. (This is literally the extent of Apple’s explanation: “Returns the result of running a script.”)

Let’s explore this mysterious method with a couple of examples.

A trivial example of how to use stringByEvaluatingJavaScriptFromString is to get the title of the HTML document:

NSString *title = [webView stringByEvaluatingJavaScriptFromString:@"document.title"];

You would typically place this line of code in webViewDidFinishLoad.

This technique is not limited to one-liners, or accessing simple properties. Here’s an example of two lines of JavaScript code executed in order, as you would expect:

[webView stringByEvaluatingJavaScriptFromString:@"var field = document.getElementById('field_2');"
						 "field.value='Multiple statements - OK';"];

You can also call JavaScript functions this way. And if you want to call a JavaScript function that does not already exist in the web page that you’re downloading, you can “inject” it yourself with this technique:

[webView stringByEvaluatingJavaScriptFromString:@"var script = document.createElement('script');"
						 "script.type = 'text/javascript';"
						 "script.text = \"function myFunction() { "
							"var field = document.getElementById('field_3');"
							"field.value='Calling function - OK';"

[webView stringByEvaluatingJavaScriptFromString:@"myFunction();"];

In essence I’m using Objective C to create a string which represents JavaScript which which when executed adds a JavaScript function to the HTML DOM. Apologies for the multiple meta levels… Let me try to untangle this line by line.

Line 1 : First we create a <script> element using JavaScript.
Line 2 : Set the type of the <script> element to text/javascript.
Line 3-6 : Set the content of the <script> element to the JavaScript function that you want to inject.
Line 7 : Add the new <script> element as a child to the <head> element of the HTML DOM.
Line 9 : Call the new JavaScript function.

Bonus tip: You can break up NSString constants over multiple lines in Xcode for increased readability. Just end the line with a double-quote character and begin the next line with a double-quote character. At compile time these lines will be joined into one string. So the string that begins with “var script” on Line 1 is one continuous string ending with “appendChild(script);” on Line 7.

Although it’s not critical for the discussion above, here’s the HTML that the example JavaScript refers to:

    <meta name="viewport" content="width=320; initial-scale=1.0; maximum-scale=1.0"/>
    <p>This is the UIWebView</p>
      <input id="field_1" type="text" name="value" /><br/>
      <input id="field_2" type="text" name="value" /><br/>
      <input id="field_3" type="text" name="value" /><br/>

You can use this generic technique to add JavaScript to any web page that you’re downloading and displaying in a UIWebView. The technique and the possibilities are similar to what you can do with the Greasemonkey plugin for Firefox.

written by Nick \\ tags: , ,

Oct 30

For a recent project I encountered the following requirements:

  1. Download and store HTML documents that are rendered using the UIWebView class.
  2. Download and store the external documents referenced in the HTML, like images, so that they, along with the HTML documents, are available in non-networked situations.
  3. Use the same HTML documents and external documents as the body of in-app generated Emails.

The challenge was to devise a scheme whereby the same HTML documents can be used for both rendering content within the app under non-networked situations and to provide the exact same user experience within the Email body; a clear Catch-22!

There are a number of possible approaches (see page 10 of Nick’s presentation, but the one we choose was to use complete URL’s for all externally referenced documents within the HTML (for example, all images are referenced on a CDN). We choose this route as it provides the simplest way to support the Email requirement.

Next we had to figure out a way to modify these URL’s dynamically to reference the same external documents located locally on the device. To do this we use some features of UIWebView and Javascript.

First we set up the UIWebView and implement the webViewDidFinishLoad delegate method to kick off the Javascript. Below you can see we take advantage of the webview’s stringByEvaluatingJavaScriptFromString() method to call our Javascript event listener useLocalPaths().

- (void)webViewDidFinishLoad:(UIWebView *)webView {
  DLog(@"Local URLs used = %@",[webView stringByEvaluatingJavaScriptFromString: @"window.useLocalPaths()"]);

The Javascript for the event listener is the following:

<script type="text/javascript">
  window.addEventListener('load',function() {
    window.useLocalPaths = function() {
      var images = document.getElementsByTagName('img');
      for (var i=0; i<images.length; i++) {
        var url = images[i].src;
        if (url.length>0) {
          var bits = url.split('/');
          images[i].src = bits[bits.length-1];
      return true;

In this case, this event listener queries for all image tags and modifies the src attribute by stripping all of the URL except for the image file name. For example if our image tag looked like this,

<img src="" class="logo" alt="Pervasent" />

it would be come this

<img src="pervasent.jpg" class="logo" alt="Pervasent" />

and thus achieves our goal.

It’s worth noting that the general approach of using UIWebView + Javascript is quite handy and obviously has many possible applications.

Update: If you’re having timing issues with this approach, you may want to take a look at this technique: How To Inject JavaScript Functions Into A UIWebView

written by Jess \\ tags: ,