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: , ,