iOS retrieving javascript variables from UIWebView

Question

I have an html file that has a <div> and that I am loading into a UIWebView and I need to know how many lines of text are in the div, so I can check that with javascript:

<script>
function countLines() {
    var divHeight = document.getElementById('myDiv').offsetHeight;
    var lineHeight = parseInt(document.getElementById('myDiv').style.lineHeight);
    var lines = divHeight / lineHeight;
    alert("Lines: " + lines);
}
</script>

and it does alert the variable "lines"

In Objective-C how can I retrieve this variable from my UIWebView and then use it in my code?

Thankyou!

John Cromartie · Accepted Answer · 2013-01-15 19:21:49Z

8

The UIWebView method stringByEvaluatingJavaScriptFromString: is the sole interface to the JavaScript environment. It evaluates and returns a string representation of the value of a JavaScript expression you pass to it. So, for example:

// simplified expression, no function needed
NSString *expr = @"document.getElementById('myDiv').offsetHeight / document.getElementById('myDiv').style.lineHeight";
// now pass it to the web view and parse the result
int lines = [[self.webView stringByEvaluatingJavaScriptFromString:expr] intValue];

edited Jan 15, 2013 at 19:21

answered Jan 15, 2013 at 16:58

John Cromartie

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Lefteris · Accepted Answer · 2013-01-15 18:49:50Z

3

This is easy:

NSString *jsString = @"function countLines() {var divHeight = document.getElementById('myDiv').offsetHeight;var lineHeight = parseInt(document.getElementById('myDiv').style.lineHeight);var lines = divHeight / lineHeight;return lines;};countLines();";

NSString *responseString = [MywebView stringByEvaluatingJavaScriptFromString:jsString];

And you should have the value in the responseString variable, which you can convert from a NSString to number :-)

edited Jan 15, 2013 at 18:49

answered Jan 15, 2013 at 16:57

Lefteris

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3 Comments

John Cromartie Over a year ago

The <script> tags will cause this to fail, and there's no need to define a function just to call it. It could be an IIFE or a simple expression.

Lefteris Over a year ago

You might need to call the function at a custom JavaScript event and that's why I kept it...

Lefteris Over a year ago

@JohnCromartie You are correct about the script tags. Editing to remove them

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