iPhone SDK Tip: Firing custom events when a link is clicked in a UIWebView

Well I have been asked this a few times and I can see how it would be a very helpful addition to theUIWebView’s feature set. What I am talking about is adding the ability to intercept the event generated when a user selects a link in a webpage being displayed with a UIWebView, this will then allow you to perform any action that you want.
When would you use this I hear you say? Well one person asked me if I knew how to append the users current location onto all HTTP requests? and another just wanted to know how to perform custom actions from an embedded UIWebView.

Once again I will be extending the “Build your very own Web Browser!”, if you haven’t already completed it then I suggest that you head over there now and spend 5 minutes reading it and then downloading the project files at the end.
Start by opening up the WebBrowserTutorialAppDelegate.h file and editing the @interface line to read:

@interface WebBrowserTutorialAppDelegate : NSObject  {

What we have done is to make the main AppDelegate a delegate for the UIWebView as well.
Now we need to set our webView to have the main AppDelegate as its delegate, you can do this by opening up WebBrowserTutorialAppDelegate.m and putting the following line just inside theapplicationDidFinishLaunching function:

webView.delegate = self;

That is all pretty self explanatory, it just sets the delegate of our webView to self, which in this case is our main application delegate.
Now we are pretty much done, we just need to add the function to catch the link clicks. To do this we need to add a new function, copy the content below to the WebBrowserTutorialAppDelegate.m file:

- (BOOL)webView:(UIWebView*)webView 

shouldStartLoadWithRequest:(NSURLRequest*)request 

navigationType:(UIWebViewNavigationType)navigationType {
 NSURL *url = request.URL;
 NSString *urlString = url.absoluteString;
 NSLog(urlString);
 return YES;
}

This function will catch all requests and allow you to either manipulate them and pass them on or to perform your own custom action and stop the event from bubbling.
The first line gets the URL of the request, this is the contents inside the href attribute in the anchor tag.
The next line converts the URL to a string so we can log it out. You can access many parts of the NSURL, here are some of them and brief description of what they do.

* absoluteString – An absolute string for the URL. Creating by resolving the receiver’s string against its base.
* absoluteURL – An absolute URL that refers to the same resource as the receiver. If the receiver is already absolute, returns self.
* baseURL – The base URL of the receiver. If the receiver is an absolute URL, returns nil.
* host – The host of the URL.
* parameterString – The parameter string of the URL.
* password – The password of the URL (i.e. http://user:pass@www.test.com would return pass)
* path – Returns the path of a URL.
* port – The port number of the URL.
* query – The query string of the URL.
* relativePath – The relative path of the URL without resolving against the base URL. If the receiver is an absolute URL, this method returns the same value as path.
* relativeString – string representation of the relative portion of the URL. If the receiver is an absolute URL this method returns the same value as absoluteString.
* scheme – The resource specifier of the URL (i.e. http, https, file, ftp, etc).
* user – The user portion of the URL.

Then the third line simply logs the URL to the console, so you will new to open up the console while you run this in the simulator to see the results.
Finally the forth line returns YES, this will allow the UIWebView to follow the link, if you would just like to catch a link and stop the UIWebView from following it then simply return NO.