How can I inject a user script on iOS?

Question

I'm working on Fahrii, which is, essentially, a userscript enabled mobile browser. At the moment, I'm having trouble invoking the scripts on a web page that's loaded into a UIWebView. I'd like to keep this public APIs if possible. If not, a proof of concept would at least be nice, so I can get a better understanding of how this works. I'm trying to do it in the webViewDidFinishLoad: method.

I've tried injecting by reading out the webView's content (using JavaScript) and then loading it back with loadHTML:baseURL:, but that causes infinite recursion. (A completed load causes a script to be injected, which is in turn causing a completed "load".)

Next, I've tried something like this:

    [self.browser stringByEvaluatingJavaScriptFromString:[NSString stringWithFormat:@"document.body.innerHTML = %@", originalHTMLWithScriptsInjected]];

The script seems to be injected, but not run.

The only way I got this to work was to use the ASIHTTPRequest library to make the actual requests, but then I ended up having trouble with login forms, besides for which, I have to make each request twice in that case.

I might be doing something that's either impossible, or I'm just doing it the wrong away. So, how can I invoke a user script on an existing UIWebView with content loaded from the web?

Edit:

Here's some more code, including the changes proposed by @rich:

 //
 //  Run the remaining scripts
 //

NSMutableString *scriptTextToInject = [[NSMutableString alloc]init];

//
//  Add each script into the page
//

for (Userscript *script in scriptsToExecute) {

    //
    //  Read the userscript
    //

    NSString *scriptToLoad = [NSString stringWithContentsOfURL:[NSURL URLWithString:script.pathToScript] encoding:NSUTF8StringEncoding error:&error];

    NSLog(@"Script to load: %@", scriptToLoad);

    [scriptTextToInject appendFormat:@"%@\n\n",scriptToLoad];
}

NSString *documentHeadBefore = [self.browser stringByEvaluatingJavaScriptFromString:@"document.getElementsByTagName('head')[0].innerHTML"];

//
//  Inject the scripts
//

NSString *scriptWrappedInATag = [NSString stringWithFormat:@"window.onload() = function(){var script = document.createElement('script');\n"
                                 "script.charset = 'UTF-8'"
                                 "script.setAttribute(\"type\",\"text/javascript\");\n"
                                 "text = \"function u(){\n"
                                 "%@"
                                 "}\";\n"
                                 "document.getElementsByTagName('head')[0].appendChild(script);}", scriptTextToInject];
NSLog(@"Scripts wrapped in a tag: %@", scriptWrappedInATag);

NSString *runResults = [self.browser stringByEvaluatingJavaScriptFromString:@"u();"];

NSString *documentHeadAfter = [self.browser stringByEvaluatingJavaScriptFromString:@"document.getElementsByTagName('head')[0].innerHTML"];

NSLog(@"Before: %@ \n\n\n\n After: %@", documentHeadBefore, documentHeadAfter);

Answer 1

An example of doing just this is below. I had the same problem until I realized that the js needed to be injected into the page. To properly inject the js you need to write it into the head of the dom as shown below.

- (void)webViewDidFinishLoad:(UIWebView *)webView {
 [webView stringByEvaluatingJavaScriptFromString:@"var script = document.createElement('script');"  
 "script.type = 'text/javascript';"  
 "script.text = \"function myFunction() { "  
 "window.scrollTo(100,100)"
 "}\";"  
 "document.getElementsByTagName('head')[0].appendChild(script);"];  

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