UIWebView stringByEvaluatingJavaScriptFromString in background - objective-c

Question

In an iOS app, I'm running a fairly large script on a UIWebView using stringByEvaluatingJavaScriptFromString (large in terms of the length of the javascript string). There is a brief pause after calling the javascript causing other elements on the screen to hiccup for a moment.
Placing the javascript call in a function called in the background with self performSelectorInBackground breaks the application. Is there a safe way to call run this on a background thread or otherwise prevent the interface from pausing?

Answer 1

No, Webviews and the Webkit JavaScript engine are both single-threaded and cannot be used on a background thread.
A better option is to split up your JavaScript into discrete execution blocks and pipeline them using a JavaScript timer, like this (JS code, not Obj-C):
var i = 0;
var operation = function() {
switch (i) {
case 0:
//do first part of code
break;
case 1:
//do second part of code
break;
case 2:
//do third part of code
break;
etc...
}
//prepare to execute next block
i++;
if (i < TOTAL_PARTS) {
setTimeout(operation, 0);
}
};
operation();
That will prevent your script from blocking user interaction while it executes

Answer 2

Well, I was doing the same thing. I had to run a synchronous ajax request which was freezing my UI. So this is how I fixed it :
__block NSString *message;
dispatch_queue_t q = dispatch_queue_create("sign up Q", NULL);
dispatch_async(q, ^{
NSString *function = [[NSString alloc] initWithFormat: #"signup(\'%#\',\'%#\',\'%#\')",self.email.text,self.password.text,self.name.text];
dispatch_async(dispatch_get_main_queue(), ^{
NSString *result = [self.webView stringByEvaluatingJavaScriptFromString:function];
NSLog(#"%#",result);
if ([result isEqualToString:#"1"]) {
message = [NSString stringWithFormat:#"Welcome %#",self.name.text];
[self.activityIndicator stopAnimating];
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
}
else {
message = [NSString stringWithFormat:#"%# is a registered user",self.name.text];
[self.activityIndicator stopAnimating];
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
}
UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:#"Message" message:message delegate:self cancelButtonTitle:#"Okay" otherButtonTitles: nil];
[alertView show];
});
});
The logic is simple. Go to a new thread, and from within that, dispatch to the main queue and then do the JS work and everything worked like a charm for me...

Answer 3

Anything you do with a UIWebView must be done on the main thread. It's a UI element, so this is why performSelectorInBackground breaks your app.

Answer 4

You could try putting that call into an NSOperation. Since you are working with a UI element, be sure to use the [NSOperationQueue mainQueue].