stringByEvaluatingJavaScriptFromString ios Hangs UI

Question

I have to call this method stringByEvaluatingJavaScriptFromString every second to update the UI of an Webview. My UI gets hang for a while when ever the method gets called. The following code snippet didn't solved my problem. Thanks in advance.

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    dispatch_async(dispatch_get_main_queue(), ^{

    [webView stringByEvaluatingJavaScriptFromString:string];

    });
    });

stringByEvaluatingJavaScriptFromString blocks the thread that it runs on. — Piepants
– Piepants, Commented May 6, 2014 at 19:39
Therefore you need to make your java script quicker, or broken down into smaller quick steps — Piepants
– Piepants, Commented May 6, 2014 at 19:56

random · Accepted Answer · 2014-05-06 19:20:20Z

0

Some searching around shows that wrapping in GCD block could potentially be a bug and doing this following helps:

[webView performSelectorOnMainThread:@selector(stringByEvaluatingJavaScriptFromString:) withObject:string waitUntilDone:NO];

However, stringByEvaluatingJavaScriptFromString: has to be called on the main thread which if your string variable is lengthy js there maybe no way to smooth it out from the obj-c side. You could try optimizing your js code.

answered May 6, 2014 at 19:20

random

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6 Comments

Piepants Over a year ago

How would this stop the gui from hanging?

random Over a year ago

Depending on how complex your js code is it won't. What I saw was that there was a bug filed in wrapping the call in blocks was causing a longer UI lock. stackoverflow.com/questions/11593900/…

Piepants Over a year ago

Useful to know. However I think the op has found the call to stringByEvalutingJavaScriptFromString was slow and so he wrapped it in dispatch_async because he thought that makes it run asynchronously and thus would stop the gui freezing. "The following code snippet didn't solved my problem" i.e. the code is an attempt at a solution to solve code that is already hanging

random Over a year ago

The problem is that stringByEvalutingJavaScriptFromString is part of UIKit which has to be called from the main thread. I saw some other hacks to try and get around it but nothing pretty. Also, I though the OP said the accepted answer fixed his problem??

random Over a year ago

Out of curiosity did you try changing it to performSelectorOnMainThread ?? I'm curious if it did anything at all

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