Access the JavaScriptCore engine of a UIWebView

Question

I just discovered a new framework available in iOS7: JavaScriptCore.

It looks awesome, but how can I access the runtime/context of a UIWebView?

Answer 1

There is no official mechanism for this. But there are two approaches I know about to get the JSContext. I probably wouldn't use either in a shipping app.

1. Use KVC to access internal UIWebView properties. Explained in detail in this blog post: http://blog.impathic.com/post/64171814244/true-javascript-uiwebview-integration-in-ios7

JSContext *ctx = [webView valueForKeyPath:@"documentView.webView.mainFrame.javaScriptContext"];

2. Add a magic method to NSObject via a category. Explained in more detail, here: https://github.com/TomSwift/UIWebView-TS_JavaScriptContext

@implementation NSObject (magic)
- (void) webView: (id) unused didCreateJavaScriptContext: (JSContext*) ctx forFrame: (id) frame
{
    // ...
}
@end

Answer 2

You might like to look at the WWDC 2013 Session Videos on the developer web-site (https://developer.apple.com/wwdc/videos/) and in particular 'Integrating JavaScript into Native Apps' which to quote the description "Introducing a new Objective-C API to JavaScriptCore. iOS developers can now integrate scripting into their apps without having to bundle custom language interpreters. This API builds on top of the existing C API to JavaScriptCore available on Mac, and makes programming with JavaScript much easier and less error-prone." There is a section on using it with WebView e.g. using: -webView:didCreateJavaScriptContext:forFrame:

Answer 3

JavaScriptCore exists so that developers have a JavaScript runtime to use from Objective-C. The only way I'm aware of to access the UIWebView runtime is through the stringByEvaluatingJavaScriptFromString: method, which obviously isn't very flexible.