Objective-C calling javascript in UIWebView calling back Objective-C; threading issues - ios

Question

I have a UIWebView-based application that is storing some state. I have a native tab bar and upper bar however. What I need is that when I click on the native bar, I persist the data stored in the UIWebView.
To do this, I called evaluateJavaScriptByString to create a JSON object and it does a callback to objective-C via the UIWebViewDelegate protocol.
However, I find that the callback is asynchronous and hence my main transaction happens before the data is loaded.
Any idea how this problem can be solved?
Update: To explain the problem better:-
I have an HTML page with a form that someone puts some data into.
My Tab bar is native.
On the click of a button on the tab bar, I want the data from HTML saved to an Objective-C model, and the view should change to some other view.
What is happening is that once I click the button, the javascript call happens to the page (to create a JSON string to send to Objective-C) and this data does get saved to the obj-c model. However, this happens asynchronously. So my screen changes before the data is loaded into the model. If I refresh the next screen it shows the correct data. I was wondering if there was a way around this.
Note, I know how to call OBjective-C functions from WebViews. We are using JSOBjBridge for that anyway.

Answer 1

Just add something like this #"some_var = MAKE_JSON(); window.location = \"myapp://callback/\" + escape(some_var)" at the end of your javascript code, that you transfer to evaluateJavaScriptByString:
In UIWebView's delegate implement webView:shouldStartLoadWithRequest:navigationType: and catch all urls with myapp:// prefix like this:
- (BOOL) webView: (UIWebView *) webView
shouldStartLoadWithRequest: (NSURLRequest *) request
navigationType: (UIWebViewNavigationType) navigationType {
NSString *url = #"myapp://";
NSString *path = request.mainDocumentURL.relativePath;
NSString *callbackURL = #"callback/";
if ( [path hasPrefix:url] ) {
path = [path substringFromIndex:[url length]];
if ( [relPath hasPrefix:callbackURL] ) {
NSString *json = [path substringFromIndex:[callbackURL length]];
//TODO: Work with json
}
return NO;
}
return [super webView:webView shouldStartLoadWithRequest:request navigationType: navigationType];
}
I didn't test this code, I just written it from scratch, but it should work, I already did it in this way.