UIWebView JSContext

gistfile1.txt

- (void)observeJSContext
{
    CFRunLoopRef runLoop = CFRunLoopGetCurrent();
    
    // This is a idle mode of RunLoop, when UIScrollView scrolls, it jumps into "UITrackingRunLoopMode"
    // and won't perform any cache task to keep a smooth scroll.
    CFStringRef runLoopMode = kCFRunLoopDefaultMode;
    
    CFRunLoopObserverRef observer = CFRunLoopObserverCreateWithHandler
    (kCFAllocatorDefault, kCFRunLoopBeforeWaiting, true, 0, ^(CFRunLoopObserverRef observer, CFRunLoopActivity _) {
        JSContext *context = [_web valueForKeyPath:@"documentView.webView.mainFrame.javaScriptContext"];
        if (![_context isEqual:context]) {
            CFRunLoopRemoveObserver(runLoop, observer, runLoopMode);
            _context = context;
            _context[@"iOS"] = self;
            void(^alert)(NSString *) = ^(NSString *message) {
                NSLog(@"%@", message);
            };
            _context[@"Window"][@"prototype"][@"alert"] = alert;
        }
    });
    
    CFRunLoopAddObserver(runLoop, observer, runLoopMode);
}

- (BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)request navigationType:(UIWebViewNavigationType)navigationType
{
    [self observeJSContext];
}