Update on Remote View Controllers

It’s time for another update on remote view controllers. For context, please read part 1 and part 2 of this series first.

Recreating Remote Views in iOS 5

Someone calling himself iKy1e noticed something in my logs that I paid little attention to: the _UIRemoteView that sits at the bottom of a remote view controller hierarchy uses a CALayerHost as its underlying Core Animation layer. According to iKy1e, a CALayerHost can not only display contents from another process; it also allows user interaction to pass from the host app to the remote process. In other words, this class is the key to transmitting both user actions and graphics across process boundaries.

With his knowledge of CALayerHost, iKy1e managed to recreate a very similar remote view architecture on iOS 5 (jailbreak required). As a replacement for XPC, he uses Apple’s private CPDistributedNotificationCenter, which also allows communication between processes. The implementation isn’t perfect (for example, the keyboard doesn’t appear in the remote process; you can type blindly, however) but nevertheless it’s a very impressive demo.


With everything we have learned about remote view controllers, we should now be able to present a web view in a remote process. The hope is that _UIWebViewController is the soon-to-be-public entry point. When we create an instance, it should in turn set up an internal XPC connection and manage a _UIRemoteWebViewController for us. The code is straightforward:

- (IBAction)openWebView:(id)sender
    _UIWebViewController *controller = [[_UIWebViewController alloc] init];
    controller.delegate = self;

    NSURL *url = [NSURL URLWithString:@"http://google.com"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    [controller loadRequest:request];

    [self presentViewController:controller animated:YES completion:^{
        // Log the view hierarchy here to see what's going on
        NSLog(@"View hierarchy: %@", [controller.view recursiveDescription]);

Sadly, this does not work. We seem to be on the right track, though. The log messages in the swizzled +requestViewController:­fromServiceWithBundleIdentifier:­connectionHandler: method prove that _UIWebViewController calls the method with seemingly the correct arguments:

self: _UIRemoteWebViewController
arg1: __NSCFConstantString _UIServiceWebViewController
arg2: __NSCFConstantString com.apple.WebViewService
arg3: __NSStackBlock__ <__NSStackBlock__: 0x2fdaa6bc>
Return Value: _UIAsyncInvocation <_UIAsyncInvocation: 0x1e0376f0>

We would expect to receive a valid _UIRemoteViewController in the connectionHandler block. Instead, the block arguments are nil and an NSError with a rather unhelpful error message:

blockArg1: (null) (null)
blockArg2: NSError Error Domain=XPCObjectsErrorDomain Code=2 "The operation couldn’t be completed. (XPCObjectsErrorDomain error 2.)"
Failed to get remote view controller with error: Error Domain=XPCObjectsErrorDomain Code=2 "The operation couldn’t be completed. (XPCObjectsErrorDomain error 2.)"

This at least confirms my earlier assumption that the second parameter of the connectionHandler block is an error object.

The only explanation I can think of why the above code does not work is that Apple has not yet completed the implementation of _UIRemoteWebViewController and its corresponding XPC service. That’s certainly a possibility, though I wonder why Apple would include code in the shipping version of iOS 6 that is so far from finished that Apple does not even use it itself.

I’d love to hear from you if you have any other idea what I might be doing wrong.