How do I UnSnap a Snapped app?

Question

I registered to the OnSizeChanged event of my Page, like that:

private void OnSizeChanged(object sender, SizeChangedEventArgs e)
{
    ApplicationViewState myViewState = ApplicationView.Value;
    if (myViewState == ApplicationViewState.Snapped)
    {
         Windows.UI.ViewManagement.ApplicationView.TryUnsnap();                
    }
}  

I'm tyring to set the application-view to Filled/Portrait state when the user trying (manually....) to resize it to snapped view. but the TryUnsnap method fails and it stays in snapped state...

Help!

Thanks.

Answer 1

To understand TryUnsnap() we need to understand the 2 types of Windows 8 events:

1. Programmatic events

   Programmatic events do not require the user to do anything. For example the Loaded event of a Page or the Tick event of a Timer.

2. User-initiated events

   User-initiated events require the user to do something. For example the Click event of a Button or the Tapped event of a Control.

The important part

Depending on the type of event, only certain Windows 8 APIs can be called. Adding a Secondary Tile, for example. And (as you might have guessed) un-Snapping an app.

That means you can call those APIs all you want from programmatic events but they will never deliver the results you desire. Unsnap in the StateChanged event, and it will fail for this reason. Unsnap in the Button.Click event, and it will succeed for this reason.

The rationale behind this behavior is the user experience. If the app can change it's 'orientation' on the user without the user's interaction then the behavior of the app becomes both confusing and unpredictable. Windows 8 is a pro-user operating system. When you discover developer 'constraints', 99% of the time it is this philosophy behind it.

Let me demonstrate:

If you attached to the StateChanged event, your code would look like this:

this.ApplicationViewStates.CurrentStateChanged += (s, args) =>
{
    System.Diagnostics.Debug.WriteLine("After StateChanged: {0}", this.ApplicationViewStates.CurrentState.Name);
    if (this.ApplicationViewStates.CurrentState == this.Snapped)
    {
        System.Diagnostics.Debug.WriteLine("Before Unsnap: {0}", this.ApplicationViewStates.CurrentState.Name);
        Unsnap();
    }
};

However, the resulting output (in the debugger) would look like this:

After StateChanged: FullScreenLandscape
After StateChanged: Snapped
Before Unsnap: Snapped
After TryUnsnap: Snapped

This is frustrating for a developer who does not understand the difference between programmatic and user-initiated events in Windows 8. The API appears to "not work" when, in fact, it works perfectly. Just not like they want it to.

If you attached to the Click event, your code would look like this:

MyButton.Click += (s, args) =>
{
    System.Diagnostics.Debug.WriteLine("After Button.Click: {0}", this.ApplicationViewStates.CurrentState.Name);
    if (this.ApplicationViewStates.CurrentState == this.Snapped)
    {
        System.Diagnostics.Debug.WriteLine("Before Unsnap: {0}", this.ApplicationViewStates.CurrentState.Name);
        Unsnap();
    }
};

Then, the resulting output would look like this:

After Button.Click: Snapped
Before Unsnap: Snapped
After TryUnsnap: Snapped
After StateChanged: FullScreenLandscape

This gets you what you want, but it brings up an important point. See how After TryUnsnap the state REMAINS "Snapped"? The transition of a Visual State from one to another is not a synchronous event. Calling for a change takes an unpredictable amount of time. It's probably done with a dispatch post, but I would have to check to be sure.

Having said all that, the state does change. And, after the change the CurrentStateChanged event is raised and you can handle the new Snapped state. By the way, it does not matter if there is another snapped app, this works either way.

The MSDN docs say it only works when it is in the foreground. This is pretty stupid since user interaction can't occur on a background app, and background apps have their threads suspended anyway. But, to be fair to MSDN, this API does not work when your app is in the background - whatever that's worth.

I hope this helps clear it up.

And now to your question:

You want to go from Snapped to Portrait? Of course in Portrait, Snapped is not possible so this is not a possibility for you to code. You want to go from Snapped to Filled as soon as the app is snapped. The event raised from the Snapped action is a programmatic event. As a result, you have to lure the user into doing something in your UI first. So, no you can't do what you are asking. You can't Unsnap() until the user interacts with your app somehow (like a button click event).

Oh, and here's the Unsnap() method if you wanted to reference all my code. I am not doing anything special, but you might be interested:

void Unsnap()
{
    if (Windows.UI.ViewManagement.ApplicationView.TryUnsnap())
        // successfully unsnapped
        System.Diagnostics.Debug.WriteLine("After TryUnsnap: {0}", this.ApplicationViewStates.CurrentState.Name);
    else
        // un-successfully unsnapped
        System.Diagnostics.Debug.WriteLine("After TryUnsnap: {0}", this.ApplicationViewStates.CurrentState.Name);
}

Have a great day and best of luck!

Answer 2

            var CurrentSnappedState = ApplicationView.Value;
            if (CurrentSnappedState == ApplicationViewState.Snapped && !ApplicationView.TryUnsnap())
            {
                return;
            }

Should do the trick. Remember that you can still snap the page, but when you try to do anything in the snapped page you will be redirected to the fullview.