/// <summary>
/// Provides storage for session state associated with the specified <see cref="AlternativeFrame"/>.
/// Frames that have been previously registered with <see cref="RegisterFrame"/> have
/// their session state saved and restored automatically as a part of the global
/// <see cref="SessionState"/>. Frames that are not registered have transient state
/// that can still be useful when restoring pages that have been discarded from the
/// navigation cache.
/// </summary>
/// <remarks>Apps may choose to rely on <see cref="LayoutAwarePage"/> to manage
/// page-specific state instead of working with frame session state directly.</remarks>
/// <param name="frame">The instance for which session state is desired.</param>
/// <returns>A collection of state subject to the same serialization mechanism as
/// <see cref="SessionState"/>.</returns>
public static Dictionary<String, Object> SessionStateForFrame(AlternativeFrame frame)
{
var frameState = (Dictionary<String, Object>)frame.GetValue(FrameSessionStateProperty);
if (frameState == null)
{
var frameSessionKey = (String)frame.GetValue(FrameSessionStateKeyProperty);
if (frameSessionKey != null)
{
// Registered frames reflect the corresponding session state
if (!_sessionState.ContainsKey(frameSessionKey))
{
_sessionState[frameSessionKey] = new Dictionary<String, Object>();
}
frameState = (Dictionary<String, Object>)_sessionState[frameSessionKey];
}
else
{
// Frames that aren't registered have transient state
frameState = new Dictionary<String, Object>();
}
frame.SetValue(FrameSessionStateProperty, frameState);
}
return frameState;
}
/// <summary>
/// Registers a <see cref="AlternativeFrame"/> instance to allow its navigation history to be saved to
/// and restored from <see cref="SessionState"/>. Frames should be registered once
/// immediately after creation if they will participate in session state management. Upon
/// registration if state has already been restored for the specified key
/// the navigation history will immediately be restored. Subsequent invocations of
/// <see cref="RestoreAsync"/> will also restore navigation history.
/// </summary>
/// <param name="frame">An instance whose navigation history should be managed by
/// <see cref="SuspensionManager"/></param>
/// <param name="sessionStateKey">A unique key into <see cref="SessionState"/> used to
/// store navigation-related information.</param>
public static void RegisterFrame(AlternativeFrame frame, String sessionStateKey)
{
if (frame.GetValue(FrameSessionStateKeyProperty) != null)
{
throw new InvalidOperationException("Frames can only be registered to one session state key");
}
if (frame.GetValue(FrameSessionStateProperty) != null)
{
throw new InvalidOperationException("Frames must be either be registered before accessing frame session state, or not registered at all");
}
// Use a dependency property to associate the session key with a frame, and keep a list of frames whose
// navigation state should be managed
frame.SetValue(FrameSessionStateKeyProperty, sessionStateKey);
_registeredFrames.Add(new WeakReference<AlternativeFrame>(frame));
// Check to see if navigation state can be restored
RestoreFrameNavigationState(frame);
}