public async Task <bool> GoBack()
{
if (_isNavigating)
{
throw new InvalidOperationException("Navigation already in progress.");
}
_isNavigating = true;
this.CanNavigate = false;
this.CanGoBack = false;
try
{
await this.WaitForLoadedAsync();
AlternativePage currentPage = null;
var currentJe = this.BackStack.Pop();
var je = BackStack.Peek();
if (_currentPagePresenter != null)
{
currentPage = (AlternativePage)_currentPagePresenter.Content;
var cancelArgs =
new AlternativeNavigatingCancelEventArgs(
NavigationMode.Back,
je.Type);
await currentPage.OnNavigatingFromInternal(cancelArgs);
if (cancelArgs.Cancel)
{
this.BackStack.Push(currentJe);
return(false);
}
}
AlternativePage newPage;
ContentPresenter newPagePresenter;
if (_preloadedPageCache.ContainsKey(je))
{
newPagePresenter = _preloadedPageCache[je];
newPage = (AlternativePage)newPagePresenter.Content;
_preloadedPageCache.Remove(je);
}
else
{
newPage = (AlternativePage)Activator.CreateInstance(je.Type);
newPage.Frame = this;
newPagePresenter = new ContentPresenter {
Style = PagePresenterStyle
};
newPagePresenter.Content = newPage;
_pagePresentersPanel.Children.Add(newPagePresenter);
}
newPagePresenter.Opacity = 0.005;
await UnloadPreloadedPages();
var args = new AlternativeNavigationEventArgs(
newPage.Content, NavigationMode.New, je.Parameter, je.Type);
await newPage.OnNavigatingToInternal(args);
await newPagePresenter.WaitForLoadedAsync();
await newPagePresenter.WaitForNonZeroSizeAsync();
newPagePresenter.Opacity = 1.0;
if (this.PageTransition != null)
{
await this.PageTransition.TransitionBackward(_currentPagePresenter, newPagePresenter);
}
if (currentPage != null)
{
await currentPage.OnNavigatedFromInternal(args);
_pagePresentersPanel.Children.Remove(_currentPagePresenter);
}
_currentPagePresenter = newPagePresenter;
await newPage.OnNavigatedToInternal(args);
return(true);
}
finally
{
_isNavigating = false;
this.CanNavigate = true;
this.CanGoBack = this.BackStack.Count > 1;
}
}