/**
* Switches the current image one to the left. The means changing the index of the current image within
* the cache and also updating one of the cache positions. Because the cache always holds the same number of
* images before and the same number after the current image, moving the current image left means that one of
* cache positions after the current image will need to store an image before the current one.
*
* Once this is done, the cached images will be loaded according to priority. From highest to lowest priority,
* this is the current image, the ones after it, and the ones before it. This corresponds to how likely it is
* the user will view each image.
*/
private async void MoveCacheLeft()
{
// First, move the current image index one to the left
_imageCacheCurrIndex--;
if (_imageCacheCurrIndex < 0)
{
_imageCacheCurrIndex += CACHE_LEN;
}
// Which position in the cache is now stale and should be replaced
int cacheUpdateIndex = _imageCacheCurrIndex - BACK_CACHE_NUM;
if (cacheUpdateIndex < 0)
{
cacheUpdateIndex += CACHE_LEN;
}
// What image in the gallery will replace the stale cache position
int galleryUpdateIndex = CurrentIndex - BACK_CACHE_NUM;
if (galleryUpdateIndex < 0)
{
galleryUpdateIndex += _galleryItems.Count();
}
// Update the cache position with the new image
Photo p = _galleryItems[galleryUpdateIndex];
_imageCache[cacheUpdateIndex].Photo = p;
// If the current image has been loaded (i.e., there isn't an old image
// still loaded), either the preview or the full resolution, then update the
// view to the user. Otherwise, when the image loads, the view will update.
if (CurrentImageViewModel.PreviewLoaded())
{
OnPropertyChanged("CurrentImage");
}
// Update sidebar first so that it's not waiting on the image load to update
CurrentImageChanged();
// Then load the new cache position's image
await Task.Run(() => { _imageCache[cacheUpdateIndex].UpdateImage(); });
}
/// <summary>
/// Called when this page is no longer visible to the user. This cancels any image loading operations that might be going on.
/// </summary>
public override void NavigatorLostFocus()
{
CurrentImageViewModel.CancelAllLoads();
}
