protected override void OnDetachingCore(LayoutContext context)
{
base.OnDetachingCore(context);
// clear any state
context.LayoutState = null;
}
protected override void OnAttachedCore(LayoutContext context)
{
base.OnAttachedCore(context);
var state = context.LayoutState as ActivityFeedLayoutState;
if (state == null)
{
// Store any state we might need since (in theory) the layout could be in use by multiple elements simultaneously
// In reality for the Xbox Activity Feed there's probably only a single instance.
context.LayoutState = new ActivityFeedLayoutState();
}
}
protected override Size ArrangeLayoutCore(Size finalSize, LayoutContext context)
{
// walk through the cache of containers and arrange
var state = context.LayoutState as ActivityFeedLayoutState;
if (state == null)
{
throw new InvalidOperationException();
}
foreach (var key in state.Containers.Keys)
{
var container = state.Containers[key].Item1;
var rect = state.Containers[key].Item2;
container.Arrange(rect);
}
return(finalSize);
}
protected override Size MeasureLayoutCore(Size availableSize, LayoutContext context)
{
// In the Xbox scenario there isn't a user grabbing the window and resizing it. Otherwise, it
// might be useful to skip a full layout here and return a cached size until some condition is met
// such as having enough room to fit another column. That would require extra work like
// tracking some additional state, comparing the virtualizing rect with the cached value, or setting
// a timeout to invalidate the layout and resetting the timer while the available width is changing.
var virtualizingContext = context as VirtualizingLayoutContext;
if (virtualizingContext == null)
{
throw new InvalidOperationException("This layout requires a virtualizing host like Repeater");
}
if (this.MinItemSize == Size.Empty)
{
var firstElement = virtualizingContext.GetElementAt(0);
firstElement.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
// setting the member value directly to skip invalidating layout
this._minItemSize = firstElement.DesiredSize;
}
// ideal item width that will expand/shrink to fill available space
double desiredItemWidth = Math.Max(this.MinItemSize.Width, (availableSize.Width - this.ItemSpacing * 3) / 4);
var state = context.LayoutState as ActivityFeedLayoutState;
var realizationRect = virtualizingContext.GetRealizationRect();
// estimate the extent by finding the last item and getting its bottom/right position
var extentHeight = ((int)(context.ItemCount / 3) - 1) * (this.MinItemSize.Height + this.RowSpacing) + this.MinItemSize.Height;
// Determine which rows need to be realized
var firstRow = Math.Max((int)(realizationRect.Y / (this.MinItemSize.Height + this.RowSpacing)) - 1, 0);
var lastRow = Math.Min((int)(realizationRect.Bottom / (this.MinItemSize.Height + this.RowSpacing)) + 1, (int)(context.ItemCount / 3));
// Determine which items fall on those rows and determine the rect for each item
SortedDictionary <int, Tuple <UIElement, Rect> > positions = new SortedDictionary <int, Tuple <UIElement, Rect> >();
Rect[] rectArray = new Rect[3];
rectArray[0].Height = rectArray[1].Height = rectArray[2].Height = this.MinItemSize.Height;
for (int rowIndex = firstRow; rowIndex < lastRow; rowIndex++)
{
var yoffset = rowIndex * (this.MinItemSize.Height + this.RowSpacing);
// initialize Y positions of the rects
rectArray[0].Y = rectArray[1].Y = rectArray[2].Y = yoffset;
var firstItemIndex = rowIndex * 3;
var wnn = firstItemIndex % 3 == 0 && firstItemIndex % 6 != 0;
var nnw = firstItemIndex % 3 == 0 && firstItemIndex % 6 == 0;
// Update the rects to what is required for a given row
if (wnn)
{
// Left tile (wide)
rectArray[0].X = 0;
rectArray[0].Width = (desiredItemWidth * 2 + this.ItemSpacing);
// Middle tile (narrow)
rectArray[1].X = rectArray[0].Right + this.ItemSpacing;
rectArray[1].Width = desiredItemWidth;
// Right tile (narrow)
rectArray[2].X = rectArray[1].Right + this.ItemSpacing;
rectArray[2].Width = desiredItemWidth;
}
else if (nnw)
{
// Left tile (narrow)
rectArray[0].X = 0;
rectArray[0].Width = desiredItemWidth;
// Middle tile (narrow)
rectArray[1].X = rectArray[0].Right + this.ItemSpacing;
rectArray[1].Width = desiredItemWidth;
// Right tile (wide)
rectArray[2].X = rectArray[1].Right + this.ItemSpacing;
rectArray[2].Width = desiredItemWidth * 2 + this.ItemSpacing;
}
UIElement container = null;
for (int i = 0; i < 3; i++)
{
var index = firstItemIndex + i;
if (state.Containers.ContainsKey(index))
{
container = state.Containers[index].Item1;
var rect = rectArray[index % 3];
positions.Add(index, new Tuple <UIElement, Rect>(container, rect));
state.Containers.Remove(index);
}
else
{
// GetElementAt will return a new element each time it is called in a virtualizing
// context so in any given layout pass it should only be called once for a given
// index and only for those that need to be measured again. In this layout we
// only call measure on the items that are being realized for the first time or
// have just been recycled.
container = virtualizingContext.GetElementAt(index);
container.Measure(new Size(rectArray[i].Width, rectArray[i].Height));
positions.Add(index, new Tuple <UIElement, Rect>(container, rectArray[i]));
}
}
}
// clear anything that wasn't used
foreach (var item in state.Containers)
{
virtualizingContext.ClearElement(state.Containers[item.Key].Item1);
}
// store the containers we used for use on the next layout pass
state.Containers = positions;
// Report a desired size for the layout
return(new Size(desiredItemWidth * 4 + this.ItemSpacing * 2, extentHeight));
}
