private void ReconfigureViewportPropagation(bool isInternal = false, IFrameworkElement_EffectiveViewport?child = null)
{
if (IsLoaded && IsEffectiveViewportEnabled)
{
if (_parentViewportUpdatesSubscription == null)
{
TRACE_EFFECTIVE_VIEWPORT("Enabling effective viewport propagation.");
var parent = this.GetVisualTreeParent();
if (parent is IFrameworkElement_EffectiveViewport parentFwElt)
{
_parentViewportUpdatesSubscription = parentFwElt.RequestViewportUpdates(isInternal, this);
}
else
{
global::System.Diagnostics.Debug.Assert(IsVisualTreeRoot);
// We are the root of the visual tree, we update the effective viewport
// in order to initialize the _parentViewport of children.
PropagateEffectiveViewportChange(isInitial: true, isInternal: isInternal);
}
}
else if (child != null)
{
TRACE_EFFECTIVE_VIEWPORT("New child requested viewport propagation which has already been enabled, forwarding current viewport to it.");
// We are already subscribed, the parent won't send any update (and our _parentViewport is expected to be up-to-date).
// But if this "reconfigure" was made for a new child (child != null), we have to initialize its own _parentViewport.
var parentViewport = GetParentViewport();
var viewport = GetEffectiveViewport(parentViewport);
child.OnParentViewportChanged(isInitial: true, isInternal: true, this, viewport);
}
}
else
{
#if CHECK_LAYOUTED
if (!IsLoaded)
{
_isLayouted = false;
}
#endif
if (_parentViewportUpdatesSubscription != null)
{
TRACE_EFFECTIVE_VIEWPORT("Disabling effective viewport propagation.");
_parentViewportUpdatesSubscription.Dispose();
_parentViewportUpdatesSubscription = null;
_parentViewport = ViewportInfo.Empty;
}
}
}
private ViewportInfo GetEffectiveViewport(ViewportInfo parentViewport)
{
global::System.Diagnostics.Debug.Assert(parentViewport.Reference == this || parentViewport.Reference == null);
if (IsVisualTreeRoot)
{
var slot = LayoutInformation.GetLayoutSlot(this);
return(new ViewportInfo(this, slot, Rect.Infinite));
}
if (IsScrollPort)
{
var viewport = parentViewport.Clip;
// Pseudo intersect:
// A normal intersect would return `Empty` as soon as the `viewport` and the `scrollport` does not overlap,
// but on UWP the viewport is `Empty` only if the `viewport` is below the `scrollport` (for vertical scroll).
// It means for an item in a list which is not yet visible on load (while vertical offset is 0), its EffVP will be empty.
// Then when you scroll, its effVP will be something like [slot.Width,10 @ slot.X,slot.Y] when its first top 10px becomes visible.
// Then if you continue to scroll and the item goes above the 'scrollport', its effVP will be [slot.Width,slot.Height @ slot.X,slot.Y]
// while a normal intersect would be `Empty` (which is logic as the item is no longer visible at all,
// but it make sense to not request to not go back to empty so element won't unload its content when scrolling up)!
// However, nested scroll host has to do a real intersect.
if (viewport.Right <= 0 ||
viewport.Bottom <= 0)
{
return(new ViewportInfo(this, Rect.Empty));
}
// The visible window of the SCP
// TODO: We should constrains the clip only on axis on which we can scroll
var scrollport = new Rect(
new Point(ScrollOffsets.X, ScrollOffsets.Y),
LayoutInformation.GetLayoutSlot(this).Size);
if (viewport.IsInfinite)
{
viewport = scrollport;
}
else
{
viewport.Intersect(scrollport);
}
return(new ViewportInfo(this, viewport));
}
return(parentViewport);
}
void IFrameworkElement_EffectiveViewport.OnParentViewportChanged(
bool isInitial, // Update is intended to initiate the _parentViewport and should be "public" only if not due to a new event listener while tree is live (e.g. load)
bool isInternal, // Indicates that this update is due to a new event handler
IFrameworkElement_EffectiveViewport parent, // We propagate the parent to avoid costly lookup
ViewportInfo viewport) // The viewport of the parent, expressed in parent's coordinates
{
if (!IsEffectiveViewportEnabled)
{
// We do not keep the _parentViewport up-to-date if not needed.
// It's expected to the root parent to update its children when propagation activated.
return;
}
if (!isInitial && viewport == _parentViewport)
{
return;
}
_parentViewport = viewport;
PropagateEffectiveViewportChange(isInitial, isInternal);
}
