static int CalculateCenterItemIndex(int firstVisibleItemIndex, int lastVisibleItemIndex, LinearLayoutManager linearLayoutManager)
{
// This can happen if a layout pass has not happened yet
if (firstVisibleItemIndex == -1)
{
return(firstVisibleItemIndex);
}
var keyValuePairs = new Dictionary <int, int>();
for (var i = firstVisibleItemIndex; i <= lastVisibleItemIndex; i++)
{
var view = linearLayoutManager.FindViewByPosition(i);
var rect = new Rect();
view.GetLocalVisibleRect(rect);
keyValuePairs[i] = rect.Height();
}
var center = keyValuePairs.Values.Sum() / 2.0;
foreach (var keyValuePair in keyValuePairs)
{
center -= keyValuePair.Value;
if (center <= 0)
{
return(keyValuePair.Key);
}
}
return(firstVisibleItemIndex);
}
/**
* Returns true iff the {@link RecyclerView} is scrolled to the
* top of its content (i.e. its first item is completely visible).
*/
private static bool IsRvScrolledToTop(RecyclerView rv)
{
LinearLayoutManager lm = (LinearLayoutManager)rv.GetLayoutManager();
return(lm.FindFirstVisibleItemPosition() == 0 &&
lm.FindViewByPosition(0).Top == 0);
}
public virtual int GetRowPositionOffset()
{
Android.Views.View child =
mRowHeaderLayoutManager.FindViewByPosition(mRowHeaderLayoutManager.FindFirstVisibleItemPosition());
if (child != null)
{
return(child.Left);
}
return(0);
}
public override void OnDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state)
{
base.OnDrawOver(c, parent, state);
int itemCount = parent.GetAdapter().ItemCount;
// center horizontally, calculate width and subtract half from center
float totalLength = mIndicatorItemLength * itemCount;
float paddingBetweenItems = Math.Max(0, itemCount - 1) * mIndicatorItemPadding;
float indicatorTotalWidth = totalLength + paddingBetweenItems;
float indicatorStartX = (parent.Width - indicatorTotalWidth) / 2F;
float indicatorPosY = parent.Height - mIndicatorHeight / 2F;
drawInactiveIndicators(c, indicatorStartX, indicatorPosY, itemCount);
// find active page (which should be highlighted)
LinearLayoutManager layoutManager = (LinearLayoutManager)parent.GetLayoutManager();
int activePosition = layoutManager.FindLastVisibleItemPosition();
if (activePosition == RecyclerView.NoPosition)
{
return;
}
// find offset of active page (if the user is scrolling)
View activeChild = layoutManager.FindViewByPosition(activePosition);
int left = activeChild.Left;
int width = activeChild.Width;
// on swipe the active item will be positioned from [-width, 0]
// interpolate offset for smooth animation
float progress = mInterpolator.GetInterpolation(left * -1 / (float)width);
drawHighlights(c, indicatorStartX, indicatorPosY, activePosition, progress, itemCount);
}
/// <summary>
/// This method calculates the current scroll position and its offset to help new attached
/// recyclerView on window at that position and offset
/// </summary>
/// <seealso cref="GetScrollPosition()"/>
/// <seealso cref="GetScrollPositionOffset()"/>
private void RenewScrollPosition(RecyclerView recyclerView)
{
LinearLayoutManager layoutManager = (LinearLayoutManager)recyclerView.GetLayoutManager();
mScrollPosition = layoutManager.FindFirstCompletelyVisibleItemPosition();
// That means there is no completely visible Position.
if (mScrollPosition == -1)
{
mScrollPosition = layoutManager.FindFirstVisibleItemPosition();
// That means there is just a visible item on the screen
if (mScrollPosition == layoutManager.FindLastVisibleItemPosition())
{
}
else
{
// in this case we use the position which is the last & first visible item.
// That means there are 2 visible item on the screen. However, second one is not
// completely visible.
mScrollPosition = mScrollPosition + 1;
}
}
mScrollPositionOffset = layoutManager.FindViewByPosition(mScrollPosition).Left;
}
public override bool Fling(int velocityX, int velocityY)
{
if (ItemCount < 1)
{
return(true);
}
FlingEventArgs flingEventArgs;
if (velocityX > 0)
{
flingEventArgs = new FlingEventArgs(FlingDirection.Right, CurrentItem);
if (CurrentItem == ItemCount - 1)
{
return(true);
}
}
else
{
flingEventArgs = new FlingEventArgs(FlingDirection.Left, CurrentItem);
}
if (BeforeFling != null)
{
BeforeFling(this, flingEventArgs);
}
LinearLayoutManager linearLayoutManager = (LinearLayoutManager)GetLayoutManager();
//these four variables identify the views you see on screen.
int lastVisibleView = linearLayoutManager.FindLastVisibleItemPosition();
int firstVisibleView = linearLayoutManager.FindFirstVisibleItemPosition();
View firstView = linearLayoutManager.FindViewByPosition(firstVisibleView);
View lastView = linearLayoutManager.FindViewByPosition(lastVisibleView);
//these variables get the distance you need to scroll in order to center your views.
//my views have variable sizes, so I need to calculate side margins separately.
//note the subtle difference in how right and left margins are calculated, as well as
//the resulting scroll distances.
int leftMargin = (screenWidth - lastView.Width) / 2;
int rightMargin = (screenWidth - firstView.Width) / 2 + firstView.Width;
int leftEdge = lastView.Left;
int rightEdge = firstView.Right;
int scrollDistanceLeft = leftEdge - leftMargin;
int scrollDistanceRight = rightMargin - rightEdge;
//if(user swipes to the left)
if (velocityX > 0)
{
CurrentItem = lastVisibleView - 1;
SmoothScrollBy(scrollDistanceLeft, 0);
}
else
{
CurrentItem = firstVisibleView - 1;
SmoothScrollBy(-scrollDistanceRight, 0);
}
if (OnFling != null)
{
OnFling(this, flingEventArgs);
}
return(true);
}