如何制作GridView快照滚动

Question

Is there a way to make a GridView snap to a row while scrolling?

It seems that applying the known fixes for a ListView does not suffice for a GridView .

Answer 1

Note: This code was tested for API 23

Note: listPaddingTop == 11 , and listPaddingBottom == 10 for my GridView . Couldn't find where this is set, may be default (didn't see it in the style referenced by Android.Resource.Attribute.GridViewStyle in the below constructor chain (which points to Widget.GridView / Widget.AbsListView in res/values/styles.xml ). Need to check source.

Note: Calling SetPadding(0,0,0,0) in constructor has no effect on listPaddingTop (Bottom).

public class SnapScrollGridView : GridView,
    //View.IOnScrollChangeListener
    AbsListView.IOnScrollListener
{        
    private bool _bSnapScroll;

    public new int FirstVisiblePosition { get; set; }

    // ctor chain
    public SnapScrollGridView(Context context) : this(context, null) { }
    public SnapScrollGridView(Context context, IAttributeSet attrs) : this(context, attrs, Android.Resource.Attribute.GridViewStyle) { }
    public SnapScrollGridView(Context context, IAttributeSet attrs, int defaultStyleAttr) : this(context, attrs, defaultStyleAttr,0) { }
    public SnapScrollGridView(Context context, IAttributeSet attrs, int defaultStyleAttr, int defStyleRes) : base(context, attrs, defaultStyleAttr, defStyleRes)
    {
        _bSnapScroll = false;
        FirstVisiblePosition = 0;
        //SetOnScrollChangeListener(this);
        SetOnScrollListener(this);
        // no effect on listPaddingTop (or bottom)
        // SetPadding(0, 0, 0, 0);
    }

    void AbsListView.IOnScrollListener.OnScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount)
    {
        // firstVisibleItem and visibleItemCount not accurate
    }

    protected override void OnOverScrolled(int scrollX, int scrollY, bool clampedX, bool clampedY)
    {            
        // - would be very useful, however is called after OnScrollStateChanged()
        // - may be able to refactor OnScrollStateChanged to work in conjuction with this
        base.OnOverScrolled(scrollX, scrollY, clampedX, clampedY);
    }

    void AbsListView.IOnScrollListener.OnScrollStateChanged(AbsListView view, [GeneratedEnum] ScrollState scrollState)
    {
        switch (scrollState)
        {
            case ScrollState.TouchScroll:
            case ScrollState.Fling:
                break;           

            case ScrollState.Idle:

                if (_bSnapScroll)
                {
                    _bSnapScroll = false;
                }
                else
                {
                    View item = view.GetChildAt(0);
                    int top = System.Math.Abs(item.Top);
                    int btm = System.Math.Abs(item.Bottom);

                    // OVERSCROLL TOP ("topping out")
                    // - user tried scrolling up (swiping down) past top

                    if (top == view.ListPaddingTop && view.FirstVisiblePosition == 0)
                    {
                        this.FirstVisiblePosition = 0;
                        return;
                    }

                    // OVERSCROLL BOTTOM ("bottoming out")                      
                    // - for now, user needs to hit bottom to force snap when last row wont like "normal"
                    // - this happens for certain GridView height and item/row height combinations
                    // - could smooth scroll in the same manner as below, but requires more code

                    // nRows, curRow round down (floor() not needed)
                    int nRows = view.Count / NumColumns;
                    int curRow = view.LastVisiblePosition / NumColumns;

                    if (curRow == nRows)
                    {                            
                        View item2 = view.GetChildAt(view.LastVisiblePosition-view.FirstVisiblePosition);
                        int top2 = System.Math.Abs(item2.Top);
                        int btm2 = System.Math.Abs(item2.Bottom);                            

                        if (btm2 == view.Height - view.ListPaddingBottom)
                        {
                            //view.SetSelection(view.LastVisiblePosition);
                            this.FirstVisiblePosition = view.LastVisiblePosition;
                            return;
                        }
                    }

                    // EXCEPTION CASE: smore info below
                    if (item.Top == 0)
                    {
                        view.SetSelection(0);
                        this.FirstVisiblePosition = 0;
                        return;
                    }

                    _bSnapScroll = true;

                    // Compensate for ListPaddingTop
                    // - initially gives a positive offset (11 for me),
                    //   observed when attempting to scroll upward from top
                    //   (swipe downward while at very top)
                    //   (break point will show item.Top = 11)
                    // - if scrolling down a tiny bit, item.Top lessens,
                    //   eventually becoming zero, then negative after that

                    int scrollBy = item.Top > 0
                        ? top >= btm ? btm - view.ListPaddingTop : -(view.ListPaddingTop - top)
                        : top >= btm ? btm - view.ListPaddingTop : -(view.ListPaddingTop + top);

                    // ALTERNATIVE SYNTAX - debug friendly (less cool)
                    //int scrollBy = 0;
                    //if (item.Top > 0)
                    //    scrollBy = top >= btm ? btm - view.ListPaddingTop : -(view.ListPaddingTop - top);
                    //else
                    //    scrollBy = top >= btm ? btm - view.ListPaddingTop : -(view.ListPaddingTop + top);

                    if (scrollBy <= 0)
                    {
                        this.FirstVisiblePosition = view.FirstVisiblePosition;
                    }
                    else
                    {
                        this.FirstVisiblePosition = view.FirstVisiblePosition + NumColumns;
                    }

                    view.Post(() => { view.SmoothScrollBy(scrollBy, 1000); });
                }
                break;

            default: break;
        }
    }

Example of exception case noted above

If user scrolls down, from very top, by exactly the padding amount, and SmoothScrollBy() is called with the resulting value calculated below (which is -view.ListPaddingTop since item.Top == 0 ), no scrolling occurs for some reason (leaving _bSnapScroll inconsistent since OnScrollStateChanged() subsequently misfires).

                    //if (item.Top == 0)
                    //{
                    //    view.SetSelection(0);
                    //    return;
                    //}

                    _bSnapScroll = true;

                    //int scrollBy = item.Top > 0
                    int scrollBy = item.Top >= 0
                        ? top >= btm ? btm - view.ListPaddingTop : -(view.ListPaddingTop - top)
                        : top >= btm ? btm - view.ListPaddingTop : -(view.ListPaddingTop + top);

Usage

private void GridViewAdapterLoad()
{
    int pos = ((SnapScrollGridView)_gridView).FirstVisiblePosition;

    _gridView.Adapter = new GridViewAdapter(
        Activity,
        Resource.Layout.GridItem,
        Model.GridItems);

    _gridView.SetSelection(pos);
}

Positing grid to bottom after adding a new item (to end)

private void GridViewLoad(bool bNewItem)
{
    _controller.LoadGridItems();

    _gridView.Adapter = null;

    GridViewAdapterLoad();

    if (bNewItem)
    {
        int count = _gridView.Count;            
        int row = --count / _gridView.NumColumns; // floor not needed
        int pos = row * _gridView.NumColumns;

        ((SnapScrollGridView)_gridView).FirstVisiblePosition = pos;

        _gridView.SetSelection(pos);

        // Would be sweet to scroll from current position, but isn't.
        //_gridView.Post(() => { _gridView.SmoothScrollToPosition(pos); });
    }
}