How to scroll to a specific element in ScrollRect with Unity UI?

Question

I built a registration form for a mobile game using Unity 5.1. To do that, I use Unity UI components: ScrollRect + Autolayout (Vertical layout) + Text (labels)

Answer 1

The preconditions for my version of this problem:

• The element I want to scroll to should be fully visible (with minimal clearance)
• The element is a direct child of the scrollRect's content
• Keep scoll position if element is already fully visible
• I only care about the vertical dimension

This is what worked best for me (thanks for the other inspirations):

// ScrollRect scrollRect;
// RectTransform element;
// Fully show `element` inside `scrollRect` with at least 25px clearance
scrollArea.EnsureVisibility(element, 25);

Using this extension method:

public static void EnsureVisibility(this ScrollRect scrollRect, RectTransform child, float padding=0)
{
    Debug.Assert(child.parent == scrollRect.content,
        "EnsureVisibility assumes that 'child' is directly nested in the content of 'scrollRect'");

    float viewportHeight = scrollRect.viewport.rect.height;
    Vector2 scrollPosition = scrollRect.content.anchoredPosition;

    float elementTop = child.anchoredPosition.y;
    float elementBottom = elementTop - child.rect.height;

    float visibleContentTop = -scrollPosition.y - padding;
    float visibleContentBottom = -scrollPosition.y - viewportHeight + padding;

    float scrollDelta =
        elementTop > visibleContentTop ? visibleContentTop - elementTop :
        elementBottom < visibleContentBottom ? visibleContentBottom - elementBottom :
        0f;

    scrollPosition.y += scrollDelta;
    scrollRect.content.anchoredPosition = scrollPosition;
}

Answer 2

width signifiy the width of childern in scroll rect (assuming that all childerns width is same), spacing signifies the space between childerns, index signifies the target element you want to reach

public float getSpecificItem (float pWidth, float pSpacing,int pIndex) {
    return (pIndex * pWidth) - pWidth + ((pIndex - 1) * pSpacing);
}

Answer 3

Although @maksymiuk's answer is the most correct one, as it properly takes into account anchors, pivot and all the rest thanks to InverseTransformPoint() function, it still didn't work out-of-box for me - for vertical scroller, it was changing its X position too. So I just made change to check if vertical or horizontal scroll is enabled, and not change their axis if they aren't.

public static void SnapTo( this ScrollRect scroller, RectTransform child )
{
    Canvas.ForceUpdateCanvases();

    var contentPos = (Vector2)scroller.transform.InverseTransformPoint( scroller.content.position );
    var childPos = (Vector2)scroller.transform.InverseTransformPoint( child.position );
    var endPos = contentPos - childPos;
    // If no horizontal scroll, then don't change contentPos.x
    if( !scroller.horizontal ) endPos.x = contentPos.x;
    // If no vertical scroll, then don't change contentPos.y
    if( !scroller.vertical ) endPos.y = contentPos.y;
    scroller.content.anchoredPosition = endPos;
}

Answer 4

Vertical adjustment:

[SerializeField]
private ScrollRect _scrollRect;

private void ScrollToCurrentElement()
{
    var siblingIndex = _currentListItem.transform.GetSiblingIndex();

    float pos = 1f - (float)siblingIndex / _scrollRect.content.transform.childCount;

    if (pos < 0.4)
    {
        float correction = 1f / _scrollRect.content.transform.childCount;
        pos -= correction;
    }

    _scrollRect.verticalNormalizedPosition = pos;
} 

Answer 5

A variant of Peter Morris answer for ScrollRects which have movement type "elastic". It bothered me that the scroll rect kept animating for edge cases (first or last few elements). Hope it's useful:

/// <summary>
/// Thanks to https://stackoverflow.com/a/50191835
/// </summary>
/// <param name="instance"></param>
/// <param name="child"></param>
/// <returns></returns>
public static IEnumerator BringChildIntoView(this UnityEngine.UI.ScrollRect instance, RectTransform child)
{
    Canvas.ForceUpdateCanvases();
    Vector2 viewportLocalPosition = instance.viewport.localPosition;
    Vector2 childLocalPosition = child.localPosition;
    Vector2 result = new Vector2(
        0 - (viewportLocalPosition.x + childLocalPosition.x),
        0 - (viewportLocalPosition.y + childLocalPosition.y)
    );
    instance.content.localPosition = result;

    yield return new WaitForUpdate();

    instance.horizontalNormalizedPosition = Mathf.Clamp(instance.horizontalNormalizedPosition, 0f, 1f);
    instance.verticalNormalizedPosition = Mathf.Clamp(instance.verticalNormalizedPosition, 0f, 1f);
}

It introduces a one frame delay though.

UPDATE: Here is a version which does not introduce a frame delay and it also takes scaling of the content into account:

/// <summary>
/// Based on https://stackoverflow.com/a/50191835
/// </summary>
/// <param name="instance"></param>
/// <param name="child"></param>
/// <returns></returns>
public static void BringChildIntoView(this UnityEngine.UI.ScrollRect instance, RectTransform child)
{
    instance.content.ForceUpdateRectTransforms();
    instance.viewport.ForceUpdateRectTransforms();

    // now takes scaling into account
    Vector2 viewportLocalPosition = instance.viewport.localPosition;
    Vector2 childLocalPosition = child.localPosition;
    Vector2 newContentPosition = new Vector2(
        0 - ((viewportLocalPosition.x * instance.viewport.localScale.x) + (childLocalPosition.x * instance.content.localScale.x)),
        0 - ((viewportLocalPosition.y * instance.viewport.localScale.y) + (childLocalPosition.y * instance.content.localScale.y))
    );

    // clamp positions
    instance.content.localPosition = newContentPosition;
    Rect contentRectInViewport = TransformRectFromTo(instance.content.transform, instance.viewport);
    float deltaXMin = contentRectInViewport.xMin - instance.viewport.rect.xMin;
    if(deltaXMin > 0) // clamp to <= 0
    {
        newContentPosition.x -= deltaXMin;
    }
    float deltaXMax = contentRectInViewport.xMax - instance.viewport.rect.xMax;
    if (deltaXMax < 0) // clamp to >= 0
    {
        newContentPosition.x -= deltaXMax;
    }
    float deltaYMin = contentRectInViewport.yMin - instance.viewport.rect.yMin;
    if (deltaYMin > 0) // clamp to <= 0
    {
        newContentPosition.y -= deltaYMin;
    }
    float deltaYMax = contentRectInViewport.yMax - instance.viewport.rect.yMax;
    if (deltaYMax < 0) // clamp to >= 0
    {
        newContentPosition.y -= deltaYMax;
    }

    // apply final position
    instance.content.localPosition = newContentPosition;
    instance.content.ForceUpdateRectTransforms();
}

/// <summary>
/// Converts a Rect from one RectTransfrom to another RectTransfrom.
/// Hint: use the root Canvas Transform as "to" to get the reference pixel positions.
/// </summary>
/// <param name="from"></param>
/// <param name="to"></param>
/// <returns></returns>
public static Rect TransformRectFromTo(Transform from, Transform to)
{
    RectTransform fromRectTrans = from.GetComponent<RectTransform>();
    RectTransform toRectTrans = to.GetComponent<RectTransform>();

    if (fromRectTrans != null && toRectTrans != null)
    {
        Vector3[] fromWorldCorners = new Vector3[4];
        Vector3[] toLocalCorners = new Vector3[4];
        Matrix4x4 toLocal = to.worldToLocalMatrix;
        fromRectTrans.GetWorldCorners(fromWorldCorners);
        for (int i = 0; i < 4; i++)
        {
            toLocalCorners[i] = toLocal.MultiplyPoint3x4(fromWorldCorners[i]);
        }

        return new Rect(toLocalCorners[0].x, toLocalCorners[0].y, toLocalCorners[2].x - toLocalCorners[1].x, toLocalCorners[1].y - toLocalCorners[0].y);
    }

    return default(Rect);
}

Answer 6

Yes,this is possible using coding to scroll vertically, please try this code :

//Set Scrollbar Value - For Displaying last message of content
Canvas.ForceUpdateCanvases ();
verticleScrollbar.value = 0f;
Canvas.ForceUpdateCanvases ();

This code working fine for me ,when i developed chat functionality.