positionDependentBox function

Offset positionDependentBox ({@required Size size, @required Size childSize, @required Offset target, @required bool preferBelow, double verticalOffset: 0.0, double margin: 10.0 })

Position a child box within a container box, either above or below a target point.

The container's size is described by size.

The target point is specified by target, as an offset from the top left of the container.

The child box's size is given by childSize.

The return value is the suggested distance from the top left of the container box to the top left of the child box.

The suggested position will be above the target point if preferBelow is false, and below the target point if it is true, unless it wouldn't fit on the preferred side but would fit on the other side.

The suggested position will place the nearest side of the child to the target point verticalOffset from the target point (even if it cannot fit given that constraint).

The suggested position will be at least margin away from the edge of the container. If possible, the child will be positioned so that its center is aligned with the target point. If the child cannot fit horizontally within the container given the margin, then the child will be centered in the container.

Used by Tooltip to position a tooltip relative to its parent.

The arguments must not be null.

Implementation 

Offset positionDependentBox({
  @required Size size,
  @required Size childSize,
  @required Offset target,
  @required bool preferBelow,
  double verticalOffset = 0.0,
  double margin = 10.0,
}) {
  assert(size != null);
  assert(childSize != null);
  assert(target != null);
  assert(verticalOffset != null);
  assert(preferBelow != null);
  assert(margin != null);
  // VERTICAL DIRECTION
  final bool fitsBelow = target.dy + verticalOffset + childSize.height <= size.height - margin;
  final bool fitsAbove = target.dy - verticalOffset - childSize.height >= margin;
  final bool tooltipBelow = preferBelow ? fitsBelow || !fitsAbove : !(fitsAbove || !fitsBelow);
  double y;
  if (tooltipBelow)
    y = math.min(target.dy + verticalOffset, size.height - margin);
  else
    y = math.max(target.dy - verticalOffset - childSize.height, margin);
  // HORIZONTAL DIRECTION
  double x;
  if (size.width - margin * 2.0 < childSize.width) {
    x = (size.width - childSize.width) / 2.0;
  } else {
    final double normalizedTargetX = target.dx.clamp(margin, size.width - margin);
    final double edge = margin + childSize.width / 2.0;
    if (normalizedTargetX < edge) {
      x = margin;
    } else if (normalizedTargetX > size.width - edge) {
      x = size.width - margin - childSize.width;
    } else {
      x = normalizedTargetX - childSize.width / 2.0;
    }
  }
  return Offset(x, y);
}