textViewDidChangeSelection(_:)

TextViewDidChangeSelection.swift

// Exists inside of a scrollview subclass containing an "expanding" textview that lives underneath other content.

private var previousSelection: CGRect?

func textViewDidChangeSelection(_ textView: UITextView) {
	guard textView.isFirstResponder else {
		return
	}

	var selection = textView.layoutManager.boundingRect(forGlyphRange: textView.selectedRange, in: textView.textContainer).offsetBy(dx: insets.left, dy: insets.top)

	let contentInset: UIEdgeInsets
	if #available(iOS 11.0, *) {
		contentInset = self.adjustedContentInset
	}
	else {
		contentInset = self.contentInset
	}

	let scale = UIScreen.main.scale
	let selectionOrigin = round(selection.origin.y / scale) * scale
	let selectionHeight = round(selection.size.height / scale) * scale
	let absoluteMinY = 0 - contentInset.top
	let absoluteMaxY = self.contentSize.height - self.bounds.height + contentInset.bottom
	let selectionMinY = selectionOrigin - textView.textContainerInset.top + (textView.frame.origin.y - contentInset.top)
	let selectionMaxY = (textView.frame.origin.y + selectionOrigin + selectionHeight + textView.textContainerInset.bottom) - (self.bounds.height - contentInset.bottom)

	if let previous = previousSelection {
		let expanding = selection.height > previous.height
		let contracting = selection.height < previous.height
		let moving = !expanding && !contracting

		let offsetY: CGFloat
		if expanding, selection.minY < previous.minY {
			offsetY = selectionMinY // up
		}
		else if expanding, selection.maxY > previous.maxY {
			offsetY = selectionMaxY // down
		}
		else if contracting, selection.minY > previous.minY {
			offsetY = selectionMinY // up
		}
		else if contracting, selection.maxY < previous.maxY {
			offsetY = selectionMaxY // down
		}
		else if moving, selection.minY < previous.minY {
			offsetY = selectionMinY // up
		}
		else if moving, selection.maxY > previous.maxY {
			offsetY = selectionMaxY  // down
		}
		else {
			return
		}

		if offsetY == selectionMinY, offsetY < self.contentOffset.y  {
			self.contentOffset = CGPoint(x: self.contentOffset.x, y: min(max(offsetY, absoluteMinY), absoluteMaxY))
		}
		else if offsetY == selectionMaxY, offsetY > self.contentOffset.y  {
			self.contentOffset = CGPoint(x: self.contentOffset.x, y: min(max(offsetY, absoluteMinY), absoluteMaxY))
		}
		else if selectionMinY < self.contentOffset.y  {
			self.contentOffset = CGPoint(x: self.contentOffset.x, y: min(max(selectionMinY, absoluteMinY), absoluteMaxY))
		}
		else if selectionMaxY > self.contentOffset.y  {
			self.contentOffset = CGPoint(x: self.contentOffset.x, y: min(max(selectionMaxY, absoluteMinY), absoluteMaxY))
		}
	}
	else if selectionMinY < self.contentOffset.y  {
		self.contentOffset = CGPoint(x: self.contentOffset.x, y: min(max(selectionMinY, absoluteMinY), absoluteMaxY))
	}
	else if selectionMaxY > self.contentOffset.y  {
		self.contentOffset = CGPoint(x: self.contentOffset.x, y: min(max(selectionMaxY, absoluteMinY), absoluteMaxY))
	}

	self.previousSelection = selection
}