Scrolling Programmatically and Manually

Nathan Sokalski 4,116

I have a UWP app which contains a ScrollViewer. The ScrollViewer will be scrolled both programmatically and by the user. When the ScrollViewer is resized (whether by modifying the layout of other components or the user resizing the window), I want to programmatically scroll to the current item only if that is how it was most recently scrolled. However, if the user manually scrolled to a position, I do not want to scroll (I want to leave it wherever it ended up). I thought the best way to do this would be using the LayoutUpdated event. However, because LayoutUpdated is triggered multiple times, as are events of the ScrollViewer (ViewChanged, ViewChanging, etc.) it is hard to know when to specify whether I want the ScrollViewer to be automatically updated with a size or layout change. What would be a good way to do this? Thanks.

Fay Wang - MSFT 5,196 Reputation points

2020-02-20T07:52:30.67+00:00

I'm confused about your expected behavior. Do you mean when the ScrollViewer is resized, if the most recent scrolling method was scrolling programmatically, then scroll to a specific item, but if the most recent scrolling method was scrolling by user, then don't scroll to the specific item? Can you show more details about your expected behavior and show some code snippet that you currently implement?
Nathan Sokalski 4,116 Reputation points

2020-02-20T15:44:42.823+00:00

What you said is exactly what I want, I am just having trouble figuring out how to make it happen (what events to use, and how/when to remember what the last kind of scrolling was).
Fay Wang - MSFT 5,196 Reputation points

2020-02-21T07:12:11.907+00:00

There is no directly method could judge what the last kind of scrolling was. But you could try to listen the user's behavior. For example, you can subscribe the PointerWheelChangedEvent and declare a property of bool type. If the user uses mouse to trigger the scrolling of scrollViewer and you can set the property to true. If you trigger the scrolling of scrollViewer programmatically, you can set the property to false. And when the size of window changes, according to the property to determine if scrolling the scrollviewer to the specific position.
Fay Wang - MSFT 5,196 Reputation points

2020-02-26T08:17:35.147+00:00

Hi, can this help you? Do you have other questions?
Fay Wang - MSFT 5,196 Reputation points

2020-02-27T10:54:29.987+00:00

Hi, is there any update?
Fay Wang - MSFT 5,196 Reputation points

2020-02-28T09:42:10.57+00:00

Hi, is there any update? Have you solved your issue?
Nathan Sokalski 4,116 Reputation points

2020-02-28T16:02:21.54+00:00

I don't think it really qualifies as a solution, but I managed to (at least in most cases) achieve the functionality I need. The general concept behind what I did is adding & removing the LayoutUpdated event of the ContentControl that contains the ScrollViewer (one of the important ones being the PointerPressed event when the user scrolls manually). However, because there is not one explicit place that this needs done, it was very hard to figure out where I needed to do it, and I wouldn't be surprised if at some point I realized that I forgot it in several places, ended up adding it multiple times (an event where I add it got fired multiple times before I removed it), and it is something that would be easy to forget to include in the future, or cause a problem if I modify the app. So I'm probably doing bad programming practice, but for the moment is seems to be the best I could find to accomplish what I need.
Fay Wang - MSFT 5,196 Reputation points

2020-03-02T06:05:50.117+00:00

Great, you can write your current method as an answer and mark it, it will help more people who have the same problem as you.