Add methods to control Scrollable programmatically

[yusdacra]

This PR adds methods:

• snap_to_bottom and on_scroll to Scrollable
  • snap_to_bottom takes a bool and if true makes the Scrollable snap to bottom when the user scrolls to bottom. If the contents of the Scrollable change, it will automatically snap to bottom. Whenever the user scrolls up it will stop snapping to bottom. By default a Scrollable won't snap to bottom.
  • on_scroll takes a function whose arguments are two f32s and produces a Message. First argument is to pass the current offset of the Scrollable to provide the user with "where is this Scrollable at right now" and second argument is to pass the previous offset of the Scrollable to provide the user with "where was this Scrollable before this scroll event".
• scroll_to_percentage and scroll_to_bottom to scrollable::State
  • scroll_to_percentage takes a value between 0.0 and 1.0. It marks the Scrollable to scroll to the specified percentage in the next draw call.
  • scroll_to_bottom is just a shorthand for scroll_to_percentage(1.0).

This should fix #307

I wonder what people think about this so please post your thoughts!

[agraven]

snap_to_bottom is a feature I need to make the chat client I'm building work right, it being merged would be strong cause for celebration for me.

[yusdacra]

snap_to_bottom is a feature I need to make the chat client I'm building work right, it being merged would be strong cause for celebration for me.

Haha, same. It's why I implemented this feature in the first place :D

[twitchyliquid64]

I've also found myself needing this, as I am implementing a search results list and need to scroll so that the result focused after a <tab> is visible. Plz! 🤞 🤞 🤞

[twitchyliquid64]

In the meantime here's a disgusting hack to scroll_to_bottom on iced 0.2/0.3, while we wait for this to land.

self.scroll = unsafe {
    let mut tmp = std::mem::transmute::<_, (Option<f32>, f32)>(self.scroll);
    tmp.1 = 999999.0;
    std::mem::transmute::<_, scrollable::State>(tmp)
};

(Assuming self.scroll is your scrollable::State)

Caveat: Dirty, disgusting, unsafe, brittle, use at your own risk.

[pleshevskiy]

О! That's just what I need! Thank you!

[hecrj]

I made some changes that I believe simplify everything overall.

Specifically:

• We avoid the RefCell by introducing a new Offset enum and deferring the absolute computation until the next scrolling event.
• Instead of a snap_to_bottom, we introduce a snap_to method that allows snapping to any given relative position.
• The scroll_to API remains unchanged.

I have also updated the scrollable example to showcase the new API:

Let me know what you think!

[yusdacra]

I made some changes that I believe simplify everything overall.

Specifically:

• We avoid the RefCell by introducing a new Offset enum and deferring the absolute computation until the next scrolling event.
• Instead of a snap_to_bottom, we introduce a snap_to method that allows snapping to any given relative position.
• The scroll_to API remains unchanged.

Looks nice! Deferring the computation is nice, I like that. Can it be merged soon then?

[hecrj]

@yusdacra Yes! Let's merge and fix any potential issues and implement new use cases as they come up.

[hecrj]

Thank you everyone! 🎉