Module cosmic::iced_widget::keyed

Keyed widgets can provide hints to ensure continuity.

What is continuity?

Continuity is the feeling of persistence of state.

In a graphical user interface, users expect widgets to have a certain degree of continuous state. For instance, a text input that is focused should stay focused even if the widget tree changes slightly.

Continuity is tricky in iced and the Elm Architecture because the whole widget tree is rebuilt during every view call. This is very convenient from a developer perspective because you can build extremely dynamic interfaces without worrying about changing state.

However, the tradeoff is that determining what changed becomes hard for iced. If you have a list of things, adding an element at the top may cause a loss of continuity on every element on the list!

How can we keep continuity?

The good news is that user interfaces generally have a static widget structure. This structure can be relied on to ensure some degree of continuity. iced already does this.

However, sometimes you have a certain part of your interface that is quite dynamic. For instance, a list of things where items may be added or removed at any place.

There are different ways to mitigate this during the reconciliation stage, but they involve comparing trees at certain depths and backtracking… Quite computationally expensive.

One approach that is cheaper consists in letting the user provide some hints about the identities of the different widgets so that they can be compared directly without going deeper.

The widgets in this module will all ask for a “hint” of some sort. In order to help them keep continuity, you need to make sure the hint stays the same for the same items in your user interface between view calls.

Modules

• column
  Keyed columns distribute content vertically while keeping continuity. Distribute content vertically.

Structs

• Column
  A container that distributes its contents vertically while keeping continuity.