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71 lines
4.1 KiB
Markdown
71 lines
4.1 KiB
Markdown
# The CIDER (Contenteditable-Input-Diff-Error-Reconcile) editor
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The CIDER editor is a custom editor written for Element.
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Most of the code can be found in the `/editor/` directory of the `matrix-react-sdk` project.
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It is used to power the composer main composer (both to send and edit messages), and might be used for other usecases where autocomplete is desired (invite box, ...).
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## High-level overview.
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The editor is backed by a model that contains parts.
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A part has some text and a type (plain text, pill, ...). When typing in the editor,
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the model validates the input and updates the parts.
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The parts are then reconciled with the DOM.
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## Inner workings
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When typing in the `contenteditable` element, the `input` event fires and
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the DOM of the editor is turned into a string. The way this is done has
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some logic to it to deal with adding newlines for block elements, to make sure
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the caret offset is calculated in the same way as the content string, and to ignore
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caret nodes (more on that later).
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For these reasons it doesn't use `innerText`, `textContent` or anything similar.
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The model addresses any content in the editor within as an offset within this string.
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The caret position is thus also converted from a position in the DOM tree
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to an offset in the content string. This happens in `getCaretOffsetAndText` in `dom.ts`.
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Once the content string and caret offset is calculated, it is passed to the `update()`
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method of the model. The model first calculates the same content string of its current parts,
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basically just concatenating their text. It then looks for differences between
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the current and the new content string. The diffing algorithm is very basic,
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and assumes there is only one change around the caret offset,
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so this should be very inexpensive. See `diff.ts` for details.
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The result of the diffing is the strings that were added and/or removed from
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the current content. These differences are then applied to the parts,
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where parts can apply validation logic to these changes.
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For example, if you type an @ in some plain text, the plain text part rejects
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that character, and this character is then presented to the part creator,
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which will turn it into a pill candidate part.
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Pill candidate parts are what opens the auto completion, and upon picking a completion,
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replace themselves with an actual pill which can't be edited anymore.
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The diffing is needed to preserve state in the parts apart from their text
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(which is the only thing the model receives from the DOM), e.g. to build
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the model incrementally. Any text that didn't change is assumed
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to leave the parts it intersects alone.
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The benefit of this is that we can use the `input` event, which is broadly supported,
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to find changes in the editor. We don't have to rely on keyboard events,
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which relate poorly to text input or changes, and don't need the `beforeinput` event,
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which isn't broadly supported yet.
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Once the parts of the model are updated, the DOM of the editor is then reconciled
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with the new model state, see `renderModel` in `render.ts` for this.
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If the model didn't reject the input and didn't make any additional changes,
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this won't make any changes to the DOM at all, and should thus be fairly efficient.
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For the browser to allow the user to place the caret between two pills,
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or between a pill and the start and end of the line, we need some extra DOM nodes.
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These DOM nodes are called caret nodes, and contain an invisble character, so
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the caret can be placed into them. The model is unaware of caret nodes, and they
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are only added to the DOM during the render phase. Likewise, when calculating
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the content string, caret nodes need to be ignored, as they would confuse the model.
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As part of the reconciliation, the caret position is also adjusted to any changes
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the model made to the input. The caret is passed around in two formats.
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The model receives the caret _offset_ within the content string (which includes
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an atNodeEnd flag to make it unambiguous if it is at a part and or the next part start).
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The model converts this to a caret _position_ internally, which has a partIndex
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and an offset within the part text, which is more natural to work with.
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From there on, the caret _position_ is used, also during reconciliation.
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