Rich text editor

Package ui/richtext edits attributed text, as NSTextView does on macOS and SwiftUI's TextEditor bound to an AttributedString. It supports:

• Formats: bold, italic, underline, strikethrough, inline code, fonts, sizes and colors.
• Links: opened by a click, and set from the app or with ⌘K.
• Paragraphs: headings, bulleted, numbered and task lists, quotations, blocks of code and alignment.
• Attachments: images inside lines.
• Editing: the shortcuts and the Format menu of word processors, and undo that brings formats back with the text.
• Exchange: copy and paste in rich formats, input methods that compose in the typing attributes, Markdown export, and screen readers that read the formats.

It lays out and draws only the paragraphs in view, so a long document edits as a short one.

import "github.com/egoist/mygo/ui/richtext"

// Kept by the app, as its other state.
app.doc = richtext.NewDocument(richtext.NewText("Hello"))

// In the view:
if richtext.Editor(c, app.doc).Fill().Changed() {
	app.save(app.doc.Text())
}

The notes example is a notes app with a toolbar, a link popover and the Format menu, which saves its notes as JSON.

Text

A Text is attributed text: paragraphs, each with a ParagraphStyle, made of runs, each with its Attributes. It is a value, as a string is. Its methods return a new text and leave the one they are called on as it was, sharing with it all they did not change. Keeping a version, to undo, to save later or to compare, costs a pointer, and a version can be read from any goroutine while others are made.

note := richtext.TextOf(
	richtext.Paragraph{Style: richtext.ParagraphStyle{Heading: 1}, Runs: []richtext.Run{{Text: "Groceries"}}},
	richtext.Paragraph{Style: richtext.ParagraphStyle{List: richtext.Tasks}, Runs: []richtext.Run{
		{Text: "Milk, "},
		{Text: "oat", Attributes: richtext.Attributes{Weight: 700}},
	}},
)
note = note.Format(0, 9, func(a *richtext.Attributes) { a.Color = ui.RGB(37, 99, 235) })
note = note.ReplaceString(note.Len(), note.Len(), "\nBread", richtext.Attributes{})

Offsets count runes, newlines included, as Element.TextSelection does. To read a text:

• String and PlainText give its characters;
• Paragraphs and Runs give its structure;
• AttributesAt and ParagraphStyleAt describe one rune.

To change one:

• Replace and ReplaceString change its text;
• Format changes the attributes of a range;
• FormatParagraphs changes the styles of the paragraphs a range touches.

Attributes left zero take the editor's style: its font, size and color. Code, Link and Tag (the app's own mark over a run, such as a mention or a comment) hold only what they hold: text typed at their edges does not join them.

A paragraph's style and the attributes of its newline belong to the paragraph, as in word processors: an empty paragraph remembers what typing gets there. When paragraphs are joined or split, each takes the style of the paragraph its first rune comes from, as AppKit fixes paragraph styles:

• Deleting the newline after a heading keeps the heading.
• Deleting the heading whole, its newline included, leaves the paragraph after it.
• Text pasted at the start of a paragraph brings its own style.

Lists are flat, as in Quill and Automerge: a paragraph's Indent is its level, and items next to each other make a list, numbered at each level from 1 (in letters and Roman numerals below the first).

A paragraph's ID is the app's name for it, as the key of what the app keeps about it, such as notes or a timeline. The editor does not show it, and it follows the paragraph's text through edits:

• Splitting a paragraph keeps the ID in the first half, or in the second when Enter at its start moves the paragraph down. The other half has none.
• Joining two paragraphs keeps the first's, unless the first was empty.
• Paragraphs pasted or inserted have none: Replace never takes the IDs of the text it inserts, so no two paragraphs get one ID.

Undo and redo bring IDs back with the text, and JSON saves them. The clipboard, HTML, Markdown and RTF leave them out. Give IDs as the text is made (TextOf, UnmarshalJSON), or have the document name the paragraphs that edits make, in the step of undo that makes them:

app.doc = richtext.NewDocument(chapter)
app.doc.IdentifyParagraphs(func() string { return rand.Text() })

// On save, every paragraph has an ID:
for start, p := range app.doc.Text().Paragraphs() {
	app.store.Save(p.Style.ID, start, p)
}

MarshalJSON and UnmarshalJSON save a text. Markdown and HTML export it, and ParseMarkdown, ParseHTML and ParseRTF read Markdown, HTML and RTF, as a paste does.

Documents

A Document is what an editor edits: a text, its selection with the attributes typing gets there, and the history of undo. The app keeps it from frame to frame. Its methods are safe from any goroutine, so a toolbar, a menu or a timer formats and edits it while the editor shows it. The editor follows at its next frame. Schedule changes from other goroutines with Window.Update so that a frame comes.

Commands act on the selection as the editor's shortcuts do. On a range, they change its runs or paragraphs as one step of undo. On a caret, character formats change only the typing attributes, which the next typed text gets and the next move of the caret forgets, as ⌘B does in NSTextView.

f := app.doc.Formatting() // the selection's formats, for a toolbar
bold := ui.Button(c, "B")
if f.Bold == richtext.FormatOn {
	bold.Background(t.Accent.Alpha(0.18))
}
if bold.KeepFocus().Clicked() {
	app.doc.ToggleBold()
}

Formatting gives each character format as FormatOn, FormatOff or FormatMixed, along with the link the selection is in and the style of its first paragraph. These methods change the document:

Edit runs a function on an Edit, whose Replace, ReplaceString, Format, FormatParagraphs and Select change the document as one step of undo. The selection follows what changes, as AttributedString tracks indices: a caret where text is inserted goes after it, and a range does not grow at its end.

app.doc.Edit(func(e *richtext.Edit) {
	e.ReplaceString(0, 0, "Dear Ann,\n", richtext.Attributes{})
	e.Format(0, 8, func(a *richtext.Attributes) { a.Italic = true })
})

The editor

Editor is the editor with the look of a text area: padding, a background, a border and corners. EditorBase has none of these, for an editor of the app's own look.

Changed reports the edits the user made in the editor, not those the app made through the document. EditorOptions set:

• Label and Placeholder;
• Font, the editor's font and size;
• ReadOnly, a text the user selects, copies, and clicks the links of;
• NoMarkdown, which turns the Markdown shortcuts off;
• OnLink, to open links itself instead of the system;
• OnLinkCommand, to ask for a link's URL on ⌘K, as in a popover;
• TagStyle, to style the runs that carry a Tag.
• Menu, to build the context menu in place of the editor's own, as an app with a menu of its own opens it;
• OnKey, asked about a key before the editor acts on it, as an app's popup over the text (a list of completions, placed at Document.CaretRect) takes the arrows, Enter and Escape: true takes the key;
• Spans, given the spans a paragraph is drawn as and returning them split, colored or spaced otherwise, their text the same, as an app colors a block of code by its language or spaces the letters; change SpansVersion when what it does changes, for the paragraphs to be laid out again;
• Theme, how the text looks (see Theme).

The keys are those of word processors, with Control in place of Command off macOS:

Paragraphs behave as in Notes and Google Docs:

• Enter continues a list, and on an empty item leaves it.
• Enter at the end of a heading starts a body paragraph.
• Backspace at the start of a list's item, a heading, a quotation or a block of code takes the style away before joining the paragraph to the one above.
• A click on a task's check box checks it, and a click on a link opens it: an http, https or mailto address (ui.SafeToOpen), as Markdown views open them, since other schemes and paths run programs on Windows. OnLink opens links itself instead.

The Markdown shortcuts format what is typed: at the start of a paragraph, # to ###### make headings, - , * and + bulleted lists, 1. numbered ones, [ ] and [x] tasks, > a quotation, and ``` then Enter a block of code. Around words, **bold**, *italic*, _italic_, `code` and ~~strikethrough~~ format what they hold as the closing mark is typed; an underscore inside a word, as in snake_case, stays one. Undo takes a shortcut back to what was typed.

The Format menu's roles (RoleFormatMenu, or RoleBold, RoleAlignCenter, …) format the editor with the keyboard focus. Paste and Match Style pastes plain text in the typing attributes.

Theme

EditorOptions.Theme sets how the text looks: the colors of text, muted text, links, the caret and the selection; the font, size and colors of code, within text and in blocks, and the corners and room of blocks of code; each level of heading's size, weight, color and room above and below; the first-line indent of body paragraphs and the room between them; the indentation of lists, the colors of their markers and check boxes; the bar and text of quotations; and the padding of Editor, which replaces the window theme's. What it leaves zero takes the value of DefaultTheme for the window's theme, so that a theme sets only what it changes, and follows the window into dark mode:

violet := ui.Hex("#7c3aed")
richtext.Editor(c, doc, richtext.EditorOptions{Theme: &richtext.Theme{
	Link:     violet,
	Checkbox: violet,
	Headings: [6]richtext.HeadingStyle{{Size: 2.2, Weight: 800}},
}})

Lengths are in ems, times the editor's font size, so that a theme scales with Font, but for the padding and the quotation bar's width, in DIPs. Changing the theme lays the text out again. The colors the editor shows go to screen readers with its runs.

Body paragraphs (not headings, items of lists or code) run together by default, as in notes. For prose, set them apart in either of two ways:

• FirstLineIndent indents the first line of a body paragraph, as printed fiction does (about 1.2 ems). The first paragraph after a heading, a list, a block of code, an empty paragraph or one aligned otherwise (a centered scene break, "* * *") isn't indented, nor is the first of a quotation, as nothing needs to set it apart. Only paragraphs aligned at the start are indented.
• ParagraphSpacing puts room between two body paragraphs, as prose on screens does. Headings keep their own room above and below.

richtext.EditorBase(c, app.doc, richtext.EditorOptions{Theme: &richtext.Theme{
	FirstLineIndent: 1.2,
}})

The indent is no character: the caret, selection, clicks and arrow keys skip over it, as in a word processor.

Paragraphs on screen

Document.VisibleParagraphs returns the paragraphs in view in the editor showing the document: each one's index, the rune it starts at, and the top and height of its lines in DIPs from the top of the editor's element, scrolled. Read it as the view builds to line a gutter up with the paragraphs, as a timeline, notes in the margin or paragraph numbers. Put the gutter beside the editor with the same top, and draw at its top plus each paragraph's:

boxes := app.doc.VisibleParagraphs()
ui.Row(c).Fill().AlignItems(ui.Stretch).Children(func() {
	ui.Box(c).Width(40).Clip().Draw(func(p *ui.Painter, r ui.Rect) {
		for _, b := range boxes {
			p.Fill(ui.Rect{X: r.X + 16, Y: r.Y + b.Top, W: 4, H: b.Height}, t.Accent, 2)
		}
	})
	richtext.EditorBase(c, app.doc).Grow(1)
})

The boxes are where the editor paints the paragraphs in that frame, since it lays them out as they are read. When a frame moves them from where they were read, as the editor scrolls, its width changes or it reveals the caret, it builds another, so the gutter follows without the app watching the scrolling. The room above and below a paragraph (a heading's margins, the spacing between paragraphs) is left out of its box, so a mark at its top lines up with its first line. VisibleParagraphs lays out only what is in view, as painting does, so it costs as little in a long document as in a short one. It returns nil until an editor showed the document.

The caret on screen

Document.CaretRect returns where the editor shows the caret, focused or not: a line as high as the caret's, in DIPs from the top left of the editor's element, scrolled, and whether it is in view. Read it as the view builds to place a popup at the caret, as completions after a / or an @, whose arrows, Enter and Escape OnKey takes. Put the popup in a box with the editor, placed at the bottom of the caret:

caret, ok := app.doc.CaretRect()
ui.Box(c).Fill().Children(func() {
	richtext.EditorBase(c, app.doc, richtext.EditorOptions{OnKey: app.completionKey}).Fill()
	if app.completing && ok {
		ui.Column(c).Absolute().Left(caret.X).Top(caret.Y + caret.H + 4).Children(func() {
			for _, s := range app.completions {
				ui.Text(c, s)
			}
		})
	}
})

As with the paragraphs, the caret is where the editor paints it in that frame: one that moves it from where it was read, as the user types, the editor scrolls or its width changes, builds another, so the popup follows the caret. ok is false until an editor showed the document, and while the caret is scrolled out of view, where the popup hides.

Scrolling

The editor scrolls itself: with the wheel, its scroll bar, a selection dragged to its edges, and to keep the caret in view. Document.ScrollBy and Document.ScrollToParagraph scroll the editor that showed the document last, as far as its text goes, and ask for a frame to paint it:

An app that drags something of its own over the text, as a block by a handle beside it, scrolls the editor while the pointer is near its top or bottom edge, as the editor does while the user drags a selection there:

// y is the pointer's height in the editor's element, h the element's.
if app.dragging {
	switch y, h := app.dragY, app.editorH; {
	case y < 24:
		app.doc.ScrollBy(y - 24)
		c.After(16 * time.Millisecond)
	case y > h-24:
		app.doc.ScrollBy(y - (h - 24))
		c.After(16 * time.Millisecond)
	}
}

Both lay out what is in view as they scroll, so that VisibleParagraphs and CaretRect read after them give where the paragraphs and the caret went, and both do nothing until an editor showed the document.

The editor keeps its place by the first paragraph in view: what grows above it, as a paragraph laid out for the first time, moves nothing on screen, and what grows below its top moves what is below. While the editor has the keyboard focus and is scrolled down its text, it keeps its place by the caret's paragraph instead, when that is in view, as browsers prefer the focused editable element (CSS Scroll Anchoring's priority candidates): what grows between the top of the view and the caret, as the room EditorOptions.Space leaves or an edit, moves the text above it up, and the line being typed stays. An editor scrolled to its top stays there.

Typing at the edges of formats

Text typed takes the attributes of the character before the caret. At the start of a paragraph it takes those of the character after it, and in place of a selection those of the first character selected. These are NSTextView's rules, measured on macOS 27, and those of ProseMirror, Lexical and Quill:

• Typing at the end of bold continues bold; at its start, it does not.
• Typing at the end of a link, a code span or a tag does not continue it. Inside one, it does.
• After the text before the caret is deleted, typing gets its attributes until the caret moves: delete a bold word and type, and the text is bold.
• After Enter, typing keeps the attributes typed before it.

Attachments

An attachment is an object replacement character (U+FFFC) whose Attributes.Attachment holds an Attachment: an image, or a drawing of the app's, laid out as a box inside the line, standing on the baseline. It is selected, copied and deleted as one character, and lines break around it. A box wider than the editor is scaled down to fit.

photo, _ := ui.DecodeBitmap(data)
app.doc.InsertAttachment(&richtext.Attachment{Image: photo, Name: "Beach", Width: 240})

Without an image or a Draw function, an attachment shows its Name in a box, as an image not loaded yet. HTML and Markdown write an attachment's URL.

Copy and paste

Copying puts the selection on the clipboard in three formats:

• the editor's own (richtext.Format, JSON), which MyGo apps paste whole, attachments included within the app;
• HTML;
• plain text.

Pasting reads the richest format the clipboard holds: the editor's own, then HTML, then RTF, then Markdown (transfer.Markdown, which Markdown views and apps copy), then plain text. A Markdown view's selection pastes with its formatting.

• HTML, as browsers, mail, Word and Google Docs copy it, keeps its headings, lists, task lists, quotations, blocks of code, alignment, bold, italic, underline, strikethrough, code and links.
• RTF, as TextEdit, Pages and WordPad copy it without HTML, keeps its paragraphs, alignment, lists, bold, italic, underline, strikethrough and links. Fonts, sizes and colors are left out, as Notes leaves them, so that what is pasted takes the editor's. Paste and Match Style (⇧⌥⌘V) pastes plain text in the typing attributes.

Input methods

An input method composes at the caret in the typing attributes, as AppKit and Qt show it, underlined while it composes. The whole composition is one step of undo with what it replaced.

Accessibility

Screen readers read the editor as a text field, with the formats of its runs:

• VoiceOver reads AXAttributedStringForRange: fonts, colors, underlines, strikethroughs and links.
• Orca reads ATK's run attributes.
• Narrator reads UI Automation's Text pattern, by characters, formats, words and lines, with the font, weight, italics, lines, colors and heading level of each run, and the mixed value where they vary.

Screen readers move the selection too.

Large documents

The text is a rope of chunks with a persistent sequence of paragraphs beside it, so an edit copies what it touches and the path to it, and costs O(log n). The editor lays out only the paragraphs in view, keeps the layouts of those near it, and estimates the heights of the others. Typing a character in a document of 100,000 formatted paragraphs, and drawing the frame on the CPU, takes about as long as in one of 1,000 (BenchmarkTypeFrame in ui/richtext).

Markdown

Text.Markdown writes CommonMark with GitHub's extensions:

• headings, lists, task lists, quotations and fenced blocks of code;
• **bold**, *italic*, ~~strikethrough~~, `code`, links and images;
• underlines as <u>, and as <strong>, <em> and <del> the marks that Markdown's delimiters cannot open or close where they are, as bold "foo." right before "bar".

Alignment, fonts and colors have no Markdown and are left out.

ParseMarkdown reads it back, and what a model or another app wrote: CommonMark with GitHub's extensions and footnotes, with the parser of Markdown views (ui/markdown/parser). A list item's paragraphs after its first are indented under it, and a table's rows are paragraphs of their cells apart by tabs. Math stays as written, as the editor does not show it. What Markdown writes reads back the same.