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Research: Multiple cursors and selections
ScanuNicco edited this page Sep 25, 2021
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This document describes how we intend to implement multiple selections and cursors in Brackets based on the implementation in CodeMirror v4.
This section sets out general rules for how edit operations should operate on multiple selections and/or cursors. In the following rules, "selection" refers to both cursors and range selections.
The default behavior for most operations should be:
- For non-line-oriented operations:
- Do a batch operation that contains the result of acting on each selection independently.
- Track each selection through the edit (so that if the edit to one of the selections would change the selection, that new selection is substituted for the original in the resulting multiple selection).
- If some of the resulting selections overlap, merge them.
- For line-oriented operations (e.g. Duplicate Line, Indent), where the exact location of the selection on the line isn't meaningful:
- Operate once on each line that intersects any of the selections (i.e., don't operate on a line twice even if there are two selections within the line).
- But still track each selection through the edit as if the selections were made independently (i.e., if there are two selections within the line, the resulting selection should contain updated locations for both of them).
- As in (1), if some of the resulting selections overlap, merge them.
- For operations that really only make sense to operate on one selection, or if one selection needs to be designated as special (e.g. when opening multiple inline editors, which one gets the focus), use the "primary" selection as returned by
getSelection()(see table below).
This doesn't list all the APIs for actual edit operations that will change in order to implement multiple selection behavior - this is just intended to capture the core functionality in the Brackets Editor class that callers will need to access in order to make their own edit operations work properly with multiple selections.
In general, Brackets has not exposed most of CodeMirror's own editing APIs through methods on Editor, so in many cases you'll still need to talk to editor._codeMirror in order to perform edits. The API changes here are really just intended to address cases where we already had an API in Editor that was specific to single selections, and that either needs to be augmented or have a parallel API created for multiple selections.
The strategy Brackets is taking is the same as CodeMirror:
- add new APIs to get/set multiple selections
- in general, make existing single-selection APIs operate on the "primary" selection if there are multiple selections
The following table describes changes in behavior for existing selection-related APIs as well as new methods for multiple selections. Unless otherwise specified, "selections" means either a selection range or a cursor (which is represented by a selection range whose start and end are equal). The "primary" selection is generally the last selection made by the user, although any selection can be set as primary by using setSelections().
| Method | New Behavior |
|---|---|
hasSelection() |
When multiple selections are active, returns true if any selection is non-empty. |
getCursorPos() |
When multiple selections are active, returns the position of the cursor within the primary selection. Also now takes an argument specifying whether you want the start, end, head, or anchor of the primary selection. |
setCursorPos() |
No new behavior - always removes the current selection and replaces it with a new single cursor. |
getSelection() |
When multiple selections are active, returns only the primary selection. Note: this is different from CodeMirror's getSelection(), which is equivalent to our getSelectedText(). |
getSelections() |
New method. Returns an array of all active selections. In general, you should use this instead of getSelection(). |
getSelectedText() |
By default, returns only the contents of the primary selection. Has a new parameter, allSelections, which if true will return the concatenated contents of all selections. |
setSelection() |
No new behavior - always removes the current selection and replaces it with a new single selection. |
setSelections() |
New method. Like setSelection(), but takes an array of selections instead of a single start/end, and allows specifying one of the selections as primary (defaulting to the last one). If center is specified, centers on the primary selection. |
centerOnCursor() |
When multiple selections are active, centers on the primary selection. |
getModeForSelection()getLanguageForSelection()
|
If the beginning/end of each selection has the same mode/language, returns that, otherwise null. |
convertToLineSelections() |
New method. Helper function for line-oriented operations that expands all selections within a multiple selection to encompass whole line ranges. If several of the selections would contribute to overlapping line selections, they will be merged into a single range, but the original selections are mapped to the resulting line range so they can be properly tracked through edits. |
Document.doMultipleEdits() |
New method. Helper function that makes it easier to implement edits that iterate over multiple selections, by making it so you can treat each edit in isolation rather than having to worry about position changes due to other edits. |
In the table below, operations listed as "default" follow the rules above. Anything not marked "Not working" appears to be working as desired in the current CMv4 branch.
| Operation | Desired Behavior | Notes |
|---|---|---|
| Making selections | ||
| Multiple cursors (contiguous) | Hold down Alt, then click and drag vertically | On Windows, Sublime uses Shift-RMB or MMB, not Alt. Ace is the same as CM. Our plan is to stick with the CM implementation. |
| Multiple cursors (discontiguous) | Click first cursor, then Cmd/Ctrl-click to set other cursors | |
| Multiple selections (contiguous - rectangular) | Hold down Alt, then click and drag a rectangle | |
| Multiple selections (discontiguous) | Make first selection, then Cmd/Ctrl-click and drag to set other selections. You can also Cmd/Ctrl-Alt-drag to add rectangular selections. | |
| Shift-Alt-Up/Down | Add Line to Selection - adds the line above or below as another selection | |
| Ctrl/Cmd-Alt-L | Split into Lines - takes a block of lines and splits it into individual selections, one per line | |
| Ctrl/Cmd-B | Quick Add Next - if word not selected, expands selection to word; if word selected, adds next occurrence of word as another selection | |
| Ctrl/Cmd-Shift-B | Skip Next Match - removes the last "Quick Add Next" match from the selection and adds the next one | |
| Alt-F3 / Cmd-Ctrl-G | Find All - selects current word, then adds selections for each match in the current document | |
| Ctrl/Cmd-U | Undo Selection - undoes the last selection change (for both single and multiple selections) | |
| Ctrl/Cmd-Shift-U | Redo Selection | |
| Esc | Switch to single selection | Note: Sublime collapses to the first selection, not the last (primary) one. Ace matches CM's behavior. We're going to stick with CM's behavior for now, but can revisit if we get enough user feedback. |
| Status Bar | Show "n selections" when there are multiple selections | |
| Cursor navigation | ||
| Arrow key (multiple cursors only) | Default | |
| Arrow key (multiple selections) | Collapse selection in direction of arrow | Note: In the case of up/down arrow, Ace and Sublime also apply the navigation (even in the single selection case), whereas CM just collapses the selection. I think keeping the CM behavior is fine. |
| Shift-arrow key | Default (extend each selection in direction of arrow; merge selections that collide) | |
| Shift-Cmd-Right/Left (Mac)/Shift-Alt-Right/Left key, Shift-Home/End (Win) | Default (extend each selection to end of its line) | Note: on Win, Sublime maps Alt-arrow to move by word, not end of line |
| Shift-Alt-Right/Left (Mac)/Shift-Ctrl-Right/Left (Win) | Default (extend each selection by word; merge selections that collide) | |
| Page up/down | Do page up/down and collapse selection to single cursor | Note: on Win, CMv4 currently collapses selection only, but this is true for single select as well. This also happens in CMv3 -- introduced in Sprint 36. See issue #6811. |
| Edits | ||
| Typing: multiple cursors/selections | Default (delete selection if any, then insert typed char) | |
| Typing: combination of cursors and selections | Default | |
| Delete: multiple cursors | Default (delete char before each cursor) | Edge cases occur when deletions can cause cursors to collide. Should test to make sure there's no bad behavior. |
| Delete: multiple selections | Default (delete content and collapse each selection to cursor) | Edge cases occur when deletions can cause cursors to collide. Should test to make sure there's no bad behavior. |
| Delete: combination of cursors and selections | Only delete selections - no operation on cursors. | |
| Tab | See description in #7022 | |
| Shift-Tab | Default (unindent all lines) | |
| Indent/Unindent | Default | |
| Copy | Concatenate each selection, separated by newlines, and copy to clipboard. Cursors count as empty selections, so add an extra newline. | This is the current cmv4 behavior. Sublime ignores cursors in this case (extra newlines are not generated). Ace's behavior is like CM's. |
| Cut | Like copy, but delete afterwards. | Note that in Sublime, if there are cursors in the selection, the lines containing those cursors are deleted, because that's the behavior in the single selection case. Brackets doesn't do that, so it makes sense to stay consistent with ourselves. Ace's behavior is like CM's. |
| Paste | Default (paste the clipboard into each selection/cursor, deleting selections first, and leaving cursor after pasted content) | |
| Duplicate Line | First, duplicate any lines containing at least one cursor, then duplicate any range selections | Note that this behavior is slightly different from Sublime in the case where there is a cursor and a range on the same line, but I don't think that's a common case and the correct behavior is arguable. |
| Delete Line | Default | |
| Open Line Above/Below | Default | |
| Move Line Up/Down | Default (except that contiguous lines should move together) | |
| Toggle Line Comment | Default | |
| Toggle Block Comment | Default | |
| Other gestures | ||
| Quick Edit/Quick Docs | Open on the primary selection. | Note that we were originally planning to open one inline editor on each selection, but it turned out to require API changes to Quick Edit and we didn't think it was worth doing right now. |
| Find (what should be in the query field) | Contents of the last (primary) selection | Ace's behavior matches this. Note: Sublime doesn't copy the selection to the query string on Cmd-F, but it does do so on Cmd-E, and it uses the contents of the last selection. |
| Find All | Add a button to the Find dialog that creates a multiple selection from all matches on the current query. | Not implemented for the initial story - tracked on a separate backlog item |
| Find Next/Find Previous (where should they start from) | Start from the last (primary) selection | Ace's behavior matches this. Note: Sublime seems to start from the selection farthest along in the document, but it seems more consistent to start from the last selection made. |
| Live Preview Highlight in HTML/CSS (do we highlight all selections?) | Default (highlight all items containing any selection) | |
| Highlight Active Line (for multiple cursors) | Default (highlight any line that contains a cursor) | |
| Code Hints | Operate on primary selection only |
- Unit tests: all passing now.
- Selections around hard tabs: should be working.
- Edits on multiple selections/cursors within inline editors: should be working.
- Edits on multiple selections/cursors in live HTML: Seems to work fine for basic cases. I would guess there might be performance issues for lots of changes at once, but that doesn't seem like a big deal. Also, if there's a syntax error we only show the first one, but that seems fine.
- Edits on multiple selections/cursors in live CSS: Seems to work fine for basic cases.
- Add API functions to Editor
- Implement any behavior above that's not implemented - lots of cases but mostly straightforward
- Create new unit tests for multiple cursor/selection cases for all appropriate functionality
- Fix existing unit tests that are broken with v4
- Scenario testing
- File bugs against popular extensions about multi-selection support