Implement asynchronous transform actions (#4419)

Close #4418

Example:

    fzf --bind 'focus:bg-transform-header(sleep 2; date; echo {})'

CHANGELOG.md

@@ -1,6 +1,24 @@
 CHANGELOG
 =========
 
+0.63.0
+------
+- Added background variants of transform actions with `bg-` prefix that run asynchronously in the background
+  ```sh
+  GETTER='curl -s http://metaphorpsum.com/sentences/1'
+  fzf --style full --border --preview : \
+      --bind "focus:bg-transform-header:$GETTER" \
+      --bind "focus:+bg-transform-footer:$GETTER" \
+      --bind "focus:+bg-transform-border-label:$GETTER" \
+      --bind "focus:+bg-transform-preview-label:$GETTER" \
+      --bind "focus:+bg-transform-input-label:$GETTER" \
+      --bind "focus:+bg-transform-list-label:$GETTER" \
+      --bind "focus:+bg-transform-header-label:$GETTER" \
+      --bind "focus:+bg-transform-footer-label:$GETTER" \
+      --bind "focus:+bg-transform-ghost:$GETTER" \
+      --bind "focus:+bg-transform-prompt:$GETTER"
+  ```
+
 0.62.0
 ------
 - Relaxed the `--color` option syntax to allow whitespace-separated entries (in addition to commas), making multi-line definitions easier to write and read

man/man1/fzf.1

@@ -1805,6 +1805,9 @@ A key or an event can be bound to one or more of the following actions.
     \fBup\fR                           \fIctrl\-k  ctrl\-p  up\fR
     \fByank\fR                         \fIctrl\-y\fR
 
+Each \fBtransform*\fR action has a corresponding \fBbg\-transform*\fR
+variant that runs the command in the background.
+
 .SS ACTION COMPOSITION
 
 Multiple actions can be chained using \fB+\fR separator.
@@ -1929,6 +1932,14 @@ e.g.
                     echo "change\-header:Invalid selection"'
     \fR
 
+.SS TRANSFORM IN THE BACKGROUND
+
+Transform actions are synchronous, meaning fzf becomes unresponsive while the
+command runs. To avoid this, each \fBtransform*\fR action has a corresponding
+\fBbg\-transform*\fR variant that runs in the background. Unless you need to
+chain multiple transform actions where later ones depend on earlier results,
+prefer using the \fBbg\fR variant.
+
 .SS PREVIEW BINDING
 
 With \fBpreview(...)\fR action, you can specify multiple different preview

src/actiontype_string.go

@@ -113,48 +113,64 @@ func _() {
 	_ = x[actTransformPrompt-102]
 	_ = x[actTransformQuery-103]
 	_ = x[actTransformSearch-104]
-	_ = x[actSearch-105]
+	_ = x[actBgTransform-105]
-	_ = x[actPreview-106]
+	_ = x[actBgTransformBorderLabel-106]
-	_ = x[actPreviewTop-107]
+	_ = x[actBgTransformGhost-107]
-	_ = x[actPreviewBottom-108]
+	_ = x[actBgTransformHeader-108]
-	_ = x[actPreviewUp-109]
+	_ = x[actBgTransformFooter-109]
-	_ = x[actPreviewDown-110]
+	_ = x[actBgTransformHeaderLabel-110]
-	_ = x[actPreviewPageUp-111]
+	_ = x[actBgTransformFooterLabel-111]
-	_ = x[actPreviewPageDown-112]
+	_ = x[actBgTransformInputLabel-112]
-	_ = x[actPreviewHalfPageUp-113]
+	_ = x[actBgTransformListLabel-113]
-	_ = x[actPreviewHalfPageDown-114]
+	_ = x[actBgTransformNth-114]
-	_ = x[actPrevHistory-115]
+	_ = x[actBgTransformPointer-115]
-	_ = x[actPrevSelected-116]
+	_ = x[actBgTransformPreviewLabel-116]
-	_ = x[actPrint-117]
+	_ = x[actBgTransformPrompt-117]
-	_ = x[actPut-118]
+	_ = x[actBgTransformQuery-118]
-	_ = x[actNextHistory-119]
+	_ = x[actBgTransformSearch-119]
-	_ = x[actNextSelected-120]
+	_ = x[actSearch-120]
-	_ = x[actExecute-121]
+	_ = x[actPreview-121]
-	_ = x[actExecuteSilent-122]
+	_ = x[actPreviewTop-122]
-	_ = x[actExecuteMulti-123]
+	_ = x[actPreviewBottom-123]
-	_ = x[actSigStop-124]
+	_ = x[actPreviewUp-124]
-	_ = x[actFirst-125]
+	_ = x[actPreviewDown-125]
-	_ = x[actLast-126]
+	_ = x[actPreviewPageUp-126]
-	_ = x[actReload-127]
+	_ = x[actPreviewPageDown-127]
-	_ = x[actReloadSync-128]
+	_ = x[actPreviewHalfPageUp-128]
-	_ = x[actDisableSearch-129]
+	_ = x[actPreviewHalfPageDown-129]
-	_ = x[actEnableSearch-130]
+	_ = x[actPrevHistory-130]
-	_ = x[actSelect-131]
+	_ = x[actPrevSelected-131]
-	_ = x[actDeselect-132]
+	_ = x[actPrint-132]
-	_ = x[actUnbind-133]
+	_ = x[actPut-133]
-	_ = x[actRebind-134]
+	_ = x[actNextHistory-134]
-	_ = x[actToggleBind-135]
+	_ = x[actNextSelected-135]
-	_ = x[actBecome-136]
+	_ = x[actExecute-136]
-	_ = x[actShowHeader-137]
+	_ = x[actExecuteSilent-137]
-	_ = x[actHideHeader-138]
+	_ = x[actExecuteMulti-138]
-	_ = x[actBell-139]
+	_ = x[actSigStop-139]
-	_ = x[actExclude-140]
+	_ = x[actFirst-140]
-	_ = x[actExcludeMulti-141]
+	_ = x[actLast-141]
+	_ = x[actReload-142]
+	_ = x[actReloadSync-143]
+	_ = x[actDisableSearch-144]
+	_ = x[actEnableSearch-145]
+	_ = x[actSelect-146]
+	_ = x[actDeselect-147]
+	_ = x[actUnbind-148]
+	_ = x[actRebind-149]
+	_ = x[actToggleBind-150]
+	_ = x[actBecome-151]
+	_ = x[actShowHeader-152]
+	_ = x[actHideHeader-153]
+	_ = x[actBell-154]
+	_ = x[actExclude-155]
+	_ = x[actExcludeMulti-156]
+	_ = x[actAsync-157]
 }
 
-const _actionType_name = "actIgnoreactStartactClickactInvalidactBracketedPasteBeginactBracketedPasteEndactCharactMouseactBeginningOfLineactAbortactAcceptactAcceptNonEmptyactAcceptOrPrintQueryactBackwardCharactBackwardDeleteCharactBackwardDeleteCharEofactBackwardWordactCancelactChangeBorderLabelactChangeGhostactChangeHeaderactChangeFooteractChangeHeaderLabelactChangeFooterLabelactChangeInputLabelactChangeListLabelactChangeMultiactChangeNthactChangePointeractChangePreviewactChangePreviewLabelactChangePreviewWindowactChangePromptactChangeQueryactClearScreenactClearQueryactClearSelectionactCloseactDeleteCharactDeleteCharEofactEndOfLineactFatalactForwardCharactForwardWordactKillLineactKillWordactUnixLineDiscardactUnixWordRuboutactYankactBackwardKillWordactSelectAllactDeselectAllactToggleactToggleSearchactToggleAllactToggleDownactToggleUpactToggleInactToggleOutactToggleTrackactToggleTrackCurrentactToggleHeaderactToggleWrapactToggleMultiLineactToggleHscrollactTrackCurrentactToggleInputactHideInputactShowInputactUntrackCurrentactDownactUpactPageUpactPageDownactPositionactHalfPageUpactHalfPageDownactOffsetUpactOffsetDownactOffsetMiddleactJumpactJumpAcceptactPrintQueryactRefreshPreviewactReplaceQueryactToggleSortactShowPreviewactHidePreviewactTogglePreviewactTogglePreviewWrapactTransformactTransformBorderLabelactTransformGhostactTransformHeaderactTransformFooteractTransformHeaderLabelactTransformFooterLabelactTransformInputLabelactTransformListLabelactTransformNthactTransformPointeractTransformPreviewLabelactTransformPromptactTransformQueryactTransformSearchactSearchactPreviewactPreviewTopactPreviewBottomactPreviewUpactPreviewDownactPreviewPageUpactPreviewPageDownactPreviewHalfPageUpactPreviewHalfPageDownactPrevHistoryactPrevSelectedactPrintactPutactNextHistoryactNextSelectedactExecuteactExecuteSilentactExecuteMultiactSigStopactFirstactLastactReloadactReloadSyncactDisableSearchactEnableSearchactSelectactDeselectactUnbindactRebindactToggleBindactBecomeactShowHeaderactHideHeaderactBellactExcludeactExcludeMulti"
+const _actionType_name = "actIgnoreactStartactClickactInvalidactBracketedPasteBeginactBracketedPasteEndactCharactMouseactBeginningOfLineactAbortactAcceptactAcceptNonEmptyactAcceptOrPrintQueryactBackwardCharactBackwardDeleteCharactBackwardDeleteCharEofactBackwardWordactCancelactChangeBorderLabelactChangeGhostactChangeHeaderactChangeFooteractChangeHeaderLabelactChangeFooterLabelactChangeInputLabelactChangeListLabelactChangeMultiactChangeNthactChangePointeractChangePreviewactChangePreviewLabelactChangePreviewWindowactChangePromptactChangeQueryactClearScreenactClearQueryactClearSelectionactCloseactDeleteCharactDeleteCharEofactEndOfLineactFatalactForwardCharactForwardWordactKillLineactKillWordactUnixLineDiscardactUnixWordRuboutactYankactBackwardKillWordactSelectAllactDeselectAllactToggleactToggleSearchactToggleAllactToggleDownactToggleUpactToggleInactToggleOutactToggleTrackactToggleTrackCurrentactToggleHeaderactToggleWrapactToggleMultiLineactToggleHscrollactTrackCurrentactToggleInputactHideInputactShowInputactUntrackCurrentactDownactUpactPageUpactPageDownactPositionactHalfPageUpactHalfPageDownactOffsetUpactOffsetDownactOffsetMiddleactJumpactJumpAcceptactPrintQueryactRefreshPreviewactReplaceQueryactToggleSortactShowPreviewactHidePreviewactTogglePreviewactTogglePreviewWrapactTransformactTransformBorderLabelactTransformGhostactTransformHeaderactTransformFooteractTransformHeaderLabelactTransformFooterLabelactTransformInputLabelactTransformListLabelactTransformNthactTransformPointeractTransformPreviewLabelactTransformPromptactTransformQueryactTransformSearchactBgTransformactBgTransformBorderLabelactBgTransformGhostactBgTransformHeaderactBgTransformFooteractBgTransformHeaderLabelactBgTransformFooterLabelactBgTransformInputLabelactBgTransformListLabelactBgTransformNthactBgTransformPointeractBgTransformPreviewLabelactBgTransformPromptactBgTransformQueryactBgTransformSearchactSearchactPreviewactPreviewTopactPreviewBottomactPreviewUpactPreviewDownactPreviewPageUpactPreviewPageDownactPreviewHalfPageUpactPreviewHalfPageDownactPrevHistoryactPrevSelectedactPrintactPutactNextHistoryactNextSelectedactExecuteactExecuteSilentactExecuteMultiactSigStopactFirstactLastactReloadactReloadSyncactDisableSearchactEnableSearchactSelectactDeselectactUnbindactRebindactToggleBindactBecomeactShowHeaderactHideHeaderactBellactExcludeactExcludeMultiactAsync"
 
-var _actionType_index = [...]uint16{0, 9, 17, 25, 35, 57, 77, 84, 92, 110, 118, 127, 144, 165, 180, 201, 225, 240, 249, 269, 283, 298, 313, 333, 353, 372, 390, 404, 416, 432, 448, 469, 491, 506, 520, 534, 547, 564, 572, 585, 601, 613, 621, 635, 649, 660, 671, 689, 706, 713, 732, 744, 758, 767, 782, 794, 807, 818, 829, 841, 855, 876, 891, 904, 922, 938, 953, 967, 979, 991, 1008, 1015, 1020, 1029, 1040, 1051, 1064, 1079, 1090, 1103, 1118, 1125, 1138, 1151, 1168, 1183, 1196, 1210, 1224, 1240, 1260, 1272, 1295, 1312, 1330, 1348, 1371, 1394, 1416, 1437, 1452, 1471, 1495, 1513, 1530, 1548, 1557, 1567, 1580, 1596, 1608, 1622, 1638, 1656, 1676, 1698, 1712, 1727, 1735, 1741, 1755, 1770, 1780, 1796, 1811, 1821, 1829, 1836, 1845, 1858, 1874, 1889, 1898, 1909, 1918, 1927, 1940, 1949, 1962, 1975, 1982, 1992, 2007}
+var _actionType_index = [...]uint16{0, 9, 17, 25, 35, 57, 77, 84, 92, 110, 118, 127, 144, 165, 180, 201, 225, 240, 249, 269, 283, 298, 313, 333, 353, 372, 390, 404, 416, 432, 448, 469, 491, 506, 520, 534, 547, 564, 572, 585, 601, 613, 621, 635, 649, 660, 671, 689, 706, 713, 732, 744, 758, 767, 782, 794, 807, 818, 829, 841, 855, 876, 891, 904, 922, 938, 953, 967, 979, 991, 1008, 1015, 1020, 1029, 1040, 1051, 1064, 1079, 1090, 1103, 1118, 1125, 1138, 1151, 1168, 1183, 1196, 1210, 1224, 1240, 1260, 1272, 1295, 1312, 1330, 1348, 1371, 1394, 1416, 1437, 1452, 1471, 1495, 1513, 1530, 1548, 1562, 1587, 1606, 1626, 1646, 1671, 1696, 1720, 1743, 1760, 1781, 1807, 1827, 1846, 1866, 1875, 1885, 1898, 1914, 1926, 1940, 1956, 1974, 1994, 2016, 2030, 2045, 2053, 2059, 2073, 2088, 2098, 2114, 2129, 2139, 2147, 2154, 2163, 2176, 2192, 2207, 2216, 2227, 2236, 2245, 2258, 2267, 2280, 2293, 2300, 2310, 2325, 2333}
 
 func (i actionType) String() string {
 	if i < 0 || i >= actionType(len(_actionType_index)-1) {

src/constants.go

@@ -29,6 +29,10 @@ const (
 	maxPatternLength  = 1000
 	maxMulti          = math.MaxInt32
 
+	// Background processes
+	maxBgProcesses          = 30
+	maxBgProcessesPerAction = 3
+
 	// Matcher
 	numPartitionsMultiplier = 8
 	maxPartitions           = 32

src/options.go

@@ -1435,7 +1435,7 @@ const (
 
 func init() {
 	executeRegexp = regexp.MustCompile(
-		`(?si)[:+](become|execute(?:-multi|-silent)?|reload(?:-sync)?|preview|(?:change|transform)-(?:query|prompt|(?:border|list|preview|input|header|footer)-label|header|footer|search|nth|pointer|ghost)|transform|change-(?:preview-window|preview|multi)|(?:re|un|toggle-)bind|pos|put|print|search)`)
+		`(?si)[:+](become|execute(?:-multi|-silent)?|reload(?:-sync)?|preview|(?:change|bg-transform|transform)-(?:query|prompt|(?:border|list|preview|input|header|footer)-label|header|footer|search|nth|pointer|ghost)|bg-transform|transform|change-(?:preview-window|preview|multi)|(?:re|un|toggle-)bind|pos|put|print|search)`)
 	splitRegexp = regexp.MustCompile("[,:]+")
 	actionNameRegexp = regexp.MustCompile("(?i)^[a-z-]+")
 }
@@ -1892,6 +1892,36 @@ func isExecuteAction(str string) actionType {
 		return actTransformQuery
 	case "transform-search":
 		return actTransformSearch
+	case "bg-transform":
+		return actBgTransform
+	case "bg-transform-list-label":
+		return actBgTransformListLabel
+	case "bg-transform-border-label":
+		return actBgTransformBorderLabel
+	case "bg-transform-preview-label":
+		return actBgTransformPreviewLabel
+	case "bg-transform-input-label":
+		return actBgTransformInputLabel
+	case "bg-transform-header-label":
+		return actBgTransformHeaderLabel
+	case "bg-transform-footer-label":
+		return actBgTransformFooterLabel
+	case "bg-transform-footer":
+		return actBgTransformFooter
+	case "bg-transform-header":
+		return actBgTransformHeader
+	case "bg-transform-ghost":
+		return actBgTransformGhost
+	case "bg-transform-nth":
+		return actBgTransformNth
+	case "bg-transform-pointer":
+		return actBgTransformPointer
+	case "bg-transform-prompt":
+		return actBgTransformPrompt
+	case "bg-transform-query":
+		return actBgTransformQuery
+	case "bg-transform-search":
+		return actBgTransformSearch
 	case "search":
 		return actSearch
 	}
@@ -2857,7 +2887,13 @@ func parseOptions(index *int, opts *Options, allArgs []string) error {
 				return err
 			}
 			if opts.ListBorderShape == tui.BorderLine {
-				return errors.New("list border cannot be 'line'")
+				if hasArg {
+					// '--list-border line' is not allowed
+					return errors.New("list border cannot be 'line'")
+				}
+				// This is when '--style full:line' is previously specified and
+				// '--list-border' is specified without an argument.
+				opts.ListBorderShape = tui.BorderRounded
 			}
 		case "--no-list-border":
 			opts.ListBorderShape = tui.BorderNone

src/terminal.go

@@ -388,6 +388,10 @@ type Terminal struct {
 	startChan          chan fitpad
 	killChan           chan bool
 	serverInputChan    chan []*action
+	callbackChan       chan func()
+	bgQueue            map[action][]func()
+	bgSemaphore        chan struct{}
+	bgSemaphores       map[action]chan struct{}
 	keyChan            chan tui.Event
 	eventChan          chan tui.Event
 	slab               *util.Slab
@@ -489,6 +493,7 @@ const (
 	actBackwardDeleteCharEof
 	actBackwardWord
 	actCancel
+
 	actChangeBorderLabel
 	actChangeGhost
 	actChangeHeader
@@ -505,6 +510,7 @@ const (
 	actChangePreviewWindow
 	actChangePrompt
 	actChangeQuery
+
 	actClearScreen
 	actClearQuery
 	actClearSelection
@@ -561,6 +567,7 @@ const (
 	actHidePreview
 	actTogglePreview
 	actTogglePreviewWrap
+
 	actTransform
 	actTransformBorderLabel
 	actTransformGhost
@@ -576,6 +583,23 @@ const (
 	actTransformPrompt
 	actTransformQuery
 	actTransformSearch
+
+	actBgTransform
+	actBgTransformBorderLabel
+	actBgTransformGhost
+	actBgTransformHeader
+	actBgTransformFooter
+	actBgTransformHeaderLabel
+	actBgTransformFooterLabel
+	actBgTransformInputLabel
+	actBgTransformListLabel
+	actBgTransformNth
+	actBgTransformPointer
+	actBgTransformPreviewLabel
+	actBgTransformPrompt
+	actBgTransformQuery
+	actBgTransformSearch
+
 	actSearch
 	actPreview
 	actPreviewTop
@@ -613,6 +637,7 @@ const (
 	actBell
 	actExclude
 	actExcludeMulti
+	actAsync
 )
 
 func (a actionType) Name() string {
@@ -623,10 +648,34 @@ func processExecution(action actionType) bool {
 	switch action {
 	case actTransform,
 		actTransformBorderLabel,
+		actTransformGhost,
 		actTransformHeader,
+		actTransformFooter,
+		actTransformHeaderLabel,
+		actTransformFooterLabel,
+		actTransformInputLabel,
+		actTransformListLabel,
+		actTransformNth,
+		actTransformPointer,
 		actTransformPreviewLabel,
 		actTransformPrompt,
 		actTransformQuery,
+		actTransformSearch,
+		actBgTransform,
+		actBgTransformBorderLabel,
+		actBgTransformGhost,
+		actBgTransformHeader,
+		actBgTransformFooter,
+		actBgTransformHeaderLabel,
+		actBgTransformFooterLabel,
+		actBgTransformInputLabel,
+		actBgTransformListLabel,
+		actBgTransformNth,
+		actBgTransformPointer,
+		actBgTransformPreviewLabel,
+		actBgTransformPrompt,
+		actBgTransformQuery,
+		actBgTransformSearch,
 		actPreview,
 		actChangePreview,
 		actRefreshPreview,
@@ -773,7 +822,7 @@ func mayTriggerPreview(opts *Options) bool {
 	for _, actions := range opts.Keymap {
 		for _, action := range actions {
 			switch action.t {
-			case actPreview, actChangePreview, actTransform:
+			case actPreview, actChangePreview, actTransform, actBgTransform:
 				return true
 			}
 		}
@@ -987,6 +1036,10 @@ func NewTerminal(opts *Options, eventBox *util.EventBox, executor *util.Executor
 		startChan:          make(chan fitpad, 1),
 		killChan:           make(chan bool),
 		serverInputChan:    make(chan []*action, 100),
+		callbackChan:       make(chan func(), maxBgProcesses),
+		bgQueue:            make(map[action][]func()),
+		bgSemaphore:        make(chan struct{}, maxBgProcesses),
+		bgSemaphores:       make(map[action]chan struct{}),
 		keyChan:            make(chan tui.Event),
 		eventChan:          make(chan tui.Event, 6), // start | (load + result + zero|one) | (focus) | (resize)
 		tui:                renderer,
@@ -2578,7 +2631,9 @@ func (t *Terminal) printPrompt() {
 
 	before, after := t.updatePromptOffset()
 	if len(before) == 0 && len(after) == 0 && len(t.ghost) > 0 {
-		w.CPrint(tui.ColGhost, t.ghost)
+		maxWidth := util.Max(1, w.Width()-t.promptLen-1)
+		runes, _ := t.trimRight([]rune(t.ghost), maxWidth)
+		w.CPrint(tui.ColGhost, string(runes))
 		return
 	}
 
@@ -4291,6 +4346,75 @@ func (t *Terminal) captureLines(template string) string {
 	return t.executeCommand(template, false, true, true, false, "")
 }
 
+func (t *Terminal) captureAsync(a action, firstLineOnly bool, callback func(string)) {
+	_, list := t.buildPlusList(a.a, false)
+	command, tempFiles := t.replacePlaceholder(a.a, false, string(t.input), list)
+	item := func() {
+		cmd := t.executor.ExecCommand(command, false)
+		cmd.Env = t.environ()
+
+		out, _ := cmd.StdoutPipe()
+		reader := bufio.NewReader(out)
+		var output string
+		if err := cmd.Start(); err == nil {
+			if firstLineOnly {
+				output, _ = reader.ReadString('\n')
+				output = strings.TrimRight(output, "\r\n")
+			} else {
+				bytes, _ := io.ReadAll(reader)
+				output = string(bytes)
+			}
+			cmd.Wait()
+		}
+		removeFiles(tempFiles)
+
+		t.callbackChan <- func() { callback(output) }
+	}
+	queue, prs := t.bgQueue[a]
+	if !prs {
+		queue = []func(){}
+	}
+	queue = append(queue, item)
+	t.bgQueue[a] = queue
+}
+
+func (t *Terminal) dispatchAsync() {
+Loop:
+	for a, queue := range t.bgQueue {
+		delete(t.bgQueue, a)
+		if len(queue) == 0 {
+			continue
+		}
+
+		semaphore, prs := t.bgSemaphores[a]
+		if !prs {
+			semaphore = make(chan struct{}, maxBgProcessesPerAction)
+			t.bgSemaphores[a] = semaphore
+		}
+		for _, item := range queue {
+			select {
+			// Acquire local semaphore
+			case semaphore <- struct{}{}:
+			default:
+				// Failed to acquire local semaphore, putting only the last one back to the queue
+				t.bgQueue[a] = queue[len(queue)-1:]
+				continue Loop
+			}
+			todo := item
+			go func() {
+				// Acquire global semaphore
+				t.bgSemaphore <- struct{}{}
+
+				todo()
+				// Release local semaphore
+				<-semaphore
+				// Release global semaphore
+				<-t.bgSemaphore
+			}()
+		}
+	}
+}
+
 func (t *Terminal) executeCommand(template string, forcePlus bool, background bool, capture bool, firstLineOnly bool, info string) string {
 	line := ""
 	valid, list := t.buildPlusList(template, forcePlus)
@@ -5089,11 +5213,27 @@ func (t *Terminal) Loop() error {
 		barrier <- true
 		needBarrier = false
 	}
+
+	// These variables are defined outside the loop to be accessible from closures
+	events := []util.EventType{}
+	changed := false
+	var newNth *[]Range
+	req := func(evts ...util.EventType) {
+		for _, event := range evts {
+			events = append(events, event)
+			if event == reqClose || event == reqQuit {
+				looping = false
+			}
+		}
+	}
+
+	// The main event loop
 	for loopIndex := int64(0); looping; loopIndex++ {
 		var newCommand *commandSpec
-		var newNth *[]Range
 		var reloadSync bool
-		changed := false
+		events = []util.EventType{}
+		changed = false
+		newNth = nil
 		beof := false
 		queryChanged := false
 		denylist := []int32{}
@@ -5110,6 +5250,7 @@ func (t *Terminal) Loop() error {
 
 		var event tui.Event
 		actions := []*action{}
+		callbacks := []func(){}
 		select {
 		case event = <-t.keyChan:
 			needBarrier = true
@@ -5141,6 +5282,20 @@ func (t *Terminal) Loop() error {
 					}
 				}
 			}
+		case callback := <-t.callbackChan:
+			event = tui.Invalid.AsEvent()
+			actions = append(actions, &action{t: actAsync})
+			callbacks = append(callbacks, callback)
+		DrainCallback:
+			for {
+				select {
+				case callback = <-t.callbackChan:
+					callbacks = append(callbacks, callback)
+					continue DrainCallback
+				default:
+					break DrainCallback
+				}
+			}
 		}
 
 		t.mutex.Lock()
@@ -5155,15 +5310,6 @@ func (t *Terminal) Loop() error {
 		previousInput := t.input
 		previousCx := t.cx
 		t.lastKey = event.KeyName()
-		events := []util.EventType{}
-		req := func(evts ...util.EventType) {
-			for _, event := range evts {
-				events = append(events, event)
-				if event == reqClose || event == reqQuit {
-					looping = false
-				}
-			}
-		}
 		updatePreviewWindow := func(forcePreview bool) {
 			t.resizeWindows(forcePreview, false)
 			req(reqPrompt, reqList, reqInfo, reqHeader, reqFooter)
@@ -5238,9 +5384,29 @@ func (t *Terminal) Loop() error {
 			//   actions to allow changing the query even when the input is hidden
 			//     e.g. fzf --no-input --bind 'space:show-input+change-query(foo)+hide-input'
 			currentInput := t.input
+			capture := func(firstLineOnly bool, callback func(string)) {
+				if a.t >= actBgTransform {
+					// bg-transform-*
+					t.captureAsync(*a, firstLineOnly, callback)
+				} else if a.t >= actTransform {
+					// transform-*
+					if firstLineOnly {
+						callback(t.captureLine(a.a))
+					} else {
+						callback(t.captureLines(a.a))
+					}
+				} else {
+					// change-*
+					callback(a.a)
+				}
+			}
 		Action:
 			switch a.t {
 			case actIgnore, actStart, actClick:
+			case actAsync:
+				for _, callback := range callbacks {
+					callback()
+				}
 			case actBecome:
 				valid, list := t.buildPlusList(a.a, false)
 				if valid {
@@ -5333,15 +5499,17 @@ func (t *Terminal) Loop() error {
 					t.previewed.version = 0
 					req(reqPreviewRefresh)
 				}
-			case actTransformPrompt:
+			case actTransformPrompt, actBgTransformPrompt:
-				prompt := t.captureLine(a.a)
+				capture(true, func(prompt string) {
-				t.promptString = prompt
+					t.promptString = prompt
-				t.prompt, t.promptLen = t.parsePrompt(prompt)
+					t.prompt, t.promptLen = t.parsePrompt(prompt)
-				req(reqPrompt)
+					req(reqPrompt)
-			case actTransformQuery:
+				})
-				query := t.captureLine(a.a)
+			case actTransformQuery, actBgTransformQuery:
-				t.input = []rune(query)
+				capture(true, func(query string) {
-				t.cx = len(t.input)
+					t.input = []rune(query)
+					t.cx = len(t.input)
+				})
 			case actToggleSort:
 				t.sort = !t.sort
 				changed = true
@@ -5399,119 +5567,102 @@ func (t *Terminal) Loop() error {
 				}
 				t.multi = multi
 				req(reqList, reqInfo)
-			case actChangeNth, actTransformNth:
+			case actChangeNth, actTransformNth, actBgTransformNth:
-				expr := a.a
+				capture(true, func(expr string) {
-				if a.t == actTransformNth {
+					// Split nth expression
-					expr = t.captureLine(a.a)
+					tokens := strings.Split(expr, "|")
-				}
+					if nth, err := splitNth(tokens[0]); err == nil {
-
+						// Changed
-				// Split nth expression
+						newNth = &nth
-				tokens := strings.Split(expr, "|")
+					} else {
-				if nth, err := splitNth(tokens[0]); err == nil {
+						// The default
-					// Changed
+						newNth = &t.nth
-					newNth = &nth
+					}
-				} else {
+					// Cycle
-					// The default
+					if len(tokens) > 1 {
-					newNth = &t.nth
+						a.a = strings.Join(append(tokens[1:], tokens[0]), "|")
-				}
+					}
-				// Cycle
+					if !compareRanges(t.nthCurrent, *newNth) {
-				if len(tokens) > 1 {
+						changed = true
-					a.a = strings.Join(append(tokens[1:], tokens[0]), "|")
+						t.nthCurrent = *newNth
-				}
+						t.forceRerenderList()
-				if !compareRanges(t.nthCurrent, *newNth) {
+					}
-					changed = true
+				})
-					t.nthCurrent = *newNth
-					t.forceRerenderList()
-				}
 			case actChangeQuery:
 				t.input = []rune(a.a)
 				t.cx = len(t.input)
-			case actChangeHeader, actTransformHeader:
+			case actChangeHeader, actTransformHeader, actBgTransformHeader:
-				header := a.a
+				capture(false, func(header string) {
-				if a.t == actTransformHeader {
+					if t.changeHeader(header) {
-					header = t.captureLines(a.a)
+						if t.headerWindow != nil {
-				}
+							// Need to resize header window
-				if t.changeHeader(header) {
+							req(reqFullRedraw)
-					if t.headerWindow != nil {
+						} else {
-						// Need to resize header window
+							req(reqHeader, reqList, reqPrompt, reqInfo)
+						}
+					} else {
+						req(reqHeader)
+					}
+				})
+			case actChangeFooter, actTransformFooter, actBgTransformFooter:
+				capture(false, func(footer string) {
+					if t.changeFooter(footer) {
 						req(reqFullRedraw)
 					} else {
-						req(reqHeader, reqList, reqPrompt, reqInfo)
+						req(reqFooter)
 					}
-				} else {
+				})
-					req(reqHeader)
+			case actChangeHeaderLabel, actTransformHeaderLabel, actBgTransformHeaderLabel:
-				}
+				capture(true, func(label string) {
-			case actChangeFooter, actTransformFooter:
+					t.headerLabelOpts.label = label
-				footer := a.a
+					t.headerLabel, t.headerLabelLen = t.ansiLabelPrinter(label, &tui.ColHeaderLabel, false)
-				if a.t == actTransformFooter {
+					req(reqRedrawHeaderLabel)
-					footer = t.captureLines(a.a)
+				})
-				}
+			case actChangeFooterLabel, actTransformFooterLabel, actBgTransformFooterLabel:
-				if t.changeFooter(footer) {
+				capture(true, func(label string) {
-					req(reqFullRedraw)
+					t.footerLabelOpts.label = label
-				} else {
+					t.footerLabel, t.footerLabelLen = t.ansiLabelPrinter(label, &tui.ColFooterLabel, false)
-					req(reqFooter)
+					req(reqRedrawFooterLabel)
-				}
+				})
-			case actChangeHeaderLabel, actTransformHeaderLabel:
+			case actChangeInputLabel, actTransformInputLabel, actBgTransformInputLabel:
-				label := a.a
+				capture(true, func(label string) {
-				if a.t == actTransformHeaderLabel {
+					t.inputLabelOpts.label = label
-					label = t.captureLine(a.a)
+					if t.inputBorder != nil {
-				}
+						t.inputLabel, t.inputLabelLen = t.ansiLabelPrinter(label, &tui.ColInputLabel, false)
-				t.headerLabelOpts.label = label
+						req(reqRedrawInputLabel)
-				t.headerLabel, t.headerLabelLen = t.ansiLabelPrinter(label, &tui.ColHeaderLabel, false)
+					}
-				req(reqRedrawHeaderLabel)
+				})
-			case actChangeFooterLabel, actTransformFooterLabel:
+			case actChangeListLabel, actTransformListLabel, actBgTransformListLabel:
-				label := a.a
+				capture(true, func(label string) {
-				if a.t == actTransformFooterLabel {
+					t.listLabelOpts.label = label
-					label = t.captureLine(a.a)
+					if t.wborder != nil {
-				}
+						t.listLabel, t.listLabelLen = t.ansiLabelPrinter(label, &tui.ColListLabel, false)
-				t.footerLabelOpts.label = label
+						req(reqRedrawListLabel)
-				t.footerLabel, t.footerLabelLen = t.ansiLabelPrinter(label, &tui.ColFooterLabel, false)
+					}
-				req(reqRedrawFooterLabel)
+				})
-			case actChangeInputLabel, actTransformInputLabel:
+			case actChangeBorderLabel, actTransformBorderLabel, actBgTransformBorderLabel:
-				label := a.a
+				capture(true, func(label string) {
-				if a.t == actTransformInputLabel {
+					t.borderLabelOpts.label = label
-					label = t.captureLine(a.a)
+					if t.border != nil {
-				}
+						t.borderLabel, t.borderLabelLen = t.ansiLabelPrinter(label, &tui.ColBorderLabel, false)
-				t.inputLabelOpts.label = label
+						req(reqRedrawBorderLabel)
-				if t.inputBorder != nil {
+					}
-					t.inputLabel, t.inputLabelLen = t.ansiLabelPrinter(label, &tui.ColInputLabel, false)
+				})
-					req(reqRedrawInputLabel)
+			case actChangePreviewLabel, actTransformPreviewLabel, actBgTransformPreviewLabel:
-				}
+				capture(true, func(label string) {
-			case actChangeListLabel, actTransformListLabel:
+					t.previewLabelOpts.label = label
-				label := a.a
+					if t.pborder != nil {
-				if a.t == actTransformListLabel {
+						t.previewLabel, t.previewLabelLen = t.ansiLabelPrinter(label, &tui.ColPreviewLabel, false)
-					label = t.captureLine(a.a)
+						req(reqRedrawPreviewLabel)
-				}
+					}
-				t.listLabelOpts.label = label
+				})
-				if t.wborder != nil {
+			case actTransform, actBgTransform:
-					t.listLabel, t.listLabelLen = t.ansiLabelPrinter(label, &tui.ColListLabel, false)
+				capture(false, func(body string) {
-					req(reqRedrawListLabel)
+					if actions, err := parseSingleActionList(strings.Trim(body, "\r\n")); err == nil {
-				}
+						// NOTE: We're not properly passing the return value here
-			case actChangeBorderLabel, actTransformBorderLabel:
+						doActions(actions)
-				label := a.a
+					}
-				if a.t == actTransformBorderLabel {
+				})
-					label = t.captureLine(a.a)
-				}
-				t.borderLabelOpts.label = label
-				if t.border != nil {
-					t.borderLabel, t.borderLabelLen = t.ansiLabelPrinter(label, &tui.ColBorderLabel, false)
-					req(reqRedrawBorderLabel)
-				}
-			case actChangePreviewLabel, actTransformPreviewLabel:
-				label := a.a
-				if a.t == actTransformPreviewLabel {
-					label = t.captureLine(a.a)
-				}
-				t.previewLabelOpts.label = label
-				if t.pborder != nil {
-					t.previewLabel, t.previewLabelLen = t.ansiLabelPrinter(label, &tui.ColPreviewLabel, false)
-					req(reqRedrawPreviewLabel)
-				}
-			case actTransform:
-				body := t.captureLines(a.a)
-				if actions, err := parseSingleActionList(strings.Trim(body, "\r\n")); err == nil {
-					return doActions(actions)
-				}
 			case actChangePrompt:
 				t.promptString = a.a
 				t.prompt, t.promptLen = t.parsePrompt(a.a)
@@ -5933,10 +6084,12 @@ func (t *Terminal) Loop() error {
 				override := []rune(a.a)
 				t.inputOverride = &override
 				changed = true
-			case actTransformSearch:
+			case actTransformSearch, actBgTransformSearch:
-				override := []rune(t.captureLine(a.a))
+				capture(true, func(query string) {
-				t.inputOverride = &override
+					override := []rune(query)
-				changed = true
+					t.inputOverride = &override
+					changed = true
+				})
 			case actEnableSearch:
 				t.paused = false
 				changed = true
@@ -6276,30 +6429,26 @@ func (t *Terminal) Loop() error {
 						}
 					}
 				}
-			case actChangeGhost, actTransformGhost:
+			case actChangeGhost, actTransformGhost, actBgTransformGhost:
-				ghost := a.a
+				capture(true, func(ghost string) {
-				if a.t == actTransformGhost {
+					t.ghost = ghost
-					ghost = t.captureLine(a.a)
+					if len(t.input) == 0 {
-				}
+						req(reqPrompt)
-				t.ghost = ghost
-				if len(t.input) == 0 {
-					req(reqPrompt)
-				}
-			case actChangePointer, actTransformPointer:
-				pointer := a.a
-				if a.t == actTransformPointer {
-					pointer = t.captureLine(a.a)
-				}
-				length := uniseg.StringWidth(pointer)
-				if length <= 2 {
-					if length != t.pointerLen {
-						t.forceRerenderList()
 					}
-					t.pointer = pointer
+				})
-					t.pointerLen = length
+			case actChangePointer, actTransformPointer, actBgTransformPointer:
-					t.pointerEmpty = strings.Repeat(" ", t.pointerLen)
+				capture(true, func(pointer string) {
-					req(reqList)
+					length := uniseg.StringWidth(pointer)
-				}
+					if length <= 2 {
+						if length != t.pointerLen {
+							t.forceRerenderList()
+						}
+						t.pointer = pointer
+						t.pointerLen = length
+						t.pointerEmpty = strings.Repeat(" ", t.pointerLen)
+						req(reqList)
+					}
+				})
 			case actChangePreview:
 				if t.previewOpts.command != a.a {
 					t.previewOpts.command = a.a
@@ -6451,6 +6600,10 @@ func (t *Terminal) Loop() error {
 		if reload {
 			reloadRequest = &searchRequest{sort: t.sort, sync: reloadSync, nth: newNth, command: newCommand, environ: t.environ(), changed: changed, denylist: denylist, revision: t.merger.Revision()}
 		}
+
+		// Dispatch queued background requests
+		t.dispatchAsync()
+
 		t.mutex.Unlock() // Must be unlocked before touching reqBox
 
 		if reload {

test/test_core.rb

@@ -1939,4 +1939,29 @@ class TestCore < TestInteractive
     tmux.send_keys %(echo -en "foo\n" | fzf --read0 --no-multi-line), :Enter
     tmux.until { |lines| assert_includes lines, '> foo␊' }
   end
+
+  def test_async_transform
+    time = Time.now
+    tmux.send_keys %(
+      seq 100 | #{FZF} --style full --border --preview : \
+          --bind 'focus:bg-transform-header(sleep 0.5; echo th.)' \
+          --bind 'focus:+bg-transform-footer(sleep 0.5; echo tf.)' \
+          --bind 'focus:+bg-transform-border-label(sleep 0.5; echo tbl.)' \
+          --bind "focus:+bg-transform-preview-label(sleep 0.5; echo tpl.)" \
+          --bind 'focus:+bg-transform-input-label(sleep 0.5; echo til.)' \
+          --bind 'focus:+bg-transform-list-label(sleep 0.5; echo tll.)' \
+          --bind 'focus:+bg-transform-header-label(sleep 0.5; echo thl.)' \
+          --bind 'focus:+bg-transform-footer-label(sleep 0.5; echo tfl.)' \
+          --bind 'focus:+bg-transform-prompt(sleep 0.5; echo tp.)' \
+          --bind 'focus:+bg-transform-ghost(sleep 0.5; echo tg.)'
+    ).strip, :Enter
+    tmux.until do |lines|
+      assert lines.any_include?('100/100')
+      %w[th tf tbl tpl til tll thl tfl tp tg].each do
+        assert lines.any_include?("#{it}.")
+      end
+    end
+    elapsed = Time.now - time
+    assert elapsed < 2
+  end
 end