Add tab completion to textinputs

This adds tab completion to textinput components. They can be configured
with a completion function. This function is called when the user
presses <tab>. The first completion is initially shown to the user
inserted into the text. Repeated presses of <tab> or <backtab> cycle
through the completions list. The completions list is invalidated when
any other non-tab-like key is pressed.

Also changed is some logic for current completion generation so that
all available commands are returned when <tab> is pressed with no
current text and similarly for arguments of commands.

lib/ui/textinput.go

@@ -5,20 +5,23 @@ import (
 	"github.com/mattn/go-runewidth"
 )
 
-// TODO: Attach history and tab completion providers
+// TODO: Attach history providers
 // TODO: scrolling
 
 type TextInput struct {
 	Invalidatable
-	cells    int
-	ctx      *Context
-	focus    bool
-	index    int
-	password bool
-	prompt   string
-	scroll   int
-	text     []rune
-	change   []func(ti *TextInput)
+	cells         int
+	ctx           *Context
+	focus         bool
+	index         int
+	password      bool
+	prompt        string
+	scroll        int
+	text          []rune
+	change        []func(ti *TextInput)
+	tabcomplete   func(s string) []string
+	completions   []string
+	completeIndex int
 }
 
 // Creates a new TextInput. TextInputs will render a "textbox" in the entire
@@ -42,6 +45,12 @@ func (ti *TextInput) Prompt(prompt string) *TextInput {
 	return ti
 }
 
+func (ti *TextInput) TabComplete(
+	tabcomplete func(s string) []string) *TextInput {
+	ti.tabcomplete = tabcomplete
+	return ti
+}
+
 func (ti *TextInput) String() string {
 	return string(ti.text)
 }
@@ -161,6 +170,41 @@ func (ti *TextInput) backspace() {
 	}
 }
 
+func (ti *TextInput) nextCompletion() {
+	if ti.completions == nil {
+		if ti.tabcomplete == nil {
+			return
+		}
+		ti.completions = ti.tabcomplete(ti.StringLeft())
+		ti.completeIndex = 0
+	} else {
+		ti.completeIndex++
+		if ti.completeIndex >= len(ti.completions) {
+			ti.completeIndex = 0
+		}
+	}
+	if len(ti.completions) > 0 {
+		ti.Set(ti.completions[ti.completeIndex] + ti.StringRight())
+	}
+}
+
+func (ti *TextInput) previousCompletion() {
+	if ti.completions == nil || len(ti.completions) == 0 {
+		return
+	}
+	ti.completeIndex--
+	if ti.completeIndex < 0 {
+		ti.completeIndex = len(ti.completions) - 1
+	}
+	if len(ti.completions) > 0 {
+		ti.Set(ti.completions[ti.completeIndex] + ti.StringRight())
+	}
+}
+
+func (ti *TextInput) invalidateCompletions() {
+	ti.completions = nil
+}
+
 func (ti *TextInput) onChange() {
 	for _, change := range ti.change {
 		change(ti)
@@ -176,32 +220,52 @@ func (ti *TextInput) Event(event tcell.Event) bool {
 	case *tcell.EventKey:
 		switch event.Key() {
 		case tcell.KeyBackspace, tcell.KeyBackspace2:
+			ti.invalidateCompletions()
 			ti.backspace()
 		case tcell.KeyCtrlD, tcell.KeyDelete:
+			ti.invalidateCompletions()
 			ti.deleteChar()
 		case tcell.KeyCtrlB, tcell.KeyLeft:
+			ti.invalidateCompletions()
 			if ti.index > 0 {
 				ti.index--
 				ti.ensureScroll()
 				ti.Invalidate()
 			}
 		case tcell.KeyCtrlF, tcell.KeyRight:
+			ti.invalidateCompletions()
 			if ti.index < len(ti.text) {
 				ti.index++
 				ti.ensureScroll()
 				ti.Invalidate()
 			}
 		case tcell.KeyCtrlA, tcell.KeyHome:
+			ti.invalidateCompletions()
 			ti.index = 0
 			ti.ensureScroll()
 			ti.Invalidate()
 		case tcell.KeyCtrlE, tcell.KeyEnd:
+			ti.invalidateCompletions()
 			ti.index = len(ti.text)
 			ti.ensureScroll()
 			ti.Invalidate()
 		case tcell.KeyCtrlW:
+			ti.invalidateCompletions()
 			ti.deleteWord()
+		case tcell.KeyTab:
+			if ti.tabcomplete != nil {
+				ti.nextCompletion()
+			} else {
+				ti.insert('\t')
+			}
+			ti.Invalidate()
+		case tcell.KeyBacktab:
+			if ti.tabcomplete != nil {
+				ti.previousCompletion()
+			}
+			ti.Invalidate()
 		case tcell.KeyRune:
+			ti.invalidateCompletions()
 			ti.insert(event.Rune())
 		}
 	}