Some small quality of life improvements

- Added Ctrl+A and Ctrl+E for moving around the edit bar a.la. Emacs
- Added the "yank" and "paste" command to copy and paste a single cell value.  These are mapped to "y" and "p".
- Modified the edit bar so that entering commands is the only case where typing backspace on an empty edit bar will cancel out of edit mode

commandmap.go

@@ -276,7 +276,10 @@ func (cm *CommandMapping) RegisterViewCommands() {
 	})
 
 	cm.Define("enter-command", "Enter command", "", func(ctx *CommandContext) error {
-		ctx.Frame().Prompt(PromptOptions{Prompt: ":"}, func(res string) error {
+		ctx.Frame().Prompt(PromptOptions{
+			Prompt: ":",
+			CancelOnEmptyBackspace: true,
+		}, func(res string) error {
 			return cm.Eval(ctx, res)
 		})
 		return nil
@@ -300,7 +303,15 @@ func (cm *CommandMapping) RegisterViewCommands() {
 		grid := ctx.Frame().Grid()
 		cellX, cellY := grid.CellPosition()
 
-		if _, isRwModel := ctx.ModelVC().Model().(RWModel); isRwModel {
+		if _, isRwModel := ctx.ModelVC().Model().(RWModel); !isRwModel {
+			return errors.New("Model is read-only")
+		}
+
+		if len(ctx.Args()) == 1 {
+			if err := ctx.ModelVC().SetCellValue(cellY, cellX, ctx.Args()[0]); err != nil {
+				return err
+			}
+		} else {
 			ctx.Frame().Prompt(PromptOptions{
 				Prompt:       "> ",
 				InitialValue: grid.Model().CellValue(cellX, cellY),
@@ -314,6 +325,29 @@ func (cm *CommandMapping) RegisterViewCommands() {
 		}
 		return nil
 	})
+	cm.Define("yank", "Yank cell value", "", func(ctx *CommandContext) error {
+		grid := ctx.Frame().Grid()
+		cellX, cellY := grid.CellPosition()
+
+		// TODO: allow ranges
+		ctx.Session().pasteBoard.SetCellValue(0, 0, grid.Model().CellValue(cellX, cellY))
+
+		return nil
+	})
+	cm.Define("paste", "Paste cell value", "", func(ctx *CommandContext) error {
+		grid := ctx.Frame().Grid()
+		cellX, cellY := grid.CellPosition()
+
+		// TODO: allow ranges
+		if _, isRwModel := ctx.ModelVC().Model().(RWModel); !isRwModel {
+			return errors.New("Model is read-only")
+		}
+		if err := ctx.ModelVC().SetCellValue(cellY, cellX, ctx.Session().pasteBoard.CellValue(0, 0)); err != nil {
+			return err
+		}
+
+		return nil
+	})
 
 	cm.Define("save", "Save current file", "", func(ctx *CommandContext) error {
 		wSource, isWSource := ctx.Session().Source.(WritableModelSource)
@@ -378,6 +412,9 @@ func (cm *CommandMapping) RegisterViewKeyBindings() {
 	cm.MapKey('/', cm.Command("search"))
 	cm.MapKey('n', cm.Command("search-next"))
 
+	cm.MapKey('y', cm.Command("yank"))
+	cm.MapKey('p', cm.Command("paste"))
+
 	cm.MapKey('0', cm.Command("clear-row-marker"))
 	cm.MapKey('1', cm.Command("mark-row-red"))
 	cm.MapKey('2', cm.Command("mark-row-green"))

frame.go

@@ -108,11 +108,14 @@ func (frame *Frame) Error(err error) {
 type PromptOptions struct {
 	Prompt                 string
 	InitialValue           string
+	CancelOnEmptyBackspace bool
 }
 
 // Prompt the user for input.  This switches the mode to entry mode.
 func (frame *Frame) Prompt(options PromptOptions, callback func(res string) error) {
+	frame.textEntry.Reset()
 	frame.textEntry.Prompt = options.Prompt
+	frame.textEntry.CancelOnEmptyBackspace = options.CancelOnEmptyBackspace
 	frame.textEntry.SetValue(options.InitialValue)
 
 	frame.textEntry.OnCancel = frame.exitEntryMode

session.go

@@ -15,6 +15,7 @@ type Session struct {
 	Commands        *CommandMapping
 	UIManager       *ui.Ui
 	modelController *ModelViewCtrl
+	pasteBoard		RWModel
 
 	LastSearch *regexp.Regexp
 }
@@ -28,6 +29,7 @@ func NewSession(uiManager *ui.Ui, frame *Frame, source ModelSource) *Session {
 		Commands:        NewCommandMapping(),
 		UIManager:       uiManager,
 		modelController: NewGridViewModel(model),
+		pasteBoard: 	 NewSingleCellStdModel(),
 	}
 
 	frame.SetModel(&SessionGridModel{session.modelController})

ui/stdcomps.go

@@ -57,6 +57,11 @@ type TextEntry struct {
 	value         string
 	cursorOffset  int
 	displayOffset int
+	isDirty 	  bool
+
+	// CancelOnEmptyBackspace will cancel the text entry prompt if no other
+	// key was pressed and the prompt was empty.
+	CancelOnEmptyBackspace bool
 
 	// Called when the user presses Enter
 	OnEntry func(val string)
@@ -65,6 +70,10 @@ type TextEntry struct {
 	OnCancel func()
 }
 
+func (te *TextEntry) Reset() {
+	te.isDirty = false
+}
+
 func (te *TextEntry) Remeasure(w, h int) (int, int) {
 	return w, 1
 }
@@ -116,16 +125,18 @@ func (te *TextEntry) KeyPressed(key rune, mod int) {
 		te.moveCursorBy(-1)
 	} else if key == KeyArrowRight {
 		te.moveCursorBy(1)
-	} else if key == KeyHome {
+	} else if (key == KeyHome) || (key == KeyCtrlA) {
 		te.moveCursorTo(0)
-	} else if key == KeyEnd {
+	} else if (key == KeyEnd) || (key == KeyCtrlE) {
 		te.moveCursorTo(len(te.value))
 	} else if (key == KeyBackspace) || (key == KeyBackspace2) {
 		if mod&ModKeyAlt != 0 {
 			te.backspaceWhile(unicode.IsSpace)
 			te.backspaceWhile(func(r rune) bool { return !unicode.IsSpace(r) })
 		} else if te.cursorOffset == 0 {
+			if te.CancelOnEmptyBackspace && !te.isDirty {
 				te.cancelAndExit()
+			}
 		} else {
 			te.backspace()
 		}
@@ -171,6 +182,7 @@ func (te *TextEntry) backspaceWhile(guard func(r rune) bool) {
 // Kill the line.  If the cursor is at the end of the line, kill to the start.
 // Otherwise, trim the line.
 func (te *TextEntry) killLine() {
+	te.isDirty = true
 	if te.cursorOffset < len(te.value) {
 		te.value = te.value[:te.cursorOffset]
 	} else {
@@ -181,6 +193,7 @@ func (te *TextEntry) killLine() {
 
 // Inserts a rune at the cursor position
 func (te *TextEntry) insertRune(key rune) {
+	te.isDirty = true
 	if te.cursorOffset >= len(te.value) {
 		te.value += string(key)
 	} else {
@@ -191,6 +204,7 @@ func (te *TextEntry) insertRune(key rune) {
 
 // Remove the character at a specific position
 func (te *TextEntry) removeCharAtPos(pos int) {
+	te.isDirty = true
 	if (pos >= 0) && (pos < len(te.value)) {
 		te.value = te.value[:pos] + te.value[pos+1:]
 	}