Add FormItem field validation

application.go

@@ -228,6 +228,11 @@ EventLoop:
 					a.Stop()
 				}
 
+				// Allow a validation to short-circuit input handling
+				if !a.ValidatePrimitive(p, event) {
+					continue
+				}
+
 				// Pass other key events to the currently focused primitive.
 				if p != nil {
 					if handler := p.InputHandler(); handler != nil {
@@ -503,3 +508,52 @@ func (a *Application) QueueEvent(event tcell.Event) *Application {
 	a.events <- event
 	return a
 }
+
+// ValidatePrimitive will call a validation function specific to
+// a FormItem set using the SetValidateFunc() methods when one of
+// the field exit keys — Enter, Tab, Backtab, or Escape — is used.
+// If the validation function returns false the focus will stay on
+// the non-valid form field.
+func (a *Application) ValidatePrimitive(p Primitive, event *tcell.EventKey) bool {
+	switch event.Key() {
+	case tcell.KeyEnter, tcell.KeyTab, tcell.KeyBacktab, tcell.KeyEscape:
+		// We will check validation below
+	default:
+		// No need to check
+		return true
+	}
+	switch p.(type) {
+	case *InputField:
+		i := p.(*InputField)
+		if i.valid == nil {
+			break
+		}
+		return i.valid(i, event)
+
+	case *DropDown:
+		dd := p.(*DropDown)
+		if dd.valid == nil {
+			break
+		}
+		return dd.valid(dd, event)
+
+	case *Checkbox:
+		cb := p.(*Checkbox)
+		if cb.valid == nil {
+			break
+		}
+		return cb.valid(cb, event)
+
+	default:
+		b, ok := p.(*Box)
+		if !ok {
+			break
+		}
+		if b.valid == nil {
+			break
+		}
+		return b.valid(p, event)
+
+	}
+	return true
+}

box.go

@@ -58,6 +58,11 @@ type Box struct {
 
 	// An optional function which is called before the box is drawn.
 	draw func(screen tcell.Screen, x, y, width, height int) (int, int, int, int)
+
+	// A callback function to be called when one of the field exit keys — Enter,
+	// Tab, Backtab, or Escape — is used. If this callback returns false it will
+	// bypass the input handler and leave the focus on the non-valid field.
+	valid func(Primitive, *tcell.EventKey) bool
 }
 
 // NewBox returns a Box without a border.
@@ -231,6 +236,14 @@ func (b *Box) SetTitleAlign(align int) *Box {
 	return b
 }
 
+// SetValidateFunc sets a callback to be called when one of the field exit keys —
+// Enter, Tab, Backtab, or Escape — is used. If this callback returns false it
+// will stop the input handler from moving focus to a different field.
+func (b *Box) SetValidateFunc(handler func(Primitive, *tcell.EventKey) bool) *Box {
+	b.valid = handler
+	return b
+}
+
 // Draw draws this primitive onto the screen.
 func (b *Box) Draw(screen tcell.Screen) {
 	// Don't draw anything if there is no space.

checkbox.go

@@ -42,6 +42,11 @@ type Checkbox struct {
 	// A callback function set by the Form class and called when the user leaves
 	// this form item.
 	finished func(tcell.Key)
+
+	// A callback function to be called when one of the field exit keys — Enter,
+	// Tab, Backtab, or Escape — is used. If this callback returns false it will
+	// bypass the input handler and leave the focus on the non-valid field.
+	valid func(*Checkbox, *tcell.EventKey) bool
 }
 
 // NewCheckbox returns a new input field.
@@ -142,6 +147,14 @@ func (c *Checkbox) SetFinishedFunc(handler func(key tcell.Key)) FormItem {
 	return c
 }
 
+// SetValidateFunc sets a callback to be called when one of the field exit keys —
+// Enter, Tab, Backtab, or Escape — is used. If this callback returns false it
+// will stop the input handler from moving focus to a different field.
+func (c *Checkbox) SetValidateFunc(handler func(*Checkbox, *tcell.EventKey) bool) *Checkbox {
+	c.valid = handler
+	return c
+}
+
 // Draw draws this primitive onto the screen.
 func (c *Checkbox) Draw(screen tcell.Screen) {
 	c.Box.Draw(screen)
@@ -254,11 +267,16 @@ type CheckboxArgs struct {
 	// returns the event to be forwarded to the primitive's default
 	// input handler (nil if nothing should be forwarded).
 	InputCaptureFunc func(event *tcell.EventKey) *tcell.EventKey
+
+	// A callback function to be called when one of the field exit keys — Enter,
+	// Tab, Backtab, or Escape — is used. If this callback returns false it will
+	// bypass the input handler and leave the focus on the non-valid field.
+	ValidateFunc func(*Checkbox, *tcell.EventKey) bool
 }
 
 // ApplyArgs applies the values from a CheckboxArgs{} struct to the
 // associated properties of the Checkbox.
-func (c *Checkbox) ApplyArgs(args *CheckboxArgs) *Checkbox{
+func (c *Checkbox) ApplyArgs(args *CheckboxArgs) *Checkbox {
 
 	c.SetLabel(args.Label)
 	c.SetChecked(args.Checked)
@@ -288,9 +306,11 @@ func (c *Checkbox) ApplyArgs(args *CheckboxArgs) *Checkbox{
 	if args.FinishedFunc != nil {
 		c.SetFinishedFunc(args.FinishedFunc)
 	}
+	if args.ValidateFunc != nil {
+		c.SetValidateFunc(args.ValidateFunc)
+	}
 	if args.InputCaptureFunc != nil {
 		c.SetInputCapture(args.InputCaptureFunc)
 	}
 	return c
 }
-

dropdown.go

@@ -70,6 +70,11 @@ type DropDown struct {
 	// A callback function which is called when the user changes the drop-down's
 	// selection.
 	selected func(text string, index int)
+
+	// A callback function to be called when one of the field exit keys — Enter,
+	// Tab, Backtab, or Escape — is used. If this callback returns false it will
+	// bypass the input handler and leave the focus on the non-valid field.
+	valid func(*DropDown, *tcell.EventKey) bool
 }
 
 // NewDropDown returns a new drop-down.
@@ -256,6 +261,14 @@ func (d *DropDown) SetFinishedFunc(handler func(key tcell.Key)) FormItem {
 	return d
 }
 
+// SetValidateFunc sets a callback to be called when one of the field exit keys —
+// Enter, Tab, Backtab, or Escape — is used. If this callback returns false it
+// will stop the input handler from moving focus to a different field.
+func (d *DropDown) SetValidateFunc(handler func(*DropDown, *tcell.EventKey) bool) *DropDown {
+	d.valid = handler
+	return d
+}
+
 // Draw draws this primitive onto the screen.
 func (d *DropDown) Draw(screen tcell.Screen) {
 	d.Box.Draw(screen)
@@ -499,8 +512,10 @@ type DropDownArgs struct {
 	// returns the event to be forwarded to the primitive's default
 	// input handler (nil if nothing should be forwarded).
 	InputCaptureFunc func(event *tcell.EventKey) *tcell.EventKey
-}
 
+	// An optional function which is called before the box is drawn.
+	ValidateFunc func(*DropDown, *tcell.EventKey) bool
+}
 
 // ApplyArgs applies the values from a DropDownArgs{} struct to the
 // associated properties of the DropDown.
@@ -537,6 +552,9 @@ func (d *DropDown) ApplyArgs(args *DropDownArgs) *DropDown {
 	if args.FinishedFunc != nil {
 		d.SetFinishedFunc(args.FinishedFunc)
 	}
+	if args.ValidateFunc != nil {
+		d.SetValidateFunc(args.ValidateFunc)
+	}
 	if args.InputCaptureFunc != nil {
 		d.SetInputCapture(args.InputCaptureFunc)
 	}

inputfield.go

@@ -85,6 +85,11 @@ type InputField struct {
 	// A callback function set by the Form class and called when the user leaves
 	// this form item.
 	finished func(tcell.Key)
+
+	// A callback function to be called when one of the field exit keys — Enter,
+	// Tab, Backtab, or Escape — is used. If this callback returns false it will
+	// bypass the input handler and leave the focus on the non-valid field.
+	valid func(*InputField, *tcell.EventKey) bool
 }
 
 // NewInputField returns a new input field.
@@ -226,6 +231,14 @@ func (i *InputField) SetFinishedFunc(handler func(key tcell.Key)) FormItem {
 	return i
 }
 
+// SetValidateFunc sets a callback to be called when one of the field exit keys —
+// Enter, Tab, Backtab, or Escape — is used. If this callback returns false it
+// will stop the input handler from moving focus to a different field.
+func (i *InputField) SetValidateFunc(handler func(*InputField, *tcell.EventKey) bool) *InputField {
+	i.valid = handler
+	return i
+}
+
 // Draw draws this primitive onto the screen.
 func (i *InputField) Draw(screen tcell.Screen) {
 	i.Box.Draw(screen)
@@ -507,6 +520,9 @@ type InputFieldArgs struct {
 	// returns the event to be forwarded to the primitive's default
 	// input handler (nil if nothing should be forwarded).
 	InputCaptureFunc func(event *tcell.EventKey) *tcell.EventKey
+
+	// An optional function which is called before the box is drawn.
+	ValidateFunc func(*InputField, *tcell.EventKey) bool
 }
 
 // ApplyArgs applies the values from a InputFieldArgs{} or PasswordFieldArgs{}
@@ -563,10 +579,11 @@ func (i *InputField) applyInputFieldArgs(args *InputFieldArgs) *InputField {
 	if args.FinishedFunc != nil {
 		i.SetFinishedFunc(args.FinishedFunc)
 	}
+	if args.ValidateFunc != nil {
+		i.SetValidateFunc(args.ValidateFunc)
+	}
 	if args.InputCaptureFunc != nil {
 		i.SetInputCapture(args.InputCaptureFunc)
 	}
 	return i
 }
-
-

passwordfield.go

@@ -64,6 +64,9 @@ type PasswordFieldArgs struct {
 	// returns the event to be forwarded to the primitive's default
 	// input handler (nil if nothing should be forwarded).
 	InputCaptureFunc func(event *tcell.EventKey) *tcell.EventKey
+
+	// An optional function which is called before the box is drawn.
+	ValidateFunc func(*InputField, *tcell.EventKey) bool
 }
 
 // applyPasswordFieldArgs applies the values from a PasswordFieldArgs{}
@@ -99,9 +102,11 @@ func (i *InputField) applyPasswordFieldArgs(args *PasswordFieldArgs) *InputField
 	if args.FinishedFunc != nil {
 		i.SetFinishedFunc(args.FinishedFunc)
 	}
+	if args.ValidateFunc != nil {
+		i.SetValidateFunc(args.ValidateFunc)
+	}
 	if args.InputCaptureFunc != nil {
 		i.SetInputCapture(args.InputCaptureFunc)
 	}
 	return i
 }
-

validation.go

@@ -0,0 +1,7 @@
+package tview
+
+import "github.com/gdamore/tcell"
+
+type PrimitiveValidationHandler interface {
+	Validate(p Primitive, key *tcell.EventKey) bool
+}