Fix widget extending for Form

Fixes #694

widget/form.go

@@ -2,6 +2,7 @@ package widget
 
 import (
 	"fyne.io/fyne"
+	"fyne.io/fyne/internal/cache"
 	"fyne.io/fyne/layout"
 	"fyne.io/fyne/theme"
 )
@@ -51,14 +52,16 @@ func (f *Form) Append(text string, widget fyne.CanvasObject) {
 
 // AppendItem adds the specified row to the end of the Form
 func (f *Form) AppendItem(item *FormItem) {
-	// ensure we have a renderer set up
-	Renderer(f)
+	f.ExtendBaseWidget(f) // could be called before render
+
+	// ensure we have a renderer set up (that creates itemGrid)...
+	cache.Renderer(f.super())
 
 	f.Items = append(f.Items, item)
 	f.itemGrid.AddObject(f.createLabel(item.Text))
 	f.itemGrid.AddObject(item.Widget)
 
-	Refresh(f)
+	f.Refresh()
 }
 
 // MinSize returns the size that this widget should not shrink below
@@ -77,7 +80,7 @@ func (f *Form) CreateRenderer() fyne.WidgetRenderer {
 	}
 
 	if f.OnCancel == nil && f.OnSubmit == nil {
-		return Renderer(NewVBox(f.itemGrid))
+		return cache.Renderer(NewVBox(f.itemGrid))
 	}
 
 	buttons := NewHBox(layout.NewSpacer())
@@ -90,14 +93,14 @@ func (f *Form) CreateRenderer() fyne.WidgetRenderer {
 
 		buttons.Append(submit)
 	}
-	return Renderer(NewVBox(f.itemGrid, buttons))
+	return cache.Renderer(NewVBox(f.itemGrid, buttons))
 }
 
 // NewForm creates a new form widget with the specified rows of form items
 // and (if any of them should be shown) a form controls row at the bottom
 func NewForm(items ...*FormItem) *Form {
 	form := &Form{BaseWidget: BaseWidget{}, Items: items}
+	form.ExtendBaseWidget(form)
 
-	Renderer(form).Layout(form.MinSize())
 	return form
 }

widget/form_extend_test.go

@@ -0,0 +1,39 @@
+package widget
+
+import (
+	"testing"
+
+	"fyne.io/fyne/test"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type extendedForm struct {
+	Form
+}
+
+func TestForm_Extended_CreateRenderer(t *testing.T) {
+	form := &extendedForm{}
+	form.ExtendBaseWidget(form)
+	form.Items = []*FormItem{{Text: "test1", Widget: NewEntry()}}
+	assert.NotNil(t, test.WidgetRenderer(form))
+	assert.Equal(t, 2, len(form.itemGrid.Objects))
+
+	form.Append("test2", NewEntry())
+	assert.Equal(t, 4, len(form.itemGrid.Objects))
+}
+
+func TestForm_Extended_Append(t *testing.T) {
+	form := &extendedForm{}
+	form.ExtendBaseWidget(form)
+	form.Items = []*FormItem{{Text: "test1", Widget: NewEntry()}}
+	assert.Equal(t, 1, len(form.Items))
+
+	form.Append("test2", NewEntry())
+	assert.True(t, len(form.Items) == 2)
+
+	item := &FormItem{Text: "test3", Widget: NewEntry()}
+	form.AppendItem(item)
+	assert.True(t, len(form.Items) == 3)
+	assert.Equal(t, item, form.Items[2])
+}