main: abstract the nil selection into a virtual database type

db_none.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"github.com/ying32/govcl/vcl"
+)
+
+type noLoadedDatabase struct{}
+
+func (n *noLoadedDatabase) DisplayName() string {
+	return "yvbolt"
+}
+
+func (n *noLoadedDatabase) DriverName() string {
+	return "No database selected"
+}
+
+func (n *noLoadedDatabase) RootElement() *vcl.TTreeNode {
+	return nil
+}
+
+func (n *noLoadedDatabase) RenderForNav(f *TMainForm, ndata *navData) {
+	f.propertiesBox.SetText("Open a database to get started...")
+	f.contentBox.SetEnabled(false)
+	f.contentBox.Clear()
+}
+
+func (n *noLoadedDatabase) ExecQuery(query string, resultArea *vcl.TListView) {
+}
+
+func (n *noLoadedDatabase) NavChildren(ndata *navData) ([]string, error) {
+	return []string{}, nil
+}
+
+func (n *noLoadedDatabase) Keepalive(ndata *navData) {
+}

main.go

@@ -22,7 +22,8 @@ type TMainForm struct {
 	queryInput    *vcl.TMemo
 	queryResult   *vcl.TListView
 
-	dbs []loadedDatabase
+	none *noLoadedDatabase
+	dbs  []loadedDatabase
 }
 
 var (
@@ -137,7 +138,6 @@ func (f *TMainForm) OnFormCreate(sender vcl.IObject) {
 	f.propertiesBox.SetReadOnly(true)
 	f.propertiesBox.SetEnabled(false)
 	f.propertiesBox.SetBorderStyle(types.BsNone)
-	f.propertiesBox.SetText("Open a database to get started...")
 
 	dataTab := vcl.NewTabSheet(f.Tabs)
 	dataTab.SetParent(f.Tabs)
@@ -199,6 +199,9 @@ func (f *TMainForm) OnFormCreate(sender vcl.IObject) {
 	f.queryResult.BorderSpacing().SetLeft(MY_SPACING)
 	f.queryResult.BorderSpacing().SetRight(MY_SPACING)
 	f.queryResult.BorderSpacing().SetBottom(MY_SPACING)
+
+	f.none = &noLoadedDatabase{}
+	f.OnNavChange(f, nil) // calls f.none.RenderForNav and sets up status bar content
 }
 
 func (f *TMainForm) OnMnuFileOpenClick(sender vcl.IObject) {
@@ -281,17 +284,18 @@ func (f *TMainForm) OnQueryExecute(sender vcl.IObject) {
 
 func (f *TMainForm) OnNavChange(sender vcl.IObject, node *vcl.TTreeNode) {
 
-	if node.Data() == nil {
-		vcl.ShowMessage("unexpected nil data")
-		return
+	var ld loadedDatabase = f.none
+	var ndata *navData = nil
+
+	if node != nil && node.Data() != nil {
+		ndata = (*navData)(node.Data())
+		ld = ndata.ld
 	}
 
-	ndata := (*navData)(node.Data())
-
-	ndata.ld.RenderForNav(f, ndata) // Handover to the database type's own renderer function
+	ld.RenderForNav(f, ndata) // Handover to the database type's own renderer function
 
 	// We're in charge of common status bar text updates
-	f.StatusBar.SetSimpleText(ndata.ld.DisplayName() + " | " + ndata.ld.DriverName())
+	f.StatusBar.SetSimpleText(ld.DisplayName() + " | " + ld.DriverName())
 }
 
 func (f *TMainForm) OnNavExpanding(sender vcl.IObject, node *vcl.TTreeNode, allowExpansion *bool) {