Enable/disable Back and Forward buttons depending on the history.

This is a standard behavior for any history-enabled app
(like web browser). When there is no previous or next item
in the history, the appropriate button on the toolbar
is disabled.

articleview.cc

@@ -790,11 +790,23 @@ void ArticleView::updateMutedContents()
   }
 }
 
+bool ArticleView::canGoBack()
+{
+  // First entry in a history is always an empty page,
+  // so we skip it.
+  return ui.definition->history()->currentItemIndex() > 1;
+}
+
+bool ArticleView::canGoForward()
+{
+  return ui.definition->history()->canGoForward();
+}
+
 void ArticleView::back()
 {
   // Don't allow navigating back to page 0, which is usually the initial
   // empty page
-  if ( ui.definition->history()->currentItemIndex() > 1 )
+  if ( canGoBack() )
   {
     saveHistoryUserData();
     ui.definition->back();

articleview.hh

@@ -101,6 +101,9 @@ public:
   /// The function reloads content if the change affects it.
   void updateMutedContents();
 
+  bool canGoBack();
+  bool canGoForward();
+
 public slots:
 
   /// Goes back in history

mainwindow.cc

@@ -975,6 +975,8 @@ void MainWindow::iconChanged( ArticleView * view, QIcon const & icon )
 
 void MainWindow::pageLoaded( ArticleView * view )
 {
+  updateBackForwardButtons();
+
   updatePronounceAvailability();
 
   if ( cfg.preferences.pronounceOnLoadMain )
@@ -985,6 +987,7 @@ void MainWindow::pageLoaded( ArticleView * view )
 
 void MainWindow::tabSwitched( int )
 {
+  updateBackForwardButtons();
   updatePronounceAvailability();
   updateFoundInDictsList();
 }
@@ -1065,6 +1068,16 @@ void MainWindow::updateFoundInDictsList()
   }
 }
 
+void MainWindow::updateBackForwardButtons()
+{
+  if ( QWidget * cw = ui.tabWidget->currentWidget() )
+  {
+    ArticleView & view = dynamic_cast< ArticleView & >( *( cw ) );
+    navBack->setEnabled(view.canGoBack());
+    navForward->setEnabled(view.canGoForward());
+  }
+}
+
 void MainWindow::updatePronounceAvailability()
 {
   bool pronounceEnabled = ui.tabWidget->count() > 0 &&
@@ -1555,6 +1568,8 @@ void MainWindow::showTranslationFor( QString const & inWord,
   history.addItem( History::Item( group, inWord.trimmed() ) );
   history.save();
 
+  updateBackForwardButtons();
+
   #if 0
   QUrl req;
 

mainwindow.hh

@@ -126,6 +126,8 @@ private:
 
   void updateFoundInDictsList();
 
+  void updateBackForwardButtons();
+
   /// Updates word search request and active article view in response to
   /// muting or unmuting dictionaries, or showing/hiding dictionary bar.
   void applyMutedDictionariesState();