Update previews when turned on in prefs (#9507)

Previews are now generated when previews are turned on in
preferences. This change ensures that when users activate previews
for the first time, they are not confused by no previews showing up
(a restart of LyX or a triggering of each individual preview would
be required).

There was a previously attempted fix for #9507 at 390ae054 which was
reverted at 358745d0 for performance reasons: it updated previews
after every preference change and updating previews is costly (even
if the cache signals there are no changes needed).

This implementation is consistent with what we do for updating the
system fonts in preferences.

src/BufferList.cpp

@@ -344,6 +344,15 @@ void BufferList::recordCurrentAuthor(Author const & author)
 }
 
 
+void BufferList::updatePreviews()
+{
+	BufferStorage::iterator it = bstore.begin();
+	BufferStorage::iterator end = bstore.end();
+	for (; it != end; ++it)
+		(*it)->updatePreviews();
+}
+
+
 int BufferList::bufferNum(FileName const & fname) const
 {
 	FileNameList const buffers(fileNames());

src/BufferList.h

@@ -115,6 +115,8 @@ public:
 	//@{
 	/// reset current author for all buffers
 	void recordCurrentAuthor(Author const & author);
+	/// update previews for all buffers, e.g. for Prefs update
+	void updatePreviews();
 	/// Call changed() on all buffers, internal or not
 	void changed(bool update_metrics) const;
 	/// emergency save for all buffers

src/frontends/qt4/GuiPrefs.cpp

@@ -1272,10 +1272,16 @@ void PrefDisplay::applyRC(LyXRC & rc) const
 			rc.preview = LyXRC::PREVIEW_OFF;
 			break;
 		case 1:
-			rc.preview = LyXRC::PREVIEW_NO_MATH;
+			if (rc.preview != LyXRC::PREVIEW_NO_MATH) {
+				rc.preview = LyXRC::PREVIEW_NO_MATH;
+				form_->updatePreviews();
+			}
 			break;
 		case 2:
-			rc.preview = LyXRC::PREVIEW_ON;
+			if (rc.preview != LyXRC::PREVIEW_ON) {
+				rc.preview = LyXRC::PREVIEW_ON;
+				form_->updatePreviews();
+			}
 			break;
 	}
 
@@ -3205,7 +3211,8 @@ void PrefIdentity::updateRC(LyXRC const & rc)
 /////////////////////////////////////////////////////////////////////
 
 GuiPreferences::GuiPreferences(GuiView & lv)
-	: GuiDialog(lv, "prefs", qt_("Preferences")), update_screen_font_(false)
+	: GuiDialog(lv, "prefs", qt_("Preferences")), update_screen_font_(false),
+	  update_previews_(false)
 {
 	setupUi(this);
 
@@ -3315,6 +3322,7 @@ bool GuiPreferences::initialiseParams(string const &)
 	movers_ = theMovers();
 	colors_.clear();
 	update_screen_font_ = false;
+	update_previews_ = false;
 
 	updateRC(rc_);
 	// Make sure that the bc is in the INITIAL state
@@ -3359,6 +3367,12 @@ void GuiPreferences::dispatchParams()
 		update_screen_font_ = false;
 	}
 
+        if (update_previews_) {
+		// resets flag in case second apply in same dialog
+		theBufferList().updatePreviews();
+		update_previews_ = false;
+	}
+
 	// The Save button has been pressed
 	if (isClosing())
 		dispatch(FuncRequest(LFUN_PREFERENCES_SAVE));
@@ -3377,6 +3391,12 @@ void GuiPreferences::updateScreenFonts()
 }
 
 
+void GuiPreferences::updatePreviews()
+{
+	update_previews_ = true;
+}
+
+
 QString GuiPreferences::browsebind(QString const & file) const
 {
 	return browseLibFile("bind", file, "bind", qt_("Choose bind file"),

src/frontends/qt4/GuiPrefs.h

@@ -101,6 +101,9 @@ public:
 	/// update the screen fonts after change
 	void updateScreenFonts();
 
+	/// update the previews after change
+	void updatePreviews();
+
 	LyXRC & rc() { return rc_; }
 	Converters & converters() { return converters_; }
 	Formats & formats() { return formats_; }
@@ -123,6 +126,7 @@ private:
 	std::vector<std::string> colors_;
 
 	bool update_screen_font_;
+	bool update_previews_;
 };