From 5c7ff48e9fde7d88b09d658593946339ad4cd938 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Thu, 22 May 2014 21:24:51 +0530
Subject: [PATCH] Live CSS: Handle updates more gracefully when the cursor is
 in a CSS editor or a <style> tag

We prevent incorrect data from being returned by ensuring that updates
are not done while the preview panel is being refreshed. We also only
update data after the event loop has run for some time after a refresh.
---
 src/calibre/gui2/tweak_book/live_css.py | 39 +++++++++++++++++++++----
 src/calibre/gui2/tweak_book/preview.py  |  2 ++
 2 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/src/calibre/gui2/tweak_book/live_css.py b/src/calibre/gui2/tweak_book/live_css.py
index c4a9db0da7..84dc9c9a3e 100644
--- a/src/calibre/gui2/tweak_book/live_css.py
+++ b/src/calibre/gui2/tweak_book/live_css.py
@@ -297,12 +297,16 @@ class LiveCSS(QWidget):
     def __init__(self, preview, parent=None):
         QWidget.__init__(self, parent)
         self.preview = preview
-        preview.refreshed.connect(self.update_data)
+        self.preview_is_refreshing = False
+        preview.refresh_starting.connect(self.preview_refresh_starting)
+        preview.refreshed.connect(self.preview_refreshed)
         self.apply_theme()
         self.setAutoFillBackground(True)
         self.update_timer = QTimer(self)
         self.update_timer.timeout.connect(self.update_data)
         self.update_timer.setSingleShot(True)
+        self.update_timer.setInterval(500)
+        self.now_showing = (None, None, None)
 
         self.stack = s = QStackedLayout(self)
         self.setLayout(s)
@@ -322,6 +326,15 @@ class LiveCSS(QWidget):
         sc.setWidgetResizable(True)
         s.addWidget(sc)
 
+    def preview_refresh_starting(self):
+        self.preview_is_refreshing = True
+
+    def preview_refreshed(self):
+        self.preview_is_refreshing = False
+        # We must let the event loop run otherwise the webview will return
+        # stale data in read_data()
+        self.start_update_timer()
+
     def apply_theme(self):
         f = self.font()
         f.setFamily(tprefs['editor_font_family'] or default_font_family())
@@ -344,13 +357,29 @@ class LiveCSS(QWidget):
         self.stack.setCurrentIndex(0)
 
     def show_data(self, editor_name, sourceline, tags):
+        if self.preview_is_refreshing:
+            return
         if sourceline is None:
             self.clear()
         else:
             data = self.read_data(sourceline, tags)
             if data is None or len(data['computed_css']) < 1:
-                self.clear()
-                return
+                if editor_name == self.current_name and (editor_name, sourceline, tags) == self.now_showing:
+                    # Try again in a little while in case there was a transient
+                    # error in the web view
+                    self.start_update_timer()
+                    return
+                if self.now_showing == (None, None, None) or self.now_showing[0] != self.current_name:
+                    self.clear()
+                    return
+                # Try to refresh the data for the currently shown tag instead
+                # of clearing
+                editor_name, sourceline, tags = self.now_showing
+                data = self.read_data(sourceline, tags)
+                if data is None or len(data['computed_css']) < 1:
+                    self.clear()
+                    return
+            self.now_showing = (editor_name, sourceline, tags)
             data['html_name'] = editor_name
             self.box.show_data(data)
             self.stack.setCurrentIndex(1)
@@ -391,7 +420,7 @@ class LiveCSS(QWidget):
         self.start_update_timer()
 
     def update_data(self):
-        if not self.is_visible:
+        if not self.is_visible or self.preview_is_refreshing:
             return
         editor_name = self.current_name
         ed = editors.get(editor_name, None)
@@ -403,7 +432,7 @@ class LiveCSS(QWidget):
 
     def start_update_timer(self):
         if self.is_visible:
-            self.update_timer.start(1000)
+            self.update_timer.start()
 
     def stop_update_timer(self):
         self.update_timer.stop()
diff --git a/src/calibre/gui2/tweak_book/preview.py b/src/calibre/gui2/tweak_book/preview.py
index 7c877518e7..757347b73e 100644
--- a/src/calibre/gui2/tweak_book/preview.py
+++ b/src/calibre/gui2/tweak_book/preview.py
@@ -450,6 +450,7 @@ class Preview(QWidget):
     split_requested = pyqtSignal(object, object, object)
     split_start_requested = pyqtSignal()
     link_clicked = pyqtSignal(object, object)
+    refresh_starting = pyqtSignal()
     refreshed = pyqtSignal()
 
     def __init__(self, parent=None):
@@ -569,6 +570,7 @@ class Preview(QWidget):
             parse_worker.add_request(self.current_name)
             # Tell webkit to reload all html and associated resources
             current_url = QUrl.fromLocalFile(current_container().name_to_abspath(self.current_name))
+            self.refresh_starting.emit()
             if current_url != self.view.url():
                 # The container was changed
                 self.view.setUrl(current_url)