Reduce flickering

2 files changed, 20 insertions, 2 deletions

diff --git a/ttystatus/area.py b/ttystatus/area.py
index 574ea1b..370f422 100644
--- a/ttystatus/area.py
+++ b/ttystatus/area.py
@@ -28,6 +28,23 @@ class AreaManager(object):
         width, _ = self._terminal.get_size()
         return width - 1
 
+    def prepare_to_overwrite(self, displayed_message):
+        '''Prepare to overwrite currently displayed message.
+
+        This is like clear, but it only moves the cursor to the top of
+        the message, but does not clear all the lines. This is helpful
+        for avoiding flickering.
+
+        The cursor is assumed to be on the last line of the displayed
+        message.
+
+        '''
+
+        num_lines = len(displayed_message.split('\n'))
+        up = self._terminal.get_up_sequence()
+        cr = self._terminal.get_carriage_return_sequence()
+        self._terminal.write(up * (num_lines - 1) + cr)
+
     def make_space(self, num_lines):
         '''Make space for a message needing a given number of lines.
 
@@ -56,7 +73,7 @@ class AreaManager(object):
         lines = message.split('\n')
 
         parts = [up * (len(lines) - 1)]
-        for i, line in enumerate(message.split('\n')):
+        for i, line in enumerate(lines):
             if i > 0:
                 parts.append(down)
             parts.append(cr)
diff --git a/ttystatus/messager.py b/ttystatus/messager.py
index 91206bb..3a2da02 100644
--- a/ttystatus/messager.py
+++ b/ttystatus/messager.py
@@ -84,7 +84,8 @@ class Messager(object):
         '''
 
         if self.enabled and (force or self.time_to_write()):
-            self.clear()
+            if self._displayed_message is not None:
+                self._area.prepare_to_overwrite(self._displayed_message)
             num_lines = len(message.split('\n'))
             self._area.make_space(num_lines)
             self._area.display(message)