Add support for popup windows.

README.md

@@ -100,6 +100,7 @@ there is a workround if it's impossible to upgrade. See
 5. [Window class](./README.md#5-window-class)  
  5.1 [Constructor](./README.md#51-constructor)  
  5.2 [Class method](./README.md#52-class-method)  
+ 5.3 [Popup windows](./README.md#53-popup-windows)  
 6. [Label widget](./README.md#6-label-widget) Single line text display.  
 7. [LED widget](./README.md#7-led-widget) Display Boolean values.  
 8. [Checkbox widget](./README.md#8-checkbox-widget) Enter Boolean values.  
@@ -912,6 +913,7 @@ Followed by keyword-only args
  * `draw_border=True`
  * `bgcolor=None` Background color, default black.
  * `fgcolor=None` Foreground color, default white.
+ * `writer=None` See Popups below.
 
 ## 5.2 Class method
 
@@ -925,6 +927,14 @@ callbacks to the user window constructor args. These may be called by widgets
 to send data to the calling screen. Note that widgets on the screen below will
 not be updated until the window has closed.
 
+## 5.3 Popup windows
+
+In general `Screen` and `Window` instances need at least one active widget.
+There is a special case of a popup window which typically displays status data,
+possibly with a progress meter. A popup has no user controls and is closed by
+user code. A popup is created by passing a `Writer` (or `CWriter`) to the
+constructor and is closed by issuing the `close()` static method.
+
 ###### [Contents](./README.md#0-contents)
 
 # 6. Label widget

gui/core/ugui.py

@@ -1,7 +1,7 @@
 # ugui.py Micropython GUI library
 
 # Released under the MIT License (MIT). See LICENSE.
-# Copyright (c) 2019-2021 Peter Hinch
+# Copyright (c) 2019-2022 Peter Hinch
 
 # Requires uasyncio V3
 
@@ -21,7 +21,7 @@ display = None  # Singleton instance
 ssd = None
 
 gc.collect()
-__version__ = (0, 1, 3)
+__version__ = (0, 1, 4)
 
 # Null function
 dolittle = lambda *_ : None
@@ -490,8 +490,12 @@ class Window(Screen):
             cls._value = val
         return cls._value
 
+    @staticmethod
+    def close():  # More intuitive name for popup window
+        Screen.back()
+
     def __init__(self, row, col, height, width, *, draw_border=True,
-                 bgcolor=None, fgcolor=None):
+                 bgcolor=None, fgcolor=None, writer=None):
         Screen.__init__(self)
         self.row = row
         self.col = col
@@ -500,6 +504,8 @@ class Window(Screen):
         self.draw_border = draw_border
         self.fgcolor = fgcolor if fgcolor is not None else color_map[FG]
         self.bgcolor = bgcolor if bgcolor is not None else color_map[BG]
+        if writer is not None:  # Special case of popup message
+            DummyWidget(writer, self)  # Invisible active widget
 
     def _do_open(self, old_screen):
         dev = display.usegrey(False)
@@ -516,7 +522,6 @@ class Window(Screen):
         y = self.row
         return x, y, x + w, y + h, w, h
 
-
 # Base class for all displayable objects
 class Widget:
 
@@ -622,7 +627,7 @@ class Widget:
             y = self.row - 2
             w = self.width + 4
             h = self.height + 4
-            if self.has_focus():
+            if self.has_focus() and not isinstance(self, DummyWidget):
                 color = color_map[FOCUS]
                 if hasattr(self, 'precision') and self.precision and self.prcolor is not None:
                     color = self.prcolor
@@ -747,3 +752,11 @@ class LinearIO(Widget):
     def leave(self):  # Control has lost focus
         self.precise(False)
 
+# The dummy enables popup windows by satisfying the need for at least one active
+# widget on a screen. It is invisible and is drawn by Window constructor before
+# any user labels..
+class DummyWidget(Widget):
+    
+    def __init__(self, writer, window):
+        super().__init__(writer, window.row + 1, window.col + 1, 4, 4,
+                 window.fgcolor, window.bgcolor, False, None, True)