From 93ebee6ddc3214303a18287528e9afc30adeb46b Mon Sep 17 00:00:00 2001
From: Antony Lee <anntzer.lee@gmail.com>
Date: Wed, 27 Jun 2018 15:16:26 +0200
Subject: [PATCH] Add private API retrieving the current event loop and backend
 GUI info.

Work towards implementation of backend switching.

Note that the API is kept private for now as the lack of extensibility
is a bit unsatisfying; perhaps we'll figure out a better way to do it
if other custom implementers (e.g., code editors?) get interested in
looking into it.

The `required_event_loop` variable could be made private for now, or
not...
---
 doc/api/next_api_changes/2018-06-27-AL.rst |  7 +++
 lib/matplotlib/backends/__init__.py        | 50 ++++++++++++++++++++++
 lib/matplotlib/backends/_backend_tk.py     |  1 +
 lib/matplotlib/backends/backend_gtk3.py    |  1 +
 lib/matplotlib/backends/backend_macosx.py  |  1 +
 lib/matplotlib/backends/backend_qt4.py     |  2 +-
 lib/matplotlib/backends/backend_qt5.py     |  1 +
 lib/matplotlib/backends/backend_wx.py      |  1 +
 src/_macosx.m                              | 15 +++++++
 9 files changed, 78 insertions(+), 1 deletion(-)

diff --git a/doc/api/next_api_changes/2018-06-27-AL.rst b/doc/api/next_api_changes/2018-06-27-AL.rst
index 74c32d357386..51a8171bf02d 100644
--- a/doc/api/next_api_changes/2018-06-27-AL.rst
+++ b/doc/api/next_api_changes/2018-06-27-AL.rst
@@ -4,3 +4,10 @@ Changes to backend loading
 Failure to load backend modules (``macosx`` on non-framework builds and
 ``gtk3`` when running headless) now raises `ImportError` (instead of
 `RuntimeError` and `TypeError`, respectively.
+
+Third-party backends that integrate with an interactive framework are now
+encouraged to define the ``required_interactive_framework`` global value to one
+of the following values: "qt5", "qt4", "gtk3", "wx", "tk", or "macosx". This
+information will be used to determine whether it is possible to switch from a
+backend to another (specifically, whether they use the same interactive
+framework).
diff --git a/lib/matplotlib/backends/__init__.py b/lib/matplotlib/backends/__init__.py
index e506d3100927..2467a4235373 100644
--- a/lib/matplotlib/backends/__init__.py
+++ b/lib/matplotlib/backends/__init__.py
@@ -1,5 +1,7 @@
 import importlib
 import logging
+import os
+import sys
 import traceback
 
 import matplotlib
@@ -14,6 +16,54 @@
     if not line.startswith('  File "<frozen importlib._bootstrap'))
 
 
+def _get_running_interactive_framework():
+    """
+    Return the interactive framework whose event loop is currently running, if
+    any, or "headless" if no event loop can be started, or None.
+
+    Returns
+    -------
+    Optional[str]
+        One of the following values: "qt5", "qt4", "gtk3", "wx", "tk",
+        "macosx", "headless", ``None``.
+    """
+    QtWidgets = (sys.modules.get("PyQt5.QtWidgets")
+                 or sys.modules.get("PySide2.QtWidgets"))
+    if QtWidgets and QtWidgets.QApplication.instance():
+        return "qt5"
+    QtGui = (sys.modules.get("PyQt4.QtGui")
+             or sys.modules.get("PySide.QtGui"))
+    if QtGui and QtGui.QApplication.instance():
+        return "qt4"
+    Gtk = (sys.modules.get("gi.repository.Gtk")
+           or sys.modules.get("pgi.repository.Gtk"))
+    if Gtk and Gtk.main_level():
+        return "gtk3"
+    wx = sys.modules.get("wx")
+    if wx and wx.GetApp():
+        return "wx"
+    tkinter = sys.modules.get("tkinter")
+    if tkinter:
+        for frame in sys._current_frames().values():
+            while frame:
+                if frame.f_code == tkinter.mainloop.__code__:
+                    return "tk"
+                frame = frame.f_back
+    try:
+        from matplotlib.backends import _macosx
+    except ImportError:
+        pass
+    else:
+        # Note that the NSApp event loop is also running when a non-native
+        # toolkit (e.g. Qt5) is active, but in that case we want to report the
+        # other toolkit; thus, this check comes after the other toolkits.
+        if _macosx.event_loop_is_running():
+            return "macosx"
+    if sys.platform.startswith("linux") and not os.environ.get("DISPLAY"):
+        return "headless"
+    return None
+
+
 def pylab_setup(name=None):
     """
     Return new_figure_manager, draw_if_interactive and show for pyplot.
diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
index 4fb68a835fb7..caa29e446de0 100644
--- a/lib/matplotlib/backends/_backend_tk.py
+++ b/lib/matplotlib/backends/_backend_tk.py
@@ -994,6 +994,7 @@ def trigger(self, *args):
 
 @_Backend.export
 class _BackendTk(_Backend):
+    required_interactive_framework = "tk"
     FigureManager = FigureManagerTk
 
     @classmethod
diff --git a/lib/matplotlib/backends/backend_gtk3.py b/lib/matplotlib/backends/backend_gtk3.py
index 68af0c19a301..41a72c1cbdf4 100644
--- a/lib/matplotlib/backends/backend_gtk3.py
+++ b/lib/matplotlib/backends/backend_gtk3.py
@@ -976,6 +976,7 @@ def error_msg_gtk(msg, parent=None):
 
 @_Backend.export
 class _BackendGTK3(_Backend):
+    required_interactive_framework = "gtk3"
     FigureCanvas = FigureCanvasGTK3
     FigureManager = FigureManagerGTK3
 
diff --git a/lib/matplotlib/backends/backend_macosx.py b/lib/matplotlib/backends/backend_macosx.py
index f10c1e2a69c0..ab7601f63bcd 100644
--- a/lib/matplotlib/backends/backend_macosx.py
+++ b/lib/matplotlib/backends/backend_macosx.py
@@ -185,6 +185,7 @@ def set_message(self, message):
 
 @_Backend.export
 class _BackendMac(_Backend):
+    required_interactive_framework = "macosx"
     FigureCanvas = FigureCanvasMac
     FigureManager = FigureManagerMac
 
diff --git a/lib/matplotlib/backends/backend_qt4.py b/lib/matplotlib/backends/backend_qt4.py
index 15bbb1f76e91..f51066f7d57d 100644
--- a/lib/matplotlib/backends/backend_qt4.py
+++ b/lib/matplotlib/backends/backend_qt4.py
@@ -7,4 +7,4 @@
 
 @_BackendQT5.export
 class _BackendQT4(_BackendQT5):
-    pass
+    required_interactive_framework = "qt4"
diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
index ddbdf56e8177..545724b26ff6 100644
--- a/lib/matplotlib/backends/backend_qt5.py
+++ b/lib/matplotlib/backends/backend_qt5.py
@@ -1109,6 +1109,7 @@ def exception_handler(type, value, tb):
 
 @_Backend.export
 class _BackendQT5(_Backend):
+    required_interactive_framework = "qt5"
     FigureCanvas = FigureCanvasQT
     FigureManager = FigureManagerQT
 
diff --git a/lib/matplotlib/backends/backend_wx.py b/lib/matplotlib/backends/backend_wx.py
index 7c684e76cdd9..657e543f45c3 100644
--- a/lib/matplotlib/backends/backend_wx.py
+++ b/lib/matplotlib/backends/backend_wx.py
@@ -1930,6 +1930,7 @@ def OnPrintPage(self, page):
 
 @_Backend.export
 class _BackendWx(_Backend):
+    required_interactive_framework = "wx"
     FigureCanvas = FigureCanvasWx
     FigureManager = FigureManagerWx
     _frame_class = FigureFrameWx
diff --git a/src/_macosx.m b/src/_macosx.m
index 2cc9182aa383..c3ebac8c35c1 100644
--- a/src/_macosx.m
+++ b/src/_macosx.m
@@ -2789,6 +2789,16 @@ - (int)index
 }
 @end
 
+static PyObject*
+event_loop_is_running(PyObject* self)
+{
+    if ([NSApp isRunning]) {
+        Py_RETURN_TRUE;
+    } else {
+        Py_RETURN_FALSE;
+    }
+}
+
 static PyObject*
 show(PyObject* self)
 {
@@ -3038,6 +3048,11 @@ static bool verify_framework(void)
 }
 
 static struct PyMethodDef methods[] = {
+   {"event_loop_is_running",
+    (PyCFunction)event_loop_is_running,
+    METH_NOARGS,
+    "Return whether the NSApp main event loop is currently running."
+   },
    {"show",
     (PyCFunction)show,
     METH_NOARGS,