Move draw_if_interactive logic to new_figure_manager_given_figure.

Currently, Matplotlib only ever calls draw_if_interactive at the end of
`plt.figure()` (and upon figure unpickling), i.e. this is a mechanism to
further customize handling of newly generated figures.  (Prior to
Matplotlib 1.5, draw_if_interactive was also called at the end of all
pyplot functions to trigger a figure redraw, but this is now handled by
the stale attribute.)

In order to simplify the backend API ("what is the API that a backend
module must/can provide"), I am planning to deprecate (on Matplotlib's
side) the ability for backends to provide a draw_if_interactive function
(forcing them to always do `if interactive(): draw_idle()`).  Instead,
any relevant new-figure-customization logic can instead go into
`new_figure_manager_given_figure`.  This PR implements this change for
matplotlib-inline, and should be fully back-compatible all the way back
to Matplotlib 1.5.  I would like to make these changes first on the side
of the clients (i.e., the third-party backends), to catch any possible
problems with the intended change on Matplotlib's side.

matplotlib_inline/backend_inline.py

@@ -4,13 +4,11 @@
 # Distributed under the terms of the BSD 3-Clause License.
 
 import matplotlib
-from matplotlib.backends.backend_agg import (  # noqa
-    new_figure_manager,
-    FigureCanvasAgg,
-    new_figure_manager_given_figure,
-)
 from matplotlib import colors
+from matplotlib.backends import backend_agg
+from matplotlib.backends.backend_agg import FigureCanvasAgg
 from matplotlib._pylab_helpers import Gcf
+from matplotlib.figure import Figure
 
 from IPython.core.interactiveshell import InteractiveShell
 from IPython.core.getipython import get_ipython
@@ -20,6 +18,57 @@
 from .config import InlineBackend
 
 
+def new_figure_manager(num, *args, FigureClass=Figure, **kwargs):
+    """
+    Return a new figure manager for a new figure instance.
+
+    This function is part of the API expected by Matplotlib backends.
+    """
+    return new_figure_manager_given_figure(num, FigureClass(*args, **kwargs))
+
+
+def new_figure_manager_given_figure(num, figure):
+    """
+    Return a new figure manager for a given figure instance.
+
+    This function is part of the API expected by Matplotlib backends.
+    """
+    manager = backend_agg.new_figure_manager_given_figure(num, figure)
+
+    # Hack: matplotlib FigureManager objects in interacive backends (at least
+    # in some of them) monkeypatch the figure object and add a .show() method
+    # to it.  This applies the same monkeypatch in order to support user code
+    # that might expect `.show()` to be part of the official API of figure
+    # objects.  For further reference:
+    # https://github.com/ipython/ipython/issues/1612
+    # https://github.com/matplotlib/matplotlib/issues/835
+
+    if not hasattr(figure, 'show'):
+        # Queue up `figure` for display
+        figure.show = lambda *a: display(
+            figure, metadata=_fetch_figure_metadata(figure))
+
+    # If matplotlib was manually set to non-interactive mode, this function
+    # should be a no-op (otherwise we'll generate duplicate plots, since a user
+    # who set ioff() manually expects to make separate draw/show calls).
+    if not matplotlib.is_interactive():
+        return
+
+    # ensure current figure will be drawn, and each subsequent call
+    # of draw_if_interactive() moves the active figure to ensure it is
+    # drawn last
+    try:
+        show._to_draw.remove(figure)
+    except ValueError:
+        # ensure it only appears in the draw list once
+        pass
+    # Queue up the figure for drawing in next show() call
+    show._to_draw.append(figure)
+    show._draw_called = True
+
+    return manager
+
+
 def show(close=None, block=None):
     """Show all figures as SVG/PNG payloads sent to the IPython clients.
 
@@ -56,51 +105,6 @@ def show(close=None, block=None):
 show._to_draw = []
 
 
-def draw_if_interactive():
-    """
-    Is called after every pylab drawing command
-    """
-    # signal that the current active figure should be sent at the end of
-    # execution.  Also sets the _draw_called flag, signaling that there will be
-    # something to send.  At the end of the code execution, a separate call to
-    # flush_figures() will act upon these values
-    manager = Gcf.get_active()
-    if manager is None:
-        return
-    fig = manager.canvas.figure
-
-    # Hack: matplotlib FigureManager objects in interacive backends (at least
-    # in some of them) monkeypatch the figure object and add a .show() method
-    # to it.  This applies the same monkeypatch in order to support user code
-    # that might expect `.show()` to be part of the official API of figure
-    # objects.
-    # For further reference:
-    # https://github.com/ipython/ipython/issues/1612
-    # https://github.com/matplotlib/matplotlib/issues/835
-
-    if not hasattr(fig, 'show'):
-        # Queue up `fig` for display
-        fig.show = lambda *a: display(fig, metadata=_fetch_figure_metadata(fig))
-
-    # If matplotlib was manually set to non-interactive mode, this function
-    # should be a no-op (otherwise we'll generate duplicate plots, since a user
-    # who set ioff() manually expects to make separate draw/show calls).
-    if not matplotlib.is_interactive():
-        return
-
-    # ensure current figure will be drawn, and each subsequent call
-    # of draw_if_interactive() moves the active figure to ensure it is
-    # drawn last
-    try:
-        show._to_draw.remove(fig)
-    except ValueError:
-        # ensure it only appears in the draw list once
-        pass
-    # Queue up the figure for drawing in next show() call
-    show._to_draw.append(fig)
-    show._draw_called = True
-
-
 def flush_figures():
     """Send all figures that changed