• 分解

matplotlib：一个绘图用的Python模块

backends：matplotlib中的一个模块，后端。

backend_qt5agg：backends里面的一个模块，里面有两个类：FigureCanvasQTAgg 和_BackendQT5Agg

FigureCanvasQTagg：backend_qt5agg里的一个类

• FigureCanvasQTagg应用

matplotlib绘制的图可以应用在PyQt5应用里面，下面三个import都需要：

from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure
import matplotlib.pyplot as plt

**matplotlib.backends.backend_qt5agg用来连接Matplotlib和PyQt5。**backend_qt5agg.py的原文如下：

"""
Render to qt from agg.
"""

import ctypes

from matplotlib.transforms import Bbox

from .. import cbook
from .backend_agg import FigureCanvasAgg
from .backend_qt5 import (
    QtCore, QtGui, QtWidgets, _BackendQT5, FigureCanvasQT, FigureManagerQT,
    NavigationToolbar2QT, backend_version)
from .qt_compat import QT_API


class FigureCanvasQTAgg(FigureCanvasAgg, FigureCanvasQT):

    def __init__(self, figure):
        # Must pass 'figure' as kwarg to Qt base class.
        super().__init__(figure=figure)

    def paintEvent(self, event):
        """Copy the image from the Agg canvas to the qt.drawable.
        In Qt, all drawing should be done inside of here when a widget is
        shown onscreen.
        """
        if self._update_dpi():
            # The dpi update triggered its own paintEvent.
            return
        self._draw_idle()  # Only does something if a draw is pending.

        # If the canvas does not have a renderer, then give up and wait for
        # FigureCanvasAgg.draw(self) to be called.
        if not hasattr(self, 'renderer'):
            return

        painter = QtGui.QPainter(self)

        rect = event.rect()
        left = rect.left()
        top = rect.top()
        width = rect.width()
        height = rect.height()
        # See documentation of QRect: bottom() and right() are off by 1, so use
        # left() + width() and top() + height().
        bbox = Bbox(
            [[left, self.renderer.height - (top + height * self._dpi_ratio)],
             [left + width * self._dpi_ratio, self.renderer.height - top]])
        reg = self.copy_from_bbox(bbox)
        buf = cbook._unmultiplied_rgba8888_to_premultiplied_argb32(
            memoryview(reg))

        # clear the widget canvas
        painter.eraseRect(rect)

        qimage = QtGui.QImage(buf, buf.shape[1], buf.shape[0],
                              QtGui.QImage.Format_ARGB32_Premultiplied)
        if hasattr(qimage, 'setDevicePixelRatio'):
            # Not available on Qt4 or some older Qt5.
            qimage.setDevicePixelRatio(self._dpi_ratio)
        origin = QtCore.QPoint(left, top)
        painter.drawImage(origin / self._dpi_ratio, qimage)
        # Adjust the buf reference count to work around a memory
        # leak bug in QImage under PySide on Python 3.
        if QT_API in ('PySide', 'PySide2'):
            ctypes.c_long.from_address(id(buf)).value = 1

        self._draw_rect_callback(painter)

        painter.end()

    def blit(self, bbox=None):
        """Blit the region in bbox.
        """
        # If bbox is None, blit the entire canvas. Otherwise
        # blit only the area defined by the bbox.
        if bbox is None and self.figure:
            bbox = self.figure.bbox

        # repaint uses logical pixels, not physical pixels like the renderer.
        l, b, w, h = [pt / self._dpi_ratio for pt in bbox.bounds]
        t = b + h
        self.repaint(l, self.renderer.height / self._dpi_ratio - t, w, h)

    def print_figure(self, *args, **kwargs):
        super().print_figure(*args, **kwargs)
        self.draw()


@_BackendQT5.export
class _BackendQT5Agg(_BackendQT5):
    FigureCanvas = FigureCanvasQTAgg

• Reference

1. matplotlib.backends.backend_qt5agg
2. matplotlib/lib/matplotlib/backends/backend_qt5agg.py