QTreeView中选择速度慢，为什么？

Question

我最近在一个使用 PyQt 的项目中遇到了困难。 我有一个连接到 QAbstractItemModel 的 QTreeView，其中通常有数千个节点。 到目前为止，它工作正常，但今天我意识到选择很多节点非常慢。 经过一番挖掘，结果发现 QAbstractItemModel.parent() 被调用得太频繁了。 我创建了最少的代码来重现该问题：

#!/usr/bin/env python
import sys
import cProfile
import pstats

from PyQt4.QtCore import Qt, QAbstractItemModel, QVariant, QModelIndex
from PyQt4.QtGui import QApplication, QTreeView

# 200 root nodes with 10 subnodes each

class TreeNode(object):
    def __init__(self, parent, row, text):
        self.parent = parent
        self.row = row
        self.text = text
        if parent is None: # root node, create subnodes
            self.children = [TreeNode(self, i, unicode(i)) for i in range(10)]
        else:
            self.children = []

class TreeModel(QAbstractItemModel):
    def __init__(self):
        QAbstractItemModel.__init__(self)
        self.nodes = [TreeNode(None, i, unicode(i)) for i in range(200)]

    def index(self, row, column, parent):
        if not self.nodes:
            return QModelIndex()
        if not parent.isValid():
            return self.createIndex(row, column, self.nodes[row])
        node = parent.internalPointer()
        return self.createIndex(row, column, node.children[row])

    def parent(self, index):
        if not index.isValid():
            return QModelIndex()
        node = index.internalPointer()
        if node.parent is None:
            return QModelIndex()
        else:
            return self.createIndex(node.parent.row, 0, node.parent)

    def columnCount(self, parent):
        return 1

    def rowCount(self, parent):
        if not parent.isValid():
            return len(self.nodes)
        node = parent.internalPointer()
        return len(node.children)

    def data(self, index, role):
        if not index.isValid():
            return QVariant()
        node = index.internalPointer()
        if role == Qt.DisplayRole:
            return QVariant(node.text)
        return QVariant()


app = QApplication(sys.argv)
treemodel = TreeModel()
treeview = QTreeView()
treeview.setSelectionMode(QTreeView.ExtendedSelection)
treeview.setSelectionBehavior(QTreeView.SelectRows)
treeview.setModel(treemodel)
treeview.expandAll()
treeview.show()
cProfile.run('app.exec_()', 'profdata')
p = pstats.Stats('profdata')
p.sort_stats('time').print_stats()

要重现该问题，只需运行代码（进行分析）并选择树小部件中的所有节点（通过 Shift 选择或 Cmd-A）。 当您退出应用程序时，分析统计信息将显示如下内容：

Fri May  8 20:04:26 2009    profdata

         628377 function calls in 6.210 CPU seconds

   Ordered by: internal time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    4.788    4.788    6.210    6.210 {built-in method exec_}
   136585    0.861    0.000    1.182    0.000 /Users/hsoft/Desktop/slow_selection.py:34(parent)
   142123    0.217    0.000    0.217    0.000 {built-in method createIndex}
    17519    0.148    0.000    0.164    0.000 /Users/hsoft/Desktop/slow_selection.py:52(data)
   162198    0.094    0.000    0.094    0.000 {built-in method isValid}
     8000    0.055    0.000    0.076    0.000 /Users/hsoft/Desktop/slow_selection.py:26(index)
   161357    0.047    0.000    0.047    0.000 {built-in method internalPointer}
       94    0.000    0.000    0.000    0.000 /Users/hsoft/Desktop/slow_selection.py:46(rowCount)
      404    0.000    0.000    0.000    0.000 /Users/hsoft/Desktop/slow_selection.py:43(columnCount)
       94    0.000    0.000    0.000    0.000 {len}
        1    0.000    0.000    6.210    6.210 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}

此数据中奇怪的部分是调用parent()的频率：2k节点136k次！ 有人知道为什么吗？

Answer 1

尝试为您的树视图调用 setUniformRowHeights(true)：

https://doc.qt.io/qt-4.8/qtreeview.html#uniformRowHeights-prop

此外，qt 实验室还有一个名为 modeltest 的 C++ 工具。 我不确定是否有Python的东西：

https://wiki.qt.io/Model_Test

Answer 2

我将您非常好的示例代码转换为 PyQt5 并在 Qt5.2 下运行，并且可以确认这些数字仍然相似，即莫名其妙的大量调用。 例如，这里是启动报告的顶部部分，cmd-A 选择全部，滚动一页，退出：

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1   14.880   14.880   15.669   15.669 {built-in method exec_}
   196712    0.542    0.000    0.703    0.000 /Users/dcortes1/Desktop/scratch/treeview.py:36(parent)
   185296    0.104    0.000    0.104    0.000 {built-in method createIndex}
    20910    0.050    0.000    0.056    0.000 /Users/dcortes1/Desktop/scratch/treeview.py:54(data)
   225252    0.036    0.000    0.036    0.000 {built-in method isValid}
   224110    0.034    0.000    0.034    0.000 {built-in method internalPointer}
     7110    0.020    0.000    0.027    0.000 /Users/dcortes1/Desktop/scratch/treeview.py:28(index)

And while the counts are really excessive (and I have no explanation), notice the cumtime values aren't so big. Also those functions could be recoded to run faster; for example in index(), is "if not self.nodes" ever true? Similarly, notice the counts for parent() and createIndex() are almost the same, hence index.isValid() is true much more often than not (reasonable, as end-nodes are much more numerous than parent nodes). Recoding to handle that case first would cut the parent() cumtime further. Edit: on second thought, such optimizations are "rearranging the deck chairs on the titanic".