在Qt C ++中在自定义项目委托上绘画文本时的性能问题 [英] Performance issues when painting text on custom item delegates in Qt C++

问题描述

目标:创建具有自定义文本内容的项目委托，以在 QListView .

Goal: Creating an item delegate with custom textual content to be used in QListView.

问题:使用 QPainter 绘制文本重新实现 QAbstractItemDelegate 的子类明显比绘制形状和像素图慢.将基类更改为 QStyledItemDelegate 不会提高速度.

Problem: Drawing text with QPainter in a reimplementation of the paint() method of a QAbstractItemDelegate's subclass is noticably slower than drawing shapes and pixmaps. Changing the base class to QStyledItemDelegate does not improve the speed.

设置:Qt 5.9.1，MSVC 2017，已在Windows 7/10上进行了测试

Setup: Qt 5.9.1, MSVC 2017, tested on Windows 7/10

预研究:此处a>，但是在这种特殊情况下，即使不使用 QPainter :: setFont()，也会存在性能问题.关于项目委托的Qt示例并没有多大帮助，因为它们显示了如何绘制控件，而不仅仅是文本.

Pre-research: A similar bug is reported here, however in this particular case the performance issue exists even if QPainter::setFont() is not used. The Qt examples on item delegates do not help much, as they show how to draw controls, not just text.

示例:以下示例说明了该问题.显示应用程序窗口后，显示 QListView 的内容时会出现轻微但明显的延迟.在某些情况下，这种延迟会持续长达几秒钟.当 view-> setItemDelegate(new Delegate(this)); 或 painter-> drawText(0，option.rect.y()+ 18，index.data().toString()); 被注释，显示窗口和 QListView 的内容之间没有明显的延迟.

Example: The example given below illustrates the problem. After showing the application's window, the content of the QListView is shown with a slight, but noticable delay. In some cases this delay lasts up to several seconds. When either of view->setItemDelegate(new Delegate(this)); or painter->drawText(0, option.rect.y() + 18, index.data().toString()); are commented there is no observable delay between showing the window and the content of the QListView.

MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QListView>
#include <QStandardItemModel>
#include "Delegate.h"

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0);
};

#endif // MAINWINDOW_H

MainWindow.cpp

#include "MainWindow.h"

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
    QStandardItemModel *model = new QStandardItemModel(this);
    QListView *view = new QListView(this);

    view->setModel(model);
    view->setItemDelegate(new Delegate(this));

    for (int n = 0; n < 100; n++) { model->appendRow(new QStandardItem("Test " +  QString::number(n))); }

    setCentralWidget(view);
}

Delegate.h

#ifndef DELEGATE_H
#define DELEGATE_H

#include <QAbstractItemDelegate>
#include <QPainter>

class Delegate : public QAbstractItemDelegate
{
    Q_OBJECT
public:
    explicit Delegate(QObject *parent = nullptr);

    void paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const;
    QSize sizeHint(const QStyleOptionViewItem &option, const QModelIndex &index) const;
};

#endif // DELEGATE_H

Delegate.cpp

#include "Delegate.h"

Delegate::Delegate(QObject *parent) : QAbstractItemDelegate(parent)
{

}

void Delegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    painter->drawText(0, option.rect.y() + 18, index.data().toString());
    painter->drawLine(0, option.rect.y() + 19, option.rect.width(), option.rect.y() + 19);
}

QSize Delegate::sizeHint(const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    return QSize(0, 20);
}

推荐答案

目前，我已经创建了

For now I've created a bug report. I will update this post with any further info I receive.

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