QT高级编程实战指南

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"qt开发初级入门" Qt开发是一个深入且广泛的领域，尤其对于初学者来说，了解其基础和关键概念至关重要。Qt是一个强大的跨平台开发框架，由Qt Company创建，允许开发者使用C++语言构建GUI（图形用户界面）应用以及非GUI程序，如命令行工具和服务。Qt库提供了丰富的类和功能，涵盖了界面设计、事件处理、网络通信、数据存储等多个方面。在Qt开发中，自定义控件是提高应用特性的常用手段。你可以通过扩展已有的Qt控件（如QSpinBox）或直接继承自QWidget来创建自己的控件。例如，你可以创建一个16进制的旋钮控件（HexSpinBox），使得用户可以方便地输入和显示16进制数值。这通常涉及到重写父类的方法，添加额外的功能，以及可能的UI定制。 SIGNAL和SLOT机制是Qt事件驱动编程的核心。它们提供了一种安全的方式来连接对象间的信号（当特定事件发生时触发）和槽（执行相应操作的函数）。这种方式避免了硬编码的回调函数，使得代码更加灵活和可维护。理解信号和槽的工作原理以及如何在代码中正确使用它们是掌握Qt编程的关键。 Qt的事件系统是另一个核心特性，它允许程序响应用户的交互，如鼠标点击、键盘输入等。通过编写事件处理器，开发者可以定制对各种事件的响应。同时，QT也支持多进程通讯，如使用QCOP协议和FIFO（先进先出）机制来实现不同进程间的数据交换，这对于大型应用的模块化设计非常有用。在图形绘制方面，Qt提供了丰富的API来进行2D和3D绘图。你可以创建自定义的画布，使用QPainter类进行绘图操作，学习基本的绘图概念，如坐标系统、路径、刷子和画笔等，以实现复杂的视觉效果。多线程编程在现代软件中是必不可少的，尤其是在需要并行处理任务的场景下。Qt提供了对多线程的支持，开发者可以创建和管理线程，以提高程序的运行效率。然而，由于环境限制，这部分内容可能不会在初级入门阶段详细讲解。 Qt还支持输入法和插件功能。输入法组件允许应用与各种输入设备和输入法引擎进行交互，而插件系统则允许你扩展应用的功能，使其能够加载和使用第三方模块。网络编程在Qt中也很强大，可以轻松处理HTTP、TCP/IP、UDP等网络协议。Qt的网络模块提供了如QNetworkAccessManager等类，用于发送HTTP请求、接收数据、处理套接字等任务。最后，Qt支持XML解析，提供了基于SAX和DOM的API。SAX适用于处理大型XML文件，而DOM则更适合对整个文档结构进行操作。 Qt开发初级入门涵盖了大量的主题，从基础的控件使用到高级的系统集成，都是开发者需要逐步掌握的技能。通过学习这些知识点，你可以构建出功能丰富、跨平台的桌面和移动应用程序。

QT 高级编程

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without worrying about the other pixels.

Unfortunately, the two-step approach is also a major source of flicker. For example,

if the user resizes the widget, the widget is first cleared in its entirety, and then the

pixels are painted. The flicker is even worse if the window system shows the

contents of the window as it is resized, because then the widget is repeatedly erased

and painted.

图表 6 无预览缩放控件防止抖动

The WStaticContents flag used to implement the IconEditor widget is one

solution to this problem, but it can only be used for widgets whose content is

independent of the size of the widget. Such widgets are rare. Most widgets tend to

stretch their contents to consume all the available space. They need to be

completely repainted when they are resized. We can still avoid flicker, but the

solution is slightly more complicated.

The first rule to avoid flicker is to construct the widget with the WNoAutoErase flag.

This flag tells Qt not to erase the widget before a paint event. The old pixels are then

left unchanged, and any newly revealed pixels are undefined.

图表 7 缩放一个 WNoAutoErase 控件

When using WNoAutoErase, it is important that the paint handler sets all the pixels

explicitly. Any pixel that is not set in the paint event will keep its previous value,

which isn't necessarily the background color.

The second rule to avoid flicker is to paint every pixel just once. The easiest way to

implement this requirement is to draw the whole widget in an offscreen pixmap and

to copy the pixmap onto the widget in one go. Using this approach, it doesn't matter

if some pixels are painted multiple times because the painting takes place off-screen.

This is double buffering.

Adding double buffering to a custom widget to eliminate flicker is straightforward.

Suppose the original paint event handler looks like this:

void MyWidget::paintEvent(QPaintEvent *)

{

QPainter painter(this);

QT 高级编程

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drawMyStuff(&painter);

}

The double-buffered version looks like this:

void MyWidget::paintEvent(QPaintEvent *event)

{

static QPixmap pixmap;

QRect rect = event->rect();

QSize newSize = rect.size().expandedTo(pixmap.size());

pixmap.resize(newSize);

pixmap.fill(this, rect.topLeft());

QPainter painter (&pixmap, this);

painter.translate(-rect.x(), -rect.y());

drawMyStuff(&painter);

bitBlt(this, rect.x(), rect.y(), &pixmap, 0, 0,

rect.width(), rect.height());

}

First, we resize a QPixmap to be at least as large as the bounding rectangle of the

region to repaint. (A "region" is very often either a rectangle or an L-shaped area,

but it can be arbitrarily complex.) We make the QPixmap a static variable to avoid

repeatedly allocating and deallocating it. For the same reason, we never shrink the

QPixmap; the calls to QSize::expandedTo() and QPixmap::resize() ensure that

it is always large enough. After resizing, we fill the QPixmap with the widget's erase

color or background pixmap using QPixmap::fill(). The second argument to

fill() specifies which point in the widget the QPixmap's top-left pixel corresponds

to. (This makes a difference if the widget is to be erased using a pixmap instead of

a uniform color.)

The QPixmap class is similar to both QImage and QWidget. Like a QImage, it stores

an image, but the color depth and possibly the colormap are aligned with the display,

rather like a hidden QWidget. If the window system is running in 8-bit mode, all

widgets and pixmaps are restricted to 256 colors, and Qt automatically maps 24-bit

color specifications onto 8-bit colors. (Qt's color allocation strategy is controlled by

calling QApplication::setColorSpec().)

Next, we create a QPainter to operate on the pixmap. By passing the this pointer

to the constructor, we tell QPainter to adopt some of the widget's settings, such as

its font. We translate the painter to paint the correct rectangle into the pixmap,

before we perform the drawing using the QPainter as usual.

Finally, we copy the pixmap to the widget using the bitBlt() global function, whose

name stands for "bit-block transfer".

Double buffering is not only useful for avoiding flicker. It is beneficial if the widget's

rendering is complex and needed repeatedly. We can then store a pixmap

permanently with the widget, always ready for the next paint event, and copy the

pixmap to the widget whenever we receive a paint event. It is especially helpful

when we want to do small modifications, such as drawing a rubber band, without

recomputing the whole widget's rendering over and over.

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