Qt开发疑难解答：400+技术问题详尽解析

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"《QT维基百科：技术常见问题解答》是一份详尽的资源，专为QT开发者设计，收录了超过400个在开发过程中可能会遇到的关键问题和解决方案。这些问题涵盖了广泛的领域，包括但不限于插件加载与库分离、代码贡献流程、多线程操作、图形视图中的交互、Makefile管理、许可证选择、脚本引擎访问私有成员、窗口调整、辅助工具使用、Mac平台控件样式、菜单管理、文本编辑器功能、抗锯齿设置以及qtApp与qApp的区别。这份文档旨在帮助开发者解决实际开发过程中的疑难杂症，提升开发效率和理解QT的深层次功能。" 1. QPluginLoadervsQLibrary: 当代码分割成多个DLL时，开发者需要决定何时使用QPluginLoader加载插件，何时直接使用QLibrary。这涉及到模块化和性能优化的选择，需要根据项目需求和跨平台兼容性来决定。 2. 代码贡献流程：Qt社区鼓励用户参与贡献，该部分详细解释了如何提交bug报告、修复、新特性请求以及遵循项目的开发规范和提交指南。 3. 跨线程调用: 了解如何在Qt中正确地在主线程和子线程之间调用QObject的方法，这对于实现多线程应用程序至关重要，包括信号槽机制的使用。 4. 图形视图旋转：通过QGraphicsView和鼠标交互实现元素的旋转，涉及到图形渲染和用户界面交互的设计技巧。 5. Makefile配置：针对Qt在Eclipse集成环境中的Makefile，解释了dist目标缺失的原因，以及如何正确构建和打包项目。 6. Qt的LGPL许可：探讨了Qt选择LGPL许可证的背景，这对于理解开源软件的版权法律和社区合作至关重要。 7. 脚本引擎接口：讲解如何使对象的非槽函数和非属性成员对脚本引擎可见，方便脚本编程和动态调用。 8. 窗口调整响应：讨论如何确保当子窗口被程序性地调整大小时，父窗口也随之调整，保证布局的一致性。 9. Qt Assistant问题：如果启动助手或文档加载失败，此部分提供了排查和解决此类问题的步骤和建议。 10. Mac平台字体处理：阐述了为何在Mac上，明确设置的QLabel字体会优先于继承设置，涉及UI设计中的细节和平台特定行为。 11. 菜单管理：介绍如何移除菜单栏中的某个菜单项，这是UI设计和可定制性的体现。 12. QTextEdit功能：关于文本编辑器的长度限制设置，这对于处理用户输入和文本处理应用很有帮助。 13. 抗锯齿与线条显示：解释了为什么在不启用抗锯齿时，宽度为1的线条可能会消失，以及如何处理这种视觉效果。 14. qtApp与qApp选择：讨论了在不同场景下使用qtApp全局对象和qApp的区别，以优化内存管理和应用程序行为。《QT维基百科：技术常见问题解答》是每个QT开发者必备的参考资料，它深入浅出地解答了开发过程中的各种疑难问题，有助于开发者提升技能和工作效率。

7/10/2018 Technical FAQ - Qt Wiki

https://wiki.qt.io/Technical_FAQ#How_can_I_convert_a_QString_to_char.2A_and_vice_versa.3F 16/150

If you created your pixel data using GDI functions its likely that you ended up with pixels in the form 0x00rrggbb as GDI will unset the alpha channel of all

pixels it touches. You can work around this by manually patching up the alpha channel, afterwards or by only using GDI functions that support a proper

alpha channel, like AlphaBlend.

Is it possible to have transparent Qt widgets on top of a QGLWidget?

Qt provides a way to paint to an OpenGL context, but this is in the end painted separately from Qt and the two painting systems are not synchronized, so Qt

cannot compose (semi-)transparent images on top of a QGLWidget. At best, you can have opaque widgets on top of the QGLWidget, but even then you

may experience some flicker due to the painting being asynchronous.

Where do I find the resource editor in the integrated Qt Designer when using the Qt Eclipse Integration?

The Qt Eclipse Integration comes with a full editor for resource files. Just double click on the resource file in the Projects view. The integrated Qt Designer

doesn't come with an extra resource editor.

Is there a way to make a menu in a QMainWindow that automatically redirects standard commands to the current widget ?

There is no automatic way to redirect the commands in a QMainWindow's http://doc.qt.io/qt-5/qmainwindow.html menu to the current widget. If you

would like e.g the Copy command to be redirected to the current widget, then you can listen for the QApplication::focusChanged() http://doc.qt.io/qt-

5/qapplication.html#focusChanged signal and connect that signal to a slot which determines which actions should be executed for the current widget.

How do I find out why my mouse and keyboard work in QVFb, but not on my target system?

Keyboard and mouse problems should always be debugged on the Linux frame buffer not in QVFb as QVFb uses X keyboard and mouse drivers.

To debug on the Linux frame buffer, run the application in a console on your desktop system, with the option: -display LinuxFb instead of -display QVFb.

How can I specify the spacing between the text and the icon for an item in a view?

In order to specify the margin between the decoration and the text for an item in a view, you can subclass the style and reimplement pixelMetric()

http://doc.qt.io/qt-5/qstyle.html#pixelMetric to return the value you want for PM_FocusFrameHMargin http://doc.qt.io/qt-5/qstyle.html#PixelMetric-enum

and/or PM_FocusFrameVMargin. Alternatively, you can reimplement paint() http://doc.qt.io/qt-5/qstyleditemdelegate.html#paint for the item delegate to

draw the margins as you want.

The following example demonstrates how this can be done by reimplementing pixelMetric():

#include <QtGui>

class Style: public QWindowsStyle

{

public:

Style() {}

int pixelMetric(PixelMetric metric, const QStyleOption *option = 0,

const QWidget *widget = 0) const

{

if (metric == PM_FocusFrameHMargin) {

return 10;

} else {

return QWindowsStyle::pixelMetric(metric, option, widget);

}

};

int main(int argc, char **argv)

{

QApplication app(argc, argv);

QTreeWidget tree;

tree.setStyle(new Style());

QPixmap pix(12, 12);

pix.fill(Qt::red);

QTreeWidgetItem *item1 = new QTreeWidgetItem(&tree);

item1->setText(0, "first");

item1->setIcon(0, QIcon(pix));

QTreeWidgetItem *item2 = new QTreeWidgetItem(&tree);

item2->setText(0, "second");

item2->setIcon(0, QIcon(pix));

tree.show();

return app.exec();

}

Does qmake have support for pkg-config?

Yes, simply add link_pkgconfig to the CONFIG entry in your .pro file and then add pkg-config configurations to the PKGCONFIG variable, e.g:

7/10/2018 Technical FAQ - Qt Wiki

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CONFIG +=link_pkgconfig

TEMPLATE = app

TARGET =

DEPENDPATH += .

INCLUDEPATH += .

PKGCONFIG = gtk+-2.0

# Input

SOURCES += main.cpp

See the documentation http://doc.qt.io/qt-5/qmake-project-files.html#configuration-features.

How can I align the text and icons in an itemview?

You can align the text and icons in your itemview by subclassing the itemview and reimplementing viewOptions() http://doc.qt.io/qt-

5/qabstractitemview.html#viewOptions to return the alignment you want for the QStyleOptionViewItem http://doc.qt.io/qt-5/qstyleoptionviewitem.html.

The example below demonstrates how this can be done.

#include <QtGui>

class ListWidget : public QListWidget

{

public:

ListWidget()

{

QListWidgetItem *item1 = new QListWidgetItem("one");

QListWidgetItem *item2 = new QListWidgetItem("two");

QListWidgetItem *item3 = new QListWidgetItem("three");

addItem(item1);

addItem(item2);

addItem(item3);

}

QStyleOptionViewItem viewOptions() const

{

QStyleOptionViewItem item;

item.init(this);

item.displayAlignment = Qt::AlignRight;

return item;

}

};

int main(int argc, char** argv)

{

QApplication app(argc, argv);

ListWidget listWidget;

listWidget.show();

return app.exec();

}

How can I change the margins of my QTextDocument?

In order to specify the margins used by the whole document you can follow these steps:

1. Get the root frame that contains the document using QTextDocument::rootFrame() http://doc.qt.io/qt-5/qtextdocument.html#rootFrame

2. Then get the QTextFrameFormat through QTextFrame::frameFormat() http://doc.qt.io/qt-5/qtextframe.html#frameFormat.

3. Then call QTextFrameFormat::setMargin() http://doc.qt.io/qt-5/qtextframeformat.html#setMargin with the desired value.

In order to specify individual margins (independently) for each block of text, you can use the QTextBlockFormat http://doc.qt.io/qt-

5/qtextblockformat.html#setTopMargin set margin functions.

The example below demonstrates how to set margins for the entire document and also how to set the margins for individual blocks.

#include <QtGui>

int main(int argc, char** argv)

{

QApplication app(argc, argv);

QTextEdit edit;

edit.setText("The QTextEdit widget is an advanced editor that supports "

"formatted rich text. ");

edit.append("It can be used to display HTML and other rich document formats.");

QTextDocument * doc = edit.document();

QTextCursor cursor(doc);

cursor.movePosition(QTextCursor::End, QTextCursor::MoveAnchor);

//cursor.movePosition(QTextCursor::Start, QTextCursor::KeepAnchor);

#if 1

QTextBlockFormat tbf = cursor.blockFormat();

tbf.setLeftMargin(100);

tbf.setTopMargin(100);

cursor.setBlockFormat(tbf);

#else

QTextFrame *tf = doc->rootFrame();

QTextFrameFormat tff = tf->frameFormat();

tff.setMargin(50);

tf->setFrameFormat(tff);

#endif

edit.show();

return app.exec();

}

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Is is possible to run Qt applications on Windows XP Embedded ?

This should work since the Windows API is the same for Windows XP and Windows XP Embedded and since they are binary compatible, but it is not

supported or tested by us.

I use a shortcut for interacting with a widget in my QTest::keyClick() function and the test fails. What can be wrong?

The problem is likely to be that you have not called show() http://doc.qt.io/qt-5/qwidget.html#show on your widget. If the widget is not visible, the shortcut

does not actually behave correctly. You wouldn't typically have a shortcut that works for a non visible widget, so you should show your widget to get this

test to pass.

How can I change the icon for the sort indicator and its position in a QHeaderView?

In order to change the icon and position for the sort indicator in a QHeaderView http://doc.qt.io/qt-5/qheaderview.html, you can subclass the style and

reimplement drawPrimitive() http://doc.qt.io/qt-5/qstyle.html#drawPrimitive to draw the sort indicator using the icon and position you want.

#include <QtGui>

class Style: public QWindowsStyle

{

public:

Style () {}

void drawPrimitive(PrimitiveElement element, const QStyleOption *option,

QPainter *painter, const QWidget *widget = 0) const

{

if (element == PE_IndicatorHeaderArrow ) {

const QStyleOptionHeader *headerOption =

qstyleoption_cast<const QStyleOptionHeader *>(option);

int x, y, width, height;

option->rect.getRect(&x, &y, &width, &height);

QPixmap pixmap;

if (headerOption->sortIndicator == QStyleOptionHeader::SortUp)

pixmap.load("arrow_up.png");

else

pixmap.load("arrow_down.png");

painter->save();

painter->drawPixmap(x+width,y+height,pixmap);

painter->restore();

} else

QWindowsStyle::drawPrimitive(element, option, painter, widget);

}

};

int main(int argc, char **argv)

{

QApplication app(argc, argv);

QTableWidget table(2,2);

QTableWidgetItem *item1 = new QTableWidgetItem("1");

QTableWidgetItem *item2 = new QTableWidgetItem("2");

table.setItem(0,0, item1);

table.setItem(1,0, item2);

table.setSortingEnabled(true);

app.setStyle(new Style());

table.show();

return app.exec();

}

Alternatively, you can use a stylesheet to achieve this. See the documentation:

http://doc.qt.io/qt-5/stylesheet-examples.html#customizing-qheaderview

When creating a library with Qt/Windows and using it in an application, then there is a link error regarding symbols which

are in the library

When seeing errors like the following:

MyObj.obj : error LNK2019: unresolved external symbol public: static

struct QMetaObject const MyObj::staticMetaObject"

(?staticMetaObject@MyObj@@2UQMetaObject@@B) referenced in function class

MyObj * __cdecl qobject_cast<class MyObj *>(class QObject *)"

it is probably a result of the linker not finding the symbols from the library. You need to make sure that the symbols in your library are properly exported

when the library is created. Subsequently imported when you are linking against the library, so you should have something like the following in a header

file in your library:

#if defined TEST

#define TEST_COMMON_DLLSPEC __declspec(dllexport)

#else

#define TEST_COMMON_DLLSPEC __declspec(dllimport)

#endif

7/10/2018 Technical FAQ - Qt Wiki

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and use it in the classes that you wish to make available to the application like:

class TEST_COMMON_DLLSPEC MyObj : public QObject { ... };

Then add to your library's .pro file the following line so it knows that the symbols need to be exported in this case:

DEFINES += TEST

See the following wiki page http://wiki.qt.io/How_to_create_a_library_with_Qt_and_use_it_in_an_application for more information and a complete

example.

How can I enable Edit and Continue in my Qt application?

Edit and Continue is not enabled by default for Qt projects since project files, created with qmake -tp vc contain the compiler switch */Zi*. To enable edit

& continue, the right switch would be */ZI*.

To enable Edit & Continue in your .pro file, add the following to the .pro file:

QMAKE_CFLAGS_DEBUG += -ZI

QMAKE_CXXFLAGS_DEBUG += -ZI

Alternatively, you can enable Edit & Continue for your project inside Visual Studio, as follows:

open the project settings

select C/C++ -> General"

set Debug Information Format to Program Database for Edit & Continue (/ZI)"

Is there a way to specify build dependencies for Visual Studio 2003 and 2005?

There are a couple of precautions you need to be aware of for the automatic dependency generation to work properly in MS Solution files. The attached

example demonstrates how this can be done by following the steps below.

The dependencies are generated automatically, if

1a) there is a Lib/DLL project with the target name (the .lib is used, not the .dll of course) being that of what is used on link line of an other project in your

solution

1b) there is a Exe project with the target name being that of what is used in a custom build-step of an other project in your solution

2) you don't use paths in the TARGET variable (use DESTDIR/DLLDESTDIR for that), like TARGET=$(SOME_VARIABLE)/myLib, won't work

3) if you have a special location for your libs, you specify the -Lmy/library/path and LIBS += mylib, instead of just using LIBS += my/library/path/mylib

4) the leaf projects are created before you generate the solution file. (You can use the recursive flag for qmake to do this, like qmake -tp vc -r

[yourproject.pro]"