QLabel 不断扩展更新。答案

【问题标题】：QLabel keeps expanding on update.QLabel 不断扩展更新。
【发布时间】：2015-04-22 03:18:46
【问题描述】：

我正在开发一个说明shannon-fano-coding 的程序。我使用 QLabel 来显示构成编码的二叉树的图像（底部的第三个选项卡名为“Baum”）。我的问题是，每当我更新图像时，它的高度都会扩展 1px。

主窗口.ui：

<layout class="QVBoxLayout" name="verticalLayout_9">
 <item>
  <widget class="QLabel" name="treeView">
   <property name="sizePolicy">
    <sizepolicy hsizetype="Expanding" vsizetype="Expanding">
     <horstretch>0</horstretch>
     <verstretch>0</verstretch>
    </sizepolicy>
   </property>
   <property name="text">
    <string>TextLabel</string>
   </property>
  </widget>
 </item>
</layout>

mainwindow.cpp ctor：

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    codec = new SFCodec(ui->inputField);
    QWidget::showMaximized();

    QImage temp(ui->treeView->width(), ui->treeView->height(), QImage::Format_ARGB32);
    temp.fill(QColor(255,255,255));
    ui->treeView->setPixmap(QPixmap::fromImage(temp));
    ui->treeView->show();

}

mainwindow.cpp 函数调用：

ui->treeView->setPixmap(QPixmap::fromImage(codec->getTreeView(ui->treeView->width(), ui->treeView->height())));

SFCodec.cpp绘图函数：

QImage SFCodec::getTreeView(int width, int height)
{
    qDebug() << width << " " << height;
    int treeWidth = width-50,                           //the tree should have 25px spacing on each side
        treeHeight = height-50,
        max_step_x = treeWidth/4, //max step length (i.e. the first step big step from the root)
        step_x = max_step_x,  //the current step length in x-direction
        step_y = 0,
        depth = 0,      //max depth of the tree
        length = 0;     //helper variable to calculate the depth

    QPoint p1(treeWidth/2,5),p2(0,0);           //Lines are drawn between two points. These are the two points...
    QImage image(width, height, QImage::Format_ARGB32); //new image with the right dimensions

    image.fill(QColor(255,255,255,255));    //filled in with white (NOTE: the format is ARGB so the first '255' is the alpha channel)
    if(index.isEmpty())                     //no input text -> no code -> no tree
        return image;

    QPainter painter(&image);   //Qt device that does all the painting

    painter.setPen(QPen(QColor(0,0,0)));    //paints in black
    painter.setBrush(QBrush(QColor(Qt::color0), Qt::NoBrush));
    painter.setRenderHint(QPainter::Antialiasing);  //activate antialiasing since performance isn't an issue

    //calculate depth by finding the symbol with the longest code (each digit in the code is one level of depth)
    std::for_each(index.begin(), index.end(),[&](Symbol sym)
    {
        length = sym.getCode().length();
        if(length > depth)
            depth = length;
    });

    //we have treeHeight many pixels and want to fill them with depth many levels. so the step length in y direction has to be this long:
    step_y = treeHeight/depth;


    //paint the path for each symbol
    //note that we always start from the root so some lines are painted multiple times
    //this could be prevented by a recursive call which would be less readable and
    //(probably) not significantly faster.
    std::for_each(index.begin(), index.end(),[&](Symbol sym)
    {
        p1 = QPoint(treeWidth/2,5);  //reset to starting position
        step_x = max_step_x;        //and step length
        QString temp = sym.getCode();
        do{
           if(temp.at(0) == '1')
                p2 = p1 + QPoint(step_x,step_y);
            else
                p2 = p1 + QPoint(-step_x,step_y);
            step_x = step_x/2;

            painter.drawLine(p1,p2);
            p1 = p2;

            temp.remove(0,1);
        }while(temp.size());
        p1 = p1 + QPoint(-5,15);
        painter.drawText(p1, sym.getSym());
    });
    painter.end();
    return image;
}

我很确定这与布局有关，因为我的代码总是使用标签的 width() 和 height() 函数来获取值，而且我也不会改变任何东西。

我知道这里有数以千计的关于布局和调整大小的线程，我浏览了其中的不少，但没有一个小部件可以自行扩展或类似的东西。

如果您想为项目贡献任何东西（这是为了我大学的计算机科学讲座），请在相应的 github 页面上进行。我是一名电气工程师，所以我的编码自然不符合标准。

【问题讨论】：

标签： c++ qt layout resize

【解决方案1】：

QLabels 默认展开以保存其内容。如果您需要不同的行为，则需要以不同的方式设置 QLabel::sizePolicy。

void setSizePolicy(QSizePolicy::Policy horizontal, QSizePolicy::Policy vertical) 可让您分别调整水平和垂直属性。

您可以在enum QSizePolicy::Policy 中找到不同的可能设置。

从您的描述看来，您需要使用QSizePolicy::Fixed。

【讨论】：