Why are my QLabels not dynamically created when creation is called with QSlider?-CodePudding

I am new to QtCreator. I am trying to make a slider and dynamically remove and recreate labels when I move the slider. I am able to create labels dynamically when I call it from constructor, but for some reason when I do the same via moving he slider it does not work.

I intend that everytime slider is moved all the labels are deleted and the created fresh. I am not sure if it is the deletion prosess that messes this or something else. When I run this in debug mode it seems that new QLabel* are inserted into labels_? vector, but for some reason they are not visible.

Here is my code:

mainwindow.cpp:

#include "mainwindow.hh"
#include "ui_mainwindow.h"
#include <QLabel>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    ui->horizontalSlider->setRange(1,10);
    ui->horizontalSlider->setValue(5);

    connect(ui->horizontalSlider, &QSlider::sliderMoved, this, &MainWindow::move_slider);

    // This works
    //create_label(50);

    // This works
//    for (auto i = 0 ; i < ui->horizontalSlider->value(); i  ) {
//        labels_.push_back(create_label(i*item_width_));
//    }
}

MainWindow::~MainWindow()
{
    delete ui;
 }

QLabel* MainWindow::create_label(int x)
{
    QLabel* label = new QLabel(ui->frame);
    label->setGeometry(x, 0, item_width_, item_width_);
    label->setText(QString::number(ui->horizontalSlider->value()));
    return label;
}

// This method is called when silder moves. It seems to remove labels,
// but does not make new labels.
void MainWindow::move_slider()
{
    // Remove labels and reset
    for (QLabel* label: labels_) {
        delete label;
    }
    labels_.clear();

    // Create labels
    for (auto i = 0 ; i < ui->horizontalSlider->value(); i  ) {
        labels_.push_back(create_label(i*item_width_));
    }
}

mainwindow.hh

#ifndef MAINWINDOW_HH
#define MAINWINDOW_HH

#include <QMainWindow>
#include <QLabel>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    int item_width_ = 20;

    std::vector<QLabel*> labels_;

    QLabel* create_label(int x);

    void move_slider();

    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_HH

contents of labels_ after moving slider as seen in debug window:

EDIT: I tried to disconect slider from move_slider() and instead created PushButton and connected it with:

//connect(ui->horizontalSlider, &QSlider::sliderMoved, this, MainWindow::move_slider);
connect(ui->pushButton, &QPushButton::clicked, this, &MainWindow::move_slider);

This also calls the method on clicked and removes labels out of sight, but does not return them.

Another thing I tried is just running this in constructor without any button presses or slider movement:

// Create labels
for (auto i = 0 ; i < ui->horizontalSlider->value(); i  ) {
    labels_.push_back(create_label(i*item_width_));
}

// Remove labels and reset
for (QLabel* label: labels_) {
    delete label;
}
labels_.clear();

// Create labels again
for (auto i = 0 ; i < ui->horizontalSlider->value(); i  ) {
    labels_.push_back(create_label(i*item_width_));
}

It shows the labels correctly. So I think this my deletion should work (at least this line above seems to work in constructor), but simply either being part of either a method or constructor seems to be key difference in wheter or not new labels can be shown, even though it shouldn't.

CodePudding user response：

Quote from the Qt Docs of QWidget constructor:

If you add a child widget to an already visible widget you must explicitly show the child to make it visible.

Just add label->setVisible(true); to your create_label method:

QLabel* MainWindow::create_label(int x)
{
    QLabel* label = new QLabel(ui->frame);
    label->setGeometry(x, 0, item_width_, item_width_);
    label->setText(QString::number(ui->horizontalSlider->value()));
    label->setVisible(true);  // Add this
    return label;
}

It works in the constructor, because your window is not visible at that moment.