Qt - There is a bug in QPropertyAnimation?

Question

I face a very serious situation. By writing this question I hope that really professionals will express their opinion regarding to the problem I am going to describe. I have reported a bug in https://bugreports.qt.io/ :

I have created QPropertyAnimation for maximumWidth property of QTextEdit and it does not work (it immediately changes state from starting state to the end state), though it works for minimumWidth property. Please see the attached code.

And have attached .h and .cpp files. See those files here (files are named new.h and new.cpp).

And I got the follwing response:

MaximumWidth is not the property you want to animate. It holds the maximum width that the widget can have, it's related to layouting and so on. Changing the maximumWidth (as well as the minimumWidth) does not necessarily trigger a relayout and repaint. You should animate the size.

Please explain me whether it is a bug or no? Please tell me how the minimumWith property is being animated but when it concerns to the maximumWidth property, then I should not work and that is OK? I just don't get their point... Please explain.

PS I have written this code because I wanted to close by animation the right QTextEdit and be sure that when I resize the main window, where the button and two QTextEdit are, the closed QTextEdit does not being restored.

Answer 1

Did you check the actual value of maximumWidth? You don't seem to set a specific maximumWidth in your code.

The default value for maximumWidth is 16777215, and you set a duration of 1 msec. for the closing animation. Fading from 16777215 to 3 in 1 msec. would look like "instant", I guess.

Answer 2

I don't think it is a bug; I'd call it "undefined behavior". That means that if you try to animate minimumWidth, nobody can tell you for sure what is supposed to happen, and maybe the code has some optimizations or corner cases where sometimes it works, others it doesn't.

Anyway, minimumWidth and maximumWidth aren't supposed to be used to define what the size of a QWidget is, only what it must not exceed; ie they weren't designed to do what you are trying to do, so it can be called a bug. If you want to animate, you have to use a deterministic approach, which in this case is using the geometry property.

Answer 3

这不是错误，您从错误报告中得到的响应很好地解释了代码和解决方案的问题。

Answer 4

Dear Sofahamster I have changed my code to the code below and it works fine. Thanks for your hint!

Header file

class MyWidget : public QWidget
{

    Q_OBJECT

    QTextEdit       *m_textEditor1;
    QTextEdit       *m_textEditor2;
    QPushButton     *m_pushButton;
    QHBoxLayout     *m_layout;
    QVBoxLayout     *m_buttonLayout;

    int              m_deltaX;
    bool             m_isClosed;


public:

    MyWidget(QWidget * parent = 0);
    ~MyWidget(){}

    void resizeEvent( QResizeEvent * event );

private slots:
    void closeOrOpenTextEdit2(bool isClosing);

};

Source file

MyWidget::MyWidget(QWidget * parent):QWidget(parent),m_deltaX(0)
{

  m_pushButton = new QPushButton(this);
  m_pushButton->setText(">");
  m_pushButton->setCheckable(true);
  m_pushButton->setFixedSize(16,16);
  connect(m_pushButton, SIGNAL(clicked(bool)), this, SLOT(closeOrOpenTextEdit2(bool)));

  m_textEditor1 = new QTextEdit(this);
  m_textEditor1->setText("AAAAA AAAAAAAAAAA AAAAAAAAAAA  AAAAAAA AAAAAAAAAAA AAAAAAAAAAA  AA");

  m_textEditor2 = new QTextEdit(this);

  m_buttonLayout = new QVBoxLayout();
  m_buttonLayout->addWidget(m_pushButton);
  m_buttonLayout->addItem( new QSpacerItem(1, 1, QSizePolicy::Minimum, QSizePolicy::Expanding) );


  m_layout = new QHBoxLayout;
  m_layout->addWidget(m_textEditor1, 10);
  m_layout->addSpacing(15);
  m_layout->addLayout(m_buttonLayout);
  m_layout->setSpacing(0);
  m_layout->addWidget(m_textEditor2, 4);

  setLayout(m_layout);
  resize(800,500);
}

void MyWidget::closeOrOpenTextEdit2(bool isClosing)
{
    m_isClosed = isClosing;
    QPropertyAnimation *animation1 = new QPropertyAnimation(m_textEditor2, "maximumWidth");

    if(isClosing) //close the second textEdit
    {
        m_textEditor2->setMaximumWidth(m_textEditor2->width());

        int textEdit2_start = m_textEditor2->maximumWidth();

        m_deltaX = textEdit2_start;
        int textEdit2_end = 3;



        animation1->setDuration(500);
        animation1->setStartValue(textEdit2_start);
        animation1->setEndValue(textEdit2_end);


        m_pushButton->setText("<");

    }
    else //open
    {


        int textEdit2_start = m_textEditor2->maximumWidth();
        int textEdit2_end = m_deltaX;


        animation1->setDuration(500);
        animation1->setStartValue(textEdit2_start);
        animation1->setEndValue(textEdit2_end);


        m_pushButton->setText(">");

    }

    animation1->start();

}


void MyWidget::resizeEvent( QResizeEvent * event )
{
    if(!m_isClosed)
        m_textEditor2->setMaximumWidth( QWIDGETSIZE_MAX );
}