I need to resize the window only with a 1:1 aspect ratio. Tried to implement in different ways, but it turns out recursion.
The last thing I stopped at:

widget.cpp:
Qt Code: 
  1. #include "widget.h"
  2. #include "ui_widget.h"
  3. #include "QResizeEvent"
  4. #include "filterobject.h"
  5.  
  6. Widget::Widget(QWidget *parent) :
  7.     QWidget(parent),
  8.     ui(new Ui::Widget)
  9. {
  10.     ui->setupUi(this);
  11. }
  12.  
  13. Widget::~Widget()
  14. {
  15.     delete ui;
  16. }
  17.  
  18. bool FilterObject::eventFilter(QObject *watched, QEvent *event) {
  19.     if(watched != target){//checks for correct target object.
  20.         return false;
  21.     }
  22.  
  23.     if(event->type() != QEvent::Resize){//and correct event
  24.        return false;
  25.     }
  26.     QResizeEvent *resEvent = static_cast<QResizeEvent*>(event);//then sets correct event type
  27.  
  28.     goalHeight = 7*resEvent->size().width()/16;//calculates height, 7/16 of width in my case
  29.     if(target->height()!=goalHeight){
  30.         target->setFixedHeight(goalHeight);
  31.     }
  32.  
  33.     return true;
  34. };
  35.  
  36. void QWidget::resizeEvent(QResizeEvent *event){
  37.    FilterObject *filter = new FilterObject();
  38.    QWidget *targetWidget = new QWidget();//let it be target object
  39.    filter->target=targetWidget;
  40.    targetWidget->installEventFilter(filter);
  41. }
To copy to clipboard, switch view to plain text mode 
filterobject.h:

Qt Code: 
  1. #include <QWidget>
  2.  
  3. class FilterObject:public QObject{
  4. public:
  5.     QWidget *target = nullptr;//it holds a pointer to target object
  6.     int goalHeight=0;
  7.     FilterObject(QObject *parent=nullptr):QObject(parent){}//uses QObject constructor
  8.     bool eventFilter(QObject *watched, QEvent *event) override;//and overrides eventFilter function
  9. };
To copy to clipboard, switch view to plain text mode 
I think that
Qt Code: 
  1. if (event-> type ()! = QEvent :: Resize)
To copy to clipboard, switch view to plain text mode 
code does not work, because the function does not continue.