QScrollArea: clearing the layout of the inner widget

[mattc]

Hello,
I created a scroll area and called setWidget() to set the "inner" widget. The inner widget has a layout that I want to clear from time to time. In the documentation for QLayout::takeAt() I found this:

http://doc.trolltech.com/4.5/qlayout.html#takeAt

Unfortunately, it does not work (it looks like a redraw problem). What works is keeping a separate list of added widgets, and deleting them manually. I can't understand what's going on. I'm posting a complete test class showing the problem:

#include <QWidget>
#include <QVBoxLayout>
#include <QPushButton>
#include <QScrollArea>
#include <QLabel>
#include <QList>
 
class ScrollAreaTest : public QWidget
{
  Q_OBJECT
 
public:
	ScrollAreaTest(QWidget* parent = 0):
		QWidget(parent)
	{
		// buttons
		QPushButton* btnAdd = new QPushButton("Add label");
		connect(btnAdd, SIGNAL(clicked()), this, SLOT(onBtnAdd()));
 
		QPushButton* btnClear1 = new QPushButton("Clear Layout (mode 1)");
		connect(btnClear1, SIGNAL(clicked()), this, SLOT(onBtnClear1()));
 
		QPushButton* btnClear2 = new QPushButton("Clear Layout (mode 2)");
		connect(btnClear2, SIGNAL(clicked()), this, SLOT(onBtnClear2()));
 
		// scroll area
		QScrollArea* myScrollArea = new QScrollArea();
		myScrollArea->setWidgetResizable(true);
		myScrollArea->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
		myScrollArea->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);
 
		// scroll area "inner" widget
		QWidget* innerWidget = new QWidget();
		innerWidgetLayout = new QVBoxLayout();
		innerWidget->setLayout(innerWidgetLayout);
 
		// set scroll area inner widget
		myScrollArea->setWidget(innerWidget);
 
		// main layout
		QVBoxLayout* layout = new QVBoxLayout();
		layout->addWidget(btnAdd);
		layout->addWidget(btnClear1);
		layout->addWidget(btnClear2);
		layout->addWidget(myScrollArea);
		setLayout(layout);
	}
 
private:
	QVBoxLayout* innerWidgetLayout;
	QList<QLabel*> addedLabels;
 
private slots:
 
	void onBtnAdd()
	{
		QLabel* l = new QLabel("Hello there!");
		innerWidgetLayout->addWidget(l);
 
		addedLabels.push_back(l);  // I'm keeping a list of added labels (see onBtnClear2)
	}
 
	void onBtnClear1() // this is from the docs and doesn't work
	{
		QLayoutItem *child;
 
		while ((child = innerWidgetLayout->takeAt(0)) != 0)
			delete child;
 
		/*
		innerWidgetLayout->activate();
		innerWidgetLayout->update();
		update();
		*/
	}
 
	void onBtnClear2() // this works
	{
		for (QList<QLabel*>::iterator it = addedLabels.begin(); it != addedLabels.end(); ++it)
			delete *it;
 
		addedLabels.clear();
	}
};

#include <QWidget>
#include <QVBoxLayout>
#include <QPushButton>
#include <QScrollArea>
#include <QLabel>
#include <QList>

class ScrollAreaTest : public QWidget
{
  Q_OBJECT
  
public:
	ScrollAreaTest(QWidget* parent = 0):
		QWidget(parent)
	{
		// buttons
		QPushButton* btnAdd = new QPushButton("Add label");
		connect(btnAdd, SIGNAL(clicked()), this, SLOT(onBtnAdd()));

		QPushButton* btnClear1 = new QPushButton("Clear Layout (mode 1)");
		connect(btnClear1, SIGNAL(clicked()), this, SLOT(onBtnClear1()));

		QPushButton* btnClear2 = new QPushButton("Clear Layout (mode 2)");
		connect(btnClear2, SIGNAL(clicked()), this, SLOT(onBtnClear2()));
		
		// scroll area
		QScrollArea* myScrollArea = new QScrollArea();
		myScrollArea->setWidgetResizable(true);
		myScrollArea->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
		myScrollArea->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);

		// scroll area "inner" widget
		QWidget* innerWidget = new QWidget();
		innerWidgetLayout = new QVBoxLayout();
		innerWidget->setLayout(innerWidgetLayout);

		// set scroll area inner widget
		myScrollArea->setWidget(innerWidget);

		// main layout
		QVBoxLayout* layout = new QVBoxLayout();
		layout->addWidget(btnAdd);
		layout->addWidget(btnClear1);
		layout->addWidget(btnClear2);
		layout->addWidget(myScrollArea);
		setLayout(layout);
	}

private:
	QVBoxLayout* innerWidgetLayout;
	QList<QLabel*> addedLabels;
    
private slots:

	void onBtnAdd()
	{
		QLabel* l = new QLabel("Hello there!");
		innerWidgetLayout->addWidget(l);

		addedLabels.push_back(l);  // I'm keeping a list of added labels (see onBtnClear2)
	}

	void onBtnClear1() // this is from the docs and doesn't work
	{
		QLayoutItem *child;

		while ((child = innerWidgetLayout->takeAt(0)) != 0)
			delete child;

		/*
		innerWidgetLayout->activate();
		innerWidgetLayout->update();
		update();
		*/
	}

	void onBtnClear2() // this works
	{
		for (QList<QLabel*>::iterator it = addedLabels.begin(); it != addedLabels.end(); ++it)
			delete *it;

		addedLabels.clear();
	}
};

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Thank you

[mattc]

I solved changing

QLayoutItem *child;
 while ((child = layout->takeAt(0)) != 0) {
     ...
     delete child;
 }

QLayoutItem *child;
 while ((child = layout->takeAt(0)) != 0) {
     ...
     delete child;
 }

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to

QLayoutItem *child;
 while ((child = layout->takeAt(0)) != 0) {
     ...
     delete child->widget();  // delete Layout Item's underlying widget
 }

QLayoutItem *child;
 while ((child = layout->takeAt(0)) != 0) {
     ...
     delete child->widget();  // delete Layout Item's underlying widget
 }

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