QLayout that keeps fixed aspect ratio of it's only widget.

fixed_aspect_ratio_qlayout.h

/*
Copyright (c) 2017 Pavel Perina

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/// \file	fixed_aspect_ratio_qlayout.h
/// \brief	Header only implementation of AspectRatioSingleItemLayout

#ifndef ASPECTRATIO_SINGLEITEM_LAYOUT_H
#define ASPECTRATIO_SINGLEITEM_LAYOUT_H

#include <QtWidgets/QLayout>


/// \brief Layout that keeps single item and maintains given aspect ratio
class AspectRatioSingleItemLayout : public QLayout {
public:

	/// \brief Constructor
	AspectRatioSingleItemLayout(QWidget *parent = nullptr, double aspectRatio = 16.0/9.0)
		: QLayout(parent)
		, m_aspectRatio(aspectRatio)
		, m_item(nullptr)
	{
	}
	
	/// \brief Destructor
	/// \warning Let's hope we're responsible for deleting item
	~AspectRatioSingleItemLayout()
	{
		delete m_item;
	}
	
	/// \brief Set aspect ratio to keep
	/// \todo Invalidate geometry
	void setAspectRatio(double aspectRatio)
	{
		m_aspectRatio = aspectRatio;
	}

	/// \brief Return number of items
	int count() const override
	{
		return m_item != nullptr ? 1 : 0;
	}
	
	/// \brief Return item at given index, nullptr if index is out of range
	QLayoutItem *itemAt(int i) const override
	{
		return i == 0 ? m_item : nullptr;
	}
	
	/// \brief Take item at index. Now caller is responsible for deletion, we no longer own it
	QLayoutItem *takeAt(int) override
	{
		QLayoutItem *retval = m_item;
		m_item = nullptr;
		return retval;
	}

	/// \brief Returns directions where we want to expand beyond sizeHint()
	Qt::Orientations expandingDirections() const override
	{
		// we'd like grow beyond sizeHint()
		return  Qt::Horizontal | Qt::Vertical;
	}
	
	/// \brief We want to limit height based on width
	bool hasHeightForWidth() const override
	{
		return false;
	}
	
	/// \brief We want that much height for given \a width
	int heightForWidth(int width) const override
	{
		int height = (width - 2 * margin()) / m_aspectRatio + 2 * margin();
		return height;
	}
	
	/// \brief Set geometry of our only widget
	void setGeometry(const QRect &rect) override
	{
		QLayout::setGeometry(rect);
		if (m_item) {
			QWidget *wdg = m_item->widget();
			int availW = rect.width() - 2 * margin();
			int availH = rect.height() - 2 * margin();
			int w, h;
			h = availH;
			w = h * m_aspectRatio;
			if (w > availW) {
				// fill width
				w = availW;
				h = w / m_aspectRatio;
				int y;
				if (m_item->alignment() & Qt::AlignTop)
					y = margin();
				else if (m_item->alignment() & Qt::AlignBottom)
					y = rect.height() - margin() - h;
				else
					y = margin() + (availH - h) / 2;
				wdg->setGeometry(rect.x() + margin(), rect.y() + y, w, h);
			}
			else {
				int x;
				if (m_item->alignment() & Qt::AlignLeft)
					x = margin();
				else if (m_item->alignment() & Qt::AlignRight)
					x = rect.width() - margin() - w;
				else
					x = margin() + (availW - w) / 2;
				wdg->setGeometry(rect.x() + x, rect.y() + margin(), w, h);
			}
		}
	}
	
	QSize sizeHint() const override
	{
		const int margins = 2 * margin();
		return m_item ? m_item->sizeHint() + QSize(margins, margins) : QSize(margins, margins);
	}

	QSize minimumSize() const override 
	{
		const int margins = 2 * margin();
		return m_item ? m_item->minimumSize() + QSize(margins, margins) : QSize(margins,margins);
	}

	/// \brief Add item, this removes existing from container
	void addItem(QLayoutItem *item) override
	{
		delete m_item;
		m_item = item;
		item->setAlignment(0);
	}

private:

	QLayoutItem *m_item; // at most one :-)

	double m_aspectRatio;

};

#endif // ASPECTRATIO_SINGLEITEM_LAYOUT_H

QLayout that keeps fixed aspect ratio of it's only widget.

This may be useful when you need layouts keeping widgets at fixed aspect ratio frequently. Like images or videos.
I guess it's easier to handle resize event and place widgets manually for a few cases.

It's far from perfect, as of 2017-05-31 it can't handle margins properly for example.

Sample usage:

	assert(m_rendererWidget == nullptr);
	AspectRatioSingleItemLayout *dummyLayout = new AspectRatioSingleItemLayout(m_ui->wdg3d); // create layout as parent of wdg3d, which is used as place holder
	dummyLayout->setContentsMargins(0, 0, 0, 0);
	m_ui->wdg3d->setLayout(dummyLayout); // set layout of wdg3d to newly created
	m_rendererWidget = renderer->createRendererWidget(m_ui->wdg3d); // create renderer widget as parent of wdg3d
	dummyLayout->addWidget(m_rendererWidget); // add widget into layout, it's aspect ratio will be kept
	dummyLayout->setAlignment(m_rendererWidget, Qt::AlignLeft | Qt::AlignTop);

Hi, could you please add a license? I would like to use this layout in a closed-source, non-commercial project and I wonder whether I am allowed to do so. Thanks.

Ok, I've added MIT license.

Here is a Python version:
https://gist.github.com/sjdv1982/75899d10e6983b878f63083e3c47b39b

Hi,

I am using the QWebEngineView for html5 + hybrid application.
I need to apply to keep aspect ratio 9:4 for the QWebEngineView when resize the window.

---

int main(int argc, char *argv[]) {
....
QWebEngineView *view = new QWebEngineView();
view->setUrl(QUrl(QStringLiteral("http://localhost/qttest/index.html")));
view->resize(990, 701);
....
}

How to apply your sample for QWebEngineView??
I tried as like below. But, it dose not applied when I run application and resize window.

AspectRatioSingleItemLayout *dummyLayout = new AspectRatioSingleItemLayout(view);
dummyLayout->setContentsMargins(0, 0, 0, 0);
view->setLayout(dummyLayout);
dummyLayout->addWidget(view);

---

Please help me...
Thanks a lot !

Sorry for belated reply.
You can't have view, add dummyLayout into it and then add view into dummyLayout. You need something like placeholderWidget->aspectRatioLayout->view
placeHolder widget is empty resizeable widget, e.g. from Qt Designer.
view is your widget that will have fixed aspect ratio, e.g. image, live camera view, ...

I've added image for clarification.

PS: don't forget this: setAspectRatio ((double) width / height).

Thank you @pavel-perina! I spent so much time trying to fix the aspect ratio of a video and finally this is something that works.

@pavel-perina Can it be used in QGridLayout? Look at the effect in this video I recorded. It should be customized Layout
https://www.youtube.com/watch?v=PcZdCChcwPQ

It looks like proportional scaling, in a Layout