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Here we simply create a plain widget object. The [http://doc.qt.nokia.com/latest/qwidget.html Qt::Widget] class is the base class of all user interface objects. The widget is the atom of the user interface: It receives mouse, keyboard and other events from the window system, and paints a representation of itself on the screen. A widget is clipped by its parent and by the widgets in front of it.‎

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A widget that isn't embedded in a parent widget, like this particular widget, is called a window. Usually, windows have their own window frame and taskbar entry, provided by the window system. A widget without a parent widget is always an independent window. Its initial position on the screen is controlled by the window system.‎

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We set the window's width to 200 pixels and its height to 120 pixels.‎

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quit = Qt::PushButton.new('Quit', window)‎

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A child is born. This [http://doc.qt.nokia.com/latest/qpushbutton.html Qt::PushButton] is created with a parent widget ('''<tt>window</tt>'''). A child widget is always displayed within its parent's area. When displayed, it is clipped by its parent's bounds. By default, it is rooted at the top-left corner of its parent, at position (0, 0).‎

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The [http://doc.qt.nokia.com/latest/qwidget.html#setgeometry Qt::Widget::setGeometry()] function takes four arguments: The first two arguments are the x and y coordinates of the button's top-left corner. The coordinates are relative to the parent widget. The last two arguments are the button's width and height. The result is a button that extends from position (10, 40) to position (190, 80).‎

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When a parent widget is shown, it will call show for all its children (except those that were explicitly hidden using [http://doc.qt.nokia.com/latest/qwidget.html#hide Qt::Widget::hide()]).‎

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=== Running the Application ===‎

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The button no longer fills the entire window. Instead, it stays at position (10, 40) within the window and with a size of (180, 40), because of the [http://doc.qt.nokia.com/latest/qwidget.html#geometry-prop Qt::Widget::setGeometry()] call.‎

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=== Exercises ===‎

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Try resizing the window. How does the button change? What happens to the button's height if you run the program with a bigger font? What happens if you try to make the window really small?‎

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[[Category:Ruby]]‎

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