The UI file format used by Qt Designer is described by the XML schema presented below, which we include for your convenience. Be aware that the format may change in future Qt releases.
<?xml version="1.0" encoding="utf-8" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="ui" type="UI"/>
<xs:complexType name="UI">
<xs:all>
<xs:element name="author" type="xs:string" minOccurs="0" />
<xs:element name="comment" type="xs:string" minOccurs="0" />
<xs:element name="exportmacro" type="xs:string" minOccurs="0" />
<xs:element name="class" type="xs:string" />
<xs:element name="widget" type="Widget" />
<xs:element name="layoutdefault" type="LayoutDefault" minOccurs="0" />
<xs:element name="layoutfunction" type="LayoutFunction" minOccurs="0" />
<xs:element name="pixmapfunction" type="xs:string" minOccurs="0" />
<xs:element name="customwidgets" type="CustomWidgets" minOccurs="0" />
<xs:element name="tabstops" type="TabStops" minOccurs="0" />
<xs:element name="images" use="deprecated" />
<xs:element name="includes" type="Includes" minOccurs="0" />
<xs:element name="resources" type="Resources" minOccurs="0" />
<xs:element name="connections" type="Connections" minOccurs="0" />
<xs:element name="designerdata" type="DesignerData" minOccurs="0" />
<xs:element name="slots" type="Slots" minOccurs="0" />
<xs:element name="buttongroups" type="ButtonGroups" minOccurs="0" />
</xs:all>
<xs:attribute name="version" type="xs:string" />
<xs:attribute name="language" type="xs:string" />
<xs:attribute name="displayname" type="xs:string" />
<xs:attribute name="idbasedtr" type="xs:boolean" />
<xs:attribute name="connectslotsbyname" type="xs:boolean" />
<!-- Legacy attribute generated by uic3 -->
<xs:attribute name="stdsetdef" type="xs:integer" />
<!-- Legacy attribute generated by the VS integration -->
<xs:attribute name="stdSetDef" type="xs:integer" />
</xs:complexType>
<!-- include begin -->
<xs:complexType name="Includes">
<xs:sequence>
<xs:element name="include" type="Include" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="Include" mixed="true" >
<xs:attribute name="location" type="xs:string" />
<xs:attribute name="impldecl" type="xs:string" />
</xs:complexType>
<!-- include end -->
<!-- resource begin -->
<xs:complexType name="Resources">
<xs:sequence>
<xs:element name="include" type="Resource" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="name" type="xs:string" />
</xs:complexType>
<xs:complexType name="Resource">
<xs:attribute name="location" type="xs:string" />
</xs:complexType>
<!-- resource end -->
<!-- actions begin -->
<xs:complexType name="ActionGroup">
<xs:sequence maxOccurs="unbounded">
<xs:element name="action" type="Action" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="actiongroup" type="ActionGroup" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="property" type="Property" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="attribute" type="Property" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="name" type="xs:string" />
</xs:complexType>
<xs:complexType name="Action">
<xs:sequence maxOccurs="unbounded" >
<xs:element name="property" type="Property" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="attribute" type="Property" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="name" type="xs:string" />
<xs:attribute name="menu" type="xs:string" />
</xs:complexType>
<xs:complexType name="ActionRef">
<xs:attribute name="name" type="xs:string" />
</xs:complexType>
<!-- actions end -->
<!-- button groups begin -->
<xs:complexType name="ButtonGroup">
<xs:sequence maxOccurs="unbounded">
<xs:element name="property" type="Property" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="attribute" type="Property" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="name" type="xs:string" />
</xs:complexType>
<xs:complexType name="ButtonGroups">
<xs:sequence>
<xs:element name="buttongroup" type="ButtonGroup" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<!-- button groups end -->
<!-- custom widget support begin -->
<xs:complexType name="CustomWidgets">
<xs:sequence>
<xs:element name="customwidget" type="CustomWidget" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="Header" mixed="true" >
<xs:attribute name="location" type="xs:string" /> <!-- local or global -->
</xs:complexType>
<xs:complexType name="CustomWidget">
<xs:all>
<xs:element name="class" type="xs:string" />
<xs:element name="extends" type="xs:string" />
<xs:element name="header" type="Header" />
<xs:element name="sizehint" type="Size" minOccurs="0" />
<xs:element name="addpagemethod" type="xs:string" minOccurs="0" />
<xs:element name="container" type="xs:integer" minOccurs="0" />
<xs:element name="sizepolicy" use="deprecated" />
<xs:element name="pixmap" type="xs:string" minOccurs="0" />
<xs:element name="script" use="deprecated" />
<xs:element name="properties" use="deprecated" />
<xs:element name="slots" type="Slots" minOccurs="0" />
<xs:element name="propertyspecifications" type="PropertySpecifications" minOccurs="0" />
</xs:all>
</xs:complexType>
<!-- custom widget support end -->
<xs:complexType name="LayoutDefault">
<xs:attribute name="spacing" type="xs:integer" />
<xs:attribute name="margin" type="xs:integer" />
</xs:complexType>
<xs:complexType name="LayoutFunction">
<xs:attribute name="spacing" type="xs:string" />
<xs:attribute name="margin" type="xs:string" />
</xs:complexType>
<xs:complexType name="TabStops">
<xs:sequence>
<xs:element name="tabstop" type="xs:string" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="Layout">
<xs:sequence maxOccurs="unbounded" >
<xs:element name="property" type="Property" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="attribute" type="Property" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="item" type="LayoutItem" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="class" type="xs:string" />
<xs:attribute name="name" type="xs:string" />
<xs:attribute name="stretch" type="xs:string" />
<xs:attribute name="rowstretch" type="xs:string" />
<xs:attribute name="columnstretch" type="xs:string" />
<xs:attribute name="rowminimumheight" type="xs:string" />
<xs:attribute name="columnminimumwidth" type="xs:string" />
</xs:complexType>
<xs:complexType name="LayoutItem">
<xs:choice>
<xs:element name="widget" type="Widget" />
<xs:element name="layout" type="Layout" />
<xs:element name="spacer" type="Spacer" />
</xs:choice>
<xs:attribute name="row" type="xs:integer" />
<xs:attribute name="column" type="xs:integer" />
<xs:attribute name="rowspan" type="xs:integer" />
<xs:attribute name="colspan" type="xs:integer" />
<xs:attribute name="alignment" type="xs:string" />
</xs:complexType>
<!-- item view begin -->
<xs:complexType name="Row">
<xs:sequence>
<xs:element name="property" type="Property" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="Column">
<xs:sequence>
<xs:element name="property" type="Property" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="Item">
<xs:sequence maxOccurs="unbounded">
<xs:element name="property" type="Property" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="item" type="Item" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="row" type="xs:integer" />
<xs:attribute name="column" type="xs:integer" />
</xs:complexType>
<!-- item view end -->
<xs:complexType name="Widget">
<!-- Order does not matter (cannot use xs:all here
because of unbounded row, column, item -->
<xs:sequence>
<xs:element name="class" type="xs:string" minOccurs="0" maxOccurs="unbounded" /> <!-- unbounded kept for compatibility reasons -->
<xs:element name="property" type="Property" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="script" use="deprecated" />
<xs:element name="widgetdata" use="deprecated" />
<xs:element name="attribute" type="Property" minOccurs="0" maxOccurs="unbounded" />
<!-- item view begin -->
<xs:element name="row" type="Row" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="column" type="Column" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="item" type="Item" minOccurs="0" maxOccurs="unbounded" />
<!-- item view end -->
<xs:element name="layout" type="Layout" minOccurs="0" maxOccurs="unbounded" /> <!-- unbounded kept for compatibility reasons -->
<xs:element name="widget" type="Widget" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="action" type="Action" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="actiongroup" type="ActionGroup" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="addaction" type="ActionRef" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="zorder" type="xs:string" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="class" type="xs:string" />
<xs:attribute name="name" type="xs:string" />
<xs:attribute name="native" type="xs:boolean" />
</xs:complexType>
<xs:complexType name="Spacer">
<xs:sequence>
<xs:element name="property" type="Property" maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="name" type="xs:string" />
</xs:complexType>
<xs:complexType name="Color">
<xs:all>
<xs:element name="red" type="xs:integer" />
<xs:element name="green" type="xs:integer" />
<xs:element name="blue" type="xs:integer" />
</xs:all>
<xs:attribute name="alpha" type="xs:integer" />
</xs:complexType>
<xs:complexType name="GradientStop">
<xs:sequence>
<xs:element name="color" type="Color" />
</xs:sequence>
<xs:attribute name="position" type="xs:double" />
</xs:complexType>
<xs:complexType name="Gradient">
<xs:sequence>
<xs:element name="gradientStop" type="GradientStop" maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="startx" type="xs:double" />
<xs:attribute name="starty" type="xs:double" />
<xs:attribute name="endx" type="xs:double" />
<xs:attribute name="endy" type="xs:double" />
<xs:attribute name="centralx" type="xs:double" />
<xs:attribute name="centraly" type="xs:double" />
<xs:attribute name="focalx" type="xs:double" />
<xs:attribute name="focaly" type="xs:double" />
<xs:attribute name="radius" type="xs:double" />
<xs:attribute name="angle" type="xs:double" />
<xs:attribute name="type" type="xs:string" />
<xs:attribute name="spread" type="xs:string" />
<xs:attribute name="coordinatemode" type="xs:string" />
</xs:complexType>
<xs:complexType name="Brush">
<xs:choice>
<xs:element name="color" type="Color" />
<xs:element name="texture" type="Property" />
<xs:element name="gradient" type="Gradient" />
</xs:choice>
<xs:attribute name="brushstyle" type="xs:string" />
</xs:complexType>
<xs:complexType name="ColorRole">
<xs:sequence>
<xs:element name="brush" type="Brush" />
</xs:sequence>
<xs:attribute name="role" type="xs:string" />
</xs:complexType>
<xs:complexType name="ColorGroup">
<xs:sequence maxOccurs="unbounded">
<xs:element name="colorrole" type="ColorRole" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="color" type="Color" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="Palette">
<xs:all>
<xs:element name="active" type="ColorGroup" />
<xs:element name="inactive" type="ColorGroup" />
<xs:element name="disabled" type="ColorGroup" />
</xs:all>
</xs:complexType>
<xs:complexType name="Font">
<xs:all>
<xs:element name="family" type="xs:string" minOccurs="0" />
<xs:element name="pointsize" type="xs:integer" minOccurs="0" />
<xs:element name="weight" type="xs:integer" minOccurs="0" />
<xs:element name="italic" type="xs:boolean" minOccurs="0" />
<xs:element name="bold" type="xs:boolean" minOccurs="0" />
<xs:element name="underline" type="xs:boolean" minOccurs="0" />
<xs:element name="strikeout" type="xs:boolean" minOccurs="0" />
<xs:element name="antialiasing" type="xs:boolean" minOccurs="0" />
<xs:element name="stylestrategy" type="xs:string" minOccurs="0" />
<xs:element name="kerning" type="xs:boolean" minOccurs="0" />
</xs:all>
</xs:complexType>
<xs:complexType name="Point">
<xs:all>
<xs:element name="x" type="xs:integer" />
<xs:element name="y" type="xs:integer" />
</xs:all>
</xs:complexType>
<xs:complexType name="Rect">
<xs:all>
<xs:element name="x" type="xs:integer" />
<xs:element name="y" type="xs:integer" />
<xs:element name="width" type="xs:integer" />
<xs:element name="height" type="xs:integer" />
</xs:all>
</xs:complexType>
<xs:complexType name="Locale">
<xs:attribute name="language" type="xs:string" />
<xs:attribute name="country" type="xs:string" />
</xs:complexType>
<xs:complexType name="SizePolicy">
<xs:all>
<xs:element name="hsizetype" type="xs:integer" minOccurs="0" />
<xs:element name="vsizetype" type="xs:integer" minOccurs="0" />
<xs:element name="horstretch" type="xs:integer" />
<xs:element name="verstretch" type="xs:integer" />
</xs:all>
<xs:attribute name="hsizetype" type="xs:string" />
<xs:attribute name="vsizetype" type="xs:string" />
</xs:complexType>
<xs:complexType name="Size">
<xs:all>
<xs:element name="width" type="xs:integer" />
<xs:element name="height" type="xs:integer" />
</xs:all>
</xs:complexType>
<xs:complexType name="Date">
<xs:all>
<xs:element name="year" type="xs:integer" />
<xs:element name="month" type="xs:integer" />
<xs:element name="day" type="xs:integer" />
</xs:all>
</xs:complexType>
<xs:complexType name="Time">
<xs:all>
<xs:element name="hour" type="xs:integer" />
<xs:element name="minute" type="xs:integer" />
<xs:element name="second" type="xs:integer" />
</xs:all>
</xs:complexType>
<xs:complexType name="DateTime">
<xs:all>
<xs:element name="hour" type="xs:integer" />
<xs:element name="minute" type="xs:integer" />
<xs:element name="second" type="xs:integer" />
<xs:element name="year" type="xs:integer" />
<xs:element name="month" type="xs:integer" />
<xs:element name="day" type="xs:integer" />
</xs:all>
</xs:complexType>
<xs:complexType name="StringList">
<xs:attribute name="notr" type="xs:string" />
<xs:attribute name="comment" type="xs:string" />
<xs:attribute name="extracomment" type="xs:string" />
<xs:attribute name="id" type="xs:string" />
<xs:sequence>
<xs:element name="string" type="xs:string" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="ResourcePixmap" mixed="true">
<xs:attribute name="resource" type="xs:string" />
<xs:attribute name="alias" type="xs:string" />
</xs:complexType>
<xs:complexType name="ResourceIcon" mixed="true">
<xs:all>
<xs:element name="normaloff" type="ResourcePixmap" minOccurs="0" />
<xs:element name="normalon" type="ResourcePixmap" minOccurs="0" />
<xs:element name="disabledoff" type="ResourcePixmap" minOccurs="0" />
<xs:element name="disabledon" type="ResourcePixmap" minOccurs="0" />
<xs:element name="activeoff" type="ResourcePixmap" minOccurs="0" />
<xs:element name="activeon" type="ResourcePixmap" minOccurs="0" />
<xs:element name="selectedoff" type="ResourcePixmap" minOccurs="0" />
<xs:element name="selectedon" type="ResourcePixmap" minOccurs="0" />
</xs:all>
<xs:attribute name="theme" type="xs:string" />
<xs:attribute name="resource" type="xs:string" /> <!-- pre 4.4 legacy support -->
</xs:complexType>
<xs:complexType name="String" mixed="true">
<xs:attribute name="notr" type="xs:string" />
<xs:attribute name="comment" type="xs:string" />
<xs:attribute name="extracomment" type="xs:string" />
<xs:attribute name="id" type="xs:string" />
</xs:complexType>
<xs:complexType name="PointF">
<xs:all>
<xs:element name="x" type="xs:double" />
<xs:element name="y" type="xs:double" />
</xs:all>
</xs:complexType>
<xs:complexType name="RectF">
<xs:all>
<xs:element name="x" type="xs:double" />
<xs:element name="y" type="xs:double" />
<xs:element name="width" type="xs:double" />
<xs:element name="height" type="xs:double" />
</xs:all>
</xs:complexType>
<xs:complexType name="SizeF">
<xs:all>
<xs:element name="width" type="xs:double" />
<xs:element name="height" type="xs:double" />
</xs:all>
</xs:complexType>
<xs:complexType name="Char">
<xs:sequence>
<xs:element name="unicode" type="xs:integer" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="Url">
<xs:sequence>
<xs:element name="string" type="String" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="Property">
<xs:choice>
<xs:element name="bool" type="xs:string" />
<xs:element name="color" type="Color" />
<xs:element name="cstring" type="xs:string" />
<xs:element name="cursor" type="xs:integer" />
<xs:element name="cursorshape" type="xs:string" />
<xs:element name="enum" type="xs:string" />
<xs:element name="font" type ="Font" />
<xs:element name="iconset" type="ResourceIcon"/>
<xs:element name="pixmap" type="ResourcePixmap" />
<xs:element name="palette" type="Palette" />
<xs:element name="point" type="Point" />
<xs:element name="rect" type="Rect" />
<xs:element name="set" type="xs:string" />
<xs:element name="locale" type="Locale" />
<xs:element name="sizepolicy" type="SizePolicy" />
<xs:element name="size" type="Size" />
<xs:element name="string" type="String" />
<xs:element name="stringlist" type="StringList" />
<xs:element name="number" type="xs:integer" />
<xs:element name="float" type="xs:float" />
<xs:element name="double" type="xs:double" />
<xs:element name="date" type="Date" />
<xs:element name="time" type="Time" />
<xs:element name="datetime" type="DateTime" />
<xs:element name="pointf" type="PointF" />
<xs:element name="rectf" type="RectF" />
<xs:element name="sizef" type="SizeF" />
<xs:element name="longlong" type="xs:long" />
<xs:element name="char" type="Char" />
<xs:element name="url" type="Url" />
<xs:element name="uint" type="xs:unsignedInt" />
<xs:element name="ulonglong" type="xs:unsignedLong" />
<xs:element name="brush" type="Brush" />
</xs:choice>
<xs:attribute name="name" type="xs:string" />
<xs:attribute name="stdset" type="xs:integer" />
</xs:complexType>
<xs:complexType name="Connections">
<xs:sequence>
<xs:element name="connection" type="Connection" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="Connection">
<xs:all>
<xs:element name="sender" type="xs:string" />
<xs:element name="signal" type="xs:string" />
<xs:element name="receiver" type="xs:string" />
<xs:element name="slot" type="xs:string" />
<xs:element name="hints" type="ConnectionHints" minOccurs="0" />
</xs:all>
</xs:complexType>
<xs:complexType name="ConnectionHints">
<xs:sequence>
<xs:element name="hint" type="ConnectionHint" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="ConnectionHint">
<xs:all>
<xs:element name="x" type="xs:integer"/>
<xs:element name="y" type="xs:integer"/>
</xs:all>
<xs:attribute name="type" type="xs:string" />
</xs:complexType>
<xs:complexType name="DesignerData">
<xs:sequence>
<xs:element name="property" type="Property" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="Slots">
<xs:sequence maxOccurs="unbounded">
<xs:element name="signal" type="xs:string" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="slot" type="xs:string" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="PropertySpecifications">
<xs:sequence maxOccurs="unbounded">
<xs:element name="tooltip" type="PropertyToolTip" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="stringpropertyspecification" type="StringPropertySpecification" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="PropertyToolTip">
<xs:attribute name="name" type="xs:string" />
</xs:complexType>
<xs:complexType name="StringPropertySpecification">
<xs:attribute name="name" type="xs:string" />
<xs:attribute name="type" type="xs:string" />
<xs:attribute name="notr" type="xs:string" />
</xs:complexType>
</xs:schema>
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Licensed under the GNU Free Documentation License, Version 1.3.
https://doc.qt.io/qt-5.15/designer-ui-file-format.html