path: root/doc/src/tutorials/declarative.qdoc

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/*!
    \page tutorials-declarative-contacts.html
    \startpage {index.html}{Qt Reference Documentation}
    \nextpage {tutorials/declarative/contacts/part1}{Chapter 1}

    \title Declarative UI Tutorial
    \ingroup howto
    \ingroup tutorials
    \brief An introduction to using Qt Declarative UI to put together a
    simple animated application.

    \omit 
    At the time of writing the tutorial Declarative UI was still under
    development.  It is extremely likely that an update will be required
    prior to 4.6 release.
    \endomit

    This tutorial gives an introduction to using the Qt Declarative UI
    animation framework.

    In this process we will learn about some of the basics of using
    Declarative UI, such as

    \list
    \o Basic drawing
    \o States and Transitions
    \o Reuse of components
    \o Models and Views
    \endlist

    An existing knowledge of Qt is not required.

    The tutorial's source code is located in Qt's
    \c examples/declarative/tutorials/contacts directory.
    It is split up into a number of sub directories, and within each
    sub directory the files are numbered in an order of increasing features.

    The code in this example is not compiled, but interpreted at run time.
    This means you should use the duiviewer application provided with
    Qt to run the examples.

    \list 1
    \o \l{tutorials/declarative/contacts/part1}{Drawing and Animation}
    \o \l{tutorials/declarative/contacts/part2}{Reuse of QML components}
    \o \l{tutorials/declarative/contacts/part3}{Models, Views and Delegates}
    \o \l{tutorials/declarative/contacts/part4}{Other Tricks}
    \endlist
*/

/*!
    \page tutorials-declarative-contacts-part1.html
    \contentspage {Declarative UI Tutorial}{Contents}
    \nextpage {tutorials/declarative/contacts/part2}{Chapter 2}
    \example tutorials/declarative/contacts/part1
    \title Drawing and Animation
    \tableofcontents

    The first part of this tutorial covers basic drawing of elements on the
    screen and causing them to animate.  The file 1_Drawing_and_Animation.qml
    loads and displays each of the five stages of this tutorial in a single
    window.  For now you don't need to worry about the contents of
    1_Drawing_and_Animation.qml.

    \section1 Drawing

    In this first chapter we will build a button that indicates something
    can be removed and asks for confirmation.  When clicked it will expand
    from a small button with a trash can icon, to a wide button with a
    confirm icon on the left, the text "Remove" in the middle, and a
    cancel icon on the right.

    \image declarative-removebutton.png

    Because Declarative UI is declarative, you don't pass instructions on
    what to paint in a sequential manner as you may be used to.  Instead
    elements and how they appear on the screen are declared in much the
    same was as elements on a web page are declared.

    We will start by drawing a simple red rectangle with rounded corners.

    \image declarative-roundrect.png

    \code
    <Rect id="removeButton"
        width="30" height="30"
        color="red"
        radius="5"/>
    \endcode

    This is the simplest of QML components.  It describes a rectangle with
    some simple properties.  In QML all components start with a capital
    letter, and their properties with lower case letters.  Properties
    can either be declared as XML attributes or as children of the 
    component element.  The above rectangle could equally be written
    
    \code
    <Rect id="removeButton" color="red">
        <width>30</width>
        <height>30</height>
        <radius>5</radius>
    </Rect>
    \endcode

    The rectangle component is one of the more simple QML components.  Apart
    from the properties all QML components share, it has the properties

    \list
    \o color - The background color of the rectangle
    \o tintColor - The overlay color of the rectangle
    \o gradientColor - The color at the base of the rectangle to blend upwards
    \o pen - The description of how to draw the border of the rectangle
    \o radius - The corner radius used to draw rounded rectangles.
    \endlist

    \omit
    For more information on the Rect element, see: TODO
    \endomit

    There are also a number of properties all QML components share.  To see
    a full description of the base QML item, see {QFxItem}.  The rectangle
    drawn in the above code uses the properties;

    \list
    \o id - An identifier of the component
    \o width - the width of the component when drawn
    \o height - the height of the component when drawn
    \endlist

    All items have properties to handle their position on the screen, size,
    clipping, rotation, scale and layout in regards to other elements.  In
    the current example width and height refer to how large to draw the
    rectangle.  The identifier allows other components to refer to the
    identified component.

    Another important property of a component is its children.  All components
    have a list of children.  When drawing, first any components earlier
    siblings are drawn, then the component, then any of the components children.

    \section1 Layout

    The next step of the tutorial adds an image over the rectangle.

    \image declarative-removebutton-close.png

    \code
    <Rect id="removeButton"
        width="30" height="30"
            color="red"
        radius="5">
        <Image id="trashIcon"
            width="22" height="22"
            anchors.right="{parent.right}" anchors.rightMargin="4"
            anchors.verticalCenter="{parent.verticalCenter}"
            src="../shared/pics/trash.png"/>
    </Rect>
    \endcode

    The trashIcon image is added as a child of the Rectangle.  In this case
    the <children> tag isn't used because the default property of the
    Rect component is its children.  Some elements don't often have children
    and use some other default component, when this is the case its possible
    to explicitly list the sub component as a child as follows:

    \code
    <Rect id="removeButton"
        width="30" height="30"
            color="red"
        radius="5">
        <children>
            <Image id="trashIcon"
                width="22" height="22"
                anchors.right="{parent.right}" anchors.rightMargin="4"
                anchors.verticalCenter="{parent.verticalCenter}"
                src="../shared/pics/trash.png"/>
        </children>
    </Rect>
    \endcode

    The Image element allows loading an image file for display.  The source
    specified is a URL, and in this case refers to a portable network graphics
    file in a relative directory to where the QML file was loaded from.

    Also new in this code is the use of anchors.  In QML components can either
    have their position and size specified explicitly using x, y, width
    and height, or they can instead specify the size and position in relation
    to elements either parent or sibling elements.  The Image component uses
    a combination of both styles.  It has a fixed size, but specifies its
    position to align to the right of its parent and for its vertical center
    to align with the vertical center of its parent.  The braces "{}" are
    used to indicate that the value is not a static value, but instead a
    binding to an expression.  In this case it binds to the parent
    element, which is a special identifier that always refers to the
    parent component of a component.  The removeButton identifier can
    be used interchangeably with parent in this case, however it must
    always be a parent or sibling.  Because of this its most common to
    use the parent identifier as it makes later refactoring of code easier.

    Anchors are most useful when the size of items might change based on
    the component state or contents.

    \omit
    See TODO for full list of anchor properties.
    \endomit

    At this point the initial state of the RemoveButton is complete.  A small
    rounded rectangle with a trash icon.  The component also needs a
    description of its open state:

    \image declarative-removebutton-open.png

    This is a wider rectangle with two images and some text.  The code to
    draw this state of the button could be written as follows:

    \code
    <Rect id="removeButton"
        width="230" height="30"
        color="red"
        radius="5">
        <Image id="cancelIcon"
            width="22" height="22"
            anchors.right="{parent.right}" anchors.rightMargin="4"
            anchors.verticalCenter="{parent.verticalCenter}"
            src="../shared/pics/cancel.png"/>
        <Image id="confirmIcon"
            width="22" height="22"
            anchors.left="{parent.left}" anchors.leftMargin="4"
            anchors.verticalCenter="{parent.verticalCenter}"
            src="../shared/pics/ok.png"/>
        <Text id="text"
            anchors.verticalCenter="{parent.verticalCenter}"
            anchors.left="{confirmIcon.right}" anchors.leftMargin="4"
            anchors.right="{cancelIcon.left}" anchors.rightMargin="4"
            font.bold="true"
            color="white"
            hAlign="AlignHCenter"
            text="Remove"/>
    </Rect>
    \endcode

    The rectangle with is now wider by 200 pixels.  Also the trashIcon has
    been replaced with the confirm state children.  Normally we wouldn't
    remove the trashIcon when developing an alternate state of the RemoveButton,
    however since this is a tutorial its been done so that its easier to
    understand the alternate state we are aiming for and how it relates to
    transitioning between states.

    We also introduce the Text element, which is used to display read only
    text.  \omit see {Text} for more information on the text element \endomit
    Because we want text to fill the space between the icons, rather than
    a fixed with the left and right anchors are specified instead.  This
    means as the parent removeButton gets wider, so will the text component.
    It also means that if we animate a width change on the removeButton,
    any bindings, that is the values specified by a braced expression such as
    "{parent.left}" will be evaluated and animated as well.

    \section1 Defining States

    When designing a component with multiple states, it should be developed
    with the initial state and the changes that would be made specified
    as an additional state.  Its not possible to add new children to an
    element when changing state, only changing the properties of existing
    children.  This means that all possible child components should be included
    in the initial state, and those that should not be visible in the initial
    state should have their opacity set to zero.  Thus
    for the RemoveButton we specify the starting size of the removeButton
    and hide any items that should not initially be visible.

    The code snippet below shows what the start of the duel state specification
    might look like.

    \code
    <Rect id="removeButton"
        width="30" height="30"
            color="red"
        radius="5">
        <Image id="trashIcon"
            width="22" height="22"
            anchors.right="{parent.right}" anchors.rightMargin="4"
            anchors.verticalCenter="{parent.verticalCenter}"
            src="../shared/pics/trash.png"/>
        <Image id="cancelIcon"
            width="22" height="22"
            anchors.right="{parent.right}" anchors.rightMargin="4"
            anchors.verticalCenter="{parent.verticalCenter}"
            src="../shared/pics/cancel.png"
            opacity="0"/>
    \endcode

    The code above includes components from both states of the RemoveButton,
    but by setting opacity="0" for the cancelIcon it means that the
    components of the second state won't be drawn yet.
    The base state of a component always has an empty name, however new
    states can be added that describe how a component and its children
    should be changed.  For the RemoveButton there is only one non-base state
    required.  In this tutorial we will name it the 'opened' state.

    \code
    <states>
        <State name="opened">
            <SetProperty target="{removeButton}" property="width" value="230"/>
            <SetProperty target="{text}" property="opacity" value="1"/>
            <SetProperty target="{confirmIcon}" property="opacity" value="1"/>
            <SetProperty target="{cancelIcon}" property="opacity" value="1"/>
            <SetProperty target="{trashIcon}" property="opacity" value="0"/>
        </State>
    </states>
    \endcode

    In the opened state the width of the button itself changes from the base
    width of 30 to the new width of 230.  Also the opacity of the children
    are changed so that the trash icon is now hidden and the other elements
    are now visible.

    \section1 Changing States

    To trigger the change we will react to the 'clicked' signal of a
    MouseRegion component.

    \code
    <Image id="trashIcon"
        width="22" height="22"
        anchors.right="{parent.right}" anchors.rightMargin="4"
        anchors.verticalCenter="{parent.verticalCenter}"
        src="../shared/pics/trash.png"
        opacity="1">
        <MouseRegion
            anchors.fill="{parent}"
            onClicked="toggle()"/>
    </Image>
    \endcode

    MouseRegion components handle mouse actions within their geometry.  This
    geometry behaves the same way as painted components, such that children
    cover their parents and later siblings will cover earlier siblings and
    all the children of the earlier sibling, should they overlap.

    When a component has a signal, such as clicked, the action for the signal
    can be specified using on<SignalName>, as is done above.  In this
    case when the clicked signal is emitted by the MouseRegion component,
    a function called toggle() is called.  It might also have been written

    \code
    onClicked="removeButton.state='opened'"
    \endcode

    However in this case we are using a function because it allows multiple
    mouse regions to use the same functionality, and also makes it
    easier to specify complex behavior in response to a signal.

    The toggle() function is a new function specified as part of the remove
    button element.

    \code
    <resources>
        <Script>
            function toggle() {
                print('removeButton.toggle()');
                if (removeButton.state == 'opened') {
                    removeButton.state = '';
                } else {
                    removeButton.state = 'opened';
                }
            }
        </Script>
    </resources>
    \endcode

    Any QML component can have a set of resources specified.  One of those
    resources is any Script that might be needed.   See the
    {QtScript Module}{QtScript Module} for more information on how to write
    script code in Qt.  There are only a couple of additional items to
    note when using Script with QML components.  The first is that it
    is an xml file, that means either CDATA or other forms of escaping
    should be used if special characters are needed.  For example, 
    the expression;

    \code
    if (a && b) {}
    \endcode

    Should either be escaped as:

    \code
    if (a &amp;&amp; b) {}
    \endcode

    or enclosed in a CDATA section as

    \code
    <![CDATA[if (a && b) {}]]>
    \endcode

    The other item to note is that you can refer to identified QML components
    within the script.  Hence the function for our RemoveButton will check
    if the state is already open to determine what the new state should be.

    We also have added a print function.  This isn't required for the button
    to function, but is useful for tracking down possible bugs when
    working with QML.

    See the file RemoveButton4.qml for the full multi-state specification.

    \section1 Animation

    Currently the RemoveButton is function, but snaps between our two states.
    Fortunately making the transition between states smooth is very simple.
    We only need one more bit of code at the end of our removeButton component.

    \code
    <transitions>
        <Transition fromState="*" toState="opened" reversible="true">
            <NumericAnimation properties="opacity,x,width" duration="200"/>
        </Transition>
    </transitions>
    \endcode

    All QML components have a transitions property.  This describes how
    properties of items within the component should change.  In this case
    we specify that if the x, width or opacity of the removeButton or its
    children change due to a change in state, that they should take 200ms
    to complete their transition.

    \omit
        TODO More on types of animation
    \endomit

    In the next chapter we will show how we can use the remove button in
    other QML components.
*/