source: trunk/doc/src/declarative/qdeclarativestates.qdoc@ 860

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/*!
\page qdeclarativestates.html
\target qmlstates
\title QML States

\section1 Overview

User interfaces are designed to present different interface configurations in
different scenarios, or to modify their appearances in response to user
interaction. Often, there are a set of changes that are made concurrently, such
that the interface could be seen to be internally changing from one \e state to
another. 

This applies generally to interface elements regardless of their complexity. 
A photo viewer may initially present images in a grid, and when an image is
clicked, change to a "detailed" state where the individual image is expanded
and the interface is changed to present new options for image editing. On the
other end of the scale, when a simple button is pressed, it may change to a
"pressed" state in which its color and position is modified to give a pressed
appearance.

In QML, any object can change between different \e states to apply sets of
changes that modify the properties of relevant items. Each \e state could 
present a different configuration that could, for example:

\list
\o Show some UI elements and hide others
\o Present different available actions to the user
\o Start, stop or pause animations
\o Execute some script required in the new state
\o Change a property value for a particular item
\o Show a different view or "screen"
\endlist

Changes between states can be animated using \l {Transitions}{transitions}, as
discussed further below.

All \l {Item}-based objects have a \e {default state}, and can specify additional
states by adding new \l State objects to the item's \l {Item::}{states}
property. Each state has a \e name that is unique for all states within that
item; the default state's name is an empty string. To change the current state
of an item, set the \l {Item::}{state} property to the name of the state.

Non-Item objects can use states through the StateGroup element.


\section1 Creating states

To create a state, add a \l State object to the item's \l {Item::}{states} property,
which holds a list of states for that item.

Following is an example. Here, the \l Rectangle is initially placed in the
default (0, 0) position. It has defined an additional state named "moved", in
which a PropertyChanges object repositions the rectangle to (50, 50). Clicking
within the MouseArea changes the state to the "moved" state, thus moving the \l
Rectangle.

\snippet doc/src/snippets/declarative/states.qml 0
 
The \l State item defines all the changes to be made in the new state. It
could specify additional properties to be changed, or create additional 
PropertyChanges for other objects. It can also modify the properties of other 
objects, not just the object that owns the state. For example:

\qml
Rectangle {
    ...
    states: [
        State {
            name: "moved"
            PropertyChanges { target: myRect; x: 50; y: 50; color: "blue" }
            PropertyChanges { target: someOtherItem; width: 1000 }            
        }
    ]
}
\endqml

As a convenience, if an item only has one state, its \l {Item::}{states} 
property can be defined as a single \l State, without the square-brace list
syntax:

\qml
Item {
    ...
    states: State {
        ...
    }
}
\endqml

A \l State is not limited to performing modifications on property values. It 
can also:

\list
\o Run some script using StateChangeScript
\o Override an existing signal handler for an object using PropertyChanges
\o Re-parent an \l Item using ParentChanges
\o Modify anchor values using AnchorChanges
\endlist

The \l {declarative/animation/states}{States and Transitions example} 
demonstrates how to declare a basic set of states and apply animated 
transitions between them.


\section1 The default state

Of course, the \l Rectangle in the example above could have simply been moved
by setting its position to (50, 50) in the mouse area's \c onClicked handler.
However, aside from enabling batched property changes, one of the features of
QML states is the ability of an item to revert to its \e {default state}.
The default state contains all of an item's initial property values before 
they were modified in a state change.

For example, suppose the \l Rectangle should move to (50,50) when the mouse is
pressed, and then move back to its original position when the mouse is 
released. This can be achieved by using the \l {State::}{when} property,
like this:

\qml 
Rectangle {
    ...

    MouseArea {
        id: mouseArea
        anchors.fill: parent
    }

    states: State {
        name: "moved"; when: mouseArea.pressed
        ...
    }
}
\endqml 

The \l {State::}{when} property is set to an expression that evaluates to
\c true when the item should be set to that state. When the mouse is pressed,
the state is changed to \e moved. When it is released, the item reverts to its
\e default state, which defines all of the item's original property values.

Alternatively, an item can be explicitly set to its default state by setting its
\l {Item::}{state} property to an empty string (""). For example, instead of
using the \l {State::}{when} property, the above code could be changed to:

\qml 
Rectangle {
    ...

    MouseArea {
        anchors.fill: parent
        onPressed: myRect.state = 'moved';
        onReleased: myRect.state = '';
    }

    states: State {
        name: "moved"
        ...
    }
}
\endqml 

Obviously it makes sense to use the \l {State::}{when} property when possible
as it provides a simpler (and a better, more declarative) solution than 
assigning the state from signal handlers.


\section1 Animating state changes


State changes can be easily animated through \l {Transitions}{transitions}. A
\l Transition defines the animations that should be applied when an item
changes from one state to another.

If the above example was modified to include the following \l Transition, the
movement of the \l Rectangle would be animated:

\qml
Rectangle {
    ...

    MouseArea { ... }

    states: [
       ...
    ]
     
    transitions: [
        Transition {
            NumberAnimation { properties: "x,y"; duration: 500 }
        }
    ]
 }
\endqml  

This \l Transition defines that if any \c x or \c y properties have changed
during a state change within this item, their values should be animated over 500
milliseconds.

See the \l Transitions documentation for more information.

*/