source: trunk/doc/src/declarative/focus.qdoc@ 846

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/*!
\target qmlfocus
\page qdeclarativefocus.html
\title Keyboard Focus in QML

When a key is pressed or released, a key event is generated and delivered to the
focused QML \l Item. To facilitate the construction of reusable components
and to address some of the cases unique to fluid user interfaces, the QML items add a
\e scope based extension to Qt's traditional keyboard focus model.

\tableofcontents

\section1 Key Handling Overview

When the user presses or releases a key, the following occurs:
\list 1
\o Qt receives the key action and generates a key event.
\o If the Qt widget containing the \l QDeclarativeView has focus, the key event
is delivered to it. Otherwise, regular Qt key handling continues.
\o The key event is delivered by the scene to the QML \l Item with
\e {active focus}. If no Item has active focus, the key event is
\l {QEvent::ignore()}{ignored} and regular Qt key handling continues.
\o If the QML Item with active focus accepts the key event, propagation
stops. Otherwise the event is "bubbled up", by recursively passing it to each
Item's parent until either the event is accepted, or the root Item is reached.

If the \c {Rectangle} element in the following example has active focus and the \c A key is pressed,
it will bubble up to its parent. However, pressing the \c B key will bubble up to the root
item and thus subsequently be ignored.

\snippet doc/src/snippets/declarative/focus/rectangle.qml simple key event
\snippet doc/src/snippets/declarative/focus/rectangle.qml simple key event end

\o If the root \l Item is reached, the key event is \l {QEvent::ignore()}{ignored} and regular Qt key handling continues.

\endlist

See also the \l {Keys}{Keys attached property} and \l {KeyNavigation}{KeyNavigation attached property}.

\section1 Querying the Active Focus Item

Whether or not an \l Item has active focus can be queried through the
property \c {Item::activeFocus} property. For example, here we have a \l Text
element whose text is determined by whether or not it has active focus.

\snippet doc/src/snippets/declarative/focus/rectangle.qml active focus

\section1 Acquiring Focus and Focus Scopes

An \l Item requests focus by setting the \c focus property to \c true.

For very simple cases simply setting the \c focus property is sometimes
sufficient. If we run the following example with the \l {QML Viewer}, we see that
the \c {keyHandler} element has active focus and pressing the \c A, \c B,
or \c C keys modifies the text appropriately.

\snippet doc/src/snippets/declarative/focus/basicwidget.qml focus true

\image declarative-qmlfocus1.png

However, were the above example to be used as a reusable or imported component,
this simple use of the \c focus property is no longer sufficient.

To demonstrate, we create two instances of our previously defined component and
set the first one to have focus. The intention is that when the \c A, \c B, or
\c C keys are pressed, the first of the two components receives the event and
responds accordingly.

The code that imports and creates two MyWidget instances:
\snippet doc/src/snippets/declarative/focus/widget.qml window

The MyWidget code:
\snippet doc/src/snippets/declarative/focus/mywidget.qml mywidget

We would like to have the first MyWidget object to have the focus by setting its
\c focus property to \c true. However, by running the code, we can confirm that
the second widget receives the focus.

\image declarative-qmlfocus2.png

Looking at both \c MyWidget and \c window code, the problem is evident - there
are three elements that set the \c focus property set to \c true. The two
MyWidget sets the \c focus to \c true and the \c window component also sets the
focus. Ultimately, only one element can have keyboard focus, and the system has
to decide which element receives the focus. When the second MyWidget is created,
it receives the focus because it is the last element to set its \c focus
property to \c true.

This problem is due to visibility. The \c MyWidget component would like to have
the focus, but it cannot control the focus when it is imported or reused.
Likewise, the \c window component does not have the ability to know if its
imported components are requesting the focus.

To solve this problem, the QML introduces a concept known as a \e {focus scope}.
For existing Qt users, a focus scope is like an automatic focus proxy.
A focus scope is created by declaring the \l FocusScope element.

In the next example, a \l FocusScope element is added to the component, and the
visual result shown.

\snippet doc/src/snippets/declarative/focus/myfocusscopewidget.qml widget in focusscope

\image declarative-qmlfocus3.png


Conceptually \e {focus scopes} are quite simple.
\list
\o Within each focus scope one element may have \c {Item::focus} set to
\c true. If more than one \l Item has the \c focus property set, the
last element to set the \c focus will have the focus and the others are unset,
similar to when there are no focus scopes.
\o When a focus scope receives active focus, the contained element with
\c focus set (if any) also gets the active focus. If this element is
also a \l FocusScope, the proxying behavior continues. Both the
focus scope and the sub-focused item will have \c activeFocus property set.
\endlist

Note that, since the FocusScope element is not a visual element, the properties
of its children need to be exposed to the parent item of the FocusScope. Layouts
and positioning elements will use these visual and styling properties to create
the layout. In our example, the \c Column element cannot display the two widgets
properly because the FocusScope lacks visual properties of its own. The MyWidget
component directly binds to the \c rectangle properties to allow the \c Column
element to create the layout containing the children of the FocusScope.

So far, the example has the second component statically selected. It is trivial
now to extend this component to make it clickable, and add it to the original
application. We still set one of the widgets as focused by default.
Now, clicking either MyClickableWidget gives it focus and the other widget
loses the focus.

The code that imports and creates two MyClickableWidget instances:
\snippet doc/src/snippets/declarative/focus/clickablewidget.qml clickable window

The MyClickableWidget code:
\snippet doc/src/snippets/declarative/focus/myclickablewidget.qml clickable in focusscope

\image declarative-qmlfocus4.png

When a QML \l Item explicitly relinquishes focus (by setting its
\c focus property to \c false while it has active focus), the
system does not automatically select another element to receive focus. That is,
it is possible for there to be no currently active focus.

See the \l{declarative/keyinteraction/focus}{Keyboard Focus example} for a
demonstration of moving keyboard focus between multiple areas using FocusScope
elements.

\section1 Advanced uses of Focus Scopes

Focus scopes allow focus to allocation to be easily partitioned. Several
QML items use it to this effect.

\l ListView, for example, is itself a focus scope. Generally this isn't
noticeable as \l ListView doesn't usually have manually added visual children.
By being a focus scope, \l ListView can focus the current list item without
worrying about how that will effect the rest of the application. This allows the
current item delegate to react to key presses.

This contrived example shows how this works. Pressing the \c Return key will
print the name of the current list item.

\snippet doc/src/snippets/declarative/focus/advancedFocus.qml FocusScope delegate

\image declarative-qmlfocus5.png

While the example is simple, there are a lot going on behind the scenes. Whenever
the current item changes, the \l ListView sets the delegate's \c {Item::focus}
property. As the \l ListView is a focus scope, this doesn't affect the
rest of the application. However, if the \l ListView itself has
active focus this causes the delegate itself to receive active focus.
In this example, the root element of the delegate is also a focus scope,
which in turn gives active focus to the \c {Text} element that actually performs
the work of handling the \c {Return} key.

All of the QML view classes, such as \l PathView and \l GridView, behave
in a similar manner to allow key handling in their respective delegates.
*/