dynamicobjects.qdoc

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/*!
\page qdeclarativedynamicobjects.html
\inqmlmodule QtQuick 2
\ingroup qml-features
\contentspage {QML Features}
\previouspage {Integrating QML Code with Existing Qt UI Code}
\nextpage {Network Transparency}{Loading Resources in QML}
\title Dynamic Object Management in QML

QML provides a number of ways to dynamically create and manage QML objects.
The \l{Loader}, \l{Repeater}, \l{ListView}, \l{GridView} and \l{PathView} elements
all support dynamic object management.  Objects can also be created and managed
from C++, and this is the preferred method for hybrid QML/C++ applications
(see \l{Using QML Bindings in C++ Applications}).

QML also supports the dynamic creation of objects from within JavaScript
code. This is useful if the existing QML elements do not fit the needs of your
application, and there are no C++ components involved.

See the \l {declarative/toys/dynamicscene}{Dynamic Scene example} for a demonstration
of the concepts discussed on this page.


\section1 Creating Objects Dynamically

There are two ways to create objects dynamically from JavaScript. You can either call
\l {QML:Qt::createComponent()}{Qt.createComponent()} to dynamically create
a \l Component object, or use \l{QML:Qt::createQmlObject()}{Qt.createQmlObject()}
to create an item from a string of QML.
Creating a component is better if you have an existing component defined in a \c .qml
file, and you want to dynamically create instances of that component. Otherwise,
creating an item from a string of QML is useful when the item QML itself is generated
at runtime.


\section2 Creating a Component Dynamically

To dynamically load a component defined in a QML file, call the
\l {QML:Qt::createComponent()}{Qt.createComponent()} function on the \l{QML Global Object}.
This function takes the URL of the QML file as its only argument and creates
a \l Component object from this URL.

Once you have a \l Component, you can call its \l {Component::createObject()}{createObject()} method to create an instance of
the component. This function can take one or two arguments:
\list
\o The first is the parent for the new item. Since graphical items will not appear on the scene without a parent, it is 
  recommended that you set the parent this way. However, if you wish to set the parent later you can safely pass \c null to 
  this function. 
\o The second is optional and is a map of property-value items that define initial any property values for the item.
  Property values specified by this argument are applied to the object before its creation is finalized, avoiding
  binding errors that may occur if particular properties must be initialized to enable other property bindings.
  when certain properties have been bound to before they have been set by the code. Additionally, there are small
  performance benefits when compared to defining property values and bindings after the object is created.
\endlist

Here is an example. First there is \c Sprite.qml, which defines a simple QML component:

\snippet doc/src/snippets/declarative/Sprite.qml 0

Our main application file, \c main.qml, imports a \c componentCreation.js JavaScript file
that will create \c Sprite objects:

\snippet doc/src/snippets/declarative/createComponent.qml 0

Here is \c componentCreation.js. Notice it checks whether the component \l{Component::status}{status} is
\c Component.Ready before calling \l {Component::createObject()}{createObject()}
in case the QML file is loaded over a network and thus is not ready immediately.

\snippet doc/src/snippets/declarative/componentCreation.js vars
\codeline
\snippet doc/src/snippets/declarative/componentCreation.js func
\snippet doc/src/snippets/declarative/componentCreation.js remote
\snippet doc/src/snippets/declarative/componentCreation.js func-end
\codeline
\snippet doc/src/snippets/declarative/componentCreation.js finishCreation

If you are certain the QML file to be loaded is a local file, you could omit the \c finishCreation()
function and call \l {Component::createObject()}{createObject()} immediately:

\snippet doc/src/snippets/declarative/componentCreation.js func
\snippet doc/src/snippets/declarative/componentCreation.js local
\snippet doc/src/snippets/declarative/componentCreation.js func-end

Notice in both instances, \l {Component::createObject()}{createObject()} is called with
\c appWindow passed as an argument so that the created object will become a child of the
\c appWindow item in \c main.qml. Otherwise, the new item will not appear in the scene.

When using files with relative paths, the path should
be relative to the file where \l {QML:Qt::createComponent()}{Qt.createComponent()} is executed.

To connect signals to (or receive signals from) dynamically created objects,
use the signal \c connect() method. See
\l{QML Signal and Handler Event System#Connecting Signals to Methods and Signals}
{Connecting Signals to Methods and Signals} for more information.


\section2 Creating an Object from a String of QML

If the QML is not defined until runtime, you can create a QML item from
a string of QML using the \l{QML:Qt::createQmlObject()}{Qt.createQmlObject()} function, as in the following example:

\snippet doc/src/snippets/declarative/createQmlObject.qml 0

The first argument is the string of QML to create. Just like in a new file, you will need to
import any types you wish to use. The second argument is the parent item for the new item;
this should be an existing item in the scene. The third argument is the file path to associate
with the new item; this is used for error reporting.

If the string of QML imports files using relative paths, the path should be relative
to the file in which the parent item (the second argument to the method) is defined.


\section1 Maintaining Dynamically Created Objects

When managing dynamically created items, you must ensure the creation context
outlives the created item. Otherwise, if the creation context is destroyed first,
the bindings in the dynamic item will no longer work.

The actual creation context depends on how an item is created:

\list
\o If \l {QML:Qt::createComponent()}{Qt.createComponent()} is used, the creation context
   is the QDeclarativeContext in which this method is called
\o If \l{QML:Qt::createQmlObject()}{Qt.createQmlObject()}
   if called, the creation context is the context of the parent item passed to this method
\o If a \c {Component{}} item is defined and \l {Component::createObject()}{createObject()}
   is called on that item, the creation context is the context in which the \c Component is defined
\endlist

Also, note that while dynamically created objects may be used the same as other objects, they
do not have an id in QML.


\section1 Deleting Objects Dynamically

In many user interfaces, it is sufficient to set an item's opacity to 0 or
to move the item off the screen instead of deleting the item. If you have
lots of dynamically created items, however, you may receive a worthwhile
performance benefit if unused items are deleted.

Note that you should never manually delete items that were dynamically created
by QML elements (such as \l Loader and \l Repeater). Also, you should avoid deleting
items that you did not dynamically create yourself.

Items can be deleted using the \c destroy() method. This method has an optional
argument (which defaults to 0) that specifies the approximate delay in milliseconds
before the object is to be destroyed.

Here is an example. The \c application.qml creates five instances of the \c SelfDestroyingRect.qml
component. Each instance runs a NumberAnimation, and when the animation has finished, calls
\c destroy() on its root item to destroy itself:

\table
\row
\o \c application.qml
\o \c SelfDestroyingRect.qml

\row
\o \snippet doc/src/snippets/declarative/dynamicObjects-destroy.qml 0
\o \snippet doc/src/snippets/declarative/SelfDestroyingRect.qml 0

\endtable

Alternatively, the \c application.qml could have destroyed the created object
by calling \c object.destroy().

Note that it is safe to call destroy() on an object within that object. Objects are not destroyed the
instant destroy() is called, but are cleaned up sometime between the end of that script block and the next frame
(unless you specified a non-zero delay).

Note also that if a \c SelfDestroyingRect instance was created statically like this:

\qml
Item {
    SelfDestroyingRect {
        // ...
    }
}
\endqml

This would result in an error, since items can only be dynamically
destroyed if they were dynamically created.

Objects created with \l{QML:Qt::createQmlObject()}{Qt.createQmlObject()}
can similarly be destroyed using \c destroy():

\snippet doc/src/snippets/declarative/createQmlObject.qml 0
\snippet doc/src/snippets/declarative/createQmlObject.qml destroy
*/