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

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/*!
\page qdeclarativejavascript.html
\title Integrating JavaScript

QML encourages building UIs declaratively, using \l {Property Binding} and the
composition of existing \l {QML Elements}.  To allow the implementation of more
advanced behavior, QML integrates tightly with imperative JavaScript code.

The JavaScript environment provided by QML is stricter than that in a web browser.
In QML you cannot add, or modify, members of the JavaScript global object.  It
is possible to do this accidentally by using a variable without declaring it.  In
QML this will throw an exception, so all local variables should be explicitly
declared.

In addition to the standard JavaScript properties, the \l {QML Global Object}
includes a number of helper methods that simplify building UIs and interacting
with the QML environment.

\section1 Inline JavaScript

Small JavaScript functions can be written inline with other QML declarations.
These inline functions are added as methods to the QML element that contains
them.

\code
Item {
    function factorial(a) {
        a = parseInt(a);
        if (a <= 0)
            return 1;
        else
            return a * factorial(a - 1);
    }

    MouseArea {
        anchors.fill: parent
        onClicked: console.log(factorial(10))
    }
}
\endcode

As methods, inline functions on the root element in a QML component can be
invoked by callers outside the component.  If this is not desired, the method
can be added to a non-root element or, preferably, written in an external
JavaScript file.

\section1 Separate JavaScript files

Large blocks of JavaScript should be written in separate files. These files
can be imported into QML files using an \c import statement, in the same way
that \l {Modules}{modules} are imported.

For example, the \c {factorial()} method in the above example for \l {Inline JavaScript}
could be moved into an external file named \c factorial.js, and accessed like this:

\code
import "factorial.js" as MathFunctions
Item {
    MouseArea {
        anchors.fill: parent
        onClicked: console.log(MathFunctions.factorial(10))
    }
}
\endcode

Both relative and absolute JavaScript URLs can be imported.  In the case of a
relative URL, the location is resolved relative to the location of the
\l {QML Document} that contains the import.  If the script file is not accessible,
an error will occur.  If the JavaScript needs to be fetched from a network
resource, the QML document has a "Loading"
\l {QDeclarativeComponent::status()}{status} until the script has been
downloaded.

Imported JavaScript files are always qualified using the "as" keyword.  The
qualifier for JavaScript files must be unique, so there is always a one-to-one
mapping between qualifiers and JavaScript files.

\section2 Code-Behind Implementation Files

Most JavaScript files imported into a QML file are stateful, logic implementations
for the QML file importing them.  In these cases, for QML component instances to
behave correctly each instance requires a separate copy of the JavaScript objects
and state.

The default behavior when importing JavaScript files is to provide a unique, isolated
copy for each QML component instance.  The code runs in the same scope as the QML
component instance and consequently can can access and manipulate the objects and
properties declared.

\section2 Stateless JavaScript libraries

Some JavaScript files act more like libraries - they provide a set of stateless
helper functions that take input and compute output, but never manipulate QML
component instances directly.

As it would be wasteful for each QML component instance to have a unique copy of
these libraries, the JavaScript programmer can indicate a particular file is a
stateless library through the use of a pragma, as shown in the following example.

\code
// factorial.js
.pragma library

function factorial(a) {
    a = parseInt(a);
    if (a <= 0)
        return 1;
    else
        return a * factorial(a - 1);
}
\endcode

The pragma declaration must appear before any JavaScript code excluding comments.

As they are shared, stateless library files cannot access QML component instance
objects or properties directly, although QML values can be passed as function
parameters.

\section1 Running JavaScript at Startup

It is occasionally necessary to run some imperative code at application (or
component instance) startup.  While it is tempting to just include the startup
script as \e {global code} in an external script file, this can have severe limitations
as the QML environment may not have been fully established.  For example, some objects
might not have been created or some \l {Property Binding}s may not have been run.
\l {QML JavaScript Restrictions} covers the exact limitations of global script code.

The QML \l Component element provides an \e attached \c onCompleted property that
can be used to trigger the execution of script code at startup after the
QML environment has been completely established. For example:

\code
Rectangle {
    function startupFunction() {
        // ... startup code
    }

    Component.onCompleted: startupFunction();
}
\endcode

Any element in a QML file - including nested elements and nested QML component
instances - can use this attached property.  If there is more than one \c onCompleted()
handler to execute at startup, they are run sequentially in an undefined order.

Likewise, the \l {Component::onDestruction} attached property is triggered on
component destruction.


\section1 Property Assignment vs Property Binding

When working with both QML and JavaScript, it is important to differentiate between
QML \l {Property Binding} and JavaScript value assignment. In QML, a property 
binding is created using the \e {property: value} syntax:

\code
Rectangle {
    width: otherItem.width
}
\endcode

The \c width of the above \l Rectangle is updated whenever \c otherItem.width changes. On the other
hand, take the following JavaScript code snippet, that runs when the \l Rectangle is created:

\code
Rectangle {

    Component.onCompleted: {
        width = otherItem.width;
    }
}
\endcode

The \c width of this \l Rectangle is \e assigned the value of \c otherItem.width using the
\e {property = value} syntax in JavaScript. Unlike the QML \e {property: value} syntax, this
does not invoke QML property binding; the \c rectangle.width property is set to the value
of \c otherItem.width at the time of the assignment and will not be updated if that value
changes.

See \l {Property Binding} for more information.


\section1 QML JavaScript Restrictions

QML executes standard JavaScript code, with the following restrictions:

\list
\o JavaScript code cannot modify the global object.

In QML, the global object is constant - existing properties cannot be modified or
deleted, and no new properties may be created.

Most JavaScript programs do not intentionally modify the global object.  However,
JavaScript's automatic creation of undeclared variables is an implicit modification
of the global object, and is prohibited in QML.

Assuming that the \c a variable does not exist in the scope chain, the following code
is illegal in QML.

\code
// Illegal modification of undeclared variable
a = 1;
for (var ii = 1; ii < 10; ++ii)
    a = a * ii;
console.log("Result: " + a);
\endcode

It can be trivially modified to this legal code.

\code
var a = 1;
for (var ii = 1; ii < 10; ++ii)
    a = a * ii;
console.log("Result: " + a);
\endcode

Any attempt to modify the global object - either implicitly or explicitly - will
cause an exception.  If uncaught, this will result in an warning being printed,
that includes the file and line number of the offending code.

\o Global code is run in a reduced scope

During startup, if a QML file includes an external JavaScript file with "global"
code, it is executed in a scope that contains only the external file itself and
the global object.  That is, it will not have access to the QML objects and
properties it \l {QML Scope}{normally would}.

Global code that only accesses script local variable is permitted.  This is an
example of valid global code.

\code
var colors = [ "red", "blue", "green", "orange", "purple" ];
\endcode

Global code that accesses QML objects will not run correctly.

\code
// Invalid global code - the "rootObject" variable is undefined
var initialPosition = { rootObject.x, rootObject.y }
\endcode

This restriction exists as the QML environment is not yet fully established.
To run code after the environment setup has completed, refer to
\l {Running JavaScript at Startup}.

\o The value of \c this is currently undefined in QML

The value of \c this is undefined in QML.  To refer to any element, provide
an \c id.  For example:

\qml
Item {
    width: 200; height: 100
    function mouseAreaClicked(area) {
        console.log("Clicked in area at: " + area.x + ", " + area.y);
    }
    // This will not work because this is undefined
    MouseArea {
        height: 50; width: 200
        onClicked: mouseAreaClicked(this)
    }
    // This will pass area2 to the function
    MouseArea {
        id: area2
        y: 50; height: 50; width: 200
        onClicked: mouseAreaClicked(area2)
    }
}
\endqml

\endlist

*/