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Integrating JavaScriptQML encourages building UIs declaratively, using Property Binding and the composition of existing 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 QML Global Object includes a number of helper methods that simplify building UIs and interacting with the QML environment. Inline JavaScriptSmall JavaScript functions can be written inline with other QML declarations. These inline functions are added as methods to the QML element that contains them. 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))
}
}
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. Separate JavaScript filesLarge blocks of JavaScript should be written in separate files. These files can be imported into QML files using an import statement, in the same way that modules are imported. For example, the factorial() method in the above example for Inline JavaScript could be moved into an external file named factorial.js, and accessed like this: import "factorial.js" as MathFunctions Item { MouseArea { anchors.fill: parent onClicked: console.log(MathFunctions.factorial(10)) } } 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 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 component's status is set to "Loading" 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. (This also means qualifiers cannot be named the same as built-in JavaScript objects such as Date and Math). Code-Behind Implementation FilesMost 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. Stateless JavaScript librariesSome 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. // factorial.js .pragma library function factorial(a) { a = parseInt(a); if (a <= 0) return 1; else return a * factorial(a - 1); } 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. Importing One JavaScript File From AnotherIf a JavaScript file needs to use functions defined inside another JavaScript file, the other file can be imported using the Qt.include() function. This imports all functions from the other file into the current file's namespace. For example, the QML code below left calls showCalculations() in script.js, which in turn can call factorial() in factorial.js, as it has included factorial.js using Qt.include().
Notice that calling Qt.include() imports all functions from factorial.js into the MyScript namespace, which means the QML component can also access factorial() directly as MyScript.factorial(). Running JavaScript at StartupIt 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 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 Property Bindings may not have been run. QML JavaScript Restrictions covers the exact limitations of global script code. The QML Component element provides an attached 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: Rectangle {
function startupFunction() {
// ... startup code
}
Component.onCompleted: startupFunction();
}
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 onCompleted() handler to execute at startup, they are run sequentially in an undefined order. Likewise, the Component::onDestruction attached property is triggered on component destruction. JavaScript and Property BindingProperty bindings can be created in JavaScript by assigning the property with a function that returns the required value. See Property Assignment versus Property Binding for details. Receiving QML Signals in JavaScriptTo receive a QML signal, use the signal's connect() method to connect it to a JavaScript function. For example, the following code connects the MouseArea clicked signal to the jsFunction() in script.js:
The jsFunction() will now be called whenever MouseArea's clicked signal is emitted. See Connecting Signals to Methods and Signals for more information. QML JavaScript RestrictionsQML executes standard JavaScript code, with the following restrictions:
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