qml property alias что это

QML. Обращение из одного обьекта к свойствам другого обьекта

Всем привет.
файл Content1.qml

Задача такая, в одном файле обьявлен switch для переключения между двух режимов, которые задаются для TextField в другом файле. Я выбрал вариант просто с флагом false или true.
Первая проблема, с чем я столкнулся так это то, что они друг друга не видят.
И мне кажется я какой-то не очень удобный вариант выбрал.
Посоветуйте, кто знает, как лучше это реализовать? Или может я где-то упустил что-то.

Рекомендуем хостинг TIMEWEB

Подписчики

Объекты, которые представлены данными файлами, должны где-то объявляться. Например, в файле main.

Да, они обьявлены в main.qml

И само собою все эти файлы объявленный в qml.qrc

Здесь нужно использовать сигналы для передачи информации из одного объекта в другой. У меня есть пример с использованием сигналов в QML. Также не хватает property alias в файле Content2, который уже свяжет property bool flag.
А для данной задачи решение следующее:

Здесь нужно использовать сигналы для передачи информации

Спасибо. Все оказалось не так тривиально, как казалось на первый взгляд. Будем изучать дальше).

Комментарии

Timeweb

Позвольте мне порекомендовать вам отличный хостинг, на котором расположен EVILEG.

В течение многих лет Timeweb доказывает свою стабильность.

Для проектов на Django рекомендую VDS хостинг

Источник

Оглавление

Свойства

QML components have properties that can be read and modified by other objects. In QML, properties serve many purposes but their main function is to bind to values. Values may be a basic type, or other QML elements.

The syntax for properties is:

[default] property [: defaultValue]

Elements already possess useful properties but, to create custom properties, precede the property name with the keyword property.

QML property rules coincide with many of JavaScript’s property rules, for example, property names must begin with a lowercase letter. JavaScript reserved words are not valid property names.

Связывание свойств

Property binding is a declarative way of specifying the value of a property. Binding allows a property’s value to be expressed as an JavaScript expression that defines the value relative to other property values or data accessible in the application. The property value is automatically kept up to date if the other properties or data values change.

Property bindings are created in QML using the colon «:» before the value:

The property binding causes the width of the Rectangle to update whenever the parent‘s width changes.

QML extends a standards compliant JavaScript engine, so any valid JavaScript expression can be used as a property binding. Bindings can access object properties, make function calls and even use built-in JavaScript objects such as Date and Math.

Property Assignment versus Property Binding

When working with both QML and JavaScript, it is important to differentiate between QML property binding and JavaScript value assignment. In QML, a property binding is created using the colon «:«.

The property binding causes the width of the Rectangle to update whenever the parent‘s width changes.

Assigning a property value (using the equals sign «=«) does not create a property binding.

Instead of creating a property binding, the assignment simply sets the Rectangle width value to a number when the Component.onCompleted code is invoked.

Assigning a value to a property that is already bound will remove the previous binding. A property can only have one value at a time (a list of property is one value), and if any code explicitly re-sets this value, the property binding is removed.

There is no way to create a property binding directly from imperative JavaScript code, although it is possible to use the Binding element.

Types of Properties

Properties may bind to different types, but they are are type-safe. That is, properties only allow you to assign a value that matches the property type. For example, if a property is a real, and if you try to assign a string to it you will get an error.

Certain properties bind to more complex types such as other elements and objects.

Basic Property Types

Basic types such as int, real, and other Qt structures may be bound to properties. For a list of types, visit the QML Basic Types document.

Свойство id

Each QML object may be given a special unique property called an id. No other object within the same QML component (see QML Documents) can have the same id value. QML objects may then access an object using the id property.

A component may readily access its parent’s properties by using the parent property.

Note that an id must begin with a lower-case letter or an underscore. The id cannot contain characters other than letters, numbers, underscores, and JavaScript reserved words.

Elements and Objects as Property Values

Many properties bind to objects. For example, the Item element has a states property that can bind to State elements. This type of property binding allows elements to carry additional non-children elements. Item‘s transitions property behaves in a similar way; it can bind to Transition elements.

Care must be taken when referring to the parent of an object property binding. Elements and components that are bound to properties are not necessarily set as children of the properties’ component.

The code snippet has a Gradient element that attempts to print its parent’s width value. However, the Gradient element is bound to the gradient property, not the children property of the Rectangle. As a result, the Gradient does not have the Rectangle as its parent. Printing the value of parent.width generates an error. Printing the Rectangle object’s first child’s name will print childrectangle because the second Rectangle is bound to the children property.

For more information about the children property, please read the Default Properties section.

Присоединённые свойства

Certain objects provide additional properties by attaching properties to other objects. For example, the Keys element have properties that can attach to other QML objects to provide keyboard handling.

The element ListView provides the delegate, listdelegate, the property isCurrentItem as an attached property. The ListView.isCurrentItem attached property provides highlight information to the delegate. Effectively, the ListView element attaches the ListView.isCurrentItem property to each delegate it creates.

Attached Signal Handlers

Attached signal handlers are similar to attached properties in that they attach to objects to provide additional functionality to objects. Two prominent elements, Component and Keys element provide signal handlers as attached signal handlers.

Read the QML Signal and Handler Event System and the Keyboard Focus in QML articles for more information.

Список свойств

Some properties may accept a binding to a list property, where more than one component can bind to the property. List properties allow multiple States, Gradients, and other components to bind to a single property.

The list is enclosed in square brackets, with a comma separating the list elements. In cases where you are only assigning a single item to a list, you may omit the square brackets.

To access the list, use the index property.

The snippet code simply prints the name of the first state, FETCH.

See the list type documentation for more details about list properties and their available operations.

Сгруппированные свойства

In some cases properties form a logical group and use either the dot notation or group notation.

Grouped properties may be written both ways:

In the element documentation grouped properties are shown using the dot notation.

Property Aliases

Unlike a property definition, which allocates a new, unique storage space for the property, a property alias connects the newly declared property, called the aliasing property as a direct reference to an existing property, the aliased property. Read or write operations on the aliasing property results in a read or write operations on the aliased property, respectively.

A property alias declaration is similar to an ordinary property definition:

[default] property alias :

As the aliasing property has the same type as the aliased property, an explicit type is omitted, and the special alias keyword is before the property name. Instead of a default value, a property alias has a compulsory alias reference. Accessing the aliasing property is similar to accessing a regular property. In addition, the optional default keyword indicates that the aliasing property is a default property.

When importing the component as a Button, the buttonlabel is directly accessible through the label property.

In addition, the id property may also be aliased and referred outside the component.

The imagebutton component has the ability to modify the child Image object and its properties.

Using aliases, properties may be exposed to the top level component. Exposing properties to the top-level component allows components to have interfaces similar to Qt widgets.

Considerations for property aliases

Aliases are only activated once the component completes its initialization. An error is generated when an uninitialized alias is referenced. Likewise, aliasing an aliasing property will also result in an error.

When importing the component, however, aliasing properties appear as regular Qt properties and consequently can be used in alias references.

It is possible for an aliasing property to have the same name as an existing property, effectively overwriting the existing property. For example, the following component has a color alias property, named the same as the built-in Rectangle::color property:

Any object that use this component and refer to its color property will be referring to the alias rather than the ordinary Rectangle::color property. Internally, however, the coloredrectangle can correctly set its color property and refer to the actual defined property rather than the alias.

The TabWidget example uses aliases to reassign children to the ListView, creating a tab effect.

Default Properties

When imported, QML components will bind declared children to their designated default properties. The optional default attribute specifies a property as the default property. For example, the State element's default property is its changes property. PropertyChanges elements may simply be placed as the State's children and they will be bound to the changes property.

Similarly, the Item element's default property is its data property. The data property manages Item's children and resources properties. This way, different data types may be placed as direct children of the Item.

Reassigning a default property is useful when a component is reused. For example, the TabWidget example uses the default attribute to reassign children to the ListView, creating a tab effect.

Using the Binding Element

In some advanced cases, it may be necessary to create bindings explicitly with theBinding element.

For example, to bind a property exposed from C++ (system.brightness) to a value written in QML (slider.value), you could use the Binding element as follows:

Changing Property Values in States

The PropertyChanges element is for setting property bindings within a State element to set a property binding.

The rectangle's color property will bind to the warning component's color property when its state is set to the WARNING state.

Все остальные торговые марки являются собственностью их владельцев. Политика конфиденциальности

Лицензиаты, имеющие действительные коммерческие лицензии Qt, могут использовать этот документ в соответствии с соглашениями коммерческой лицензии Qt, поставляемой с программным обеспечением, либо, альтернативно, в соответствии с условиями, содержащимися в письменном соглашении между вами и Nokia.

Кроме того, этот документ может быть использован в соответствии с условиями GNU Free Documentation License version 1.3, опубликованной фондом Free Software Foundation.

Источник

Qt Documentation

Contents

Every QML object type has a defined set of attributes. Each instance of an object type is created with the set of attributes that have been defined for that object type. There are several different kinds of attributes which can be specified, which are described below.

Attributes in Object Declarations

An object declaration in a QML document defines a new type. It also declares an object hierarchy that will be instantiated should an instance of that newly defined type be created.

The set of QML object-type attribute types is as follows:

These attributes are discussed in detail below.

The id Attribute

Every QML object type has exactly one id attribute. This attribute is provided by the language itself, and cannot be redefined or overridden by any QML object type.

A value may be assigned to the id attribute of an object instance to allow that object to be identified and referred to by other objects. This id must begin with a lower-case letter or an underscore, and cannot contain characters other than letters, numbers and underscores.

An object can be referred to by its id from anywhere within the component scope in which it is declared. Therefore, an id value must always be unique within its component scope. See Scope and Naming Resolution for more information.

Once an object instance is created, the value of its id attribute cannot be changed. While it may look like an ordinary property, the id attribute is not an ordinary property attribute, and special semantics apply to it; for example, it is not possible to access myTextInput.id in the above example.

Property Attributes

A property is an attribute of an object that can be assigned a static value or bound to a dynamic expression. A property's value can be read by other objects. Generally it can also be modified by another object, unless a particular QML type has explicitly disallowed this for a specific property.

Defining Property Attributes

A property may be defined for a type in C++ by registering a Q_PROPERTY of a class which is then registered with the QML type system. Alternatively, a custom property of an object type may be defined in an object declaration in a QML document with the following syntax:

In this way an object declaration may expose a particular value to outside objects or maintain some internal state more easily.

Declaring a custom property implicitly creates a value-change signal for that property, as well as an associated signal handler called on

is the name of the property, with the first letter capitalized.

For example, the following object declaration defines a new type which derives from the Rectangle base type. It has two new properties, with a signal handler implemented for one of those new properties:

Valid Types in Custom Property Definitions

Any of the QML Basic Types aside from the enumeration type can be used as custom property types. For example, these are all valid property declarations:

(Enumeration values are simply whole number values and can be referred to with the int type instead.)

Some basic types are provided by the QtQuick module and thus cannot be used as property types unless the module is imported. See the QML Basic Types documentation for more details.

Note the var basic type is a generic placeholder type that can hold any type of value, including lists and objects:

Additionally, any QML object type can be used as a property type. For example:

This applies to custom QML types as well. If a QML type was defined in a file named ColorfulButton.qml (in a directory which was then imported by the client), then a property of type ColorfulButton would also be valid.

Assigning Values to Property Attributes

The value of a property of an object instance may be specified in two separate ways:

In either case, the value may be either a static value or a binding expression value.

Value Assignment on Initialization

The syntax for assigning a value to a property on initialization is:

An initialization value assignment may be combined with a property definition in an object declaration, if desired. In that case, the syntax of the property definition becomes:

An example of property value initialization follows:

Imperative Value Assignment

An imperative value assignment is where a property value (either static value or binding expression) is assigned to a property from imperative JavaScript code. The syntax of an imperative value assignment is just the JavaScript assignment operator, as shown below:

An example of imperative value assignment follows:

Static Values and Binding Expression Values

As previously noted, there are two kinds of values which may be assigned to a property: static values, and binding expression values. The latter are also known as property bindings.

The QML engine enforces the relationship between a property and its dependencies. When any of the dependencies change in value, the QML engine automatically re-evaluates the binding expression and assigns the new result to the property.

Here is an example that shows both kinds of values being assigned to properties:

Note: To assign a binding expression imperatively, the binding expression must be contained in a function that is passed into Qt.binding(), and then the value returned by Qt.binding() must be assigned to the property. In contrast, Qt.binding() must not be used when assigning a binding expression upon initialization. See Property Binding for more information.

Type Safety

Properties are type safe. A property can only be assigned a value that matches the property type.

For example, if a property is a real, and if you try to assign a string to it, you will get an error:

Likewise if a property is assigned a value of the wrong type during run time, the new value will not be assigned, and an error will be generated.

Some property types do not have a natural value representation, and for those property types the QML engine automatically performs string-to-typed-value conversion. So, for example, even though properties of the color type store colors and not strings, you are able to assign the string "red" to a color property, without an error being reported.

See QML Basic Types for a list of the types of properties that are supported by default. Additionally, any available QML object type may also be used as a property type.

Special Property Types

Object List Property Attributes

A list type property can be assigned a list of QML object-type values. The syntax for defining an object list value is a comma-separated list surrounded by square brackets:

For example, the Item type has a states property that is used to hold a list of State type objects. The code below initializes the value of this property to a list of three State objects:

If the list contains a single item, the square brackets may be omitted:

A list type property may be specified in an object declaration with the following syntax:

and, like other property declarations, a property initialization may be combined with the property declaration with the following syntax:

An example of list property declaration follows:

If you wish to declare a property to store a list of values which are not necessarily QML object-type values, you should declare a var property instead.

Grouped Properties

In some cases properties contain a logical group of sub-property attributes. These sub-property attributes can be assigned to using either the dot notation or group notation.

For example, the Text type has a font group property. Below, the first Text object initializes its font values using dot notation, while the second uses group notation:

Grouped property types are basic types which have subproperties. Some of these basic types are provided by the QML language, while others may only be used if the Qt Quick module is imported. See the documentation about QML Basic Types for more information.

Property Aliases

Property aliases are properties which hold a reference to another property. Unlike an ordinary property definition, which allocates a new, unique storage space for the property, a property alias connects the newly declared property (called the aliasing property) as a direct reference to an existing property (the aliased property).

A property alias declaration looks like an ordinary property definition, except that it requires the alias keyword instead of a property type, and the right-hand-side of the property declaration must be a valid alias reference:

Unlike an ordinary property, an alias has the following restrictions:

However, aliases to properties that are up to two levels deep will work.

For example, below is a Button type with a buttonText aliased property which is connected to the text object of the Text child:

The following code would create a Button with a defined text string for the child Text object:

Here, modifying buttonText directly modifies the textItem.text value; it does not change some other value that then updates textItem.text. If buttonText was not an alias, changing its value would not actually change the displayed text at all, as property bindings are not bi-directional: the buttonText value would have changed if textItem.text was changed, but not the other way around.

Considerations for Property Aliases

Aliases are only activated once a component has been fully initialized. An error is generated when an uninitialized alias is referenced. Likewise, aliasing an aliasing property will also result in an error.

When importing a QML object type with a property alias in the root object, however, the property appear as a regular Qt property and consequently can be used in alias references.

It is possible for an aliasing property to have the same name as an existing property, effectively overwriting the existing property. For example, the following QML type has a color alias property, named the same as the built-in Rectangle::color property:

Any object that use this type and refer to its color property will be referring to the alias rather than the ordinary Rectangle::color property. Internally, however, the rectangle can correctly set its color property and refer to the actual defined property rather than the alias.

Property Aliases and Types

Property aliases cannot have explicit type specifications. The type of a property alias is the declared type of the property or object it refers to. Therefore, if you create an alias to an object referenced via id with extra properties declared inline, the extra properties won't be accessible through the alias:

You cannot initialize inner.extraProperty from outside of this component, as inner is only an Item:

Default Properties

An object definition can have a single default property. A default property is the property to which a value is assigned if an object is declared within another object's definition without declaring it as a value for a particular property.

Declaring a property with the optional default keyword marks it as the default property. For example, say there is a file MyLabel.qml with a default property someText :

The someText value could be assigned to in a MyLabel object definition, like this:

This has exactly the same effect as the following:

However, since the someText property has been marked as the default property, it is not necessary to explicitly assign the Text object to this property.

You will notice that child objects can be added to any Item-based type without explicitly adding them to the children property. This is because the default property of Item is its data property, and any items added to this list for an Item are automatically added to its list of children.

Default properties can be useful for reassigning the children of an item. See the TabWidget Example, which uses a default property to automatically reassign children of the TabWidget as children of an inner ListView. See also Extending QML.

Required Properties

An object declaration may define a property as required, using the required keyword. The syntax is

As the name suggests, required properties must be set when an instance of the object is created. Violation of this rule will result in QML applications not starting if it can be detected statically. In case of dynamically instantiated QML components (for instance via Qt.createComponent()), violating this rule results in a warning and a null return value.

It's possible to make an existing property required with

The following example shows how to create a custom Rectangle component, in which the color property always needs to be specified.

Note: You can't assign an initial value to a required property from QML, as that would go directly against the intended usage of required properties.

Required properties play a special role in model-view-delegate code: If the delegate of a view has required properties whose names match with the role names of the view's model, then those properties will be initialized with the model's corresponding values. For more information, visit the Models and Views in Qt Quick page.

and for ways to initialize required properties from C++.

Read-Only Properties

An object declaration may define a read-only property using the readonly keyword, with the following syntax:

Read-only properties must be assigned a value on initialization. After a read-only property is initialized, it no longer possible to give it a value, whether from imperative code or otherwise.

For example, the code in the Component.onCompleted block below is invalid:

Note: A read-only property cannot also be a default property.

Property Modifier Objects

Properties can have property value modifier objects associated with them. The syntax for declaring an instance of a property modifier type associated with a particular property is as follows:

It is important to note that the above syntax is in fact an object declaration which will instantiate an object which acts on a pre-existing property.

Certain property modifier types may only be applicable to specific property types, however this is not enforced by the language. For example, the NumberAnimation type provided by QtQuick will only animate numeric-type (such as int or real ) properties. Attempting to use a NumberAnimation with non-numeric property will not result in an error, however the non-numeric property will not be animated. The behavior of a property modifier type when associated with a particular property type is defined by its implementation.

Signal Attributes

A signal is a notification from an object that some event has occurred: for example, a property has changed, an animation has started or stopped, or when an image has been downloaded. The MouseArea type, for example, has a clicked signal that is emitted when the user clicks within the mouse area.

An object can be notified through a signal handler whenever a particular signal is emitted. A signal handler is declared with the syntax on where is the name of the signal, with the first letter capitalized. The signal handler must be declared within the definition of the object that emits the signal, and the handler should contain the block of JavaScript code to be executed when the signal handler is invoked.

For example, the onClicked signal handler below is declared within the MouseArea object definition, and is invoked when the MouseArea is clicked, causing a console message to be printed:

Defining Signal Attributes

A signal may be defined for a type in C++ by registering a Q_SIGNAL of a class which is then registered with the QML type system. Alternatively, a custom signal for an object type may be defined in an object declaration in a QML document with the following syntax:

Attempting to declare two signals or methods with the same name in the same type block is an error. However, a new signal may reuse the name of an existing signal on the type. (This should be done with caution, as the existing signal may be hidden and become inaccessible.)

Here are three examples of signal declarations:

If the signal has no parameters, the "()" brackets are optional. If parameters are used, the parameter types must be declared, as for the string and var arguments for the actionPerformed signal above. The allowed parameter types are the same as those listed under Defining Property Attributes on this page.

To emit a signal, invoke it as a method. Any relevant signal handlers will be invoked when the signal is emitted, and handlers can use the defined signal argument names to access the respective arguments.

Property Change Signals

QML types also provide built-in property change signals that are emitted whenever a property value changes, as previously described in the section on property attributes. See the upcoming section on property change signal handlers for more information about why these signals are useful, and how to use them.

Signal Handler Attributes

Signal handlers are a special sort of method attribute, where the method implementation is invoked by the QML engine whenever the associated signal is emitted. Adding a signal to an object definition in QML will automatically add an associated signal handler to the object definition, which has, by default, an empty implementation. Clients can provide an implementation, to implement program logic.

Consider the following SquareButton type, whose definition is provided in the SquareButton.qml file as shown below, with signals activated and deactivated :

These signals could be received by any SquareButton objects in another QML file in the same directory, where implementations for the signal handlers are provided by the client:

See the Signal and Handler Event System for more details on use of signals.

Property Change Signal Handlers

Signal handlers for property change signal take the syntax form on

is the name of the property, with the first letter capitalized. For example, although the TextInput type documentation does not document a textChanged signal, this signal is implicitly available through the fact that TextInput has a text property and so it is possible to write an onTextChanged signal handler to be called whenever this property changes:

Method Attributes

A method of an object type is a function which may be called to perform some processing or trigger further events. A method can be connected to a signal so that it is automatically invoked whenever the signal is emitted. See Signal and Handler Event System for more details.

Defining Method Attributes

A method may be defined for a type in C++ by tagging a function of a class which is then registered with the QML type system with Q_INVOKABLE or by registering it as a Q_SLOT of the class. Alternatively, a custom method can be added to an object declaration in a QML document with the following syntax:

Methods can be added to a QML type in order to define standalone, reusable blocks of JavaScript code. These methods can be invoked either internally or by external objects.

Unlike signals, method parameter types do not have to be declared as they default to the var type.

Attempting to declare two methods or signals with the same name in the same type block is an error. However, a new method may reuse the name of an existing method on the type. (This should be done with caution, as the existing method may be hidden and become inaccessible.)

Below is a Rectangle with a calculateHeight() method that is called when assigning the height value:

If the method has parameters, they are accessible by name within the method. Below, when the MouseArea is clicked it invokes the moveTo() method which can then refer to the received newX and newY parameters to reposition the text:

Attached Properties and Attached Signal Handlers

Attached properties and attached signal handlers are mechanisms that enable objects to be annotated with extra properties or signal handlers that are otherwise unavailable to the object. In particular, they allow objects to access properties or signals that are specifically relevant to the individual object.

A QML type implementation may choose to create an attaching type in C++ with particular properties and signals. Instances of this type can then be created and attached to specific objects at run time, allowing those objects to access the properties and signals of the attaching type. These are accessed by prefixing the properties and respective signal handlers with the name of the attaching type.

References to attached properties and handlers take the following syntax form:

For example, the ListView type has an attached property ListView.isCurrentItem that is available to each delegate object in a ListView. This can be used by each individual delegate object to determine whether it is the currently selected item in the view:

An attached signal handler is referred to in the same way. For example, the Component.onCompleted attached signal handler is commonly used to execute some JavaScript code when a component's creation process has been completed. In the example below, once the ListModel has been fully created, its Component.onCompleted signal handler will automatically be invoked to populate the model:

A Note About Accessing Attached Properties and Signal Handlers

A common error is to assume that attached properties and signal handlers are directly accessible from the children of the object to which these attributes have been attached. This is not the case. The instance of the attaching type is only attached to specific objects, not to the object and all of its children.

For example, below is a modified version of the earlier example involving attached properties. This time, the delegate is an Item and the colored Rectangle is a child of that item:

Now delegateItem.ListView.isCurrentItem correctly refers to the isCurrentItem attached property of the delegate.

Enumeration Attributes

Enumerations provide a fixed set of named choices. They can be declared in QML using the enum keyword:

As shown above, enumeration types (e.g. TextType ) and values (e.g. Normal ) must begin with an uppercase letter.

More information on enumeration usage in QML can be found in the QML Basic Types enumeration documentation.

The ability to declare enumerations in QML was introduced in Qt 5.10.

В© 2021 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.

Источник