Qt is a cross-platform application framework that's used to develop UI-driven apps. Qt uses and extends C++ through a code generator called the MOC (meta-object compiler). When you build an app, the MOC reads your C++ header files looking for Qt-specific macros like Q_OBJECT, and produces the appropriate C++ meta-object code. Among other things, the meta-object code is necessary for the signals and slots mechanism.
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In addition to the MOC, Qt provides several modules that contain foundational classes that you can use to build a Cascades app, such as QtCore, QtNetwork, QtXml, QtSql, and more. For a closer look at what parts of Qt are supported in Cascades, see Support for Qt in Cascades.
QObject, the Qt base class
All objects in Cascades are derived from the QObject class. This class is the base class of all Qt objects and provides several important features. Most notably, QObject provides support for signals and slots, which is a powerful mechanism that allows objects to communicate with each other. When something happens to an object in your app (for example, a user clicks a button), the object emits a signal. You can handle that signal and respond to the event by using a function called a slot.
To illustrate some of the features and requirements of a QObject in Cascades, consider the Button control. This control represents a clickable button that you can use in your apps. The header file (button.h) for this control is located in the folder where you installed the BlackBerry 10 Native SDK, in the targetqnx6usrincludebbcascadescontrols subfolder. Let's take a closer look at some of the code in this file and what the code means.
Don't do that, since Qt doesn't support multiple derivation from QObject, not even virtual derivation. The second and subsequent QObject bases are ignored by the QObject system. Thus, to the QObject machinery, the Derived class only derives from QObject, and not from Base - thus it doesn't know about baseSignal. Qt Development General and Desktop. And a derived class. QObject::connect: No such slot QCcmStatusPanel::DistributorDepthSlot(int) Reply Quote 0. The Qt Object Model. Qt is based around the Qt object model. This architecture is what makes Qt powerful and easy to use. It is all based around the QObject class and the moc tool. By deriving classes from the QObject a number of benefits are inherited. They are listed below: Easy memory management; Signals and slots; Properties.
Class declaration
All Cascades controls inherit QObject, either directly or indirectly. The Button class inherits several intermediate classes, all of which eventually inherit QObject.
Q_OBJECT macro
To identify a class as a QObject, you need to use the Q_OBJECT macro in the private: section of the class definition. This macro is processed by the Qt meta-object compiler (MOC) and enables features such as properties, signals, and slots. To learn more about the Qt meta-object system and meta-object compiler, see The Meta-Object System on the Qt website.
Property declaration
A property is a single piece of information about an object, and is declared by using the Q_PROPERTY macro. This macro allows the property to be accessible in QML. In the Q_PROPERTY, you specify the name and type of the property. You also use keywords (such as READ and WRITE) to specify the names of functions that can manipulate the value of the property. Properties are declared in the private: section of the header file, along with the Q_OBJECT macro.
As you can see above, the Button class includes the text property, which specifies the text that appears on the button. This property is a QString (the Qt representation of a text string) and has functions called text(), setText(), and resetText() to get, set, and reset the value of the property, respectively. These functions still need to be declared later; the Q_PROPERTY macro associates them only with the property. You can also specify a signal that's emitted when the value changes by using the keyword NOTIFY. To learn more about properties and the keywords you can use, see The Property System on the Qt website.
Function declaration
You can create functions for QObject classes just as you do for any other C++ classes. The Q_SLOT macro is used before the function declaration. This macro is another special element of Qt and indicates that this function is a slot. You can connect slots to signals so that when a signal is emitted, the connected slot function is called automatically. As an alternative to the Q_SLOT macro, you might see slot functions declared in a public slots: section in a header file. In Cascades, these approaches are identical.
Another useful macro that you might see is Q_INVOKABLE. You can use this macro in front of a function declaration (in the same place as Q_SLOT above) to indicate that you want to be able to call the function from QML. This approach can be useful if you have a complicated operation (for example, a custom transition for your UI control) that you want to implement in C++. You can still keep the UI-related part (calling the function at the appropriate time) in QML, but you can implement the operation in C++ any way you want. To learn more about Q_INVOKABLE, see QML and C++ integration.
Signal declaration
You can use the Q_SIGNALS: section to declare signals that a QObject emits when certain events occur. A Button emits the textChanged() signal when its text changes, and you can handle this event in your app if necessary. You might also see signals declared in a signals: section in a header file, which means the same thing in Cascades.
Object ownership
Every QObject can be related to other objects in parent/child relationships. An object can have one parent, as well as zero or more children. A parent object owns its child objects. When an object is destroyed, its children are also destroyed, so it's important to remember which objects are related to other objects in your apps.
Scene graphs
Cascades maintains the relationships between objects internally, and transfers ownership between objects automatically when you call functions that change object ownership. To make it easier to visualize the parent/child relationships in your app, you can use a hierarchical structure called a scene graph. This graph shows the controls (also called nodes) in your app and how they're related to each other. You can't view the scene graph for your app in the Momentics IDE or anywhere else; it's just a concept that makes it easier to visualize how Cascades tracks object relationships internally.
A hierarchical structure (such as a scene graph) is commonly used in many types of apps, such as complex UI apps and games, and makes it easier to manage complex arrangements of components. Using a hierarchical structure can also increase the performance of apps, which is a major reason why it was chosen as a feature for Cascades. Finally, the hierarchical nature of scene graphs fits nicely with the parent/child relationship structure of Qt objects. To learn more about this relationship structure, see Object Trees & Ownership from the Qt documentation.
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For example, consider a Container with two Button controls.
Here's what the scene graph would look like for this arrangement: Persona 5 casino palace red panel.
Scene graphs can help you track how object ownership might affect various aspects of your app. For example, you can use a scene graph to visualize how touch events are delivered to different controls on the screen. To learn how scene graphs can help you handle touch events, see Cascades touch propagation.
Setting ownership
Object ownership can be established either when controls are created or when they're added to other controls. Both approaches result in the same parent/child relationships, so you can choose which approach you prefer to use in your apps.
For example, here's how to create a container and make it the parent for two buttons. This approach establishes object ownership when the buttons are created. The parent for each button is specified in the Button constructor, and each button becomes owned by the specified parent automatically when it's created.
Here's a second approach that establishes object ownership by adding the buttons to the container explicitly.
Wow classic equip slot macro. I have seen a diagram of equipment slot numbers before, and I have since forgotten where I ran across it. When I switch specs, I am currently using a macro to switch the weapons I wield, but I would also like to be able to switch armor with the same macro as well. This is my macro switching into PvE & combat spec: /equipslot 16 Seven-Fingered Claws. Equipping items Macro There are three commands for equipping items: /equip, /equipslot, and /equipset. /equip simply takes an item name and will equip it to the default slot as if you had right-clicked it in one of your bags. /equipslot takes an inventory slot ID and an item name, and equips the item to the specified slot.
After object ownership is established, you can change the parent of an object by adding the object to a different parent. The object is removed from the first parent and added to the new parent, and the new parent then owns the object. Here's an example:
You can also remove an object from a container without specifying a new parent for the object. However, here's when things get tricky: even though the object was removed from the container, the container is still the object's parent and still owns the object. If the container is deleted, the object is deleted as well.
This behavior can lead to errors in your app that might be difficult to recognize. For example, consider the following example of two buttons that are added to a container. The second button is then removed from the container, and after the button is removed, the container is deleted. Finally, the removed button's setText() function is called.
Because the second button was removed from the container before the container was deleted, you might expect that when the container is deleted, the button isn't deleted and you can still access it. However, the container remains the parent of the button even after remove() is called. The second button is deleted when the container is deleted (along with the first button), so the call to setText() isn't valid because the button doesn't exist anymore.
When an object is removed from its parent, or when an object is replaced by a new object (in the case of certain setter functions, such as Page::setContent()), it's removed from the visual hierarchy (that is, it's not displayed on the screen anymore). The object's parent still owns the object, but you can change the parent or delete the object at this point. If you don't change the object's parent, it's still deleted when the parent object is deleted. It's important to note that you can't change the parent of an object if the object is still part of the visual hierarchy of its parent. You need to remove it before you try to change its parent.
All QObject instances include a function called setParent(), and you can use this function to change the parent of an object (when it's allowed, as described above). When you use setParent() to change an object's parent, the function doesn't add that object to the parent's visual hierarchy; you still need to add it explicitly (for example, by calling Container::add()).
To prevent the error that occurred in the code sample above, you can add a call to setParent(0) after you remove an object from its parent. This way, the object isn't deleted when its former parent is deleted.
If you have a UIObject that does not have a parent, an app crash can occur if the app quits and the object hasn't been deleted first. Always make sure to set a parent for UIObject instances or delete them manually before your app quits.
Scene ownership
There's one important exception to the rules of object ownership. The top-level scene in your app, which is the AbstractPane object that you set as the root node using Application::setScene(), behaves differently. The app takes ownership of this AbstractPane only if the pane doesn't already have a parent. If the AbstractPane doesn't have a parent when setScene() is called, the app owns the pane and deletes it if a new scene is set. However, if the AbstractPane already has a parent, the app won't take ownership of the pane when setScene() is called.
This behavior lets you switch between different scenes in your apps but ensures that for the most basic use case (an app with just a single scene), the old scene is deleted when appropriate. If you want to switch between two scenes, you should set the parents of the AbstractPane objects explicitly using setParent() before you call setScene().
Many other functions of Cascades classes change the ownership of objects. You should check the API reference for the classes and functions that you're working with to make sure that they don't change object ownership in unexpected ways.
Object ownership in QML
In QML, object ownership is established automatically according to the structure of the QML code that you write. For example, here's how to create the same example as above, with two Button controls and a Container:
The container is the parent of the buttons, and the buttons are children of the container. When the container is destroyed, both buttons are destroyed as well.
Signals and slots
For handling events and inter-object communication, Qt uses a mechanism called signals and slots. By using this mechanism, objects can communicate with each other by emitting a signal. An object that wants to receive a message from another object can create a slot for that particular signal.
Unlike the callback technique for inter-object communication, signals and slots are type-safe and loosely coupled. An object that implements a slot must match the method signature for the signal that it can receive. This approach allows the compiler to check the type of the subscription. Apart from the data type in the signature, a slot implementation doesn't need to know anything about the signal implementation. The slot doesn't even need to receive the same number of arguments that a signal emits. Similarly, you can send signals to objects if you know which slots they implement. The framework discards signal arguments that are not implemented by the slot.
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Events in Cascades are implemented as signals. When a user interacts with your UI, the Cascades framework emits a signal. The framework delivers the signal to each slot that's connected to the signal.
Scene ownership
There's one important exception to the rules of object ownership. The top-level scene in your app, which is the AbstractPane object that you set as the root node using Application::setScene(), behaves differently. The app takes ownership of this AbstractPane only if the pane doesn't already have a parent. If the AbstractPane doesn't have a parent when setScene() is called, the app owns the pane and deletes it if a new scene is set. However, if the AbstractPane already has a parent, the app won't take ownership of the pane when setScene() is called.
This behavior lets you switch between different scenes in your apps but ensures that for the most basic use case (an app with just a single scene), the old scene is deleted when appropriate. If you want to switch between two scenes, you should set the parents of the AbstractPane objects explicitly using setParent() before you call setScene().
Many other functions of Cascades classes change the ownership of objects. You should check the API reference for the classes and functions that you're working with to make sure that they don't change object ownership in unexpected ways.
Object ownership in QML
In QML, object ownership is established automatically according to the structure of the QML code that you write. For example, here's how to create the same example as above, with two Button controls and a Container:
The container is the parent of the buttons, and the buttons are children of the container. When the container is destroyed, both buttons are destroyed as well.
Signals and slots
For handling events and inter-object communication, Qt uses a mechanism called signals and slots. By using this mechanism, objects can communicate with each other by emitting a signal. An object that wants to receive a message from another object can create a slot for that particular signal.
Unlike the callback technique for inter-object communication, signals and slots are type-safe and loosely coupled. An object that implements a slot must match the method signature for the signal that it can receive. This approach allows the compiler to check the type of the subscription. Apart from the data type in the signature, a slot implementation doesn't need to know anything about the signal implementation. The slot doesn't even need to receive the same number of arguments that a signal emits. Similarly, you can send signals to objects if you know which slots they implement. The framework discards signal arguments that are not implemented by the slot.
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Events in Cascades are implemented as signals. When a user interacts with your UI, the Cascades framework emits a signal. The framework delivers the signal to each slot that's connected to the signal.
To learn more about signals and slots, see Signals and slots.
Thread support
Qt provides support for threaded apps with the QThread class. The ability to create new threads is often necessary if your app contains resource-intensive processes that might otherwise block the UI thread. Some Qt classes, like QNetworkAccessManager, are already asynchronous in nature and don't require the creation of a separate thread to operate.
QThread
A QThread represents a separate thread of control in the app; it can share data with all the other threads within the app, but runs independently in the same way that a standalone app does. To facilitate communication between threads, signals and slots are fully supported.
Previously, the standard method for creating a QThread was by subclassing it. Now, you should use QThread by creating a worker object that is derived from QObject and implementing a slot on that worker to handle the task. The worker should also specify a signal that is emitted when the task is finished, and an optional signal that is emitted when there's an error.
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After you create the worker class, you can give QThread ownership of the worker, connect the appropriate signals and slots, and start the thread.
For more information, see Thread Support in Qt on the Qt website.
QtConcurrentRun
The QtConcurrent::run() function is another way that you can run processes asynchronously. The difference from QThread is that it's designed to run just a single function. For more information on QtConcurrent, see QtConcurrentRun on the Qt website.
