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QML and JavaScript

QML and JavaScript are the cornerstones of Cascades declarative user interface design. Both technologies, while amazingly easy to master, pack an enormous amount of punch when it comes to creating user interfaces quickly and effortlessly. QML, being a declarative language, lets you describe your user interface much like HTML would describe a web page. JavaScript then adds programmatic logic in event handlers, slots in Qt/Cascades parlance, and essentially ties your UI together with some behavior.

I've deliberately kept C++ out of the mix because I want to exclusively concentrate on QML and JavaScript for the moment, but you will see in Chapter 3 that C++ also transparently integrates with QML. As a good rule of thumb, you should always rely on C++ whenever you need to access core platform services or do some heavy lifting, such as computationally intensive tasks.

QML, despite being a small language, is nevertheless extremely flexible, and by mastering the language's nuances, you will be able to build rich and enticing user interfaces. QML is also extensible: you can add to the core system your own types or “custom controls.” You should consider this chapter as a review of the building blocks of QML, where you will learn how to assemble the language constructs and types to design your UI. Once you have mastered the basic elements of QML, you will be ready to apply them in full throttle in the following chapters. You will also have a firm grip on how Cascades uses the same language constructs for its own core controls (the topic of Chapters 4 and 5).

Syntax Basics

You have already seen in Chapter 1 an example of a QML document. I did not go into the details of explaining the QML syntax and I informally presented concepts such as properties, signals, and slots (or signal handlers, if you prefer). It is now time to dig a bit deeper and give you a description of the various QML syntactical elements.

QML Documents

A QML document is the basic building block for creating Cascades UIs. The QML syntax resembles JSON, except that you don't need to use quotes for defining attributes and that the QML language, combined with inline JavaScript, is much more expressive than JSON. Another big advantage of QML over other XML-based languages for designing UIs is that QML has been created from the ground up. The resulting language is very concise and expressive with advanced features, such as dynamic loading of components and transparent interoperability with C++ (you will see in Chapter 3 that you can very easily expose C++ objects to QML).

A QML document is a self-contained piece of QML source code that consists of the following:

n Import statements

n A root object declaration (the root object can also in turn declare children and JavaScript functions)

An example of a minimal main.qml document is given in Listing 2-1.

Listing 2-1. main.qml

import bb.cascades 1.0 Page {

}

As you saw in Chapter 1, the main.qml QML document is typically loaded during application start-up. The loading process is orchestrated behind the scenes by the QML declarative engine. When the engine encounters the import bb.cascades 1.0 statement, it will also search through its import paths for the bb.cascades namespace and load the Cascades core controls and types registered with that namespace. By the time it reaches the Page object declaration, the QML engine already knows about the Page type definition, properties and signals, and is in measure to validate the Page element within the document.

Another interesting aspect of QML documents is that they provide an extension mechanism for defining new object types. In fact, a QML document implicitly defines a new type. For example, a document called MyType.qml will implicitly define the corresponding MyType QML type. The engine will also validate custom types declarations against their definition whenever you import them in other QML documents.

 
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