java pdf 417 From XML Script to JavaScript in .NET framework

Creator PDF-417 2d barcode in .NET framework From XML Script to JavaScript

The process of converting the XML Script declarative code into JavaScript imperative code starts when a web page is loaded in the browser. If XML Script is enabled in the page, the Microsoft Ajax runtime instructs the XML Script parser to filter all the script tags with the type attribute set to text/xml-script. The XML Script parser is a JavaScript object stored in the Sys.Preview.MarkupParser variable. It exposes a group of methods for extracting and processing the XML Script code. As the XML Script blocks are extracted, they re collected in an array and processed sequentially. For each block, a sanity check is performed on its structure, to ensure that a root element called page exists. Also, the page element must have a child node called components. The parser ignores all the other tags in the page element.

The parser performs an additional check to see if a references element is declared in the page node. The references element was used in previous CTPs to provide a list of paths to script files to load in the page, but it s not supported in the latest CTP. If a references tag is found, the XML Script parser throws an error.

As the current XML Script block is processed, all the child elements of the components tag are extracted and stored in an array. These are all the client objects that need to be instantiated. The instantiation process is performed by the parseNode method, which is called by the parser on each tag to parse the markup code and create an instance of the object. First, the parseNode method needs to determine the fully qualified name of the class to instantiate. To locate the class, it extracts the tag name and the namespace prefix from the markup code. The tag name is the case-insensitive name of the class; it s turned to uppercase. The information on the namespace is retrieved from the XML namespace prefix used in the tag. Finally, the fully qualified name of the class is obtained by appending the class name to its containing namespace.

new Button();

new Label();

new TextBox();

new Control();

Figure 11.2 The XML Script parser extracts component declarations from XML Script code blocks. Then, it parses the declarative code and creates instances of the corresponding JavaScript objects.

If the class exists, the parser checks whether it exposes a static method called parseFromMarkup. This method must be defined in a class in order to be used in XML Script. It receives the markup code and is responsible for parsing it and creating a new instance of the class. This process is repeated for each XML Script block and for each tag extracted from the components node. When all the markup has been processed, all the client objects have been instantiated and can be safely accessed in the application code. This process is illustrated in figure 11.2. Luckily, you can avoid writing the logic needed to parse the markup code and create an instance of a client class. The Sys.Component class exposes and implements the parseFromMarkup method; it s a good choice to derive the custom classes from the base Sys.Component class. This way, you can take advantage of the features offered by the client component model and also use the custom class in XML Script with little effort. The only requirement is that every component must expose a type descriptor in order to be used in XML Script.

A type descriptor is an object that provides information about a particular type. In the .NET framework, you can perform reflection on objects to learn about their structure. For example, you can extract every sort of information about the fields, properties, methods, and events exposed by a class. On the server side, type descriptors can be used to provide additional reflection capabilities. On the client side, type descriptors have been introduced to achieve the same goal.