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Exploring Laszlo Classes, Attributes, and Events
Pages: 1, 2

Exploring the Linkage Between Attributes and Events

The sample code now has the necessary tool set to explore the linkage between attributes and events a bit further. The button that was used to test the alert can also be used to set the attributes that I have defined earlier. I want to show how setting those attributes will fire the corresponding onchange methods.



What follows is a method, respondToButton(), that will be called when the showAlert button is clicked.


<canvas width="800" debug="true">
<debug x="450" y="0" height="300"/>
<class name="test">
        <attribute name="a1" type="string"/>
        <attribute name="b1"/>
</class>
<!--Instantiating a class-->
<test name="testInstance">
        <!--Demonstrating an onchange for attribute event-->
      
        <method event="ona1" name="ona1Method">
                Debug.write("hey this works");
        </method>
      
</test>

<!-- Simulating an alert -->
<!-- A modal alert dialog -->
<alert name="myalert" width="100" y="100">
   hi - initial text
</alert>

<!-- A method to work the modal dialog -->
<method name="showAlert" args="displayText">
   canvas.myalert.setAttribute("text",displayText);
   canvas.myalert.open();
</method>

<!-- Testing the alert function -->
<button onclick="canvas.respondToButton()">Show Alert</button>
<method name="respondToButton" >
        //Prepare a string
        var somestring =
                "Hey, I am setting the a1 attributes value";

        //Write a debug to see it
        Debug.write(somestring);

        //Call the alert to see it
        showAlert(somestring);

        //Go ahead and set the attribute
        canvas.testInstance.setAttribute("a1",'satya');
        //The above will fire an event causing
        //"ona1Method" to execute.
        //You will see this in the debugger
</method>

</canvas>

Notice how the button click sets the attribute of the testInstance object that is declared using its class test. One common mistake while writing an LZX method is to put braces after a method's name; you don't need them.

When you run this example, clicking the button brings up the alert seen in Figure 1.

Event-firing example
Figure 3. Event-firing example

Using a Script Instead of a Method

Laszlo also allows a <script> tag to define functions and other procedural logic. To show this, let me modify the code above to include some scripting and briefly note the characteristics of an LZX script.


<canvas width="800" debug="true">
<debug x="450" y="0" height="300"/>

<!-- The previous respond method -->
<method name="respondToButton" >
        //Prepare a string
        var somestring = 
            "Hey, I am setting the a1 attributes value";
        //Write a debug to see it
        Debug.write(somestring);

        //Call the alert to see it
        showAlert(somestring);

        //Go ahead and set the attribute
        canvas.testInstance.setAttribute("a1",'satya');

        //The above will fire an event causing
        //"ona1Method" to execute.
        //You will see this in the debugger
</method>

<!-- Your method rewritten as a script -->
<script>
function respondToButtonUsingScript()
{
        var somestring = 
            "Hey, I am setting the a1 attributes value";
        Debug.write(somestring);
        showAlert(somestring);
        canvas.testInstance.setAttribute("a1",'satya');
}
</script> 
<!-- Button now calling the global script function -->
<button onclick="respondToButtonUsingScript()">
    Show Alert
</button>


<class name="test">
        <attribute name="a1" type="string"/>
        <attribute name="b1"/>
</class>
<!--Instantiating a class-->
<test name="testInstance">
        <!--Demonstrating an onchange for attribute event-->
        <method event="ona1" name="ona1Method">
                Debug.write("hey this works");
        </method>
</test>
<!-- Simulating an alert -->
<!-- A modal alert dialog -->
<alert name="myalert" width="100" y="100">
   hi - initial text
</alert>
<!-- A method to work the modal dialog -->
<method name="showAlert" args="displayText">
   canvas.myalert.setAttribute("text",displayText);
   canvas.myalert.open();
</method>
</canvas>

Laszlo's script is similar to JavaScript and follows similar, if not the same, conventions. In Laszlo, a script tag is allowed only inside of the root canvas object and nowhere else. Any code inside of the script, unless it is a function, is executed immediately. That means that a script tag can include code that is not inside of any function. I tend to call this sort of code an "inline code segment."

Needless to say, a script tag can contain function definitions. These functions are considered global. In contrast, an LZX's methods belong to the respective nodes under which they are defined. For instance, a method that is defined using a method tag inside of the root canvas belongs to the canvas object and is scoped and named accordingly.

Inside of a script, you can use Java-style (//) line comments. Because the script tag is inside of another XML tag, it is sometimes necessary to include the body of the script in a CDATA section to avoid conflict with any < or > characters.

Just like in JavaScript, if a variable is declared without a var, then that variable is considered global. It is better to declare variables with a var scope.

Related Reading

Java and XML
Solutions to Real-World Problems
By Brett McLaughlin

Where to Go from Here

What I have covered so far about Laszlo falls under language fundamentals. LZX is a fusion of three language concepts: XML, OO programming, and JavaScript. You will need to spend some time with LZX to know how these three concepts are melded together, especially in the use of instance methods. Also, in JavaScript, the terms object, dictionary, and array can be used synonymously: this idea is used more extensively in LZX. You will need to brush up on these concepts of JavaScript one more time to develop successfully in Laszlo.

I also have touched briefly on Laszlo's visual programming model, involving components and events. Obviously, there is a lot more to the visual programming side if you want to do practical programs. In particular, you will need to master the various visual components that come with Laszlo. For example, you'll find a grid control and a tree control among the list of components that you can use to build your applications.

One area I haven't even touched in this article is the data handling side of Laszlo. It uses data binding and XML navigation extensively. Briefly, Laszlo uses URLs to retrieve XML data sets that are bound to visual controls. The relationship between the UI and the data is lot closer. Data affects the UI much more substantially in Laszlo. For example, a UI component bound to an XML node that has siblings will get replicated (if needed).

Conclusion

This introductory article gave you a starting point for experimenting with Laszlo and starting to explore it. The Laszlo documentation links in the Resources section of this article are a good place to start reading more about the language elements. I also keep running notes on Laszlo as I experiment with it, and keep Laszlo sample code on my weblogging system "akc."

Resources

Satya Komatineni is the CTO at Indent, Inc. and the author of Aspire, an open source web development RAD tool for J2EE/XML.


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