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Page And Component Classes

Main article: Component Classes

What's the difference between a page and a component?

There's very little difference between the two. Pages classes must be in the root-package.pages package; components must be in the root-package.components. Pages may provide event handlers for certain page-specific events (such as activate and passivate). Components may have parameters.

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How do I store my page classes in a different package?

Tapestry is very rigid here; you can't. Page classes must go in root-package.pages, component classes in root-package.components, etc.

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Why do my instance variables have to be private?

Tapestry does a large amount of transformation to your simple POJO classes as it loads them into memory. In many cases, it must locate every read or write of an instance variable and change its behavior; for example, reading a field that is a component parameter will cause a property of the containing page or component to be read.

Limiting fields to private means that Tapestry can do the necessary processing one class at a time, as needed, at runtime. More complex Aspect Orient Programming systems such as AspectJ can perform similar transformations (and much more complex ones), but requires a dedicated build step (or the introduction of a JVM agent).

Why don't my informal parameters show up in the rendered markup?

Getting informal parameters to work is in two steps. First, you must make a call to the ComponentResources.renderInformalParameters() method, but just as importantly, you must tell Tapestry that you want the component to support informal parameters, using the SupportsInformalParameters annotation. Here's a hypothetical component that displays an image based on the value of a Image object (presumably, a database entity):

@SupportsInformalParameters
public class DBImage
{
  @Parameter(required=true)
  private Image image;

  @Inject
  private ComponentResources resources;

  boolean beginRender(MarkupWriter writer)
  {
    writer.element("img", "src", image.toClientURL(), "class", "db-image");

    resources.renderInformalParameters(writer);

    writer.end();

    return false;
  }
}

Why do I get java.lang.LinkageError when I invoke public methods of my page classes?

In Tapestry, there are always two versions of page (or component) classes. The first version is the version loaded by standard class loader: the simple POJO version that you wrote.

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Just be sure to put the interface class in a non-controlled package, such as your application's root-package (and not root-package.pages).

Which is better, using magic method names (i.e., beginRender()) or annotations (i.e. BeginRender)?

There is no single best way; this is where your taste may vary. Historically, the annotations came first, and the method naming conventions came later.

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Why do I have to inject a page? Why can't I just create one using new?

Tapestry tranforms your class at runtime. It tends to build a large constructor for the class instance. Further, an instance of the class is useless by itself, it must be wired together with its template and its sub-components.

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