org.apache.tapestry.corelib.components.Delegate

A component that does not do any rendering of its own, but will delegate to some other object that can do rendering. This other object may be a component or a org.apache.tapestry.Block (among other things).

[JavaDoc]

Component Parameters

NameTypeFlagsDefaultDefault PrefixDescription
toObjectRequiredpropThe object which will be rendered in place of the Delegate component. This is typically a specific component instance, or a org.apache.tapestry.Block.

Examples

The Delegate component allows us to be very flexible in how and what gets rendered, and in what order. In some cases, the object to be rendered may come from an entirely different page.

This example is simpler, and could easily be accomplished using an If component. We'll create a page that can be used for viewing or editting an object.

ViewAccount.tml

<html xmlns:t="http://tapestry.apache.org/schema/tapestry_5_0_0.xsd">
    <body>
        <h1>View Account</h1>

        <t:delegate to="activeBlock"/>

        <t:block id="view">
            <t:beandisplay object="account"/>

            <p><t:actionlink t:id="edit">Edit this account</t:actionlink></p>
        </t:block>

        <t:block id="edit">
            <t:beaneditform t:id="account"/>
        </t:block>
    </body>
</html>

So we end up with a display of the Account's properties, and a link to activate edit mode. In edit mode, we use the other block and show a BeanEditForm.

ViewAccount.java

public class ViewAccount
{
    @Persist
    private Account _account;

    @Persist
    private boolean _editMode;

    @Inject
    private Block _view;

    @Inject
    private Block _edit;

    @Inject
    private AccountDAO _accountDAO;

    public Account getAccount()
    {
        return _account;
    }

    public void setAccount(Account account)
    {
        _account = account;
        _editMode = false;
    }

    void onSuccess()
    {
         _accountDAO.update(_account);

        _editMode = false;
    }

    void onActionFromEdit()
    {
        _editMode = true;
    }

    public Object getActiveBlock()
    {
        return _editMode ? _edit : _view;
    }
}

The use of the @Inject annotation on a field of type Block is used to access a <t:block> element from the template. The field name, stripped of leading underscores, is matched against the block's id.

The rest is concerned with handling the form submission, turning on edit mode, and determining which block will be the one to render.

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