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In essence, a Tapestry application is a number of related pages, working together. To some degree, each page is like an application unto itself.
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This dichotomy between component event requests and render requests alleviates a number of problems in traditional web applications related to the browser back button, or to the user hitting the refresh button in their browser.
Logical Page Name Shortening
In certain cases, Tapestry will shorten the the logical name of a page. For example, the page class org.example.pages.address.CreateAddress will be given a logical name of "address/Create" (the redundant "Address" is removed as a suffix). However, this only affects how the page is referenced in URLs; the template file will still be CreateAddress.tml, whether on the classpath, or as address/CreateAddress.tml (in the web context).
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Component Event Requests & Responses
Main Article: Component Events Page Navigation
Component event requests may take the form of hyperlinks (EventLink or ActionLink) or form submissions (Form).
The value returned from an event handler method controls the response sent to the client web browser.
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The user will see the newly generated content in their browser. In addition, the URL in the browser's address bar will be a render request URL. Render request URLs are shorter and contain less application structure (for instance, they don't include component ids or event types). Render requests URLs are what your users will bookmark. The component event request URLs are transitory, meaningful only while the application is actively engaged, and not meant to be used in later sessions.
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public Object onAction(){ return null; } |
2. String response
When a string is returned, it is expected to be the logical name of a page (as opposed to the page's fully qualified class name). As elsewhere, the name of the page is case insensitive.
Again, a render request URL will be constructed and sent to the client as a redirect.
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public String onAction(){ return "Index"; } |
3. Class response
When a class is returned, it is expected to be a page class. Returning a page class from an event handler is safer for refactoring than returning a page name.
As with other response types, a render request URL will be constructed and sent to the client as a redirect.
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public Object onAction(){ return Index.class } |
4. Page response
You may also return an instance of a page, rather than the name or class of a page.
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You can also return a component within the page, but this will generate a runtime warning (unless you are doing a partial-page update via Ajax).
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@InjectPage private Index index; public Object onAction(){ return index; } |
5. HttpError
An event handler method may return a HttpError instance to send an error response to the client.
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public Object onAction(){ return new HttpError(302, "The Error message); } |
6. Link response
An event handler method may return a Link instance directly. The Link is converted into a URL and a client redirect to that URL is sent to the client.
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An event handler can also return a StreamResponse object, which encapsulates a stream to be sent directly to the client browser. This is useful for components that want to, say, generate an image or PDF and provide it to the client:
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public Object onAction(){ return new StreamResponse() { @Override public String getContentType() |
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{ return "application/pdf"; } @Override public InputStream getStream() throws IOException { return new ByteArrayInputStream(getMyPdfByteArray()); } @Override public |
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void prepareResponse(Response response) { response.setHeader("Content-Disposition", "attachment; filename=\"" + myFileName + "\""); } }; } |
8. URL response
A java.net.URL response is handled as a client redirect to an external URL. (In Tapestry 5.3.x and earlier this only works for non-Ajax requests.)
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When a page has an activation context, the values of the context are appended to the URL path. For example, in http://www.example.com/myapp/foo/bar
the "myapp" part is the servlet context (usually the name of your app), and the "foo/bar" part is the activation context, with "foo" being the first activation parameter and "bar" being the second.
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When no explicit activation context is provided, the page itself is queried for its activation context. This querying takes the form of an event trigger. The event name is "passivate" (as we'll see shortly, there's a corresponding "activate"). The return value of the method is used as the context. For example:
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public class ProductDetail { private Product product; . . . long onPassivate() { return product.getId(); } } |
The activation context may consist of a series of values, in which case the return value of the method should be an array or a List.
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Note: If you are using the tapestry-hibernate integration library and your passivate context is a Hibernate entity, then you can just use the entity itself, not its id. Tapestry will automatically extract the entity's id into the URL, and convert it back for the "activate" event handler method. |
Page activation
When a page render request arrives, the page is activated before it is rendered.
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{float:right|background=#eee|padding=0 1em} *JumpStart Demos:* [onActivate and onPassivate|http://jumpstart.doublenegative.com.au/jumpstart/examples/navigation/onactivateandonpassivate/3] [Handling A Bad Context|http://jumpstart.doublenegative.com.au/jumpstart/examples/infrastructure/handlingabadcontext/1] {float} |
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Page activation uses Tapestry's Component Event mechanism. See Component Events Page Navigation for details.
A page's activate event handler mirrors its passivate handler:
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private Product product; . . . void onActivate(long productId) { product = productDAO.getById(productId); } . . . |
Here's the relevant part: when the page renders, it is likely to include more component event request URLs (links and forms). The component event requests for those links and forms will also start by activating the page, before performing other work. This forms an unbroken chain of requests that include the same activation context.
To some degree, this same effect could be accomplished using a persistent page value, but that requires an active servlet session, and the result is not bookmarkable.
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You sometimes need to handle multiple page activation scenarios in one page class. You could create multiple activate event handler methods with different arguments (see the "Multiple Method Matches" section at Component Events Page Navigation for details), but if you do so, you should generally return true
from each to avoid having more than one activation event handler method from being called for each page request. However, a better approach is to create one method with an EventContext argument. Tapestry will populate the EventContext argument with all of the activation parameters, and the EventContext's get
method will retrieve and coerce each parameter to the desired type. For example:
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. . . void onActivate(EventContext eventContext) { String color = DEFAULT_COLOR; if (eventContext.getCount() > 0) { long productId = eventContext.get(Long.class, 0); if (eventContext.getCount() > 1) { String color = eventContext.get(String.class, 1); } product = productDAO.getById(productId, color) |
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; } else { return SelectProduct.class; // no product selected } } . . . |
Page Navigation Patterns
This combination of action links and context and page context can be put together in any number of ways.
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In this pattern, the ProductListing page uses action events and a persistent field on the ProductDetails page.
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<t:loop source="products" value="product"> <a t:type="actionlink" t:id="select" context="product.id">${product.name}</a> </t: |
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@InjectPage private ProductDetails details; Object onActionFromSelect(long productId) { details.setProductId(productId); return details; } |
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@Inject private ProductDAO dao; private Product product; @Persist private long productId; public void setProductId(long productId) { this.productId = productId; } void onActivate() { product = dao.getById(productId); } |
This is a minimal approach, perhaps good enough for a prototype.
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- It requires a session (to store the productId field between requests).
- It may fail if the ProductDetails page is accessed before a valid product id is set.
- The URL does not indicate the identity of the product; if the user bookmarks the URL and comes back later, they will trigger the previous case (no valid product id).
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Pattern 2: Component Event Requests / No Persistent Data
We can improve the previous example without changing the ProductListing page, using a passivation and activation context to avoid the session and make the links more bookmarkable.
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@Inject private ProductDAO dao; private Product product; private long productId; public void setProductId(long productId) { productId = productId; } void onActivate(long productId) { this.productId = productId; product = dao.getById(productId); |
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} long onPassivate() { return productId; } |
This change ensures that the render request URL will include the product id, i.e., "http://.../productdetails/99".
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This is the most common version of this master/detail relationship.
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<t:loop source="products" value="product"> <a t:type="pagelink" page="productdetails" context="product.id">${product.name}</a> </t:loop> |
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No code is needed to support the link. |
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@Inject private ProductDAO dao; private Product product; private long productId; void onActivate(long productId) { this.productId = productId; product = dao.getById(productId); } long onPassivate() { return productId; } |
The setProductId() method is no longer needed.
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At some point, persistent values make more sense. Tapestry has several persistence strategies available, including one that stores data in URL query parameters. See Persistent Page DataNavigation for details.