Component parameters are the primary means for a component instance and its container to communicate with each other. Parameters are used to configure component instances.
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In the following example, page
is a parameter of the pagelink
component. The page parameter tells the pagelink component which page to go to when the user clicks on the rendered hyperlink:
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<html xmlns:t="http://tapestry.apache.org/schema/tapestry_5_ |
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4.xsd"> <t:pagelink page="Index">Go Home</t:pagelink> </html> |
A component may have any number of parameters. Each parameter has a specific name, a specific Java type (which may be a primitive value), and may be optional or required.
Within a component class, parameters are declared by using the @Parameter annotation on a private field, as we'll see below.
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Parameter Bindings
In Tapestry, a parameter is not a slot into which data is pushed: it is a connection between a field of the component (marked with the @Parameter annotation) and a property or resource of the component's container. (Components can be nested, so the container can be either the page or another component.)
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The connection between a component and a property (or resource) of its container is called a binding. The binding is two-way: the component can read the bound property by reading its parameter field. Likewise, a component that updates its parameter field will update the bound property.
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The component listed below is a looping component; it renders its body a number of times, defined by its start
and end
parameters (which set the boundaries of the loop). The component can update a result
parameter bound to a property of its container; it will automatically count up or down depending on whether start
or end
is larger.
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package org.example.app.components; import org.apache.tapestry5.annotations.AfterRender; import org.apache.tapestry5.annotations.Parameter; import org.apache.tapestry5.annotations.SetupRender; public class Count { @Parameter (value="1") private int start; @Parameter(required = true) private int end; @Parameter private int result; private boolean increment; @SetupRender void initializeValues() |
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{
result = start;
increment = start < end;
}
@AfterRender
boolean next()
{
if (increment)
{
int newResult = value + 1;
if (newResult <= end)
{
result = newResult;
return false;
}
}
else
{
int newResult= value - 1;
if (newResult>= end)
{
result = newResult;
return false;
}
}
return true;
}
}
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The name of the parameter is the same as field name (except with leading "_" and "$" characters, if any, removed). Here, the parameter names are "start", "end" and "result".
The component above can be referenced in another component or page template, and its parameters bound:
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<html t:type="layout" xmlns:t="http://tapestry.apache.org/schema/tapestry_5_ |
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4.xsd"> <p> Merry Christmas: <t:count end="3"> Ho! </t:count> </p> </html> |
The end attribute is used to bind the end parameter of the Count component. Here, it is being bound to the string value "3", which is automatically coerced by Tapestry into the int value, 3.
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Component parameters may also be bound using the @Component annotation inside the component class. (Where conflicts occur, the parameters bound using the Component annotation will take precedence over parameter bindings in the template.)
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Binding Expressions
The value inside the template, "3" in the previous example, is a binding expression.
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Prefix | Description |
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asset: | The relative path to an asset file (which must exist) |
block: | The id of a block within the template |
component: | The id of another component within the same template |
context: | Context asset: path from context root |
literal: | A literal string |
nullfieldstrategy: | Used to locate a pre-defined NullFieldStrategy |
message: | Retrieves a string from the component's message catalog |
prop: | A property expression to read or update |
symbol: | Used to read one of your symbols |
translate: | The name of a configured translator |
validate: | A validator specification used to create some number of field validators |
var: | Allows a render variable of the component to be read or updated |
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For example, the following template code:
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<ul> <li t:type="loop" source="1..10" value="index">${index}</li> </ul> |
and the following Java code:
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@Property private int index; |
... could be rewritten as just:
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<ul> <li t:type="loop" source="1..10" value="var:index">${var:index}</li> </ul> |
In other words, you don't have to define a property in the Java code. The disadvantage is that render variables don't work with the property expression syntax, so you can pass around a render variable's value but you can't reference any of the value's properties.
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Render variable names are case insensitive.
Property: Bindings
Main Article: Property Expressions
The "prop:" binding prefix indicates a property expression binding.
Property expressions are used to link a parameter of a component to a property of its container. Property expressions can navigate a series of properties and/or invoke methods, as well as several other useful patterns. See Property Expressions.
The default binding prefix in most cases is "prop:", which is why it is usually omitted.
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The list of available translators is configured by the TranslatorSource service.
Asset: Bindings
Main Article: Assets
Assets bindings are used to specify Assets Component Parameters, static content served by Tapestry. By default, assets are located relative to the component class in your packaged application or module. This can be overridden by prefixing the path with "context:", in which case, the path is a context path from the root of the web application context. Because accessing context assets is relatively common, a separate "context:" binding prefix for that purpose exists (described below).
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Context: Bindings
Main Article: Assets
Context bindings are like asset bindings, but the path is always relative to the root of the web application context. This is intended for use inside templates, i.e.:
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<img src="${context:images/icon.png}"/> |
Tapestry will adjust the URL of the image so that it is processed by Tapestry, not the servlet container. It will gain a URL that includes the application's version number, it will have a far-future expires header, and (if the client supports it) its content will be compressed before being sent to the client.
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Parameters that are required must be bound. A runtime exception occurs if a component has unbound required parameters.
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public class Component{ @Parameter(required = true) private String parameter; } |
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Sometimes a parameter is marked as required, but may still be omitted if the underlying value is provided by some other means. This is the case, for example, with the Select component's value parameter, which may have its underlying value set by contributing a ValueEncoderSource. Be sure to read the component's parameter documentation carefully. Required simply enables checks that the parameter is bound, it does not mean that you must supply the binding in the template (or @Component annotation). |
Optional Parameters
Parameters are optional unless they are marked as required.
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The @Parameter annotation's value
element can be used to specify a binding expression that will be the default binding for the parameter if otherwise left unbound. Typically, this is the name of a property that that will compute the value on the fly.
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@Parameter(value="defaultMessage") // or, equivalently, @Parameter("defaultMessage") private String message; @Parameter(required=true) private int maxLength; public String getDefaultMessage(){ return String.format("Maximum field length is %d.", maxLength); } |
As elsewhere, you may use a prefix on the value. A common prefix to use is the "message:" prefix, to access a localized message.
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In rare cases, it is desirable to defeat the caching; this can be done by setting the cache() attribute of the @Parameter annotation to false.
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Don't use the ${...} syntax!
Main Article: Component Templates#Expansions Expansions
You generally should not use the Template Expansion syntax, ${...}, within component parameter bindings. Doing so results in the property inside the braces being converted to an (immutable) string, and will therefore result in a runtime exception if your component needs to update the value (whenever the default or explicit binding prefix is prop:
or var:
, since such component parameters are two-way bindings).
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The general rule is, only use the ${...} syntax in non-Tapestry-controlled locations in your template, such as in attributes of ordinary HTML elements and in plain-text areas of your template.
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Informal Parameters
Main Article: Supporting Informal Parameters
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If your component should render informal parameters, just inject the ComponentResources for your component and invoke the renderInformalParameters()
method. See See Supporting Informal Parameters for an example of how to do this.
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Parameters are not simply variables; each parameter represents a connection, or binding, between a component and a property of its container. When using the prop: binding prefix, the component can force changes into a property of its container, just by assigning a value to its own instance variable.
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<t:layout xmlns:t="http://tapestry.apache.org/schema/tapestry_5_ |
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4.xsd |
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"> <p> Countdown: <t:count start="5" end="1" result="index"> ${index} ... </t:count> </p> </t:layout> |
Because the Count component updates its result parameter (the result
field), the index property of the containing component is updated. Inside the Count's body, we output the current value of the index property, using the expansion ${index
}. The resulting output will look something like:
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<p> Countdown: 5 ... 4 ... 3 ... 2 ... 1 ... </p> |
(Though the whitespace will be quite different.)
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Inherited bindings are useful for complex components; they are often used when an inner component has a default value for a parameter, and the outer component wants to make it possible to override that default.
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<html xmlns:t="http://tapestry.apache.org/schema/tapestry_5_ |
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4.xsd"> <body> <div t:type="layout" t:menuTitle="literal:The Title"> ... </div> </body> </html> |
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<t:container xmlns:t="http://tapestry.apache.org/schema/tapestry_5_ |
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4.xsd"> <div t:type="title" t:title="inherit:menuTitle"></div> <t:body /> </t:container> |
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package org.example.app.components; import org.apache.tapestry5.annotations.Parameter; public class Title { @Parameter private String title; } |
Computed Parameter Binding Defaults
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Using this approach, the previous example may be rewritten as:
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@Parameter private String message; @Parameter(required=true) private int maxLength; @Inject private ComponentResources resources; @Inject private BindingSource bindingSource; Binding defaultMessage() { return bindingSource.newBinding("default value", resources, "basicMessage") |
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; } public String getBasicMessage() { return String.format("Maximum field length is %d.", maxLength); } |
In this example, a property expression, "basicMessage", is used to access the message dynamically.
Alternately, the previous example may be written even more succinctly as:
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@Parameter private String message; @Parameter(required=true) private int maxLength; @Inject private ComponentResources resources; String defaultMessage() { return String.format("Maximum field length is %d.", maxLength); } |
This form is more like using the "literal:" binding prefix, except that the literal value is computed by the defaultMessage() method.
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Parameter Type Coercion
Main Article: Parameter Type Coercion
Tapestry includes a mechanism for coercing types automatically. Most often, this is used to convert literal strings into appropriate values, but in many cases, more complex conversions will occur. This mechanism is used for component parameters, such as when an outer component passes a literal string to an inner component that is expecting an integer.
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In rare cases, you may want to take different behaviors based on whether a parameter is bound or not. This can be accomplished by querying the component's resources, which can be injected into the component using the @Inject annotation:
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public class MyComponent { @Parameter private int myParam; @Inject private ComponentResources |
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componentResources; @BeginRender void setup() { if ( |
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componentResources.isBound("myParam")) |
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{
. . .
}
}
}
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The above sketch illustrates the approach. Because the parameter type is a primitive type, int, it is hard to distinguish between no binding, and binding explicitly to the value 0.
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In Tapestry 5.1 and later, you may use the publishParameters attribute of the @Component annotation. List one or more parameters separated by commas: those parameters of the inner/embedded component become parameters of the outer component. You should not define a parameter field in the outer component.
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<t:container xmlns:t="http://tapestry.apache.org/schema/tapestry_5_ |
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4.xsd"> <t:pageLink t:id="link">Page Link</t:pageLink> </t:container> |
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public class ContainerComponent{ @Component(id="link", publishParameters="page") private PageLink link; } |
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<t:ContainerComponent t:id="Container" t:page="About" /> |
There are still cases where you want to use the "inherit:" binding prefix. For example, if you have several components that need to share a parameter, then you must do it the Tapestry 5.0 way: a true parameter on the outer component, and "inherit:" bindings on the embedded components. You can follow a similar pattern to rename a parameter in the outer component.
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