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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Component Parameters
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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_34.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. In most simple examples, the component's container is the page, but since components can have templates, often the container of a component is another component.(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()
{
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".Anchor
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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_34.xsd"> <p> Merry Christmas: <t:count end="3"> Ho! </t:count> </p> </html> |
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By placing a prefix in front of the value, you can change how Tapestry interprets the remainder of the expression (the part after the colon):
Prefix | Description |
|---|---|
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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Only prop: and var: binding prefixes are updateable (but you must not use the ${..} syntax here; see the next section warning below).
Each parameter has a default prefix, defined by the component, that is used when the prefix is not provided. The most common are "literal:" and "prop:".
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Components can have any number of of render variables. Render variables are named values with no specific type (they are ultimately stored in a Map). Render variables are useful for holding simple values, such as loop indices, that need to be passed from one component to another.
For example, the following template code:
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<ul>
<li t:type="loop" source="1..10" value="index">${index}</li>
</ul>
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and the following Java code:
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@Property
private int index;
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... could be rewritten as just:
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<ul>
<li t:type="loop" source="1..10" value="var:index">${var:index}</li>
</ul>
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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 s value but 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. SeeProperty Expressions.
The default binding prefix in most cases is "prop:", which is why it is usually omitted.
Validate: Bindings
Main Article: Forms and Validation
The "validate:" binding prefix is highly specialized. It allows a short string to be used to create and configure the objects that perform input validation for form control components, such as TextField and Checkbox.
The string is a comma-separated list of of validator types. These are short aliases for objects that perform the validation. In many cases, the validation is configurable in some way: for example, a validator that enforces a minimum string length needs to know what that minimum string length is. Such values are specified after an equals sign.
For example: validate:required,minLength=5 would would presumably enforce that a field requires a value, and with at least five characters.
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The "translate:" binding prefix is also related to input validatorvalidation. It is the name of a configured configured Translator, responsible for converting between server-side and client-side representations of data (for instance, between client-side strings and server-side numeric values).
The list of available translators is configured by the the TranslatorSource service service.
Asset: Bindings
Main Article: Assets
Assets bindings are used to specify Component Parameters, static content served by Tapestry. By default, assets are located Assets bindings are normally 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).
Context: Bindings
Main Article: Assets
Context bindings are like asset bindings, but the path is is always relative 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}"/>
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Required Parameters
Parameters that are required required must be 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 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). |
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You may set a default value for optional parameters using the the value element element of the @Parameter annotation. In the Count component above, the start parameter has a default value of 1. That value is used unless the start parameter is bound, in which case, the bound value supersedes the default.anchorInherited_
Parameter
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Parameter Binding Defaults
The @Parameter annotation's s value element element can be used to specify a a binding expression that 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);
}
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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.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
Some Many components support informal parameters, additional parameters beyond the formally defined parameters. Informal parameters will be rendered into the output as additional attributes on the tag rendered by the component. Generally speaking, components that have a 1:1 relationship with a particular HTML tag (such as <TextField> and <input> will support informal parameters.
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The default binding prefix for informal parameters depends on where the parameter binding is specified. If the parameter is bound inside a Java class, within the @Component annotation, then the default binding prefix is "prop:". If the parameter is bound inside the component template, then the default binding prefix is "literal:". This reflects the fact that a parameter specified in the Java class, using the annotation, is most likely a computed value, whereas a value in the template should simply be copied, as is, into the result HTML streamthe template should simply be copied, as is, into the result HTML stream.
Informal parameters (if supported) are always rendered into the output unless they are bound to a property whose value is null. If the bound property is null then the parameter will not be present at all in the rendered output.
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_34.xsd"> <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>
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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_34.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_34.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;
}
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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");
}
public String getBasicMessage()
{
return String.format("Maximum field length is %d.", maxLength);
}
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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);
}
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Note: updates to such fields are temporary; when the component finishes rendering, the field will revert to its default value.
TODO: This seems contradictory. What does it mean to update an unbound component parameter when the component is not rendering?
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.
You can easily contribute new coercions for your own purposes.
Parameter Names
By default, Tapestry converts from the field name to the parameter name, by stripping off leading "$" and "_" characters.
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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 resourcescomponentResources; @BeginRender void setup() { if (resourcescomponentResources.isBound("myParam")) { . . . } } } |
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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_34.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" />
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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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