Component Parameters
Component parameters are a critical aspect of Tapestry. It is not enough that an instance of a component class exists, it must be configured to do the right thing. Configuration is in terms of the parameters of the component.
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.
Parameters are defined by using the @Parameter annotation on a private field.
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 container is the page, but since components can have themselves have templates, sometime the container of a component is another component.
The connection 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.
This is important in a lot of cases; for example a TextField component can read and update the property bound to its value parameter. It reads the value when rendering, but updates the value when the form is submitted.
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 value parameter bound to a property of its container, it will automatically count up or down depending on whether start or end is larger.
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
private int start = 1;
@Parameter(required = true)
private int end;
@Parameter
private int value;
private boolean increment;
@SetupRender
void initializeValue()
{
value = start;
increment = start < end;
}
@AfterRender
boolean next()
{
if (increment)
{
int newValue = value + 1;
if (newValue <= end)
{
value = newValue;
return false;
}
}
else
{
int newValue = value - 1;
if (newValue >= end)
{
value = newValue;
return false;
}
}
return true;
}
}
The name of the parameter is derived from the field name (by stripping leading "_" and "$" characters). Here, the parameter names are "start", "end" and "value".
Binding Parameters
The component above can be referenced in another component or page template:
<html t:type="layout" xmlns:t="http://tapestry.apache.org/schema/tapestry_5_1_0.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.
Any number of parameters may be bound this way.
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 precendence over parameter bindings in the template.
Binding Expressions
The value inside the template, "3" in the previous example, is a binding expression.
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 value from the component's message catalog. |
prop |
A property expression to read or update. |
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. |
Most of these binding prefixes allow parameters to be bound to read-only values; for instance a parameter bound to "message:some-key" will see the message for "some-key" from its container's message catalog in the field. If the component tries to update the parameter (by setting the value of the field), a runtime exception will be thrown to indicate that the value is read-only.
Only prop: and var: binding prefixes are updateable.
Parameters have a default prefix, usually "prop:", that is used when the prefix is not provided.
A special prefix, "inherit:", is used to support Inherited Parameter Bindings.
Render Variables
Components can have any number 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 simply values, such as a loop index, that needs to be passed from one component to another.
For example:
<ul>
<li t:type="loop" source="1..10" value="index">${index}</li>
</ul>
And in the Java code:
@Property private int index;
... could be rewritten as just:
<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 variables value but you can't reference any of the value's properties.
Render variables are automatically cleared when a component finishes rendering.
Render variable names are case insensitive.
Property Expression Bindings
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. Property expressions have their own documentation.
The default binding prefix in most cases is "prop:", which is why it is usually omitted.
Validate Bindings
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 validator types. These are short aliases that 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 presumably enforce that a field requires a value, with at least five characters.
Translate Bindings
The "translate:" binding prefix is also related to input validator. It is the name of a 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 TranslatorSource service.
Informal Parameters
Some 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.
Only components whose class is annotated with @SupportsInformalParameters will support informal parameters.
Informal parameters are often used to set the CSS class of an element, or to specify client-side event handlers.
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 stream.
Asset Bindings
Assets bindings are normally relative to the component class. 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.
Context Bindings
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.:
<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.
Supporting Informal Parameters
Only components which area annotated with @SupportsInformalParameters may have informal parameters. Tapestry silently drops informal parameters that are specified for components that do not have this annotation.
To render informal parameters, inject the ComponentResources for your component and invoke the renderInformalParameters() method.
Parameters Are Bi-Directional
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.
<t:layout xmlns:t="http://tapestry.apache.org/schema/tapestry_5_1_0.xsd">
<p> Countdown:
<t:count start="5" end="1" value="index">
${index} ...
</t:count>
</p>
</t:layout>
Because the Count component updates its value parameter (the _value 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:
<p> Countdown: 5 ... 4 ... 3 ... 2 ... 1 ... </p>
(Though the whitespace will be quite different.)
The relevant part is that components can read fixed values, or live properties of their container, and can change properties of their container as well.
Required Parameters
Parameters that are required must be bound. A runtime exception occurs if a component has unbound required parameters.
Optional Parameters
Parameters which are not required, are optional.
You may set a default value for optional parameters as you would for any other field. In the Count component, the min parameter has a default value of 1. That value is used unless the min parameter is bound, in which case, the bound value supercedes the default.
Inherited Parameter Bindings
A special prefix, "inherit:" is used to identify the name of a parameter of the containing component. If the parameter is bound in the containing component, then it will be bound to the same value in the embedded component.
If the parameter is not bound in the containing component, then it will not be bound in the embedded component (and so, the embedded component may use a default binding).
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.
More examples on this coming soon.
Parameter Binding Defaults
The @Parameter annotation's value() attribute can be used to specify a binding expression that will be the default binding for the parameter is otherwise left unbound. Typically, this is the name of a property that that will compute the value on the fly.
Example:
@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.
Computed Parameter Binding Defaults
In rare cases, you may want to compute the binding to be used as a parameter default. In this case, you will provide a default binding method, a method that takes no parameters. The returned value is used to bind the parameter. The return value may be a Binding instance, or it may be a simple value (which is more often the case).
The method name is "default" plus the capitalized name of the parameter.
Using this approach, the previous example may be rewritten as:
@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);
}
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:
@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.
Obviously, this is a lot more work than simply specifying a default value as part of the @Parameter annotation. In the few real cases where this is approach is used, the default binding method will usually deduce a proper binding, typically in terms of the component's id. For example, the TextField component will deduce a value parameter that binds to a property of its container with the same name.
A default binding method will only be invoked if the @Parameter annotation does not provide a default value.
Unbound Parameters
If a parameter is not bound (and is optional), then the value may be read or updated at any time.
Updates to unbound parameters cause no side effects. In the first example, the value parameter of the Count component is not bound, and this is perfectly valid.
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 Caching
Reading a parameter value can be marginally expensive (because of type coercion). Therefore, it makes sense to cache the parameter value, at least while the component is actively rendering itself.
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.
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.
Parameter Names
By default, Tapestry converts from the field name to the parameter name, by stripping off leading "$" and "_" characters.
This can be overridden using the name() attribute of the @Parameter annotation.
Determining if Bound
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:
public class MyComponent
{
@Parameter
private int myParam;
@Inject
private ComponentResources resources;
@BeginRender
void setup()
{
if (resources.isBound("myParam"))
{
. . .
}
}
}
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.
The @Inject annotation will inject the ComponentResources for the component. These resources are the linkage between the Java class you provide, and the infrastructure Tapestry builds around your class. In any case, once the resources are injected, they can be queried.
Publishing Parameters
Often when creating new components from existing components, you want to expose some of the functionality of the embedded component, in the form of exposing parameters of the embedded components as parameters of the outer component.
In Tapestry 5.0, you would define a parameter of the outer component, and use the "inherit:" binding prefix to connect the inner component's parameter to the outer component's parameter. This is somewhat clumsy, as it involves creating an otherwise unused field just for the parameter; in practice it also leads to duplication of the documentation of the parameter.
In Tapestry 5.1, 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.
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.