Binding Model Data in Polymer Templates
Note | Use Lit templates instead Lit templates are recommended. Polymer templates are available in the next long term supported Vaadin version (LTS), but they are deprecated. |
The way model values are bound to different parts of the element tree that is defined by the template is at the core of the PolymerTemplate
API. See Creating A Simple Component Using the PolymerTemplate API for a simple example of how to set model values.
The first three sections below demonstrate how to bind text content, property values, and attribute values. The JavaScript template examples in these sections all use the following example Java template and model class
Example: PolymerBindingTemplate
template class.
Java
@Tag("my-template")
@JsModule("./com/example/my-template.js")
public class PolymerBindingTemplate extends PolymerTemplate<BindingModel> {
public PolymerBindingTemplate() {
getModel().setHostProperty("Bound property");
}
}
Example: BindingModel
model interface.
Java
public interface BindingModel extends TemplateModel {
void setHostProperty(String propertyValue);
String getHostProperty();
}
Binding Text Content
To bind text content in a JavaScript Polymer template, use the \[[*propertyName*]]
syntax inside a tag. Double square brackets ([[ ]]) indicate a one-way binding.
Example: Binding text content in a JavaScript Polymer template.
js
class MyTemplate extends PolymerElement {
static get template() {
return html`<div>[[hostProperty]]</div>`;
}
static get is() {return 'my-template'}
}
customElements.define(MyTemplate.is, MyTemplate);
Binding Property Values
To set an element property value based on a model, use the property name in the attribute format, by using the dash-case format, sometimes referred to as kebab-case, e.g. my-property-name
(not camelCase myPropertyName
).
Example: Setting an element property based on a model in a JavaScript Polymer template.
js
return html`<my-element my-property="[[hostProperty]]"></my-element>`;
- This binds to the target property,
myProperty
, on<my-element>
.
Note:
name="[[binding]]"
defines that the element property namedname
should get it’s value from the model property namedbinding
.name="binding"
(without brackets) defines that the element attribute namedname
should have the valuebinding
, regardless of any value in the model.
Polymer cannot bind certain common native element properties directly, because the binding causes issues on one or more browsers. In these cases, you can use attribute bindings instead. See Native properties that don’t support property binding for more.
Binding Attribute Values
The binding, <div something="[[hostProperty]]"></div>
, is bound to the property something
, because the property can typically be changed on the fly, while the attribute is typically used only for the initial value.
To explicitly bind to an attribute, use the attribute name followed by a dollar sign ($
).
Example: Binding to an attribute in an JavaScript Polymer template.
js
return html`<div something$="[[hostProperty]]"></div>`;
or
js
return html`<a href$="[[hostProperty]]"></a>`;
Two-way Versus One-way Bindings
Text surrounded by double curly bracket {{ }} or double square bracket [[ ]] delimiters identifies the host data that is bound:
Double curly brackets ({{ }}) indicate two-way bindings: both server-to-client and client-to-server data flow.
Double square brackets ([[ ]]) indicate one-way bindings: only server-to-client data flow.
Creating Two-way Data Bindings
For two-way data binding, data flows in both directions: client-to-server and server-to-client.
To demonstrate we create:
A two-way binding model class with a number of fields.
A Java template class that sets default values for the model, and adds listeners for
save
andreset
events.An associated JavaScript Polymer template.
Example: TwoWayBindingModel
with name
, accepted
and size
fields.
Java
public interface TwoWayBindingModel extends TemplateModel {
void setName(String name);
String getName();
void setAccepted(Boolean accepted);
Boolean getAccepted();
void setSize(String size);
String getSize();
}
Example: PolymerTwoWayBindingTemplate
Java template class that defines save
and reset
event property change listeners, and sets default values for the name
, accepted
and size
fields.
Java
@Tag("two-way-template")
@JsModule("./com/example/two-way-template.js")
public class PolymerTwoWayBindingTemplate
extends PolymerTemplate<TwoWayBindingModel> {
public PolymerTwoWayBindingTemplate() {
reset();
getElement().addPropertyChangeListener("name", event -> System.out
.println("Name is set to: " + getModel().getName()));
getElement().addPropertyChangeListener("accepted",
event -> System.out.println("isAccepted is set to: "
+ getModel().getAccepted()));
getElement().addPropertyChangeListener("size", event -> System.out
.println("Size is set to: " + getModel().getSize()));
}
@EventHandler
private void reset() {
getModel().setName("John");
getModel().setAccepted(false);
getModel().setSize("medium");
}
}
The
Element::addPropertyChangeListener
method gets immediate updates when the property values change. As an alternative, you could define an@EventHandler
method on the server side and add appropriate event handers in the template.On the client, we use the following methods to bind the model data (see JavaScript template below):
name
string to an input using:Native input element.
Polymer element
paper-input
.
accepted
boolean to a checkbox using:Native checkbox input.
Polymer element
paper-check-box
.
size
string to a select element using:Native select.
Polymer elements
paper-radio-group
andpaper-radio-button
.
Note | Native elements need to specify a custom-change event name in the annotation using the |
Example: Polymer JavaScript template.
js
import {PolymerElement,html} from '@polymer/polymer/polymer-element.js';
import '@polymer/paper-input/paper-input.js';
import '@polymer/paper-radio-button/paper-radio-button.js';
import '@polymer/paper-radio-group/paper-radio-group.js';
import '@polymer/paper-checkbox/paper-checkbox.js';
class TwoWayBinding extends PolymerElement {
static get template() {
return html`
<table>
<tr>
<td>Paper name:</td>
<td>
<paper-input value="{{name}}"></paper-input>
</td>
</tr>
<tr>
<td>Input name:</td>
<td>
<input value="{{name::input}}">
</td>
</tr>
<tr>
<td>Change name:</td>
<td>
<input value="{{name::change}}">
</td>
</tr>
<tr>
<td>Input accepted:</td>
<td>
<input type="checkbox" checked="{{accepted::change}}">
</td>
</tr>
<tr>
<td>Polymer accepted:</td>
<td>
<paper-checkbox checked="{{accepted}}"></paper-checkbox>
</td>
</tr>
<tr>
<td>Size:</td>
<td>
<paper-radio-group selected="{{size}}">
<paper-radio-button name="small">Small</paper-radio-button>
<paper-radio-button name="medium">Medium</paper-radio-button>
<paper-radio-button name="large">Large</paper-radio-button>
</paper-radio-group>
</td>
</tr>
<tr>
<td>Size:</td>
<td>
<select value="{{size::change}}">
<option value="small">Small</option>
<option value="medium">Medium</option>
<option value="large">Large</option>
</select>
</td>
</tr>
</table>
<div>
<button on-click="reset">Reset values</button>
</div>
<slot></slot>`;
}
static get is() {
return 'two-way-template';
}
}
customElements.define(TwoWayBinding.is, TwoWayBinding);
We use two-way bindings for each element.
Some elements bind to the same property. For example, when the value for
name
is changed in thepaper-input
element, the new value reflects in bothInput name
andChange name
.The two input bindings,
Input name
andChange name
, work in slightly different ways:Input name
binds using{{name::input}}
andChange name
binds using{{name::change}}
. The giventarget-change-event
lets Polymer know which event to listen to for change notifications.The functional difference is that
::input
updates during typing, and::change
updates when the value of the field changes, for example anonBlur
event or Enter key press.
Here’s the template representation in the browser:
For information on the <slot></slot>
element, see Using <slot> in PolymerTemplates for more.