Base class for a _Container widget which is responsible for laying out its children. Widgets which mixin this code must define layout() to manage placement and sizing of the children.
| Parameter | Type | Description |
|---|---|---|
| params | Object | null | Hash of initialization parameters for widget, including scalar values (like title, duration etc.) and functions, typically callbacks like onClick. The hash can contain any of the widget's properties, excluding read-only properties. |
| srcNodeRef | DOMNode | String |
Optional If a srcNodeRef (DOM node) is specified:
|
See the dijit/layout/_LayoutWidget reference documentation for more information.
Path to a blank 1x1 image.
Used by <img> nodes in templates that really get their image via CSS background-image.
Deprecated. Instead of attributeMap, widget should have a _setXXXAttr attribute for each XXX attribute to be mapped to the DOM.
attributeMap sets up a "binding" between attributes (aka properties) of the widget and the widget's DOM. Changes to widget attributes listed in attributeMap will be reflected into the DOM.
For example, calling set('title', 'hello') on a TitlePane will automatically cause the TitlePane's DOM to update with the new title.
attributeMap is a hash where the key is an attribute of the widget, and the value reflects a binding to a:
DOM node attribute
focus: {node: "focusNode", type: "attribute"} Maps this.focus to this.focusNode.focus
DOM node innerHTML
title: { node: "titleNode", type: "innerHTML" } Maps this.title to this.titleNode.innerHTML
DOM node innerText
title: { node: "titleNode", type: "innerText" } Maps this.title to this.titleNode.innerText
DOM node CSS class
myClass: { node: "domNode", type: "class" } Maps this.myClass to this.domNode.className
If the value is an array, then each element in the array matches one of the formats of the above list.
There are also some shorthands for backwards compatibility:
"focusNode" ---> { node: "focusNode", type: "attribute" }
This class name is applied to the widget's domNode and also may be used to generate names for sub nodes, for example dijitTabContainer-content.
Designates where children of the source DOM node will be placed. "Children" in this case refers to both DOM nodes and widgets. For example, for myWidget:
<div data-dojo-type=myWidget>
<b> here's a plain DOM node
<span data-dojo-type=subWidget>and a widget</span>
<i> and another plain DOM node </i>
</div>
containerNode would point to:
<b> here's a plain DOM node <span data-dojo-type=subWidget>and a widget</span> <i> and another plain DOM node </i>
In templated widgets, "containerNode" is set via a data-dojo-attach-point assignment.
containerNode must be defined for any widget that accepts innerHTML (like ContentPane or BorderContainer or even Button), and conversely is null for widgets that don't, like TextBox.
Bi-directional support, as defined by the HTML DIR attribute. Either left-to-right "ltr" or right-to-left "rtl". If undefined, widgets renders in page's default direction.
This is our visible representation of the widget! Other DOM Nodes may by assigned to other properties, usually through the template system's data-dojo-attach-point syntax, but the domNode property is the canonical "top level" node in widget UI.
This widget or a widget it contains has focus, or is "active" because it was recently clicked.
A unique, opaque ID string that can be assigned by users or by the system. If the developer passes an ID which is known not to be unique, the specified ID is ignored and the system-generated ID is used instead.
Indicates that this widget is going to call resize() on its children widgets, setting their size, when they become visible.
The document this widget belongs to. If not specified to constructor, will default to srcNodeRef.ownerDocument, or if no sourceRef specified, then to the document global
HTML title attribute.
For form widgets this specifies a tooltip to display when hovering over the widget (just like the native HTML title attribute).
For TitlePane or for when this widget is a child of a TabContainer, AccordionContainer, etc., it's used to specify the tab label, accordion pane title, etc. In this case it's interpreted as HTML.
When this widget's title attribute is used to for a tab label, accordion pane title, etc., this specifies the tooltip to appear when the mouse is hovered over that text.
Step during widget creation to copy widget attributes to the DOM according to attributeMap and _setXXXAttr objects, and also to call custom _setXXXAttr() methods.
Skips over blank/false attribute values, unless they were explicitly specified as parameters to the widget, since those are the default anyway, and setting tabIndex="" is different than not setting tabIndex at all.
For backwards-compatibility reasons attributeMap overrides _setXXXAttr when _setXXXAttr is a hash/string/array, but _setXXXAttr as a functions override attributeMap.
Reflect a widget attribute (title, tabIndex, duration etc.) to the widget DOM, as specified by commands parameter. If commands isn't specified then it's looked up from attributeMap. Note some attributes like "type" cannot be processed this way as they are not mutable.
| Parameter | Type | Description |
|---|---|---|
| attr | String | Name of member variable (ex: "focusNode" maps to this.focusNode) pointing to DOMNode inside the widget, or alternately pointing to a subwidget |
| value | String | |
| commands | Object |
Optional
|
Internal helper for directly changing an attribute value.
Directly change the value of an attribute on an object, bypassing any accessor setter. Also handles the calling of watch and emitting events. It is designed to be used by descendant class when there are two values of attributes that are linked, but calling .set() is not appropriate.
| Parameter | Type | Description |
|---|---|---|
| name | String | The property to set. |
| value | Mixed | The value to set in the property. |
Internal helper for directly changing an attribute value.
Helper function to get value for specified property stored by this._set(), i.e. for properties with custom setters. Used mainly by custom getters.
For example, CheckBox._getValueAttr() calls this._get("value").
| Parameter | Type | Description |
|---|---|---|
| name | String |
Helper function for get() and set(). Caches attribute name values so we don't do the string ops every time.
| Parameter | Type | Description |
|---|---|---|
| name | undefined |
Returns next or previous sibling
| Parameter | Type | Description |
|---|---|---|
| which | String | Either "next" or "previous" |
Get the next or previous widget sibling of child
| Parameter | Type | Description |
|---|---|---|
| child | dijit/_WidgetBase | |
| dir | int | if 1, get the next sibling if -1, get the previous sibling |
Collect metadata about this widget (only once per class, not once per instance):
- list of attributes with custom setters, storing in this.constructor._setterAttrs - generate this.constructor._onMap, mapping names like "mousedown" to functions like onMouseDown
Helper function to set new value for specified property, and call handlers registered with watch() if the value has changed.
| Parameter | Type | Description |
|---|---|---|
| name | String | |
| value | anything |
Sets the style attribute of the widget according to value, which is either a hash like {height: "5px", width: "3px"} or a plain string
Determines which node to set the style on based on style setting in attributeMap.
| Parameter | Type | Description |
|---|---|---|
| value | String | Object |
Common setup for initial children and children which are added after startup
| Parameter | Type | Description |
|---|---|---|
| child | dijit/_WidgetBase |
| Parameter | Type | Description |
|---|---|---|
| child | dijit/_WidgetBase | |
| insertIndex | Integer |
Optional
|
This method is deprecated, use get() or set() directly.
| Parameter | Type | Description |
|---|---|---|
| name | String | Object | The property to get or set. If an object is passed here and not a string, its keys are used as names of attributes to be set and the value of the object as values to set in the widget. |
| value | Object |
Optional Optional. If provided, attr() operates as a setter. If omitted, the current value of the named property is returned. |
Deprecated, will be removed in 2.0, use this.own(on(...)) or this.own(aspect.after(...)) instead.
Connects specified obj/event to specified method of this object and registers for disconnect() on widget destroy.
Provide widget-specific analog to dojo.connect, except with the
implicit use of this widget as the target object.
Events connected with this.connect are disconnected upon
destruction.
| Parameter | Type | Description |
|---|---|---|
| obj | Object | null | |
| event | String | Function | |
| method | String | Function |
A handle that can be passed to disconnect in order to disconnect before
the widget is destroyed.
var btn = new Button();
// when foo.bar() is called, call the listener we're going to
// provide in the scope of btn
btn.connect(foo, "bar", function(){
console.debug(this.toString());
});
Kick off the life-cycle of a widget
Create calls a number of widget methods (postMixInProperties, buildRendering, postCreate, etc.), some of which of you'll want to override. See http://dojotoolkit.org/reference-guide/dijit/_WidgetBase.html for a discussion of the widget creation lifecycle.
Of course, adventurous developers could override create entirely, but this should only be done as a last resort.
| Parameter | Type | Description |
|---|---|---|
| params | Object | null | Hash of initialization parameters for widget, including scalar values (like title, duration etc.) and functions, typically callbacks like onClick. The hash can contain any of the widget's properties, excluding read-only properties. |
| srcNodeRef | DOMNode | String |
Optional If a srcNodeRef (DOM node) is specified:
|
Wrapper to setTimeout to avoid deferred functions executing after the originating widget has been destroyed. Returns an object handle with a remove method (that returns null) (replaces clearTimeout).
| Parameter | Type | Description |
|---|---|---|
| fcn | Function | Function reference. |
| delay | Number |
Optional Delay, defaults to 0. |
Destroy this widget, but not its descendants. Descendants means widgets inside of this.containerNode. Will also destroy any resources (including widgets) registered via this.own().
This method will also destroy internal widgets such as those created from a template, assuming those widgets exist inside of this.domNode but outside of this.containerNode.
For 2.0 it's planned that this method will also destroy descendant widgets, so apps should not depend on the current ability to destroy a widget without destroying its descendants. Generally they should use destroyRecursive() for widgets with children.
| Parameter | Type | Description |
|---|---|---|
| preserveDom | Boolean | If true, this method will leave the original DOM structure alone. Note: This will not yet work with _TemplatedMixin widgets |
Recursively destroy the children of this widget and their descendants.
| Parameter | Type | Description |
|---|---|---|
| preserveDom | Boolean |
Optional If true, the preserveDom attribute is passed to all descendant widget's .destroy() method. Not for use with _Templated widgets. |
Destroy this widget and its descendants
This is the generic "destructor" function that all widget users should call to cleanly discard with a widget. Once a widget is destroyed, it is removed from the manager object.
| Parameter | Type | Description |
|---|---|---|
| preserveDom | Boolean |
Optional If true, this method will leave the original DOM structure alone of descendant Widgets. Note: This will NOT work with dijit._TemplatedMixin widgets. |
Destroys the DOM nodes associated with this widget.
| Parameter | Type | Description |
|---|---|---|
| preserveDom | Boolean |
Optional If true, this method will leave the original DOM structure alone during tear-down. Note: this will not work with _Templated widgets yet. |
Deprecated, will be removed in 2.0, use handle.remove() instead.
Disconnects handle created by connect.
| Parameter | Type | Description |
|---|---|---|
| handle | undefined |
Used by widgets to signal that a synthetic event occurred, ex:
myWidget.emit("attrmodified-selectedChildWidget", {}).
Emits an event on this.domNode named type.toLowerCase(), based on eventObj. Also calls onType() method, if present, and returns value from that method. By default passes eventObj to callback, but will pass callbackArgs instead, if specified. Modifies eventObj by adding missing parameters (bubbles, cancelable, widget).
| Parameter | Type | Description |
|---|---|---|
| type | String | |
| eventObj | Object |
Optional
|
| callbackArgs | Array |
Optional
|
Get a property from a widget.
Get a named property from a widget. The property may potentially be retrieved via a getter method. If no getter is defined, this just retrieves the object's property.
For example, if the widget has properties foo and bar
and a method named _getFooAttr(), calling:
myWidget.get("foo") would be equivalent to calling
widget._getFooAttr() and myWidget.get("bar")
would be equivalent to the expression
widget.bar2
| Parameter | Type | Description |
|---|---|---|
| name | undefined | The property to get. |
Returns all direct children of this widget, i.e. all widgets underneath this.containerNode whose parent is this widget. Note that it does not return all descendants, but rather just direct children. Analogous to Node.childNodes, except containing widgets rather than DOMNodes.
The result intentionally excludes internally created widgets (a.k.a. supporting widgets) outside of this.containerNode.
Note that the array returned is a simple array. Application code should not assume existence of methods like forEach().
Returns all the widgets contained by this, i.e., all widgets underneath this.containerNode. This method should generally be avoided as it returns widgets declared in templates, which are supposed to be internal/hidden, but it's left here for back-compat reasons.
Returns the index of this widget within its container parent. It returns -1 if the parent does not exist, or if the parent is not a dijit/_Container
Gets the index of the child in this container or -1 if not found
| Parameter | Type | Description |
|---|---|---|
| child | dijit/_WidgetBase |
Returns null if this is the last child of the parent, otherwise returns the next element sibling to the "right".
Returns null if this is the first child of the parent, otherwise returns the next element sibling to the "left".
Returns true if widget has child widgets, i.e. if this.containerNode contains widgets.
Return true if this widget can currently be focused and false if not
Return this widget's explicit or implicit orientation (true for LTR, false for RTL)
Widgets override this method to size and position their contents/children. When this is called this._contentBox is guaranteed to be set (see resize()).
This is called after startup(), and also when the widget's size has been changed.
| Parameter | Type | Description |
|---|---|---|
| type | String | Function | protected |
| func | Function |
Track specified handles and remove/destroy them when this instance is destroyed, unless they were already removed/destroyed manually.
The array of specified handles, so you can do for example:
var handle = this.own(on(...))[0];
Place this widget somewhere in the DOM based on standard domConstruct.place() conventions.
A convenience function provided in all _Widgets, providing a simple shorthand mechanism to put an existing (or newly created) Widget somewhere in the dom, and allow chaining.
| Parameter | Type | Description |
|---|---|---|
| reference | String | DomNode | DocumentFragment | dijit/_WidgetBase | Widget, DOMNode, DocumentFragment, or id of widget or DOMNode |
| position | String | Int |
Optional If reference is a widget (or id of widget), and that widget has an ".addChild" method, it will be called passing this widget instance into that method, supplying the optional position index passed. In this case position (if specified) should be an integer. If reference is a DOMNode (or id matching a DOMNode but not a widget), the position argument can be a numeric index or a string "first", "last", "before", or "after", same as dojo/dom-construct::place(). |
Provides a useful return of the newly created dijit._Widget instance so you can "chain" this function by instantiating, placing, then saving the return value to a variable.
// create a Button with no srcNodeRef, and place it in the body:
var button = new Button({ label:"click" }).placeAt(win.body());
// now, 'button' is still the widget reference to the newly created button
button.on("click", function(e){ console.log('click'); }));
// create a button out of a node with id="src" and append it to id="wrapper":
var button = new Button({},"src").placeAt("wrapper");
// place a new button as the first element of some div
var button = new Button({ label:"click" }).placeAt("wrapper","first");
// create a contentpane and add it to a TabContainer
var tc = dijit.byId("myTabs");
new ContentPane({ href:"foo.html", title:"Wow!" }).placeAt(tc)
Called after the parameters to the widget have been read-in, but before the widget template is instantiated. Especially useful to set properties that are referenced in the widget template.
Kicks off widget instantiation. See create() for details.
| Parameter | Type | Description |
|---|---|---|
| params | Object |
Optional
|
| srcNodeRef | DomNode | String |
| Parameter | Type | Description |
|---|---|---|
| child | dijit/_WidgetBase |
Call this to resize a widget, or after its size has changed.
When changeSize is specified, changes the marginBox of this widget and forces it to re-layout its contents accordingly. changeSize may specify height, width, or both.
If resultSize is specified it indicates the size the widget will become after changeSize has been applied.
When changeSize is null, indicates that the caller has already changed the size of the widget, or perhaps it changed because the browser window was resized. Tells widget to re-layout its contents accordingly.
If resultSize is also specified it indicates the size the widget has become.
In either mode, this method also:
| Parameter | Type | Description |
|---|---|---|
| changeSize | Object |
Optional Sets the widget to this margin-box size and position. May include any/all of the following properties: {w: int, h: int, l: int, t: int}
|
| resultSize | Object |
Optional The margin-box size of this widget after applying changeSize (if changeSize is specified). If caller knows this size and passes it in, we don't need to query the browser to get the size. {w: int, h: int}
|
Set a property on a widget
Sets named properties on a widget which may potentially be handled by a setter in the widget.
For example, if the widget has properties foo and bar
and a method named _setFooAttr(), calling
myWidget.set("foo", "Howdy!") would be equivalent to calling
widget._setFooAttr("Howdy!") and myWidget.set("bar", 3)
would be equivalent to the statement widget.bar = 3;
set() may also be called with a hash of name/value pairs, ex:
myWidget.set({
foo: "Howdy",
bar: 3
});
This is equivalent to calling set(foo, "Howdy") and set(bar, 3)
| Parameter | Type | Description |
|---|---|---|
| name | undefined | The property to set. |
| value | undefined | The value to set in the property. |
Set a property on a widget
Deprecated. Use set() instead.
| Parameter | Type | Description |
|---|---|---|
| attr | String | |
| value | anything |
Called after all the widgets have been instantiated and their dom nodes have been inserted somewhere under .
Widgets should override this method to do any initialization dependent on other widgets existing, and then call this superclass method to finish things off.
startup() in subclasses shouldn't do anything size related because the size of the widget hasn't been set yet.
Deprecated, will be removed in 2.0, use this.own(topic.subscribe()) instead.
Subscribes to the specified topic and calls the specified method of this object and registers for unsubscribe() on widget destroy.
Provide widget-specific analog to dojo.subscribe, except with the implicit use of this widget as the target object.
| Parameter | Type | Description |
|---|---|---|
| t | String | The topic |
| method | Function | The callback |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Returns a string that represents the widget.
When a widget is cast to a string, this method will be used to generate the output. Currently, it does not implement any sort of reversible serialization.
Deprecated. Override destroy() instead to implement custom widget tear-down behavior.
Deprecated, will be removed in 2.0, use handle.remove() instead.
Unsubscribes handle created by this.subscribe. Also removes handle from this widget's list of subscriptions
| Parameter | Type | Description |
|---|---|---|
| handle | Object |
Watches a property for changes
| Parameter | Type | Description |
|---|---|---|
| name | String |
Optional Indicates the property to watch. This is optional (the callback may be the only parameter), and if omitted, all the properties will be watched |
| callback | Function | The function to execute when the property changes. This will be called after the property has been changed. The callback will be called with the |this| set to the instance, the first argument as the name of the property, the second argument as the old value and the third argument as the new value. |
An object handle for the watch. The unwatch method of this object can be used to discontinue watching this property:
var watchHandle = obj.watch("foo", callback);
watchHandle.unwatch(); // callback won't be called now
This is where widgets do processing for when they stop being active, such as changing CSS classes. See onBlur() for more details.
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
This is where widgets do processing for when they are active, such as changing CSS classes. See onFocus() for more details.
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Maps on() type parameter (ex: "mousemove") to method name (ex: "onMouseMove"). If type is a synthetic event like touch.press then returns undefined.
| Parameter | Type | Description |
|---|---|---|
| type | String | Function |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Internal method called when this widget is made visible.
See onShow for details.
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Called when the widget stops being "active" because focus moved to something outside of it, or the user clicked somewhere outside of it, or the widget was hidden.
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of mouse click events.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | mouse Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Called when this widget is being displayed as a popup (ex: a Calendar popped up from a DateTextBox), and it is hidden. This is called from the dijit.popup code, and should not be called directly.
Also used as a parameter for children of dijit/layout/StackContainer or subclasses. Callback if a user tries to close the child. Child will be closed if this function returns true.
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of mouse double click events.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | mouse Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Called when the widget becomes "active" because it or a widget inside of it either has focus, or has recently been clicked.
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Called when another widget becomes the selected pane in a dijit/layout/TabContainer, dijit/layout/StackContainer, dijit/layout/AccordionContainer, etc.
Also called to indicate hide of a dijit.Dialog, dijit.TooltipDialog, or dijit.TitlePane.
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of keys being pressed down.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | key Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of printable keys being typed.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | key Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of keys being released.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | key Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of when the mouse button is pressed down.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | mouse Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of when the mouse moves onto this widget.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | mouse Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of when the mouse moves off of this widget.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | mouse Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of when the mouse moves over nodes contained within this widget.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | mouse Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of when the mouse moves off of nodes contained within this widget.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | mouse Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of when the mouse moves onto nodes contained within this widget.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | mouse Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Connect to this function to receive notifications of when the mouse button is released.
| Parameter | Type | Description |
|---|---|---|
| event | undefined | mouse Event |
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});
Called when this widget becomes the selected pane in a dijit/layout/TabContainer, dijit/layout/StackContainer, dijit/layout/AccordionContainer, etc.
Also called to indicate display of a dijit.Dialog, dijit.TooltipDialog, or dijit.TitlePane.
var btn = new Button();
// when /my/topic is published, this button changes its label to
// be the parameter of the topic.
btn.subscribe("/my/topic", function(v){
this.set("label", v);
});