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Widgets 101


  • Create a class with declare().
  • Any object created with new() will be initialised with preamble(), constructor() and postscript().

  • Take as an example SomeWidget = declare( [_WidgetBase], {} ); w = new SomeWidget( {param1: 'one', param2: 2} )

    • this.constructor( params, srcNodeRef ) is called . This will run before anything else. You can do whatever you like there
    • _WidgetBase::postscript( params, srcNodeRef ) is called
    • All postscript() does is call _Widgetbase::create(params, srcNodeRef)
    • _Widgetbase::create(params, srcNodeRef) is the real deal

_WidgetBase::create(params, srcNodeRef)

  • The following happens when you create w:
    • this.srcNodeRef, if present, is assigned to the DOM node with id srcNodeRef
    • params are mixed in with the object w.
    • CALLS: postMixinProperties()
    • is created (unless it was part of params and got mixed in)
    • The variables this.ownerDocument and this.ownerDocumentBody are set (although you can pass ownerDocument in params)
    • Widget is added to the registry
    • CALLS: buildRendering(). NOTE:
      • buildRendering() will define this.domNode, which will be the DOM of the widget. The "stock" buildRendering() from _WidgetBase will make sure this.domNode is indeed set by assigning it to srcNodeRef (if passed as parameter) or, as a last resort, create an <div> in ownerDocument (won't actually be displayed!). Most of the time, the "parent" buildRendering() will create this.domNode before calling this.inherited(arguments)
      • buildRendering also assigns DOM classes to the widget according to the parameter this.baseClass
    • CALLS: this._applyAttributes(). Basically, if you passed { color: 'blue' } as an attribute, then _setColorAttr() will be called with 'blue' as a parameter. (Note: this._setColorAttr() is what is called when you run this.set('color', 'blue'))
    • If this.domNode is different to this.srcNodeRef (see above: it might be the same if none of the inherited widgets replaced this.domNode and this.srcNodeRef was set), then this.domNode will replace outright this.srcNodeRef. This is when the magic begins.
      • NOTE: basically, if srcNodeRef is specified, the widget will always replace it at create() time!
    • this.domNode will have a VERY IMPORTANT attribute set: widgetId. This is the "link" between the DOM node and the Dojo widget
    • CALLS: postCreate()
  • NOTE: You must call this.inherited(arguments) when redefining postMixinProperties(), buildRendering(), postCreate() etc. but mustn't call it when defining the constructor()

_WidgetBase::startup(params, srcNodeRef) (and _WidgetBase::containerNode)

  • _WidgetBase::startup() is not called after _WidgetBase::create(). It's up to the developer to call it. It should only be called once the widget's DOM is in document flow, as its main goal is to contain any logic reliant on the widget's dimensions (when postCreate() is called, the DOM structure has been created but it may or may not be rendered in the document.)
  • this.placeAt() will startup() the widget if the parent has already been started.
  • The stock Widgetbase::startup() method will also call startup() for all contained widgets returned by _WidgetBase::getChildren()
    • Each widget might have a containerNode attribute, which will point to the widget's content
    • _Widgetbase::getChildren() will search for any DOM node with attribute widgetId and a corresponding widget in the widget registry (it runs registry.findWidgets(this.containerNode)).

_WidgetBase and widget placement

  • If you create a widget specifying srcNodeRef, it will be "placed" there (read above)
  • If you don't, you can use this.placeAt( dest, position ). NOTE:
    • dest can be a widget with addChild() defined: in that case, that method will be used
    • dest can be a widget without addchild() defined: in that case, the widget will be added to dest's domNode or containerNode
    • dest can be a DOM element: it will be placed there

_WidgetBase and attributes

Containers, layout, etc.


_Container.js -- base for _ContentPane AND _LayoutWidget

  • CONTRACT: It implements addChild(), removeChild(), hasChildren(), getIndexOfChild(), AND sets this.containerNode
  • It is a mixin for widgets that contain HTML and/or a set of widget children
  • addChild() adds the child to this.containerNode and calls startup() for it (if containing widget has already started). It also calls which makes sure the widget's DOM node has the right widgetId (it will work with the registry)
  • addChild() won't work well if a ContentPane is nested in layout widgets as addChild() doesn't resize() (it's unclear if the ContentPane is visible or not, and resize() doesn't work on hidden tabs). addChild() also doesn't call _checkIfSingleChild() to resize a single child widget to fill the whole ContentPane.


  • CONTRACT: Defines isLayoutContainer: this widget will call resize() on its children when they become visible, + follow doLayout
    • NOTE: Since there is no concept of "hidden" in a ContentPane, it means call resize() on its children when ContentPane is visible
  • Implements _onShow(), which is run either at startup (normal visible contentPane) or by its parent (hidden tab)
  • If it's not the child of a isLayoutContainer, will listen to the viewport's resize event and run this.resize()
  • this._isShown() cheats: will return true if it was ever shown, which is good enough
  • If doLayout:
    • If ContentPane only has a single child, the single child will be sized so that it takes the whole parent.
    • Children will be told what size they should be (will be passed dimensions in Resize)


  • It's a "sort of" a container of widget, but limited to simple widgets where everything is visible. This is due to addChild()'s limits
  • Simply WidgetBase + _Container + _ContentPaneResizeMixin (which honours the isLayoutContainer contract).

Layout widgets


  • It defines getPreviousSibling(), getNextSibling(), getIndexInParent()
  • Useful for widgets that are contained by a isLayoutContainer widget

_LayoutWidget ( a isLayoutContainer)

  • It includes Widgetbase + _Container + _Contained + functions to honour the isLayoutContainer contract
  • CONTRACT: Defines isLayoutContainer: this widget will call resize() on its children when they become visible, + follow doLayout
    • NOTE: unlike _ContentPaneMixin, children widgets might well be invisible (see: TabContainer etc.).
  • If it's not the child of a isLayoutContainer, will listen to the viewport's resize event and run this.resize()
  • When this.resize(), it will call this.layout(). It will be up to the Widget class to implement this.layout() to resize children
  • It's basically up to this.layout() to work on layout and call _onShow() on visible child (see CONTRACT)
  • It extends addChild() and removeChild() so that it adds (and remove) the right DOM classes (parent.baseClass+'-child' and parent.baseClass+'-'+child.baseClass) for theming
  • If doLayout:
    • If ContentPane only has a single child, the single child will be sized so that it takes the whole parent.
    • Children will be told what size they should be (will be passed dimensions in Resize)


Note: this is a very simplified description of what _StackContainer is, to give a rough idea of how it uses _LayoutWidget

  • It simply inherits from _LayoutWidget
  • this.layout() is a function that will resize just the selected child (as required)
  • At startup():
    • It looks through its children and sets the one with child.selected as the one to display. NOTE: might use cookies too (this.persist)
    • Sets children's class as dijitHidden (instead of dijitVisible), so everything is hidden
  • Publishes a topic'-startup'
  • In this.resize(), if it's the first time it's shown, it calls _onShow() for the child
  • In this.addChild(), it reruns this.layout() and (if it's the only child) selects it
  • In this.removeChild(), the first child is selected and this.layout() is called

Events, dojo/on, _WidgetBase, events


  • dojo/on is the one-stop system to emit and subscribe to events
  • Two main functions: on() and emit()
    • dojo/on(target, 'event', listener)
      • target can be:
        • An object with this.on() defined: dojo/on will delegate to that function. This is what happens in widgets! OR
        • An object with this.addEventListener() defined: it will use it using the same API as DOM, but it might be anything. OR
        • An object with this.attachEvent() defined (this is for retarded IE which up to v.9 didn't have this.addEventListener) OR
        • It will fail
    • dojo/on.emit(target, 'event', event)
      • target can be:
        • An object with this.dispatchEvent() defined (it's a DOM node): it will use native event emission (DOM) OR
        • An object with this['on'+event]: it will call that, and it will bubble up to parentNode if event is meant to bubble up. This is done 100% synthetically, by following parentNode attributes.
        • event can be a simple object, which is then "transformed" into a native event. E.g. it could have {bubbles:true} and nothing more

*** QUESTION: Why doesn't dojo/on.emit() get delegated to the widget just like dojo/on() is? It seems like the obvious thing to do -- with the object's this.emit() which then emits through the DOM, which will then bubble things up if requested... so that the "manual" bubbling wouldn't even be necessary?

*** QUESTION: dojo/emit(target, 'event', event) seems to call, synthetically for objects, onevent rather than onEvent. Didn't this use to be onEvent? Is capitalisation after "on" going away as a convention for Dojo 2.0?


  • Widgets also implement this.on() and this.emit(). However, everything is done through the DOM
    • this.on('success', listener)
      • TEMPORARILY, before Dojo 2 IF this['on'+success] exists, will run aspect.after(this, 'onsuccess', listener) to piggyback on widget methods (e.g. this.onClick() OR
      • Runs dojo/on(this.domNode, 'event', listener). So, it will get this.domNode hooked up to event (which will fire listener).
    • this.emit('type')
      • Calls this['on'+type], that us this.on if it exists in the widget (note the lack of capitalisation in 'event'). The event will not bubble up synthetically in any case (unlike synthetic events emitted with dojo/emit() over widgets) AND
      • Runs dojo/on.emit(this.domNode, toLowerCase('type'), event). So, it emits down to this.domNode, through the widget's DOM. NOTE THAT toLowerCase!!!

*** QUESTION: with widget.emit(), doesn't this mean that in some cases TWO callbacks will be called when emitting 'success': one will be the listener set up with widget.on('success', listener) and one will be the widget's this.onsuccess() method? Is that the behaviour to expect?

*** QUESTION: there is no logic to bubble up 'event' in _Widgetbase, I guess because the DOM will do that. But isn't this a bit inconsistent? (see question above)

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