KnockoutJS and MVVM

IntelliMeet April, 14
KnockoutJS and MVVM
Manvendra SK
Twitter: @Manvendra_SK

What Is KnockoutJS ?
Rich client-side
interactivity
MVVM pattern
Web browser support
Separates behavior and structure
Declarative bindings
Observables
6+ 2+

What Is The Problem ?
Delete Underlying
Data
View
What Is The Solution ?
Delete Underlying
Data
View

What KnockoutJS Is Not ?
Client side code
Server code
Database
KnockoutJS

What Is MVVM ?
Model View ViewModel

Core Observable Types
Computed Observable
Observable Array

How Knockout’s Observables Works ?
Changes to
Targets
Notify
Source
Changes to
Source
Notify
Targets
Two-Way
Data
Binding
Event
Binding

Observable And Binding
• Make Property on object an observable using
ko.observable() method passing default value.
• Call ko.applyBindings() method passing object to
method.
• Bind properties of ViewModel object to html elements using
data-bind attribute any binding like text. More on this
later.
• You can make the object using a constructor. *

Observable Tricks
• Any property that is declared observable is a function like
getter and setter.
• To access property use: vmObject.property()
• To set property use: vmObject.property(_newValue)
• Don’t ever use = operator to assign values. You’ll
overwrite the observable.

Interactive Binding
• We have used text binding. That was a non-interactive
binding.
• Interactive binding is when user interact with the page and
hence the binding.
• click is such a binding.
• More on this later.

Computed Observable
Define Your
Own Derived
Property
When You
Need A Value
That Doesn’t
Exist in Web
Service
Based On
Other
Properties and
Observables
Also Supports
Data Binding

Computed Observable And Binding
• Make Property on object an computed observable using
ko.computed() method passing a anonymous function
that returns computed value.
• Bind to html elements using data-bind attribute.

Observable Array
Notify When
Items Are
Added Or
Removed
Tracks Which
Objects Are In
The Array
No Notification
When Objects
In The Array
Change
Bonus: Can
Use Array
Functions

Observable Array And Binding
• Make Property on object an observable array using
ko.observableArray() method passing an array literal.
• Bind to html elements using data-bind attribute and
foreach binding.
• Binding contexts inside foreach: $root et al. More on this
later.
• mvObject.arrayProp() gives you native underlying array.

Observable Array Methods
• push(), pop()
• unshift(), shift(), slice()
• remove(), removeAll(), destroy(), destroyAll()
• sort(), reversed()
• indexOf()

Built In Bindings
Binding For
Element
Attributes
Multiple
Binding
Expressions
Built Into
Knockout

Control Flow Bindings
foreach if ifnot with
text
html visible css style attr
click value event enable disable
checked options selectedOptions hasfocus

Appearance Bindings
text

Interactive Bindings
text

Binding Contexts
• What if you need to access ViewModel object while iterating
array using foreach binding?
• $root : Top level ViewModel object.
• $data : Refers to object for the current context. Like this
keyword in JavaScript. Useful for atomic values like strings
and numbers.
• $index : Obviously index number of current item.

Binding Contexts
• $parent : Refers to the parent ViewModel object. Typically
used inside nested loops and when you need to access
properties in the outer loop.

with Binding
• Used when you want to manually declare block scope to
particular property of ViewModel.

text Binding
html Binding
visible Binding
attr Binding

click Binding
• click binding is one of the simplest interactive binding. It
just calls a method on your ViewModel when the user click
the element passing function two arguments model and
event.
• vmObject.someMethod = function(model, event) {
// do what ever you want here…
};

value Binding
• Similar to text binding. But this time user can update the
value from the View and ViewModel will update accordingly.
Thus value binding is two way binding.

event Binding
• The event binding lets you listen for arbitrary DOM events
on any HTML element.
• As it can listen for multiple events, it requires an object to
map events. Just like attr binding.
• data-bind=“event: {mouseover: someMethod,
mouseOut: someOtherMethod}”

event Binding
• The function will take two arguments called data and
event.
• vmObject.someMethod = function(data, event) {
// do what ever you want on happening
// of this event…
};

enable and disable Binding
• Used to enable or disable some form element on some
particular condition.

checked Binding
• Used to select or deselect HTML’s selectable elements like
check boxes and radio buttons.
• We can also use arrays instead of true or false value in the
observable. In this case only those check boxes will be
selected whose value attribute value exist in the array.
• Matches value attribute value of radio button to single
value in observable.

options Binding
• This binding is used to build options elements inside a
select element from an observable array.
• value binding is used to preselect or get the selected item
in the select list.
• optionsText binding is used to show the content on the
view when using objects in the observable array instead of
literals.

selectedOptions Binding
• As a select list can be made to select multiple items so to
retrieve or set all the selected items we need an array
instead of using an atomic value. Thus instead of value
binding we use selectedOptions binding.

hasfocus Binding
• Allows us to put the focus on any element.

Interacting With Server
Saving Data
Mapping
Data To
ViewModel
Loading
Data

Mapping Data To ViewModel
• Mapping plugin dynamically generates new observables.

Animation Inside Knockout
• Don’t support animation from core, as it’s a view binding
library not DOM manipulation library.
• All Knockout’s automatic updates are immediately applied
whenever underlying data changes.

List Callbacks
• Knockout is a powerful user interface library on its own, but
once you combine it with the animation capabilities of a
framework like jQuery or MooTools, you’re ready to create
truly stunning UIs with minimal markup.
• The foreach binding has two callbacks named
beforeRemove and afterAdd. These functions are
executed before an item is removed from the list or after it’s
been added to the list, respectively. This is our chance to put
in animation logic before Knockout manipulates the DOM.

List Callbacks
• These callbacks take a parameter named element which a
reference to the element which Knockout is manipulating.

Custom Bindings
• Okey, we’re having fun with foreach binding and its some
level support for plugging in animation logic. But what
about other bindings? They don’t support this already. So
Custom Bindings is answer to the question.
• We can make our own bindings by adding an object
defining the binding to ko.bindingHandlers. This also
happens to be where all of the built-in bindings are defined,
too.

Custom Bindings
• The object should have two methods namely init and
update.
• init method is called when Knockout first encounters the
binding. So this callback should define the initial state for the
view component and perform necessary setup actions like
registering event listeners.

Custom Bindings
• The update method executes whenever the associated
observable changes.
• Both methods take same two parameters namely element
and valueAccessor.
• The element parameter is the DOM element being bound,
and valueAccessor is a function that will return the
ViewModel property in question (here it is binding value).

Conclusion
• Knockout.js is a pure JavaScript library that makes it
incredibly easy to build dynamic, data-centric user
interfaces.
• We covered most of the Knockout.js API, but there are still a
number of nuances left to discover.
• Knockout.js provides plethora of extensibility opportunities
for you to explore!

KnockoutJS and MVVM

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