What is two way binding

Question

I have read lots that Backbone doesn t do two way binding but I don t exactly understand this concept   Could somebody give me an example of how two way binding works in an MVC codebase and how it does not with Backbone

User · Answer

McGarnagle has a great answer  and you ll want to be accepting his  but I thought I d mention  since you asked  how databinding works   It s generally implemented by firing events whenever a change is made to the data  which then causes listeners  e g  the UI  to be updated   Two-way binding works by doing this twice  with a bit of care taken to ensure that you don t wind up stuck in an event loop  where the update from the event causes another event to be fired    I was gonna put this in a comment  but it was getting pretty long

User · Answer

Actually emberjs supports two-way binding  which is one of the most powerful feature for a javascript MVC framework  You can check it out where it mentioning binding in its user guide   for emberjs  to create two way binding is by creating a new property with the string Binding at the end  then specifying a path from the global scope   App wife   Ember Object create     householdIncome  80000      App husband   Ember Object create     householdIncomeBinding   App wife householdIncome       App husband get  householdIncome       80000     Someone gets raise  App husband set  householdIncome   90000   App wife get  householdIncome       90000   Note that bindings don t update immediately  Ember waits until all of your application code has finished running before synchronizing changes  so you can change a bound property as many times as you d like without worrying about the overhead of syncing bindings when values are transient   Hope it helps in extend of original answer selected

User · Answer

Two-way binding means that any data-related changes affecting the model are immediately propagated to the matching view s   and that any changes made in the view s   say  by the user  are immediately reflected in the underlying model  When app data changes  so does the UI  and conversely   This is a very solid concept to build a web application on top of  because it makes the  Model  abstraction a safe  atomic data source to use everywhere within the application  Say  if a model  bound to a view  changes  then its matching piece of UI  the view  will reflect that  no matter what  And the matching piece of UI  the view  can safely be used as a mean of collecting user inputs data  so as to maintain the application data up-to-date   A good two-way binding implementation should obviously make this connection between a model and some view s  as simple as possible  from a developper point of view   It is then quite untrue to say that Backbone does not support two-way binding  while not a core feature of the framework  it can be performed quite simply using Backbone s Events though  It costs a few explicit lines of code for the simple cases  and can become quite hazardous for more complex bindings  Here is a simple case  untested code  written on the fly just for the sake of illustration    Model   Backbone Model extend   defaults      data      View   Backbone View extend   template     template  Edit the data   lt input type  text  value   lt    data   gt     gt       events        Listen for user inputs  and edit the model       change input    setData    initialize   options  - gt        Listen for model s edition  and trigger UI update      listenTo  model   change data    render    render  - gt        el html  template  model attributes           setData   e    gt      e preventDefault        model set  data     e currentTarget  value    model  new Model   view   new View  el      someEl    model  model    This is a pretty typical pattern in a raw Backbone application  As one can see  it requires a decent amount of  pretty standard  code   AngularJS and some other alternatives  Ember  Knockout     provide two-way binding as a first-citizen feature  They abstract many edge-cases under some DSL  and do their best at integrating two-way binding within their ecosystem  Our example would look something like this with AngularJS  untested code  see above     lt div ng-app  app  ng-controller  MainCtrl  gt    Edit the data     lt input name  mymodel data  ng-model  mymodel data  gt   lt  div gt    angular module  app          controller  MainCtrl     scope  - gt       scope mymodel    data        Rather short   But  be aware that some fully-fledged two-way binding extensions do exist for Backbone as well  in raw  subjective order of decreasing complexity   Epoxy  Stickit  ModelBinder     One cool thing with Epoxy  for instance  is that it allows you to declare your bindings  model attributes  lt -  view s DOM element  either within the template  DOM   or within the view implementation  JavaScript   Some people strongly dislike adding  directives  to the DOM template  such as the ng-  attributes required by AngularJS  or the data-bind attributes of Ember    Taking Epoxy as an example  one can rework the raw Backbone application into something like this         Model   Backbone Model extend   defaults      data      View   Backbone Epoxy View extend   template     template  Edit the data   lt input type  text    gt        or  using the inline form   lt input type  text  data-bind  value data    gt     bindings       input    value data     render  - gt        el html  template  model attributes         model  new Model   view   new View  el      someEl    model  model    All in all  pretty much all  mainstream  JS frameworks support two-way binding  Some of them  such as Backbone  do require some extra work to make it work smoothly  but those are the same which do not enforce a specific way to do it  to begin with  So it is really about your state of mind   Also  you may be interested in Flux  a different architecture for web applications promoting one-way binding through a circular pattern  It is based on the concept of fast  holistic re-rendering of UI components upon any data change to ensure cohesiveness and make it easier to reason about the code dataflow  In the same trend  you might want to check the concept of MVI  Model-View-Intent   for instance Cycle

User · Answer

Worth mentioning that there are many different solutions which offer two way binding and play really nicely   I have had a pleasant experience with this model binder - https   github com theironcook Backbone ModelBinder  which gives sensible defaults yet a lot of custom jquery selector mapping of model attributes to input elements   There is a more extended list of backbone extensions plugins on github

User · Answer

Two-way binding just means that   When properties in the model get updated  so does the UI  When UI elements get updated  the changes get propagated back to the model   Backbone doesn t have a  quot baked-in quot  implementation of  2  although you can certainly do it using event listeners    Other frameworks like Knockout do wire up two-way binding automagically      In Backbone  you can easily achieve  1 by binding a view s  quot render quot  method to its model s  quot change quot  event   To achieve  2  you need to also add a change listener to the input element  and call model set in the handler  Here s a Fiddle with two-way binding set up in Backbone

[javascript] What is two way binding?

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