I am new to knockout and readings it's tutorials for implementation.
I got to know about 2 different ways of writing view models in early chapters. I just need to know what is the difference between them.
I tried to figure out myself but may be not picking rights words.
Pdf i am refering to is Knockout PDF
There are several ways to create objects in javascript. The first one you're showing in your question is a literal. Another one is defining a constructor and invoking itt to get the object.
Yor AppViewModel is a constructor. If you need to use an instance of it as your viewmodel, you have to invoke the constructor, like this:
var vm = new AppViewModel();
ko.applyBindings(vm);
BTW, your fullName computed observable is incomplete. It's missing closing curly brakects, the second parameter, and closing parenthesis: }, self);
I have a JSON object that I used to create a form. This JSON object is parsed by KnockoutJS.
Now, when I modify the form, I want the JSON object to be updated according to the modifications made in the form. The thing is that I don't know in advance how the form will be like but I know in the JSON Object which fields need to be updated.
I really don't know what is the best way to procede. I know that I could reconstruct the JSON Object each time something has changed but this seems like a bad idea and a tedious process.
Is there a simple way to map each JSON Object field to form items in KnockoutJS ?
Here's a JSFiddle of what I'm currently doing:http://goo.gl/ZBaV7
Update :
I realized something interesting with this line:
<input type="text" data-bind="value: $data.value, attr : { disabled: $data.disabled }" />
I'm accessing the value directly from the array via ($data.value). Is there a way in the html to say to knockout to bind to this particular attribute in the array. I know that if the array would get reordered everything would get messed up but since I know that the only thing that can changed is this property I'm ready to take this risk ?
In other words, is there a way to manually say that when this value changes to change it in the array such as
data-bind="onChange: $data.value = this.value"
Is there a simple way to map each JSON Object field to form items in
KnockoutJS ?
Yes, If I understand what you want to do correctly. As of now, the values in your view model are not observables and won't be updated automatically as the form values change. There is a plugin to handle this mapping.
http://knockoutjs.com/documentation/plugins-mapping.html
Example: Using ko.mapping
To create a view model via the mapping plugin, replace the creation of
viewModel in the code above with the ko.mapping.fromJS function:
var viewModel = ko.mapping.fromJS(data);
This automatically creates observable properties for each of the
properties on data. Then, every time you receive new data from the
server, you can update all the properties on viewModel in one step by
calling the ko.mapping.fromJS function again:
ko.mapping.fromJS(data, viewModel);
Hopefully this helps.
If your Knockout ViewModel matches your form, you could just use the built in ks.toJSON()
http://knockoutjs.com/documentation/json-data.html
A better option, especially if your form is large or complex, is to use either the mapping or viewmodel plug-ins
http://knockoutjs.com/documentation/plugins-mapping.html
http://coderenaissance.github.io/knockout.viewmodel/
The simplest way to turn your json model into a usable knockout model is with the mapping plugin.
Alternatively, you can manually copy fields from your json model into ko.observable members of your view model, which give you more control and lets you choose to skip read-only properties.
Does anybody have a minute to take a look at a puzzling problem I'm having with Knockout JS and binding select lists?
The person's favourite color should be selected in the list by calling value: favColorId in the select list, rather than with the function-based call of value: favColorId(), something very strange is going on here, I've never used () in the past, it's also causing some other weird issues where it won't recall the value into the span (so changing the selected item does nothing).. I have tried recreating a simple sample as best I can demonstrating the issue.
http://jsfiddle.net/goneale/ph8Jw/
I have included my mapDictToArray() function but it simply converts a
javascript object into a key-value JS array. I wouldn't think that is
contributing to the problem.
Actually, that was part of the problem. The function returns a JavaScript array, not an observable array and therefor can't be used properly by Knockout. I've made the following changes to your code:
// The "mapDictToArray" makes a normal JS array, not a ko.observableArray();
// You can't simply "merge" a JS array with an observableArray(); you'll need
// some extra functionality for that: ko.utils.arrayPushAll()
// viewModel.colors(mapDictToArray(dict));
ko.utils.arrayPushAll(viewModel.colors(), mapDictToArray(dict));
// Apply the bindings *after* you've added the contents to the "colors" observable, in order to
// get the correct selected value
ko.applyBindings(viewModel);
That should do the trick (with the correct HTML without the () )!
JSFiddle.
UPDATE
I thought about my solution, but something wasn't correct. The only thing that was correct, was the part that you need to apply the bindings after you've added the contents of the colors observable. This is your fiddle, with that part moved down.
This works in your case, but you'll need to use the arrayPushAll method when there is already data inside the observableArray. This method merges, while you overwrite it when not using it (example with data inside the observable).
I have several knockout models, that I use on my website. Let's say for a given model I have a property
function modelA {
this.doSomething = function () { ... };
}
Now i want to use the doSomething() function in a different model. I would like to do this, such that I am not obligated to rewrite doSomething() in every single model that needs it. Ideally i would like it if this function becomes available as soon as ko.applyBindings(new modelX()) is called.
I know that i can have the functions inherit from a prototype, but that also leads repeated code in every single model. Is there a way to alter knockout.js to inject this property into all model objects?
EDIT:
In case there's a different approach that would achieve my desired goal, i'll elaborate on that desired goal. We use knockout-validation.js which allows me to call isValid() on every property in a model that has been extended with validation rules. I want to append a function to every model that will inspect the other properties in the model, and call isValid() on them. That way i can write a generic validation function that can be used with every model, without having to explicitly add it to the model.
You can add a custom function onto knockout's core datatypes. Alternatively you could look at extenders, and inside the extension you could add the function to an observable. Note, however, that both of these techniques address cross-cutting concerns at the "observable" level, not at the ViewModel level.
I would be curious to understand what doSomething is doing. Assuming that it has to access some observable(s) inside the ViewModel you should be able to reframe the problem so that the behaviour is attached to an observable.
I'm still learning the proper usage of Knockout and I've found myself quickly getting away from ever typing ko.observable when setting up my viewmodel and instead just defining an object literal and passing it through the mapping plugin with something like
var viewModel = ko.mapping.fromJS(data);
or at the very least, something along the lines of stuffing all of my data into an attribute on the viewModel like so
var viewModel = {
... events etc ... ,
"data": ko.mapping.fromJS(data)
}
To be honest, the main reason I've been doing this is to get around having to type ko.observable and ko.observableArray repetitively. I'm just trying to figure out if this is a good approach and if there are any downsides to dropping the specific var x = ko.observable() declaration all together. Also, I'm doing this all on load, not in response to any ajax call etc, which from what I can tell, is what the mapping plugin was designed for.
In your work with knockout, do you still declare the observables manually, one by one, or have you gone with the mapping.fromJS method that I use? Are there any specific downsides to using the mapping plugin so frequently like this?
Edit:
Specific Example
In this article, Steve sets up his viewModel by doing
var initialData = [ { ... } , { ... } ]; // json from the serializer
var viewModel = {
gifts : ko.observableArray(initialData)
};
Normally, I'd just use ko.mapping.fromJS for this situation as well, specifically to make sure the objects within the array are turned into observables as well. Looking at what he did, my approach seems like its overkill and adds a bit of unnecessary overhead.
After using Knockout for a little longer, I've noticed that the mapping plugin has some additional options that give you much more fine grained control over the mapping process.
Control type and amount of properties generated
There are several ways to accomplish this, and I'll go over some, but the end result is that you end up with a lighter result from the mapping plugin because everything isn't observable.
Basically you leave everything that you don't think will change, as a normal property and only make observables out of the specific items that you want to observe.
Make mapping omit certain properties
You can make the mapping plugin omit properties entirely from the end result by specifying things like ignore or include. Both of these accomplish the same thing, just in opposite ways.
Note: Samples are from the knockout.js mapping plugin documentation, comments added by me
Mapping Plugin Argument: include
The following snippet will omit all properties from the source object other than those passed in via the include argument.
// specify the specific properties to include as observables in the end result
var mapping = {
// only include these two properties
'include': ["propertyToInclude", "alsoIncludeThis"]
}
// viewModel will now only contain the two properties listed above,
// and they will be observable
var viewModel = ko.mapping.fromJS(data, mapping);
Mapping Plugin Argument: ignore
If you want to only omit certain properties from the source object, use the ignore argument as shown below. It will make observables from all properties in the source object except for the specified properties.
// specify the specific properties to omit from the result,
// all others will be made observable
var mapping = {
// only ignore these two properties
'ignore': ["propertyToIgnore", "alsoIgnoreThis"]
}
// viewModel will now omit the two properties listed above,
// everything else will be included and they will be an observable
var viewModel = ko.mapping.fromJS(data, mapping);
Control what properties are or are not made observable
If you need to include properties but you don't think that they will need to be made observable (for whatever reason), the mapping plugin has something that can help.
Mapping Plugin Argument: copy
If you want the mapping plugin to simply copy the plain properties and not make them observable, use this argument, as shown below.
// tell the mapping plugin to handle all other properties normally,
// but to simply copy this property instead of making it observable
var mapping = {
'copy': ["propertyToCopy"]
}
var viewModel = ko.mapping.fromJS(data, mapping);
Gain complete control over the mapping process
If you want to have 100% control over what is created in the mapping process, including the ability to put closures and subscriptions in your objects, then you want to use the "create" option.
plain result with calculated properties
Here is an example where I was mapping data from an ajax call to an object with a results property. I didn't want anything observable and I just wanted a simple generated property that would be made of the other simple properties on the object. Maybe not the most compelling example but it demonstrates the functionality.
var searchMappingConfig = {
// specific configuration for mapping the results property
"results": {
// specific function to use to create the items in the results array
"create": function (options) {
// return a new function so we can have the proper scope/value for "this", below
return new function () {
// instead of mapping like we normally would: ko.mapping.fromJS(options.data, {}, this);
// map via extend, this will just copy the properties from the returned json element to "this"
// we'll do this for a more light weight vm since every last property will just be a plain old property instead of observable
$.extend(this, options.data);
// all this to add a vehicle title to each item
this.vehicleTitle = this.Year + "<br />" + this.Make + " " + this.Model;
}, this);
};
}
}
}
subscriptions and closures and mapping, oh my
Another situation is if you want closures and subscriptions in your result. This example is too long to be included in its entirety but its for a vehicle make/model hierarchy. I wanted all the models (children) for a given make (parent) to be un-enabled if the model was un-enabled and I wanted this to be done with a subscription.
// here we are specifying the way that items in the make array are created,
// since makes has a child array (Models), we will specify the way that
// items are created for that as well
var makesModelsMappingConfig = {
// function that has the configuration for creating makes
"create": function (options) {
// return a new function so we can have the proper
// scope/value for "this", below
return new function () {
// Note: we have a parent / child relationship here, makes have models. In the
// UI we are selecting makes and then using that to allow the user to select
// models. Because of this, there is going to be some special logic in here
// so that all the child models under a given make, will automatically
// unselect if the user unselects the parent make.
// make the selected property a private variable so it can be closure'd over
var makeIsSelected = ko.protectedComputed(false);
// expose our property so we can bind in the UI
this.isSelected = makeIsSelected;
// ... misc other properties and events ...
// now that we've described/configured how to create the makes,
// describe/configure how to create the models under the makes
ko.mapping.fromJS(options.data, {
// specific configuration for the "Models" property
"Models": {
// function that has the configuration for creating items
// under the Models property
"create": function (model) {
// we'll create the isSelected as a local variable so
// that we can flip it in the subscription below,
// otherwise we wouldnt have access to flip it
var isSelected = ko.protectedComputed(false);
// subscribe to the parents "IsSelected" property so
// the models can select/unselect themselves
parentIsSelected.current.subscribe(function (value) {
// set the protected computed to the same
// value as its parent, note that this
// is just protected, not the actual value
isSelected(value);
});
// this object literal is what makes up each item
// in the Models observable array
return {
// here we're returning our local variable so
// we can easily modify it in our subscription
"isSelected": isSelected,
// ... misc properties to expose
// under the item in the Model array ...
};
}
}
}, this);
};
}
};
All in all, what I've found is that you rarely need 100% of an object that you'd pass to the plugin and you rarely need 100% of it to be observable. Dig in with the mapping configuration options and create all sorts of complex and simple objects. The idea is to only get everything you need, nothing more or less.
My suggestion to you would the same another questioned I just answered at https://stackoverflow.com/questions/7499133/mapping-deeply-hierarchical-objects-to-custom-classes-using-knockout-mapping-plug.
Your reasoning for using mapping plug-in is reasonable and the one that I use. Why type more code than you have to?
In my experience with knockout (all of 4 months), I've found that the less I do manually and let the knockout routines do their thing, the better my apps seem to run. My suggestion is try the simplest approach first. If it doesn't meet your needs, look at how the simple approach is doing it's "thing" and determine what has to change to meet your needs.
Allen, my recent learning experience with Knockout.js has been similar to yours. We work with a deep hierarchical object graph from the server and I have defined explicit instantiable view model functions which preserve the basic structure of it.
I began by defining each property explicitly as an observable on the relevant view model, but that quickly got out of hand. Also, a major reason for switching to using the mapping plugin was that we have to do frequent Ajax posts of the graph back to the server where it is merged with the persisted version, then validated on the server in such a way that numerous properties can change and collections be modified, and a new instance returned as the Ajax result where it has to be re-merged with the client representation. That became seriously difficult, and the mapping plugin helped big time by allowing the specification of identifiers for resolving adds / deletes / updates and to remap an updated graph onto the original.
It also helped in the original graph creation through the use of the "create" option for sub view models. In each view model constructor I receive a reference to the parent view model plus the data with which to construct the child view model, then create further mapping options to create grandchildren from the passed-in child data.
The only (slight) downside I recently found, as detailed in this question, is that when doing ko.mapping.toJSON it doesn't hook into any toJSON overrides you may have defined on the prototypes of your view models in order to exclude properties from serialization. I have been able to get around that by specifying ignore options in the unmapping, as recommended by Ryan Niemeyer in that post.
So in summary, I'll definitely be sticking with the mapping plugin. Knockout.js rules.
A simpler but help-full add-on could be knockout-data-projections
Currently, it does not handle js to viewmodel mappings, but it handles quite well view model to JS mappings.