Handling ‘this’ in ko.command

Update: this feature is now available as part of the ko.plus library available on GitHub and NuGet!


The problem of context – the this value – in JavaScript is one that seems to keep causing problems.  Languages with similar syntax (C#, Java) do not allow the developer to alter the value of this and so people don’t always expect that it can change.

JavaScript likes to be different though.

I don’t want to get into too much detail on how this behaves – there are already more than enough articles out there that cover the topic to a greater depth than I could (e.g. Scope in Javascript); instead, this is a post about how I have worked around the issue in my ko.command library.

Problematic Command Context

Take the following simple example usage of the command library.

function ViewModel() {
    this.value = ko.observable(123);
    this.increment = ko.command(function() {
        this.value(this.value()+1);
    });
}

var viewModel = new ViewModel();
viewModel.increment();
//viewModel.value() => 124

The increment command adds 1 to the value of an observable property on the same view model; everything is working so far, but what if we move the implementation of the command action onto the prototype?

function ViewModel() {
    this.value = ko.observable(123);
    this.increment = ko.command(this._increment);
}

ViewModel.prototype._increment = function() {
    this.value(this.value()+1);
}

We might expect this to behave in the same way as before, but now when we call increment we get an error that, after a little investigation, we can see is because the value of this within the _increment function is not set to the view model.

Now It (mostly) Just Works

This behaviour was actually down to a design decision in the earlier version of the library to set the context of the various callbacks to the command itself; only recently has this started to cause problems that have prompted me to fix it.

The updated behaviour (available for download from Github) now endeavours to “just work” wherever possible.  This means that in the scenario above there are no code changes required to use the prototype implementation.

To be specific, the command action will always be invoked in the context from which the command was called which should (in most cases) behave in a way that seems to make sense.

There are some specific scenarios where a little more work is needed though.

CanExecute Function

The canExecute property on any ko.command instance is currently implemented as a computed observable, and as explained in the Knockout documentation, computed observables can be a little tricky when dealing with the context.

The behaviour does make sense when you consider the cause: computed observables will be re-evaluated whenever a dependent observable changes, so there is no way for them to (automatically) guarantee the context in which it will be invoked.

It is, however, possible to explicitly specify a context for a computed observable, so ko.command has been extended to mimic this implementation:

function ViewModel() {
    this.value = ko.observable(123);
    this.increment = ko.command({
        context: this,
        action: this._increment,
        canExecute: this.somethingThatReliesOnScope
    });
}

In this example the value of this when running the canExecute function will always be set to the current instance of ViewModel.

Note: explicitly setting the context in this way will also override the default behaviour when invoking commands, so use it carefully!

Asynchronously-Invoked Callbacks

The callbacks to an asynchronous command can be attached using the done/fail/always functions and are invoked once the promise returned by the action has completed.

function ViewModel() {
    this.value = ko.observable(123);
    this.incrementAsync = ko.command(function() {
        var promise = $.Deferred();
        promise.resolve();
        return promise.promise();
    }).done(this._increment);
}

But in what context will they be executed?

This scenario is a little more complicated because the promise implementation itself is able to specify the context in which callbacks should be invoked (in jQuery this is achieved using resolveWith).  In this case we have 3 choices:

  1. Do nothing.  Leave the context for the callback as whatever is set by the promise
  2. Replace the promise context with the context from the command
  3. Replace the promise context, but only if it has been explicitly specified.

For the time being I decided to leave the behaviour unchanged as it feels like changing it – forcing the context back to the command context – would be breaking the expected behaviour of whoever has explicitly (or implicitly) set the context of the callback.

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Single Page Applications using Node & Knockout

This post is going to be a short walkthrough on how to use Node and KnockoutJS to create a simple single page application.

What is a Single Page Application?

…a web application or web site that fits on a single web page with the goal of providing a more fluid user experience akin to a desktop application

That’s according to Wikipedia.  For the purposes of this post, a single page application (or SPA) will mean a web application for which we only want to serve up one HTML page.

That page will then link to a couple of javascript files which, in cohort with a templating engine, will create and manipulate the content of the page.  All communication with the server will be through AJAX, and will only ever transfer JSON data – no UI content.

We will be using node to serve the page (plus scripts, styles, etc.) and to handle the API calls, while knockout will provide us with the client-side interaction and templating.

Serving the Single Page

First up: we need to configure node to serve up our single HTML page:

<html>
	<body>
		<h1>I'm a single page application!</h1>
	</body>
</html>

We’ll be adding more to that later, but let’s get node working first.

Express

We’re going to be using expressjs to implement the more interesting API calls later on, but we can make use of it here to serve up a static file for us as well.  To install express, use the node package manager by running the command below.:

npm install express

Now we need to create a javascript file – app.js – to run in node.  This file will grab a reference to express using the require function and will start listening on port 3000.

var express = require("express"),
	app = express();

//start listening on port 3000
app.listen(3000);

Let’s see what happens when we run this.  In a command prompt, browse to the folder containing app.js and enter the command below to start node.

node app.js

Next, open your favourite browser and navigate to http://localhost:3000/index.html.  You should see something like this:

cannot-get

This is express telling us that it cannot resolve the URL “/index.html”, which isn’t unreasonable – we haven’t told it how to yet.  We want express to respond with the contents of static files from the current folder (eventually including our styles and javascript), so let’s get that set up.

We do this in the app.configure method (before we call app.listen) using the express.static method and the current application folder (stored in the special __dirname node variable).

app.configure(function() {
	//tell express to serve static files from the special
	//node variable __dirname which contains the current
	//folder
	app.use(express.static(__dirname));
});

Restart the node application, refresh the browser and you should now see the content from our single page:

can-get

Conveniently, express will automatically return index.html if you don’t specify a file name, so we can get the same response from http://localhost:3000/

Creating the Page

The next step is to start building up content in the page.  We are going to need a few javascript resources – jQuery for the AJAX calls, Knockout for the view model – and I’m going to include my command pattern implementation to help with the actions.

For the page itself I’m going to pull in a page.js to contain our page-specific code, and we should probably include a stylesheet as I can’t stand Times New Roman.

Our HTML page now looks like this:

<html>
	<head>
		<title>SPA Example</title>
		<link rel="stylesheet" href="spa.css" />
	</head>
	<body>
		<h1>I'm a single page application</h1>
	</body>
	<script src="http://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.9.0.min.js"></script>
	<script src="http://ajax.aspnetcdn.com/ajax/knockout/knockout-2.2.1.js"></script>
	<script src="https://raw.github.com/stevegreatrex/JsUtils/master/JsUtils/utils.min.js"></script>
	<script src="page.js"></script>
</html>

I’m using a CDN for jQuery and knockout, and I’m pulling my command implementation direct from Github (sorry Github!). I’m assuming that both spa.css and page.js are in the same folder as index.html

Refresh the browser again (no need to restart node this time) and…
styled
Much better!

Creating the View Model

As this is just a sample application I don’t want to get too distracted by the view model – the purpose of this post is demonstrate the end-to-end rather than to focus on a specific functionality.  With that in mind, let’s use the example functionality of a basic todo app (as that seems to be the thing to do).

Our view model will start off with a list of todo items which we will store in a knockout observableArray.  Each todo item will have a name and a complete flag.

For the time being, we’ll bootstrap the collection with a few sample items.

var TodoViewModel = function(data) {
	this.name = ko.observable(data.name);
	this.complete = ko.observable(data.complete);
};

var TodoListViewModel = function() {
	this.todoItems = ko.observableArray();
};

$(function() {
	var viewModel = new TodoListViewModel();

	//insert some fake todo items for now...
	viewModel.todoItems.push(new TodoViewModel({ name: "Pending Item", complete: false }));
	viewModel.todoItems.push(new TodoViewModel({ name: "Completed Item", complete: true }));

	ko.applyBindings(viewModel);
});

The view model is now being populated but there’s still nothing to see in our view – we need to add some HTML and start working with the knockout templating engine to get things to display.

Displaying Items using Knockout Templating

With knockout, the UI is data bound to the view model in order to generate HTML.  http://knockoutjs.com/ has a wealth of documentation and examples on how to achieve this, but for this example we are going to use three bindings: foreach to iterate through each of the todo list items; text to display the name; and checked to display the completed state.

<ul data-bind="foreach: todoItems">
	<li>
		<span data-bind="text: name"></span>
		<input type="checkbox" data-bind="checked: complete" />
	</li>
</ul>

Refresh the page in a browser and you should now see something like this:

items

We now have the text and the completed state of the our two fake todo items.  That’s all well and good, but what about when you want to get real data from the server?

Getting Real Data from the Server

In a single page application, data is acquired from the server using AJAX calls and our example today will be no different.  Unfortunately, our server doesn’t support any AJAX calls at the moment, so our next step is to configure a URL that will return some data; in this case: todo list items.

Configuring API Calls using Express

We want to set up an API call on our node server that will respond with a JSON list of todo items for the URL:

GET /api/todos

To set this up in express we use the app.get method, which accepts a path as the first parameter – in this case /api/todos – and a callback as the second.

app.get("/api/todos", function(req, res) {
	//...
});

The callback will now be invoked whenever we browse to http://localhost:3000/api/todos.  The two parameters on the callback are the request and the response objects, and we now want to use the latter to send JSON data back to the client.

Ordinarily you would be getting the data from some kind of backing store, but to keep things simple I’m just going to return a few fake items using the res.json method.  Here we are passing in the HTTP response code (200 – OK) and our data, then calling the res.end method to finish the response.

res.json(200, [
	{ name: "Item 1 from server", complete: false },
	{ name: "Item 2 from server", complete: false },
	{ name: "Completed Item from server", complete: true }
]);
res.end();

Now let’s hook up our view model to access that data…

Getting the View Model Speaking to the Server

As our server now expects a GET call we can use jQuery.getJSON to load the data from the client side.  Once we have the data, all we need to do is push it into our view model to update the UI.

var TodoListViewModel = function() {
	var self = this;
	this.todoItems = ko.observableArray();

	this.refresh = ko.command(function() {
		//make a call to the server...
		return $.getJSON("/api/todos");
	}).done(function(items) {
		//...and update the todoItems collection when the call returns
		var newItems = [];
		for (var i=0; i < items.length; i++ ){
			newItems.push(new TodoViewModel(items[i]));
		}
		self.todoItems(newItems);
	});

	//refresh immediately to load initial data
	this.refresh();
};

Note that I’ve used the command pattern in this example (to get some free loading indicators and error handling) but there’s no need to do so – a regular function would suffice.

Restart node, refresh the page and you should now see the data returned from the server.

server-items

Sending Data back to the Server

We’ve managed to display data from the server, but what about if we want to save a change from the client?

Let’s add another API method that expects a PUT call to /api/todos/[id] with a body containing the JSON data.  We’ll also need to add an id property to the fake data returned by the server so that we can reference it in the URL.

The configuration of the PUT URL looks very similar to the GET configuration from earlier.

app.put("/api/todos/:todoId", function(req, res) {
    //...
});

The only difference (besides the verb) is that our URL path now includes a parameter named “todoId”, signified by the prefixed colon.  This will allow us to access the value of the ID appended to the URL through the req.params object.

Our handler will also need access to the body of the request, and to provide that we need to configure express to use its body parser:

app.use(express.bodyParser());

Now we have access to the body of the request through the req.body property.

As our server doesn’t have a real backing store, there isn’t much we can do to actually process this call.  To demonstrate that it is actually getting through we’ll just log the details to the node console and respond with a 200 - OK for the time being.

app.put("/api/todos/:todoId", function(req, res) {
	console.log(req.params.todoId + ": " + JSON.stringify(req.body, null, 4));
	res.send(200);
	res.end();
});

We now need our view model to call this method whenever the value of the complete flag is updated by the user.  First off, lets add another command that uses jQuery to make an AJAX call with the appropriate data.

var TodoViewModel = function(data) {
	// as before

	this.updateServer = ko.command(function() {
		return $.ajax({
			url: "/api/todos/" + data.id,
			type: "PUT",
			contentType: "application/json",
			data: JSON.stringify({
				id: data.id,
				name: self.name(),
				complete: self.complete()
			})
		});
	});
};

This one is a bit more verbose than the getJSON call earlier as we need to call the jQuery.ajax method directly in order to PUT data.  It is also worth noting that the JSON object being sent is derived from the updated values for the name and complete fields from the relevant observables.

We can now use the subscribe method on the observable complete flag to ensure that this update function will be automatically invoked whenever the flag changes.

this.complete.subscribe(this.updateServer);

Restart node, refresh the page, and try clicking on the check boxes.  You should see confirmation of the successful call to the server in the node window.

server-output

Wrapping Up

This has only been a very simple example, but hopefully demonstrates the architecture of a single page application and how it can be implemented using node and knockout.

Command Pattern v2 using Knockout

Update: this feature is now available as part of the ko.plus library available on GitHub and NuGet!


In an earlier post I outlined a basic implementation of a command pattern using Knockout that tracks the progress of an asynchronous operation using the jQuery.Deferred object.

Having used it for a couple of weeks I found that there were a few things that could do with improving, so here we go with Command v2!.

No more .execute()

The first version of the command would wrap the operation in an object, meaning that whenever you wanted to bind to or just run the action you had to call a .execute method on the new object:

//old code
var viewModel = {
    doSomething: new Command(function() {
        //invoke async operation        
    });
};

//invoke the command
viewModel.doSomething.execute();​

Whilst using the command I kept forgetting to add the .execute (particularly to bindings) and this quickly became annoying, so I changed the constructor function to return the execution function instead of a new object in a similar manner to ko.observable.

Note: having previously been returning this I had to ditch the traditional constructor for a factory method that is a bit more in line with the Knockout syntax.

ko.command = Utils.command = function (options) {
    //...

    //execute function (and return object
    var _execute = function () {
       //...
    },
    
    //public properties now attached to _execute instead of this
    _execute.isRunning            = _isRunning;
    _execute.done                 = _done;
    _execute.fail                 = _fail;
    _execute.always               = _always;

    return _execute;
};

After these changes the invocation of the command is much simpler:

var viewModel = {
    doSomething: ko.command(function() {
        //invoke async operation 
    });
};

//invoke the command
viewModel.doSomething();

//get the status of the operation
viewModel.doSomething.isRunning(); 

//attach a success handler
viewModel.doSomething.done(function() {
    alert("Finished");
});​

That’s much cleaner, and the helper observables and functions such as isRunning and the callback methods still function as before.

Support for canExecute

The command pattern implementation in .NET (the original inspiration for writing this) includes a CanExecute method that determines whether or not the command is currently in a suitable state to execute. For example, if the command is going to submit a form, the CanExecute implementation might check that the form is valid prior to submission.

For my JavaScript command I wanted to be able to specify a custom function to determine whether or not the command can execute, as well as always preventing execution whilst the command is running asynchronously. To do this I made use of Knockout’s computed observables to implement a canExecute property:

ko.command = Utils.command = function (options) {
    //factory method to create a $.Deferred that is already completed
    _instantDeferred = function(resolve, returnValue) {
       //
    },

    //execute function
    var _execute = function () {
        //check if we are able to execute
        if (!_canExecute()) {
            //dont attach any global handlers
            return _instantDeferred(false).promise();
        }

        //...
    },
    
    //dummy to allow us to force a re-evaluation of the computed _canExecute observable
    _forceRefreshCanExecute = ko.observable(), 

    //canExecute flag observable
    _canExecute = ko.computed(function() {
        _forceRefreshCanExecute(); //get and ignore the value

        //can execute if we're not running, and either there's no canExecute
        //function specified or it returns true
        return !_isRunning() &&
            (typeof options.canExecute === "undefined" ||
            options.canExecute.call(_execute));
    }, _execute),
    
    //invalidate canExecute
    _canExecuteHasMutated = function() {
        _forceRefreshCanExecute.notifySubscribers();
    };

    //public properties
    _execute.canExecute           = _canExecute;
    _execute.canExecuteHasMutated = _canExecuteHasMutated;

    return _execute;
};

Things of interest here:

  • If the _execute method cannot execute, it returns a $.Deferred object that has already been rejected so that any calling class that attaches a done/fail handler will not throw exceptions and can at least sensibly react
  • We are creating a dummy observable (_forceRefreshCanExecute) that is used by the canExecuteHasMutated function to force the computed canExecute observable to refresh (as described here)

Synchronous Execution

The first version relied on the action returning a promise object representing the status of the background operation, but after implementing the canExecute support I started to see a use case for commands that do not do anything in the background and simply execute synchronously.

var ExampleViewModel = function(externalValidationDefinition) {
    var _self = this;
    this.listOfItems = ko.observableArray();
    this.clearList = ko.command({
        action: function() {
            _self.listOfItems.removeAll();
        },
        canExecute: function() {
            return _self.listOfItems().length > 0;
        }
    });
};

To accommodate this type of usage I have updated the behaviour of the command so that when the action returns anything other than a promise it wraps that result in an immediately-completed jQuery.Deferred instance and then executes any registered handlers.

Other Minor Changes

errorMessage Removed

I realised that the error handling in version 1 was far too dependent on the signature of the error handler – what works for a failed AJAX request will not work for a failed HTML5 SQL request etc. – so I elected to remove the built-in error message recording and leave that down to the user.

Better Error Handling

As part of the support for synchronous code I also added improved error handling so that any error thrown directly from the action will be passed to the fail handlers.

Source

As ever, the code and unit tests are available on GitHub so help yourselves.