Learning To Program Abstractions

Monday, July 22, 2013 by K. Scott Allen

Last week Pluralsight published the 2nd course in my Learning to Program series. It is “Learning To Program - Part 2: Abstractions”, and the course focuses on some of the topics I had to unlearn to learn.

Unlearn to Learn?

unlearnWhen I started learning to program Basic and assembly languages on a TI-99 4/A, I  had no idea what computer programming was about and very few people who I could ask for help. Most of my time was spent trying to understand the syntax of the languages and putting together the right sequence of characters to make something work. I never gave a thought to algorithms, design, or code aesthetics, as the primary motivator was to color pixels on the screen and play musical notes.

I do remember being stumped trying to generate a random number using assembly language instructions. How do you get a giant calculator to produce an accidental value? This question led to hours of metaphysical thinking by a teenage boy. The whole idea of randomness with electronics never did click for me till Douglas Adams wrote about putting an atomic vector plotter inside a Brownian Motion producer.

University mostly reinforced the idea that computer programming was all about technical knowledge. So did every job interview I ever had.

It’s easy to be a software developer and not see the forest for the trees. It took me a long time to change my focus.

How This Course Is Different

In this course I wanted to give students a different perspective on programming than the one I started with by deemphasizing low level technical details and language syntax as much as possible. Instead I wanted to emphasize how to think about software decomposition and composition from the start, so a student would have some idea about the forest they live in. That’s one reason I chose Python as the programming language for this course.


Python is a beautiful, clutter free language that runs nearly everywhere. You can focus on design after learning just a few keywords and symbols. Python is also easy to work with. The REPL can provide instant feedback on both the syntax and the behavior of code.

In the course I cover everything from how to use the REPL to the different data structures available with Python. I also show how to break a program into functions and how to use a class definitions to create objects.

I talk about object oriented programming but I never talk about inheritance and polymorphism. Instead, I talk about building abstractions. Functions are abstractions, and objects are abstractions, too. When I was a student I would think that a hash table would be a perfectly reasonable abstraction for a shopping cart. I want today’s students to know the hash table can be an implementation detail and something you can encapsulate into a better abstraction for a shopping cart.

This course is about the single responsibility principle and the mantra of “design for use, not reuse”, although those words are never spoken verbatim. Knowing Big-O notation and how to work with pointers is incredibly important to build software correctly. But, I think those concepts receive too much of the spotlight in programming introductions, leaving many to believe, like I did, that you should solve every problem using lists of strings and numbers.

I hope everyone who watches enjoys the course and starts off with a better perspective on how to build software.

AngularJS: Listening for $destroy

Tuesday, July 16, 2013 by K. Scott Allen
1 comment

Angular will broadcast a $destroy event just before tearing down a scope and removing the scope from its parent.

Listening for this event is crucial for cleaning up tasks and resources that otherwise might continue to chew up memory or CPU.

As an example, the following controller continuously updates a model value in one second intervals, and these updates will continue forever, even after the controller’s view is gone and the scope is removed from its parent. Even worse, if the user is navigating back and forth to a view that loads this controller, each navigation will add another timer that runs forever.

module.controller("TestController", function($scope, $timeout) {

    var onTimeout = function() {
        $scope.value += 1;
        $timeout(onTimeout, 1000);
    $timeout(onTimeout, 1000);
    $scope.value = 0;

Listening for the $destroy event is an opportunity to halt the timer. One approach is to cancel the promise returned by $timeout.

module.controller("TestController", function($scope, $timeout) {
    var onTimeout = function() {
        $scope.value += 1;
        timer = $timeout(onTimeout, 1000);
    var timer = $timeout(onTimeout, 1000);
    $scope.value = 0;

    $scope.$on("$destroy", function() {
        if (timer) {

Chrome Workspaces: Edit Source From The Chrome Dev Tools

Monday, July 15, 2013 by K. Scott Allen

The Workspace feature of Chrome’s Developer Tools allows you to live edit the source code to a web application and have your changes saved on your local file system. The Workspace feature is now available not yet available in the stable builds of Chrome, but sounds like it will be soon. 

Getting Workspaces Setup

The first step is to open the Settings panel by clicking the “gears” icon in the lower right of the tools window. Then select “Workspace”. Under “Folders” you can select one or more folder where your source lives, or even a parent folder like your parent folder for all git repositories. The Workspace features is fairly smart when it comes to finding source code files inside.

Mapping Folders for Chrome Dev Tools Workspaces

Theoretically you could select the root directory of a drive, but I’m not clear on the implications of granting access, and after selecting a folder you’ll have to say yes to a  popup warning about making sure you “do not expose sensitive information”. Until there is a definitive document (I couldn’t find one), I’d have to assume third party extensions would be able to poke around the file system looking for plaintext passwords inside of config files.

Once a file system location is mounted, you can start to edit files in the Sources tab of the developer tools. You can open and edit any text file from the file system tree in the Navigator window of the Sources panel, even non web files like build files or C# code.

Of course the files you will be most interested in editing are the JavaScript and CSS files that are loaded into the browser from a web application. You can “map” a file loaded into the browser to a file on the file system by right-clicking the file in the Navigator window and selecting the matching file on your hard drive.

Mapping browser files to the file system

Once that step is complete, you can now start editing the JavaScript and CSS of an application and have the changes saved (Ctrl+S) to your local file system.

Where Workspaces Is Great

For editing the CSS of an application, the Workspaces experience is unsurpassed by any other development environment. You get to see your live site running in the browser with live data. The changes you make in the CSS are instantly reflected in the browser, and when the UI looks good, pressing Ctrl+S will save your changes into your CSS file. You can edit the CSS file directly, but you can also edit CSS from the Elements pane for fine-tuning, and these changes are immediately persisted. It is the ultimate WYSIWYG designer.

Editing CSS in Chrome Workspaces


Where Workspaces Is Good

You can edit other files, too, like JavaScript files, but these generally require a refresh of the browser to see the changes running. For example, you can open up a file of Jasmine tests and create a new test, but running the test requires a refresh. It would be nice to have an option of automatic re-execution in the spirit of today’s automated test runners.

Obviously  a change to compiled code, like a C# file, requires a re-build outside of the browser and dev tools. It would be nice to have some hooks to do this automatically like some text editors can do.

Where Workspaces Could Improve

As for the Chrome Dev Tools becoming an everyday all day editor + IDE, it still has a ways to go. The tools have always behaved like a gifted but insubordinate child. Hitting backspace or enter at the wrong time can wipe out a style definition (fortunately Ctrl+Z undo can bring the style back), and the UI often feels cramped, jumbled (even on a big screen), and quirky (as I’m typing, an unlabeled checkbox has appeared in the middle of nowhere). The tools also feel transient, since closing the wrong browser tab will close the associated workspace, and the editing ability is still missing some key features provided by other editors, like columnar selection, auto-completion for CSS properties, and commands or triggers to interact with external tools.

Despite these shortcomings, the Chrome dev tools are still head and shoulders above the tools in other browsers. Plus, they get better with every frequent release.

Looking Back: Python

Thursday, July 11, 2013 by K. Scott Allen

Rabbints, Campfires, PythonTrue story: My first use of Python was a little over 10 years ago.  I used Python to prevent rabbits from running into campfires and injuring themselves.

Of course the rabbits were virtual, as were the campfires. I was working with a team to create MMORPG middleware, and Python scripts were the brains for some of the lesser NPCs.

I’ve been working with Python again recently.

class Cart:

    def __init__(self):
        self.contents = dict()

    def process(self, order):        
        if order.add:            
            if not order.item in self.contents:
                self.contents[order.item] = 0
            self.contents[order.item] += 1

        if order.delete:
            if order.item in self.contents:
                self.contents[order.item] -= 1
                if self.contents[order.item] == 0:
                    del self.contents[order.item]

    def __repr__(self):
        return "Cart: {0!r}".format(self.__dict__)

It’s interesting how my opinion of Python has changed. Ten years ago the majority of my programming experience was with C, C++, and Java. My thoughts on Python were:

1. Using indentations to control block structure was weird, considering white space was insignificant everywhere else.

2. Tuples were useful.

3.  Double underscores look cool

Todays thoughts:

1. Tuples are still useful, but the REPL perhaps more so.

2. How did I miss lambdas, generators, map, filter, and reduce?

3. Double underscores are ugly, but the absence of { and } and is beautiful.

I like Python, it’s a great language. I have a better appreciation for its features today than I did 10 years ago.

AngularJS Drag and Drop Photo Directive

Wednesday, July 10, 2013 by K. Scott Allen

Continuing from previous posts on building a file input directive and a file reader service, this post contains my first try at a drag-n-drop directive that uses the file reader service to copy an image dropped from the desktop into an img element.

As always, I welcome suggestions!

Native HTML 5 drag-and-drop is easy to work with. The directive handles the dragover, dragleave, and drop events on the target element. Dragover and dragleave are mostly about manipulating classes on the element to style it as a droppable target, as well as using e.preventDefault(), which is required for the drop event to work.

  function ($parse, fileReader, resampler) {

    return {
        restrict: "EA",
        link: function (scope, element, attrs) {

            var expression = attrs.imageDrop;
            var accesor = $parse(expression);

            var onDragOver = function (e) {

            var onDragEnd = function (e) {

            var placeImage = function (imageData) {
                accesor.assign(scope, imageData);

            var resampleImage = function (imageData) {                       
                return resampler.resample(
                    imageData, element.width(),
                    element.height(), scope);

            var loadFile = function (file) {
                    .readAsDataUrl(file, scope)

            element.bind("dragover", onDragOver)
                   .bind("dragleave", onDragEnd)
                   .bind("drop", function (e) {

            scope.$watch(expression, function () {
                element.attr("src", accesor(scope));

Some of the code to ensure only images are being processed is omitted, but I do want to point out the  image resizing code. It’s wrapped by the resampler service below, which in turn uses Resampler.js from the post “100% Client Side Image Resizing”.

var resampler = function ($q) {

    var resample = function (imageData, width, height, scope) {
        var deferred = $q.defer();

        Resample(imageData, width, height, function (result) {

            scope.$apply(function () {

        return deferred.promise;


    return {
        resample: resample

module.factory("resampler", resampler);

Getting Started With OWIN, Katana, and VS2013

Tuesday, July 9, 2013 by K. Scott Allen

If you start almost any new ASP.NET project in the new preview of Visual Studio 2013, you’ll find a reference to the Owin package inside. OWIN (the Open Web Interface for .NET) is a specification designed to decouple web servers from the frameworks and applications they host. The OWIN goal is to provide a lightweight, modular, and portable platform for mixing and matching components, frameworks, and servers.

Katana is Microsoft’s implementation of OWIN components. The code is available on CodePlex

In a future post we can talk about how OWIN compares to System.Web, but first let’s get a simple example up and running from scratch.

In VS2013 you can start a new console mode application, then run the following commands in the package manager console:

install-package Microsoft.Owin.Hosting -IncludePreRelease

install-package Microsoft.Owin.Host.HttpListener –IncludePreRelease

install-package Microsoft.Owin.Diagnostics –IncludePreRelease

install-Package Owin.Extensions -IncludePrerelease

Inside the Main entry point for the console application, we can use the WebApp class to start an HTTP listener.

static void Main(string[] args)
    string uri = "http://localhost:8080/";

    using (WebApp.Start<Startup>(uri))

The Startup class used as the generic type parameter to WebApp.Start is a class we’ll have to implement, too.

public class Startup
    public void Configuration(IAppBuilder app)

IAppBuilder is an interface we can use to compose the application for Katana to host. In this setup we’ll invoke the UseWelcomePage extension method provided by Microsoft.Owin.Diagnostics. Running the console program and pointing a browser to http://localhost:8080 produces:

katana up and running

Other extension methods allow for more fine grained control over the request and response processing. For example, UseHandlerAsync from Owin.Extensions allows for a more traditional “Hello, World” response to every request.

public class Startup
    public void Configuration(IAppBuilder app)
        app.UseHandlerAsync((req, res) =>
            res.ContentType = "text/plain";
            return res.WriteAsync("Hello, World!");

So far this doesn’t appear much different than self-hosting WebAPI in a console application, but in some future posts we’ll dig a little deeper into some examples showing the loftier goals of OWIN and Katana.

Looking Back: My First C# Program

Monday, July 8, 2013 by K. Scott Allen

While digging through some directories of archived source code I found the first program I ever wrote in C#.

I’m not sure when I wrote this, but since there was a makefile in the directory I’m guessing this was still in the .NET 1.0 beta days of late 2000.



Based on the code and idea in Bill Wagner's VCDJ Fundamentals column.
This program takes piped input or a filename argument and copies all stream data 
to the clipboard. 


dir | clipboard 

clipboard clipboard.cs


using System;
using System.IO;
using System.WinForms;

class MainApp 
    public static void Main( string[] args ) 

        // The clipboard class uses COM interop. I figured this out because
        // calls to put data in the clipboard always failed and further 
        // investigation showed a failed hresult indicating no CoInitialize.
        // Here is the .NET equivalent:
        TextReader textReader;
        if (args.Length == 0)
            // take the piped input from stdin
            textReader = System.Console.In;
            // open the text file specified on command line
            File file = new File(args[0]);
            textReader = file.OpenText();
        string line;
        string allText = "";
        Boolean pipeFull = true;
                // When the pipe is empty, ReadLine throws an exception
                // instead of the documented "return a null string" behavior.
                // When reading from a file a null string is returned.
                line = textReader.ReadLine();
                if( line == null )
                    pipeFull = false;
                    allText += line; 
                    allText += "\r\n";
            catch(System.IO.IOException ex)
                if(ex.Message == "The pipe has been ended")
                    pipeFull = false;
                    throw ex;

        Clipboard.SetDataObject(allText, true);

The first thoughts that came to mind when seeing this code again were:

1) Wow, that’s a long function by today’s standards.

2) I could use this!

Before resharpering the program into shape, I did a quick search and discovered Windows now comes with such a program by default. It’s called clip. I guess I can leave the code in the archive.

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