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Event driven programming


Foundations of Programming: Fundamentals

with Simon Allardice

Video: Event driven programming

Right now all our code executes as soon as the page loads our script file. It runs through it as fast as possible and finishes. But that's not good enough anymore. What I want to be able to do is start to react to some input from the user. And by input, I don't just mean prompting them for their name, but just being able to react to a whole bunch of different things. Does the user clicks somewhere on the screen, does the user resize the window, do they move the mouse? What do they do? What I want to do is start responding to events.
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  1. 4m 15s
    1. Welcome
      1m 17s
    2. Making the most of this course
      2m 8s
    3. Using the exercise files
  2. 22m 11s
    1. What is programming?
      5m 45s
    2. What is a programming language?
      4m 48s
    3. Writing source code
      5m 34s
    4. Compiled and interpreted languages
      6m 4s
  3. 16m 29s
    1. Why JavaScript?
      4m 45s
    2. Creating your first program in JavaScript
      6m 54s
    3. Requesting input
      4m 50s
  4. 31m 38s
    1. Introduction to variables and data types
      5m 16s
    2. Understanding strong, weak, and duck-typed languages
      3m 51s
    3. Working with numbers
      5m 4s
    4. Using characters and strings
      4m 5s
    5. Working with operators
      4m 47s
    6. Properly using white space
      6m 46s
    7. Adding comments to code for human understanding
      1m 49s
  5. 24m 48s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 9s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 54s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 56s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 27s
  7. 13m 31s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 39s
  8. 19m 28s
    1. Cleaning up with string concatenation
      4m 30s
    2. Finding patterns in strings
      8m 3s
    3. Introduction to regular expressions
      6m 55s
  9. 19m 58s
    1. Working with arrays
      5m 46s
    2. Array behavior
      5m 29s
    3. Iterating through collections
      5m 18s
    4. Collections in other languages
      3m 25s
  10. 10m 50s
    1. Programming style
      5m 55s
    2. Writing pseudocode
      4m 55s
  11. 25m 55s
    1. Input/output and persistence
      3m 6s
    2. Reading and writing from the DOM
      8m 11s
    3. Event driven programming
      7m 47s
    4. Introduction to file I/O
      6m 51s
  12. 24m 25s
    1. Introduction to debugging
      5m 57s
    2. Tracing through a section of code
      7m 5s
    3. Understanding error messages
      3m 21s
    4. Using debuggers
      8m 2s
  13. 14m 16s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 28s
    3. Reviewing object-oriented languages
      2m 30s
  14. 11m 14s
    1. Memory management across languages
      5m 11s
    2. Introduction to algorithms
      4m 2s
    3. Introduction to multithreading
      2m 1s
  15. 29m 20s
    1. Introduction to languages
      1m 42s
    2. C-based languages
      4m 40s
    3. The Java world
      3m 13s
    4. .NET languages: C# and Visual Basic .NET
      6m 17s
    5. Ruby
      3m 4s
    6. Python
      2m 56s
    7. Objective-C
      4m 3s
    8. Libraries and frameworks
      3m 25s
  16. 1m 2s
    1. Where to go from here
      1m 2s

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Watch the Online Video Course Foundations of Programming: Fundamentals
4h 47m Beginner Sep 22, 2011

Viewers: in countries Watching now:

This course provides the core knowledge to begin programming in any language. Simon Allardice uses JavaScript to explore the core syntax of a programming language, and shows how to write and execute your first application and understand what's going on under the hood. The course covers creating small programs to explore conditions, loops, variables, and expressions; working with different kinds of data and seeing how they affect memory; writing modular code; and how to debug, all using different approaches to constructing software applications.

Finally, the course compares how code is written in several different languages, the libraries and frameworks that have grown around them, and the reasons to choose each one.

Topics include:
  • Writing source code
  • Understanding compiled and interpreted languages
  • Requesting input
  • Working with numbers, characters, strings, and operators
  • Writing conditional code
  • Making the code modular
  • Writing loops
  • Finding patterns in strings
  • Working with arrays and collections
  • Adopting a programming style
  • Reading and writing to various locations
  • Debugging
  • Managing memory usage
  • Learning about other languages
Developer Web
Simon Allardice

Event driven programming

Right now all our code executes as soon as the page loads our script file. It runs through it as fast as possible and finishes. But that's not good enough anymore. What I want to be able to do is start to react to some input from the user. And by input, I don't just mean prompting them for their name, but just being able to react to a whole bunch of different things. Does the user clicks somewhere on the screen, does the user resize the window, do they move the mouse? What do they do? What I want to do is start responding to events.

This is what's called event-driven programming. And the ability to do this is built into most languages now, including JavaScript. Now here is the great thing. These events are already happening and they've been happening all along. When the page is loaded, that's considered an event. When the user clicks somewhere on the screen, that's another event. When they move their mouse, that's a whole bunch of events. When they scroll the screen up and down, that's an event. If they click into a text field, that's an event. As they're typing, every key press is another event.

When they leave the form, it's another event. So these events are going on all the time. Your job as a programmer is to say, which ones do I care about? And the way that we can do this is by writing some functions and then saying, instead of just running this function when the page loads, only run it when the user clicks a button or moves their mouse. Now we don't have to keep asking, if the user is pressing their button. The operating system is going to take care of that.

We just need to say what events do we want to react to? We write what are called event handlers or event listeners. We can pick our own term there. That just means a function that's waiting for an event to happen. These events are all part of JavaScript and there are special words to describe them. And they're typically written all lowercase and they begin with the word on, like onload, onclick, onmouseover, onfocus, onblur. And what we can do is write functions and hook them up to these events.

You can have one event call multiple functions; you can have one function being called by multiple events. It's up to you how you start to wire them up. There are several ways to handle an event in JavaScript. For our purposes in this course, I'm just going to talk about one because we're trying to illustrate just a general principle here. And the basic idea in any language when working with event-driven programming is there are two pieces of information that you need: what's the event and what do you want to do. But bear in mind that with events, they can happen on different pieces of the page, different pieces of your user interface.

It's the idea of a click event, but that could be a click event on a button or a click event on an image or a click event on some other part of the screen. So the general format in JavaScript is not just the event name but element.event name. what element and what event. So you'll have things like this: window.onload, the load event of the window object. The window here represents the browser window. This is an event we can react to when the page loads. If I have say a complicated form with lots of text fields to fill out, we could have something like this: nameField.onblur.

The onblur event happens when I leave a field. Say I'm typing my e-mail address and I click out or hit the Tab key. This event will be called. This is one of the most common ones: myelement.onclick. Now what I've called myelement here? I'm presuming that I've created somewhere using the DOM. I've grabbed hold of it. I have a variable that represents an element on the page, whether that's an h1 or an image or a paragraph. Then I say .onclick and then I have to say, okay, that's the event, so what do I want to do? Well, the typical format is this. We actually create a function here.

And we've seen this before. The idea of the word function, followed by parentheses, followed by the opening and closing curly braces. Now usually when you say the word function, you also give it a name. You're creating a function giving it a name. We're not doing that here. This is what's called an anonymous function. This is actually very common in JavaScript. It might look a little odd, but all it means is a function without a name. It's synonymous. Well, we know that the function is really just a bunch of code wrapped up. That's all we're doing here. We're using the word function saying when this event happens, run whatever is inside this block of code.

I don't need to name this function because naming it would be a waste of time. I'm saying exactly what it does and I'm saying exactly when it's executed. Right here on the click event of whatever my element is. Now if you start reading more JavaScript, you're likely to see one thing that looks a little different. Usually when I declare a function, I just have the opening and closing curly braces. When you see this format, you'll typically see the semicolon after the closing curly brace. It looks a little unusual because we don't need that after a closing curly brace for an if statement or a while or even a usual function.

And in fact here, we're putting that semicolon not because this is a function, but because the entire line is a statement: myelement.onclick = this function and whatever is inside it. So let me show you an example. I've got an identical two files that I had earlier. A very basic HTML page with a headline which has the ID of mainHeading. And that's my name that I just made up for that section. And then script.js, which I brought over from the previous one.

What this is doing right now is grabbing hold of that headline by using document.getElementById. We're using the DOM here. And right now, it's actually changing that immediately. But I've decided I don't want that to happen immediately. I want to start reacting to events. I only want to call this line if they've clicked the headline. So here's what I'm going to do. I know that the first line of code that's executed grabs hold of the headline and then I'm going to say headline.onclick.

I'm going to set up an event handler for them clicking the headline. Equals, and we're going to write an anonymous function, the word function, opening and closing parentheses, and the opening and closing curly braces. I'm going to put my semicolon in here. It would work without it, but this is the best practice. Now I'm going to grab this line of code that sets the inner HTML, the actual text value of this headline. And I'm going to paste it indented inside this anonymous function.

Change this message one little bit, to "You clicked the headline." I'll save this and I'll open up the webpage that contains it. Now we're finally having code that doesn't immediately run. We're not changing the value of the headline yet. It's going to wait until we click on it. So events are actually happening. When I move the mouse, that's considered an event. If I resize this page, that's considered multiple events. But I didn't say I cared about that.

The only one I care about is the click event on the element called headline, which means the only thing that's going to react to, it's no click events anywhere else on the page. I can all over the page right now. But if I click on the headline, we'll change it. We're now reacting to that. And this is the great thing. With very little code, we're now allowing the page to call back into our script. We've got an event-driven program that's communicating two ways, between our user interface, the visible part of our webpage, and our code behind the scenes.

A simple example here, but it's illustrating a really powerful principle of event-driven programming.

Find answers to the most frequently asked questions about Foundations of Programming: Fundamentals .

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Q: Using TextEdit with Mac OS 10.9 Mavericks? 
A: If you're using the built-in TextEdit program in Mavericks to write your first examples and your code doesn't seem to be working, here's one reason why: by default, "smart quotes" are now turned on in TextEdit Preferences.
This is where TextEdit will automatically change pairs of double quotes to "smart quotes" - where the opening and closing quote are different, like a 66 and 99.
While this is fine for human eyes, programming languages don't want this - when writing code, they need to be the plain, generic straight-up-and-down quotes.
So make sure that in TextEdit > Preferences, that "Smart quotes" are unchecked.
Important! Whenever you make a change to TextEdit preferences, make sure to then completely quit out of the program (Command-Q or using TextEdit > Quit TextEdit) and then re-open it, as changes won't take effect on documents you already have open.
However, we're not finished - just because you've changed the preferences, it does **not** change any *existing* smart quotes back to "regular" quotes - it just doesn't add new ones - so make sure to go through your files for any time you wrote quotes and TextEdit may have changed them to smart quotes - look in both the JavaScript, and your HTML too, and compare to the downloadable exercise files if necessary.
If that sounds like a bit of a chore, I recommend just downloading a code editor like Sublime Text ( or TextMate ( and using that instead of TextEdit - it's only a matter of time before you'd move away from TextEdit anyway - we only used it in the course because it was built-in and a quick way to get started, but it's now become more of a inconvenience than it was before.
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