ActionScript 3.0 in Flash Professional CS5 Essential Training
Illustration by John Hersey

Resolving problems with instances in a linked class


ActionScript 3.0 in Flash Professional CS5 Essential Training

with Todd Perkins

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Video: Resolving problems with instances in a linked class

Occasionally, you'll run into problems when linking a class to a symbol. Knowing how to resolve these issues will prevent countless headaches in the future. I've mentioned before that whenever you work with instances and Timelines and object-oriented code, you could have problems. In this file here, I've connected the TrashCan symbol to the Obstacle Base Class. Let's take a look at the Obstacle Base Class.
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  1. 3m 22s
    1. Welcome
      1m 23s
    2. Using the exercise files
      1m 17s
    3. Using the function keys
  2. 23m 38s
    1. Adjusting preferences for ActionScript fonts, colors, and formatting
      3m 25s
    2. Changing Flash Player and ActionScript versions in the Publish settings
      1m 35s
    3. Reading and solving errors through the Compiler Errors window
      2m 49s
    4. Using the Actions panel buttons to add and remove comments
      2m 33s
    5. Using the Actions panel to format code
      1m 49s
    6. Using the Actions panel Toolbox
      2m 4s
    7. Seldom-used but helpful functions of the Actions panel
      3m 14s
    8. Understanding code hinting
      2m 3s
    9. Reviewing the Code Snippets panel
      3m 7s
    10. Using help
  3. 45m 50s
    1. Understanding how ActionScript 3.0 code is processed in the Flash Player
      3m 22s
    2. Understanding variables
      4m 56s
    3. Understanding functions
      9m 1s
    4. Understanding events and event listeners
      5m 47s
    5. Working with conditional statements
      9m 49s
    6. Creating arrays and vectors
      6m 50s
    7. Using the For command to create a loop
      6m 5s
  4. 49m 9s
    1. Reviewing the display list
      3m 0s
    2. Understanding instances referenced through ActionScript
      2m 27s
    3. Using dot syntax to modify properties in an instance
      3m 25s
    4. Placing objects at the center of the stage
      4m 2s
    5. Placing objects at the edges of the stage
      5m 53s
    6. Using the methods of an instance
      3m 44s
    7. Accessing the parents, children, and grandchildren of objects
      5m 50s
    8. Creating instances from Library movie clips using ActionScript
      4m 23s
    9. Adding objects to the stage and changing parents with the addChild method
      5m 45s
    10. Removing objects from the stage with the removeChild method
      4m 17s
    11. Using the numChildren property to loop through a container's child objects
      3m 17s
    12. Using the getChildByName method
      3m 6s
  5. 56m 20s
    1. Understanding timelines
      4m 15s
    2. Using common timeline navigation methods
      5m 34s
    3. Using the currentFrame and totalFrames properties
      8m 2s
    4. Controlling the timeline of an instance
      6m 41s
    5. Detecting and navigating frame labels with ActionScript
      7m 57s
    6. Solving problems when timelines and ActionScript animation collide
      4m 16s
    7. Condensing a multi-frame timeline into one frame
      8m 33s
    8. Creating a simple slide presentation app in the timeline
      7m 51s
    9. Using one event handler with multiple buttons
      3m 11s
  6. 36m 5s
    1. Creating a class using Flash templates
      4m 43s
    2. Setting a document class
      6m 51s
    3. Preparing a class to be connected to a symbol
      4m 31s
    4. Using the Symbol Properties menu to connect a symbol to a class
      4m 55s
    5. Resolving problems with instances in a linked class
      7m 53s
    6. Understanding packages
      3m 17s
    7. Working with ActionScript source paths
      3m 55s
  7. 44m 32s
    1. Viewing the finished game
    2. Viewing the FLA file
      2m 9s
    3. Creating the DragDrop and Map classes
      2m 51s
    4. Linking the draggable class to Library symbols
      2m 47s
    5. Adding drag-and-drop functionality
      3m 38s
    6. Saving and resetting an object's position
      3m 33s
    7. Giving a target drop object to the draggable objects
      13m 16s
    8. Showing a Win screen
      7m 3s
    9. Resetting the game
      8m 24s
  8. 29m 6s
    1. Loading bitmap images from the Library
      4m 6s
    2. Loading bitmap images from external files
      5m 22s
    3. Adding mouse functionality to bitmap images
      3m 31s
    4. Using a loop to load multiple images
      6m 14s
    5. Creating a simple slideshow
      8m 37s
    6. Using Flash Player 10 color management
      1m 16s
  9. 27m 13s
    1. Loading an external SWF
      4m 14s
    2. Running ActionScript code in an external SWF from its parent
      5m 30s
    3. Running parent code in a child SWF
      5m 7s
    4. Creating a timeline-based preloader to load an external SWF file
      5m 3s
    5. Displaying playback progress of a loaded SWF file
      7m 19s
  10. 40m 10s
    1. Creating plain text files
      2m 8s
    2. Loading text from an external text file
      6m 26s
    3. Loading multiple text files
      6m 43s
    4. Rendering simple HTML in a text field
      5m 51s
    5. Creating a scroll bar for a text field
      5m 29s
    6. Scrolling a text field
      4m 59s
    7. Scrolling movie clips and other objects using masks
      5m 42s
    8. Modifying TLF text properties through ActionScript
      2m 52s
  11. 23m 40s
    1. Reviewing XML and E4X syntax
      3m 29s
    2. Loading an XML file
      3m 26s
    3. Using dot syntax to access XML data
      4m 2s
    4. Using XML data to populate a DataGrid component
      7m 4s
    5. Using XML data to load image files
      5m 39s
  12. 23m 33s
    1. Loading audio from the Library
      1m 41s
    2. Loading audio from external files
      3m 41s
    3. Playing, pausing, and stopping sounds
      5m 39s
    4. Muting all audio with the SoundMixer.stopAll method
      1m 28s
    5. Tracking load progress
      2m 38s
    6. Displaying sound position
      5m 5s
    7. Adjusting volume
      3m 21s
  13. 19m 54s
    1. Touring the FLA file
      5m 57s
    2. Controlling video playing and pausing with ActionScript
      1m 56s
    3. Working with ActionScript cue points to add closed captioning
      3m 35s
    4. Displaying video playback position
      3m 44s
    5. Adjusting video volume
      4m 42s
  14. 5m 32s
    1. Using new code snippets for AIR and mobile
      1m 13s
    2. Viewing the new code snippets HUD
      1m 17s
    3. Loading assets with the new ProLoader class
      1m 8s
    4. Understanding Flash Player premium features
      1m 54s
  15. 16s
    1. Goodbye

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Watch the Online Video Course ActionScript 3.0 in Flash Professional CS5 Essential Training
7h 8m Beginner Oct 21, 2010 Updated May 23, 2012

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In ActionScript 3.0 in Flash Professional CS5 Essential Training, Todd Perkins shows Flash designers how to incorporate ActionScript code into their projects and create interactive presentations and applications. The course includes a review of ActionScript language basics and the object-oriented programming (OOP) methodology, a tour of those Flash Professional CS5 features designed for developers, such as code hinting and the Code Snippets panel, and instructions on interacting with objects in the Library and placing code on the Timeline. Exercise files are included with the course.

Topics include:
  • Understanding the building blocks of ActionScript
  • Working with the display list
  • Using dot syntax
  • Navigating the Timeline
  • Creating document classes
  • Linking classes to Library objects
  • Adding drag/drop functionality to objects
  • Creating a slide show
  • Loading and running code in an external SWF
  • Working with text
  • Accessing XML data
  • Playing audio and video with ActionScript
Developer Web
ActionScript Flash Professional
Todd Perkins

Resolving problems with instances in a linked class

Occasionally, you'll run into problems when linking a class to a symbol. Knowing how to resolve these issues will prevent countless headaches in the future. I've mentioned before that whenever you work with instances and Timelines and object-oriented code, you could have problems. In this file here, I've connected the TrashCan symbol to the Obstacle Base Class. Let's take a look at the Obstacle Base Class.

In, the constructor method adds an EventListener to listen for the ENTER_FRAME Event. Remember, the ENTER_FRAME Event executes every frame, so over and over and over again, 24 times a second or 30 times a second, depending on how fast your frame rate is for your FLA file. So every frame, I'm running this function called checkCollision. This code may be familiar to you because you used it before.

I simply copied and pasted this code from another exercise and made very small changes to it. The only change is that I added the removeEventListener line of code. So the if statement checks to see if the boarder is hitting the can. If so, the boarder goes to the second frame and plays, and removes the can from the stage, the stage EventListener is removed for the KeyboardEvent, and again, the ENTER_FRAME Listener is removed.

Now, if I were to test the file at this point, I would find a whole slew of problems. Look at all of these problems. First of all, there's an undefined property called boarder_mc, but boarder_mc is on the Timeline, as we know. Why is there a problem here? Well, first of all, boarder_mc is not inside of the TrashCan's Timeline; it's on the main Timeline. That's why this doesn't work.

So what we can do is reference this object from the TrashCan's parent. The trashcan and the boarder have the same parent, so this should work. So we'll type MovieClip, then parentheses, parent inside the parentheses, after the parentheses, a dot. So, I'm going to check that hitTestObject in there, and then can does not exist either. Remember that the obstacle is the can, so we need to change can to this.

You'll find can in the removeChild line of code as well, and it says this. Also, the removeChild line of code is going to break as well. It's going to break because we're calling it from inside of the trashcan. It's calling it to itself, and that's not going to work. You need to have the parent object remove the child. So we can do this: parent.removeChild(this).

Now, we don't have to wrap this code inside of a MovieClip in parentheses because this is a display object container already. So that has the removeChild method. So, you don't need to wrap that in MovieClip if you don't want to. But we will need to do that for boarder_mc.gotoAndPlay. So we'll type that again, MovieClip, capital M, capital C, parentheses, parent inside the parentheses, dot after the parentheses, and then we can have boarder_mc. Now let's see what we get. Test the movie.

We have just as many errors. Now, before we test the movie, I'm going to save the file. Now I'll test the file again, and I only have one error. Flash says undefined property keyPressed. So Flash is telling me there's a problem with removing this EventListener on the main Timeline, because it doesn't recognize this keyPressed function.

It's not defined here; it's defined in the main Timeline. In this case, it may be better to simply run a function on the main Timeline from here. So let's just cut this code, and then type MovieClip, capital M, capital C, in parentheses, parent, after the parentheses a dot, and then we'll run a block of code called disable. Now we haven't created that yet. We'll create it right now.

So disable, some parentheses and a semicolon, save the file, go back to Problems.fla, click the first keyframe of the actions layer and open up the Actions panel. In here, we'll create a function called disable. So function, space, disable, some parentheses, colon, void, some curly braces, and inside of the curly braces, paste the code that you just cut. Now test the movie. We don't have any errors.

We can control the skateboarder with the keyboard. When he hits the trashcan, he falls over. Then we get this, "Cannot access a property or method of a null object reference." That's inside of the checkCollision function. So let's go there and take a look at what's wrong. See if you can find out the problem. To find it out, walk through the code, line by line, and see if you can find anything that could possibly be undefined in this code.

So we have this code. It looks all right, checking against the hitTestObject which is going to be the can, we have boarder gotoAndPlay. That's not going to be null. So there won't be a problem there. Then we have parent.removeChild(this). So after the child is removed, then this code may create a problem. So we'll just select that line of code that says parent.removeChild(this), cut the code, and then paste that at the bottom of this function as the very last thing.

So, we don't take off the child until after everything else is taken care of. So let's test the movie again after we save the file. Run into the trashcan, and we don't get any errors. So, whenever you're going to remove an object from the stage, make sure it's the last thing that you have that object do. Now, as you can see, things get a lot more complicated when you start working with class files, but it's much more rewarding. Because I've already created this Obstacle code, if I go back to the main Timeline, and let's say I want to make a different type of Obstacle, like this cat, I can put the cat in the way of the skateboarder, I can right-click the cat, choose Properties, check Export for ActionScript, set the Base Class to Obstacle and then click OK.

Flash is telling me that the Cat class doesn't exist, and it's going to create it for me. I'll click OK, test the movie, make the skateboarder run into the cat, and it's an obstacle, just like the trashcan was. So I'll close the window. So by using these object-oriented techniques, it'll take a little bit more time upfront, but in the long run, you'll save large amounts of time in building your applications.

Find answers to the most frequently asked questions about ActionScript 3.0 in Flash Professional CS5 Essential Training .

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Q: Will the exercise files for this course work with Flash CS6?
A: Yes, the code should work fine. The language has not changed since CS5.
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