1 flash actionscript event handling. 2 event handling right now we know all about variables lets go...

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Flash ActionscriptEvent Handling

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Event Handling

Right now we know all about variables lets go back to our text input/output example:

Suppose we want to create an Enter button e.g. user types stuff into the input field, when finished clicks Enter, and then the text is displayed in the output field???

What we need is some way to detect that the user has clicked on the button and some way of knowing what to do when this happens…

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ActionScript and Events

ActionScript code is executed when an event occurs: E.g., when a movie clip is loaded, when a keyframe on the timeline is entered, or when the user clicks a button.

Events can be triggered either by the user or by the system. Users click mouse buttons and press keys; the system triggers events when specific conditions are met or processes completed (the SWF file loads, the timeline reaches a certain frame, a graphic finishes downloading…).

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When an event occurs, you write an event handler to respond to the event with an action.

When you are writing ActionScript code to perform event handling, there are three important elements you'll want to identify:

1. The event source

2. The event

3. The response

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The event source: Which object is the one the event is going to

happen to? E.g., which button will be clicked, or which

Loader object is loading the image? The event source is also known as the event

target, because it's the object where the event is targeted by Flash Player (where the event actually happens).

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The event: What is the thing that is going to happen, the

thing that you want to respond to? This is important to identify, because many

objects trigger several events.

The response: What step(s) do you want performed when the

event happens?

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Any time you write ActionScript code to handle events, it will include these three elements, and the code will follow this basic structure

elements in bold are placeholders you'd fill in for your specific case.

eventSource.addEventListener(EventType.EVENT_NAME, eventResponse);

function eventResponse(eventObject:EventType):void {

// Actions performed in response to the event go here. }

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First, we define a function, which is the way to specify the actions you want performed in response to the event.

Next, we call the addEventListener() method of the source object, in essence "subscribing" the function to the specified event so that when the event happens, the function's actions are carried out.

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A function provides a way for you to group actions together, with a single name that is like a shortcut name to carry out the actions.

When you're creating a function for event handling, you must choose the name for the function (named eventResponse in this case), and you must also specify one parameter (eventObject).

Specifying a function parameter is like declaring a variable, so you also have to indicate the data type of the parameter.

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There is an ActionScript class defined for each event, and the data type you specify for the function parameter is always the class associated with the particular event you want to respond to.

Finally, between the opening and closing curly brackets ({ ... }), you write the instructions you want the computer to carry out when the event happens.

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Once you've written the event-handling function, you need to tell the event source object (the object that the event happens to--for example, the button) that you want your function to be called when the event happens.

You do this by calling the addEventListener() method of that object (all objects that have events also have an addEventListener() method).

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The addEventListener() method takes two parameters: First, the name of the specific event you

want to respond to. Second, the name of your event response

function.

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Examining the Event-handling Process

What happens when you create an event listener? Let’s look at creating a listener function that is called when an object named myButton is clicked.

Here is how this code would actually work when it's running in Flash Player:

function eventResponse(event:MouseEvent):void {

//Actions performed in response to the event go here. } myButton.addEventListener(MouseEvent.CLICK, eventResponse);

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1. When the SWF file loads, Flash Player makes note of the fact that there's a function named eventResponse().

Function eventResponse (event: MouseEvent):void{

//actions performed in response to the event go here.}myButton.addEventListener(MouseEvent.CLICK, eventResponse);

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2. Flash Player then runs the code (specifically, the lines of code that aren't in a function). In this case that's only one line of code: calling the addEventListener() method of the event source object (named myButton) and passing the eventResponse function as a parameter.



myButton

listeners:function1()function2()

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a. Internally, myButton has a list of functions that are listening to each of its events, so when its addEventListener() method is called, myButton stores the eventResponse() function in its list of event listeners.



myButton

listeners:function1()function2()eventResponse()

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3. At some point, the user clicks the myButton object, triggering its click event (identified as MouseEvent.CLICK in the code).

At that point, the following occurs…

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a. Flash Player creates an object, an instance of the class associated with the event in question (MouseEvent in this example).

For many events this will be an instance of the Event class; for mouse events it will be a MouseEvent instance; and for other events it will be an instance of the class that's associated with that event.

This object that's created is known as the event object, and it contains specific information about the event that happened: what type of event it is, where it happened, and other event-specific information if applicable.

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myButton


eventObject

target:myButton type:mouseEvent.CLICK …

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b. Flash Player then looks at the list of event listeners stored by myButton.

It goes through these functions one by one, calling each function and passing the event object to the function as a parameter.

Since the eventResponse() function is one of myButton's listeners, as part of this process Flash Player calls the eventResponse() function.

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myButton


eventObject


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c. When the eventResponse() function is called, the code in that function runs, so your specified actions are carried out.



myButton


eventObject


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Event-Handling Examples

Clicking a button to start the current movie clip playing:

1. Draw a circle on the stage, convert it to a graphic symbol and give it the name ball.

2. Create a simple animation with motion tweening using ball.

3. Create a new button symbol for your ‘play’ button. Call it myButton.

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4. On your main timeline create a new layer called ‘button’ and with that layer selected drag myButton onto the stage.

5. With your button still selected go to the properties panel and Give this instance of myButton the instance name playbutton.

6. Create another new layer and call it ‘actions’.

7. Put the following code into frame 1 of your actions layer.

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Remember playButton is the instance name of the button, and this is a special name meaning "the current object":

this.stop();

function playMovie(event:MouseEvent):void {

this.play(); }

playButton.addEventListener(MouseEvent.CLICK, playMovie);

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Clicking a button to navigate to a URL

Create a new button symbol and call it myButton.

Place the button on the main timeline and give it the instance name linkButton.

Create a new layer called ‘actions’ and put the following code into frame 1…

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Remember In this case, linkButton is the instance name of the button:

function gotoFacebook(event:MouseEvent):void { var facebookURL:URLRequest = new URLRequest("http://www.facebook.com/"); navigateToURL(facebookURL); }

linkButton.addEventListener(MouseEvent.CLICK, gotoFacebook);

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Text Input Example…

1. Open a new movie and select the Text Option. Select Input Text from the drop-down menu in the properties panel and click once on the stage to insert a textfield.

2. Give your textfield the instance name entryText.

3. With the Text Option still selected insert another textfield, select Dynamic Text from the drop-down menu and give it the instance name outputText.

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4. Create a button symbol for your ‘enter’ button. Call it myButton.

5. Drag myButton onto the main timeline and give it the instance name entryButton.

6. Create a new layer called ‘actions’ and place the following code into frame 1…

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Remember entryText is an input text field, and outputText is a dynamic text field. entrybutton is the instance name of your enter button.

function updateOutput(event:MouseEvent):void {

var pressedKey:String = entryText.text; outputText.text = "You typed: " + pressedKey;

}

entryButton.addEventListener(MouseEvent.CLICK,updateOutput);

1 flash actionscript event handling. 2 event handling right now we know all about variables lets go...

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