Customized event in Flex 2
Events are a way of sharing data between two components. Events also acts as a asynchronous message suggesting that some input has arrived to the receiver of the event. For e.g. when a person clicks on the button control the event click is generated signifying that a user input in the form of a click has occurred. Once the event has occurred we need to take some action like on button click display the “hello world” in a text box. The action that we take is known as handling of the event. Or in other words we need to specify the even handler. Event handler is a function that specifies what needs to be done on that particular event.
The following code snippet makes it clear
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx=="http://www.adobe.com/2006/mxml"
layout="vertical">
<mx:Script>
<![CDATA[
//Event handler for the button click function
private function buttonEventHandler(evt:Event):void{
taTest.text="Hello World";
}
]]>
</mx:Script>
<mx:Button id="btTest" label="Click Me" click="buttonEventHandler(event)" />
<mx:TextArea id="taTest" />
</mx:Application>
buttonEventHandler is the event handler that will be called when the button is clicked. One salient point to note is the argument passed in the handler. We pass the object of type Event i.e event. The object “event” is always available in the application and we need to use it at the time of calling an event handler.
Sometimes it happens that we need to transfer some data between the two components.
For e.g. there may be a component which is responsible to get the data from the server using a httprequestserver. Component populates the data in a collection and now wants to send to another component which implements the list or data grid to display the information to the end user. The events are the way through which this can be done. The catch here is that the event thrown by one component should be available in the other component. So the component1 when it is done with populating the collection will throw an event indicating which needs to be handled in the component2 responsible to display the collection in the datagrid. It happens because of the way the events are transmitted from the component to the Application. Before getting into the transmission concept we need to understand one more concept known as event listener. Event Listener signifies that the component wants to receive a particular event and so it wants to register itself as one of the recipients.
We use the function addEventListener(event name , event handler) to do that. Now you must be wondering that the above button code did not had any such function call then how were we able to handle the event. The simple answer is that the event that we talked about was provided by the flex framework and so every component has got an implicit event listener for such events.
But the events that we are now going to see are the one that user has created himself (customized events). The customized events are used to transfer the data. These are created by the users and so the components will not be aware of them. Event Listener explicitly registers the component to listen to these customized events.
HOW TO WRITE A CUSTOMIZED EVENT
There are 3 stages in order t use a customized event
1. Create customized event
2. Dispatch the customized event
3. Listen the customized event.
Create Stage
Let us see how we actually write a customized event. We are just discussing the salient points here which are required to write a customized event. Syntactical and theoretical details are well covered in the Adobe documents.
Customized event can be declared by writing a class that inherits from Event.
package controls
{
import flash.events.Event;
public class ItemAddedEvent extends Event
{
var itemDescription:String; //an item to be added to the cart
public static const ITEMADDEDEVENT:String ="ItemAddedEvent";
public function ItemAddedEvent(description:String)
{
super(ITEMADDEDEVENT,true,true);
itemDescription=description;
}
override public function clone():Event{
return new ItemAddedEvent(itemDescription);
}
}
}
These are the steps that need to follow for defining any customized event
1. The class has to extend Event
2. Declare what ever data type you want to transfer by this event.
var itemDescription:String; //an item to be added to the cart
3. One can give a name to the event for easy reference. In our case it is ITEMADDEDEVENT. We can give multiple names to the same event. This is required when we want to use the same event in different places. For e.g. in a shopping cart application we need to perform add item to cart, delete item from the cart and update the quantity of the item in the cart. In all the operations the same item type data is required to be passed. So we can define a single customized class that favors for the transfer of a item type data. But the same event class can be used for all the 3 functions. We will give 3 names so that we remember for what all purpose we are calling this class.
public static const ADD:String = "addtocart";
public static const REMOVE:String = "removefromcart";
public static const UPDATE:String = "updatecart";
4. In the Constructor we need to call super(). The parameters required to be passed are the type of the event (ADD,REMOVE,UPDATE) and the optional parameters specifying the properties of the event.
Super(type, isbubble,iscancellable)
The second argument signifies whether the event is allowed to bubble. i.e. an event thrown by a component is allowed to be listened by the components that are up in the hierarchy.
5. clone() . This is required as per the Adobe document. We need to override the clone function and in the body just instantiate the customized event and return the object so created.
override public function clone():Event
{
return new ItemAddedEvent(itemDescription);
}
That is all to be done for a creation of an event.
Dispatch stage:
The event is thrown by the component in this stage. There is a well defined procedure to be followed to do that.
Lets take an example and do that
<?xml version="1.0" encoding="utf-8"?>
<mx:VBox xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:Metadata>
[Event(name="addItemEvent", type="controls.ItemAddedEvent")]
</mx:Metadata>
<mx:Label text="Item name:"/>
<mx:TextInput id="enteredItem"/>
<mx:Button width="104" height="28" cornerRadius="10" fillColors="[#00ff00, #00B000]" label="Add Item" fontSize="12" click="greenClickEventHandler()"/>
<mx:Script>
<![CDATA[
import controls.ItemAddedEvent;
private function greenClickEventHandler():void{
trace("Ouch! I got clicked! Let me tell this to the world.");
// throws the event and the passes the string to the customized event object
dispatchEvent(new ItemAddedEvent(enteredItem.text)); // passing the string
}
]]>
</mx:Script>
In the example
<mx:Metadata>
[Event(name="addItemEvent", type="controls.ItemAddedEvent")]
</mx:Metadata>
The metadata section is an optional section that tells that these are the events this component is going to dispatch. It is more for giving the information to the developer that this particular code dispatches some event. This is useful since the other components by looking at the metadata section can find out the events they can listen to.
We give the name of the event (addItemEvent) and the location of the class (controls.ItemAddedEvent). Note we could have also given ITEMADDEDEVENT in place of addItemEvent.
private function greenClickEventHandler():void{
trace("Ouch! I got clicked! Let me tell this to the world.");
// throws the event and the passes the string to the customized event object
dispatchEvent(new ItemAddedEvent(enteredItem.text)); // passing the string
}
When the greenClickEventHandler() is getting called on the click event the component is calling the dispatchEvent function. By calling this function the component is actually throwing the customized event. The dispatchEvent function requires the object of the Customized event class as an argument. So essentially we have passed the data structure (in this case string) to the event. This data will be available to all the component who will listen to this event.
Listening
This is the process of registering for the event. The component has to explicitly tell the application that it wants to get the event thrown by the other component and handle the data that is also being passed along with the event. This is done with the help of addEventListener function. The next section will make this concept clearer. For now we can assume that the function allows the component to receive the event and then handle it.
For e.g.
Component called
addEventListener(ItemAddedEvent.ITEMADDEDEVENT,addItemtoCartEventHandler);
to register the event and tell which handler function to call
private function addItemtoCartEventHandler(event:ItemAddedEvent):void{
sample.text+="yes a book has been added"+ event.itemDescription;
}
In the event handler pass an object (event:ItemAddedEvent) of the customized event class type and then use the data passed through the event using the object passed in the argument.(event.itemDescription)
Now the final step is to instantiate the component that will generate the event which is
<controls:newlargebutton />
The way it will work is
Instantiate ----:comp1 will throw the customized event on click----: comp2 will listen to it and call its event handler
Now lets get into the intricacies of how the events are propagated between the components.
In Flex in order to use any component, that component must be instantiated in the application(the application tag).
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="vertical">
comp 1
comp 2
…
comp n
</mx:Application>
The Application is the root of all the components. So if make a hierarchy tree it will look like
<Application>
| |
<Comp1> <comp2>
So we can say that Application is the parent node of comp1 and comp2
Again there can be a situation where we instantiate a component inside another component. Like
<hbox>
<list>
<button>
<hbox>
so list and button are getting instantiated in hbox
so in these cases the hierarchy tree will look like
<Application>
|
<comp1>
|
<comp2>
But in this case also <application> is related to comp2 in a ancestral relationship.
We need to understand this point because of the nature of event propagation. When any component generates an event it can be listened by all the components that are higher in the hierarchy. So an event generated by comp2 can be listened by comp1 and the Application. This means comp1 can have an event listener to register for the event generated by comp2, similar is the case with Application.
So if comp2 has to send any data to comp1, it will simply generate an event which will send the data to the comp1 which would be listening for this event.
There may be a situation where the components are not in the direct hierarchy but are in a sibling relation
<Application>
| |
<comp1> <comp2>
Now an event produced by comp2 will be easily listened by Application, but not by the comp1. But comp2 wanted to make comp1 listen to the event. Now events that are listened by the parent can be listened by the children also through following command
parent.addEventListener(event type,event handler)
This means that comp1 can listen to the events that reaches upto the parent. So the event sent by comp2 reaches Appication. Now this event can be heard by the comp1.
So in order to pass the events from one component to another one has to look at the way they are related to each other and depending on that call the addEventListener().
Lets look at some of the examples for understanding the concepts
E.g.1 There are two components related to each other in a parent child relationship. Comp1: It’s a VBOX containing a button and a textbox.
Comp2: A VBOX containing TextArea
Now the requirement is that in comp1 what ever is entered in textbox should be displayed in Comp2 textarea on the button click. So basically we want to send text string from comp1 to comp2. This is a classical situation where we will write a customized event to transfer the string between the two components.
The way we have arranged the components in the implementation is
<Application>
|
<comp2>
|
<comp1>
so addeventlistener will be called without parent prefix in comp1
Custom Event class (ItemAddedEvent.as)
package controls
{
import flash.events.Event;
public class ItemAddedEvent extends Event
{
var itemDescription:String; //an item to be added to the cart
public static const ITEMADDEDEVENT:String ="ItemAddedEvent";
public function ItemAddedEvent(description:String)
{
super(ITEMADDEDEVENT,true,true);
itemDescription=description;
}
override public function clone():Event{
return new ItemAddedEvent(itemDescription);
}
}
}
Comp1 (newlargebutton.mxml)
<?xml version="1.0" encoding="utf-8"?>
<mx:VBox xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:Metadata>
[Event(name="addItemEvent", type="controls.ItemAddedEvent")]
</mx:Metadata>
<mx:Label text="Item name:"/>
<mx:TextInput id="enteredItem"/>
<mx:Button width="104" height="28" cornerRadius="10" fillColors="[#00ff00, #00B000]" label="Add Item" fontSize="12" click="greenClickEventHandler()"/>
<mx:Script>
<![CDATA[
import controls.ItemAddedEvent;
private function greenClickEventHandler():void{
trace("Ouch! I got clicked! Let me tell this to the world.");
// throws the event and the passes the string to the customized event object
dispatchEvent(new ItemAddedEvent(enteredItem.text)); // passing the string
}
]]>
</mx:Script>
</mx:VBox>
Comp2: (BlindShoppingCart.mxml)
<?xml version="1.0" encoding="utf-8"?>
<mx:VBox xmlns:mx="http://www.adobe.com/2006/mxml"
xmlns:controls="controls.*"
creationComplete="init()">
<mx:Script>
<![CDATA[
import flash.events.Event;
import controls.ItemAddedEvent;
private function init():void
{
// eventlistener
addEventListener(ItemAddedEvent.ITEMADDEDEVENT,addItemtoCartEventHandler);
}
private function addItemtoCartEventHandler(event:ItemAddedEvent):void{
sample.text+="yes a book has been added"+ event.itemDescription;
}
]]>
</mx:Script>
<!—instantiated the comp1 -- >
<controls:newlargebutton addItemEvent="addItemtoCartEventHandler(event)" />
<mx:TextArea id="sample" />
</mx:VBox>
Application (greenbutton.mxml)
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
layout="vertical"
xmlns:ctrl="controls.*" >
<ctrl:BlindShoppingCart width="350" height="200" />
</mx:Application>
Same example but instead of child parent relation change it to sibling
So the instantiation of the comp1 will not happen inside comp2 instead it will happen in the Apllication
<Application>
| |
<comp1> <comp2>
Custom Event class (ItemAddedEvent.as)
package controls
{
import flash.events.Event;
public class ItemAddedEvent extends Event
{
var itemDescription:String; //an item to be added to the cart
public static const ITEMADDEDEVENT:String ="ItemAddedEvent";
public function ItemAddedEvent(description:String)
{
super(ITEMADDEDEVENT,true,true);
itemDescription=description;
}
override public function clone():Event{
return new ItemAddedEvent(itemDescription);
}
}
}
Comp1 (newlargebutton.mxml)
<?xml version="1.0" encoding="utf-8"?>
<mx:VBox xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:Metadata>
[Event(name="addItemEvent", type="controls.ItemAddedEvent")]
</mx:Metadata>
<mx:Label text="Item name:"/>
<mx:TextInput id="enteredItem"/>
<mx:Button width="104" height="28" cornerRadius="10" fillColors="[#00ff00, #00B000]" label="Add Item" fontSize="12"
click="greenClickEventHandler()"/>
<mx:Script>
<![CDATA[
import controls.ItemAddedEvent;
private function greenClickEventHandler():void{
trace("Ouch! I got clicked! Let me tell this to the world.");
// throws the event and the passes the string to the customized event object
dispatchEvent(new ItemAddedEvent(enteredItem.text)); // passing the string
}
]]>
</mx:Script>
</mx:VBox>
Comp2: (BlindShoppingCart.mxml)
<?xml version="1.0" encoding="utf-8"?>
<mx:VBox xmlns:mx="http://www.adobe.com/2006/mxml" xmlns:controls="controls.*"
creationComplete="init()">
<mx:Script>
<![CDATA[
import flash.events.Event;
import controls.ItemAddedEvent;
private function init():void
{
// eventlistener
//addEventListener(ItemAddedEvent.ITEMADDEDEVENT,addItemtoCartEventHandler); --wont work
parent.addEventListener(ItemAddedEvent.ITEMADDEDEVENT,addItemtoCartEventHandler);
}
private function addItemtoCartEventHandler(event:ItemAddedEvent):void{
sample.text+="yes a book has been added"+ event.itemDescription;
}
]]>
</mx:Script>
<mx:TextArea id="sample" />
</mx:VBox>
Application (greenbutton.mxml)
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
layout="vertical" xmlns:ctrl="controls.*" >
<ctrl:newlargebutton width="350" height="82" />
<ctrl:BlindShoppingCart width="350" />
</mx:Application>
In case the parent.addeventListener is not given , the event is not dispatched to the comp2 and so on button click the text in the text are is not written.
Since in the example we have given parent.addeventListener, so comp1 and interact to comp2 via parent and we get
