Flex FAQ 9-22-08

Question

Q: I want to use an XML data source to dynamically display
images and
data in my Flex application. I won't know how many
images/data points I
will have at design time. How can I best approach this?
A: Depending on what you're wanting to do, you can either use
a
Repeater with an Image or custom component that contains an
Image or you can
use a List based component to display an Image or custom
component with an
Image (or SwfLoader, or Loader) in it. The Help is full of
resources on
this subject. Here are some places you may want to try:
Help>Flex Start Page>Creating a Simple RIA
Help>Help Contents>User Interfaces
- Using Data Providers and Collections
- Controls
- Image
- SwfLoader
- Using Data Driven Controls
- Using Item Renderers and Item Editors
- Dynamically Repeating Controls and Containers
Here is an example that might get you running fast if you
find it easier to
learn from a working example than from Help files:
http://examples.adobe.com/flex2/inproduct/sdk/photoviewer/PhotoViewer.html
Q: I've created a custom itemRenderer component to use in a
List
based component (Datagrid, TileList, HorizontalList, etc.).
When my List
first displays, everything looks fine, but when I scroll it
or change the
dataProvider, some of the itemRenderers show values or
formatting that
aren't right. How do I fix this?
A: List-based components don't draw a renderer for every item
in the
dataProvider. Instead, they create enough to display what is
on screen now,
plus one or two more waiting in the wings. This means they
recycle the
renderers rather than creating new ones when you change
dataProvider or
scroll up and down. When you use a creationComplete event to
set up the
itemRenderer, that event doesn't happen again when the
renderer is used for
a different set of data. The solution to this is to override
the set data
protected function that most components have.
For more information, check out the following resources:
http://www.adobe.com/devnet/flex/articles/itemrenderers_pt1.html?devcon=f1
http://blogs.adobe.com/aharui/2007/03/thinking_about_item_renderers_1.html
Q: I want to run a function in my main application from
inside my
custom component. But when I try to refer to myFunction() in
that
component, I get a compile time error Call to a possibly
undefined function
myFunction. How can I fix this?
A: Your component has its own scope, so it doesn't know
anything
about the functions in the main file. You can get around this
by directly
referencing the main application scope like this:
Application.application.myFunction(). However, this makes
your component
tightly coupled, which is a quick way of saying that your
component is only
usable in an application that has a myFunction() function in
it. You're
better off dispatching an event from your component and
letting the
application decide how to handle it. For more information,
check out the
following resources:
http://www.adobe.com/devnet/flex/articles/loose_coupling.html
http://www.adobe.com/devnet/flex/articles/graduating_pt1.html
Q: I want to decide at runtime what kind of component to add
to
display my data. How can I accomplish this?
A: Use getDefinitionByName and ClassFactory to dynamically
create
your class. Just be sure that you actually have at least one
"hard"
reference to each class you intend to use, or the class might
not get
compiled into your swf. Here's a full write-up of how to do
that:
http://www.paulofierro.com/archives/520/
Q: I need to set a property or add an event listener on a
component
that is in a ViewStack/TabNavigator/Accordion. When the
component is not
the first child of the Navigator Container, I get a null
object error
(#1009). What causes this, and how can I fix it?
A: By default, the Navigator containers only create the
children of
each pane as that pane is viewed. The easy way to fix this is
to set the
creationPolicy on the Navigator to "all." However, this will
cause your
application to take longer to load. A better way to fix this
is to wait for
a later event, such as creationComplete on the component you
want to access,
or to use binding to "pull" the data into the component.
The way I handle it is to call invalidateProperties() on
change of the
ViewStack. I then override commitProperties() and call an
"initializer" for
each pane. In the body of each initializer function, I check
to see if the
selectedItem for the viewStack is the one my initalizer cares
about. If
not, I return from the function immediately. Inside that
initializer
function, I set properties and add listeners as appropriate.
Q: When my application is taller than the browser window, I
get
scrollbars in the application. I want the browser to do the
scrolling. How
can I do this?
A: In the html-template/index.template.html file change line
46 (in
the Flex 3 template) to read <body scroll="auto">
instead of the default of
<body scroll="no">.
Q: I am using URLRequest, HTTPRequest, or HTTPService to
retrieve
information from a database on a server. The first time my
application
makes a server call, it works fine. But when I call the same
page again, I
get the same data, even though I know the data has changed on
the server.
Is this a bug?
A: The browser has cached the response it got from the first
call
you made to the page. This means that you need to either
change how you are
calling the page so it appears to be a new request, or you
need to change
the page itself so that it won't be cached.
To change how you are calling the page, append a unique value
to the end of
the URL. For example, new
URLRequest('yourPage.asp?param="+(new
Date()).getTime).
If you want to change the page itself, you need to change the
response
headers. The method for changing the response headers will
vary from
language to language. For more on this see:
http://msdn.microsoft.com/en-us/library/aa923184.aspx
http://www.sun.com/books/components/Hall_ch7.pdf
http://www.w3schools.com/php/func_http_header.asp
http://livedocs.adobe.com/coldfusion/8/htmldocs/Tags_g-h_07.html#3989067
Q: I am using Modules in my Application. When I load the
first
Module, everything works fine. But when I load the second
one, I get a Type
Coercion Error failed error #1034 that looks something like
TypeError: Error
#1034: Type Coercion failed: cannot convert
com.myDomain.package::SingletonClass@fce9a89 to
com.myDomain.package::SingletonClass or TypeError: Error
#1034: Type
Coercion failed: cannot convert
mx.managers::HistoryManagerImpl@22346589 to
mx.managers.IHistoryManager. How do I fix this?
A: When you reference a singleton class, either a custom
singleton
or one of the managers in Flex, into a Module, if this is not
loaded in the
Application domain, it will load into the child domain of the
first Module
to load and other Modules will not be able to see it. To
prevent this,
either directly load your class into the main Application
before loading any
Module, or use a Runtime Shared Library with the class
included.
For more on this, see Alex Harui's presentation on Modules
from Flex 360
http://blogs.adobe.com/aharui/2007/03/modules.html
(slides 19, 24-31).
Not sure what a Singleton is? Check out
http://www.adobe.com/cfusion/communityengine/index.cfm?event=showdetails&postId=262&produc tId=2
Q: I'm using a Repeater to lay out the right number of
components on
the screen for me based on a data source. I'm trying to set
up the id for
each component dynamically like this:
<mx:Repeater id="myRepeater" dataProvider="mySource">
<mx:Button id="{'myButton'+myRepeater.currentIndex}"
label="{myRepeater.currentItem.label}" />
</mx:Repeater>
When I try to run the file, I get a compiler error
'{'myButton'+myRepeater.currentIndex}' is not a valid
identifier.
I need to be able to reference each of the repeated
components. How can I
do this?
A: If you give the component an ordinary id like this:
<mx:Button id="myButton"
label="{myRepeater.currentItem.label}" />
Flex will create an Array for you called myButton that
contains a reference
to each component the repeater created. For more information,
see
Referencing Repeated Components here:
http://livedocs.adobe.com/flex/3/html/help.html?content=repeater_3.html
Courtesy of Amy's Flex Diary.
The most recent version of this FAQ can always be found at
http://www.magnoliamultimedia.com/flex_examples/Amys_Flex_FAQ.pdf

Answer 1

"Greg Lafrance" <[email protected]> wrote in
message
news:gbr2r8$qtv$[email protected]..
> Is it possible to number the questions? Sometimes in
your posts you refer
> to Q3, but they are not numbered.
Good point :-). I get tired of counting them sometimes. I'll
try to get
that done before next week.