ESRI Holistic Lab: Flex API Day 3

I got around to asking about two things I would really like to have in the Flex API on day three: Will there be a way to change the version I am seeing, and will there be a way to change the data frame that I am viewing? The answer here was pretty much what you would expect. If the REST API will support it then the Flex API will be able to do it, which means that it isn’t currently available. That means this will still be one area where the ArcServer WebADF will outshine Flex. There are of course ways to get around this. You can create a Map Service for each version you wish to see although that will potentially cause problems with far too many Map Services. As far as data frames go, the same applies, simply create separate map services for each data frame.

Built In Tasks

Having gotten the QueryTask to work on day two I decided to take a gander at the other built in tasks on my last day in the labs. The other two currently available are the Locate and Find tasks. They work similarly to the QueryTask, using the built-in support of the REST API to do their work. Neither of these tasks is currently bug free however so I was unable to satisfactorily test them.

Debugging

This leads me to the next thing I would like to go over, debugging your flex code. Here, especially when using native REST API calls, you can take serious advantage of the Services Explorer. This is typically located at http://serverName/ArcGIS/rest/services and from here you can view any of the MapServices setup for that server. You can also see what capabilities exist both for the service and each individual layer and even test them. The explorer also has links directly to the REST API documentation which can be invaluable.

The other priceless tool for debugging is an extension to Firefox called Firebug. Firebug allows you to see requests being made over your browser. You can then see the request, header, parameters, and the response. By right clicking on a request you can copy it and then paste it into your browser’s address bar and see the result in your browser. I was able to figure out several issues I had with the QueryTask by doing this and then manipulating the variables directly within my request. I wouldn’t recommend working with the Flex API without using Firebug or something similar.

Building an Extensible Framework

The original Flex API from ArcWebServices was built on a variation of OSGi www.osgi.org/ which is a dynamic module system originally created for Java. Using such a framework allows for seamless adding and removing of modules from an application without having to take it down. You can simply drop a new module into your framework on a server while the application is running and it will be discovered and its functionality made available.

Eclipse (the base application for Flex Builder) is built on top of the OSGi Spec. The basic idea is that each module you create must implement the IBundle and IBundleActivator interface. When a new module is discovered your application then casts it to an IBundleActivator, passing it a context (IBundleContext) and retrieves all of the services (IService) from the module. Now any other part of the application can find and use the services provided by this new module. There are several freely available OSGi frameworks. Knoplerfish ( http://www.knopflerfish.org/ ) and Felix ( http://felix.apache.org/ ) are two fairly uncomplicated and well known ones.

Transmitting Data

We also spent some time on the different ways to transmit data using Flex. There are three basic ways beyond Web Services which are available: Remote Objects, messaging, and data services. Adobe provides a free Blaze DS service which allows you to do the first two. If you want to do the third though, you’ll need to shell out $25,000 for the privilege. Each of these transmission types buy you something. Using remote objects allows you to transmit actual objects as binary over the wire, speeding things up considerably over the use of web services. Messaging allows clients to subscribe to a destination point on a server and then receive any messages that come from there. Data Services allows for things like ‘active rows’ where you are given and can manipulate a row from a database table that will automatically manipulate the row on the server and let other users of that row that it has changed. There are a lot of powerful uses I can think of for this last method which is probably why it is so expensive.