Wpf: Event driven vs MVVM design pattern

I'm a beginning coder and I've been working on a WPF app for several months now and struggled to implement anything in MVVM due to time constraits to learn so I coded my program the only way to knew how: with code behind, tight coupling, and a whole bunch of
spaghetti code.
My program has grown into 20,000+ lines of messy (but working) code that is mostly event driven. The app I am building will probably be used and continue to be developed for years to come, but I will be the only one working on the code.
However, now that I have time to start learning MVVM, I'm wondering how useful it really is to follow the MVVM pattern. I'm considering refactoring my code into the MVVM pattern but everything seems 100x difficult to do with little documentation. With using
code behind and tight coupling, I can make the UI do exactly what I want it to do and everything seems like it can be coded so much faster through the event-driven methods.
How important is it to follow the MVVM design pattern for WPF apps, especially in the long run? Do you think it's a good idea for me to invest my time to redesigning this app and following an MVVM pattern?

20k lines is a lot.
That will mean maintenance will be a significant overhead.
Anything making maintenance harder will mean maintenance takes longer.
If you're going to do any serious wpf development ( and 20k lines is serious imo) then you should be using mvvm.
The thing is though, it's different.
You will need to learn it before it becomes second nature and doesn't appear to be an overhead.
It isn't true that "everything" can be coded so much faster with event driven methods.
In fact a number of things are way easier to do using mvvm rather than code behind.
If that isn't enough to persuade you.
Teams use MVVM.
Go to an interview and tell your prospective team lead you think mvvm is no good and you may as well not go to the interview.
If it becomes obvious in an interview you haven't learnt mvvm then you will not be chosen.
Teams also use automated testing.
This is nigh on impossible with code behind and one of the selling points of mvvm.
You can test without instantiating a view which means you can run thousands of fast tests in seconds.
That means you can make changes safe in the knowledge you're not breaking stuff you haven't worked on.
It means someone else in the team doesn't break your code without realising.
Since UI and code are decoupled you can makes changes to either or both rather easier - that decoupling = flexibility.
As your code base grows then re-factoring becomes MASSIVELY faster if you have automated tests in place and your layers are loosely coupled.
I don't know about this paraticular app you have because I don't know almost anything about it.
If you plan on doing professional wpf development then I suggest you re-adjust your opinions on MVVM.
Please don't forget to upvote posts which you like and mark those which answer your question.
My latest Technet article - Dynamic XAML

Similar Messages

Difference between n-layer software architecture and MVVM design pattern?

hello everyone,
I am not clear with this concepts of Software architecture pattern and design pattern.
I am developing an wpf application using MVVM. Now, my question is, does MVVM come under n-layered architecture?
or n-layer architecture and MVVM both are different?
Regards,
Rakesh.N
Rakesh murthy

"
Now my question is, i want to process the data which i get from database. where should i write that processing logic after getting the data from database? in model classes? or in view model classes?
The model is however the data comes from wherever it comes from.
If there is any processing after you get it then that would be in the viewmodel.
The viewmodel adapts your data to the view.
If it needs any adapting then the viewmodel does that, not the model.
EG here
protected async override void GetData()
ThrobberVisible = Visibility.Visible;
ObservableCollection<CustomerVM> _customers = new ObservableCollection<CustomerVM>();
var customers = await (from c in db.Customers
orderby c.CustomerName
select c).ToListAsync();
foreach (Customer cust in customers)
_customers.Add(new CustomerVM { IsNew = false, TheEntity = cust });
Customers = _customers;
RaisePropertyChanged("Customers");
ThrobberVisible = Visibility.Collapsed;
That logic is in a viewmodel.
in this context , in which layer the viewmodel and model classes are put under in n-tier architecture?
They're in two of the n layers/tiers.
Or they're in one.
Depending on how someone decides to classify a layer or a tier.
There is no absolute definition.
Why would you care?
Hope that helps.
Technet articles: Uneventful MVVM;
All my Technet Articles

Register event in producer consumer design pattern

Hello wire workers,
I am fairly green when it comes to dynamic event registration, so my understanding of it is probably quite flawed, but what I have implemented seems to work fine.
Assuming a fairly typical queued producer/consumer state machine architecture.
Up till now, I have done the dynamic event registration outside (i.e. to the left of) the UI loop, so all events get registered before the UI loop starts to run.
My question is:
How can I register events from inside one of the consumer loops? I cannot wire it...
Maybe an example will make my scenario more clear.
I have a DAQ loop (queued state machine). In the init state of this loop I want to register a digital input such that changes on a line are handled by my main event structure (in the totally separate UI loop). Is this possible?
I look forward to hearing positive answers
nrp
CLA

The best way to send data from the Consumer Loop to the Producer Loop is through User Events. What you will want to do is create a User Event whose data type is the registered DAQmx event refnum. Then when you register the DAQmx event in the Consumer Loop, fire the User Event with that new refnum and update the registration in the Producer Loop. That's probably hard to wrap your head around in words, so I'll attach a dummy example and pic below. Hope this helps!
VI Saved in LV85.
Message Edited by Jarrod S. on 04-21-2008 12:54 PM
Jarrod S.
National Instruments
Attachments:
DAQmx Event.vi ‏30 KB
DAQmx Event Pic.PNG ‏38 KB

Producer consumer design pattern

Hello,
I am trying to build a simple program that does the following:
1. when program starts, the indicator is initialized to -999
2. when Measure button is clicked, the indicator shows random number continuously.
3. when Save button is clicked, the indicator shows 1
I use the event-based producer-consumer design pattern and it works great. However, I cannot find a way to allow the CONTINUOUSLY generating of random number when Measure is clicked. Can someone shed a light on it? Thanks a lot for the help.
Attachments:
CommandList.ctl ‏6 KB
Untitled 1.vi ‏25 KB

One simple way to do it is to use the Timeout event case. You can modify the timeout value based on what events have occurred. See attached mod.
Not saying this is the best way. Kind of depends on where the program goes from here.
Attachments:
Untitled 1-2 MOD.vi ‏27 KB

Producer/Consumer Design Pattern + DAQmx + Event Structure

Dear community users,
at the moment I try to implement the Producer/Consumer Design Pattern into my current project. For this purpose I am trying to make a "minimal-example" in which I use the Producer/Consumer Design Pattern for the data acquisition and analysis (display in GraphXY, calculation etc..) process. In the Producer Loop I perform the data readings via DAQmxRead.VI and in the Consumer Loop, in this example only, I display the data in a GraphXY. At the same time I want to control the data acquisition via an Event Structure, for example Start Measurement/Stop Measurement Buttons. To configure the measurement time, for example if the user only wants to measure 150 sec, I use the Time Out Event of the Event Structure. This is a technique which is often used, when data acquisition and Event Structures are used at the same time (Excuse me if this is wrong!). Due to understand the Producer/Consumer Design Pattern I looked up in the Template\Framework of LabVIEW.
In the attachment you will find my "minimal-example", which -sadly-, not working. What should I do ?
Thank you.
Best regards,
tugrul öztürk
Attachments:
daqEngine_PCP.vi ‏35 KB

Your VI will never stop due to the Ok Button that stops the producer loop is read OUTSIDE of the loop. Add an event case for it.
Move the Start and the Ok Button inside of their respective event cases. This will allow the latch to reset when they are read.
Change the Ok Button to a stop button.
Use a Greater Or Equal instead of Equal for the measurement time comparison. What if somebody randomly changes it to 0 on you? Again, you will never stop.
Is the DAQ only supposed to read for "Measurement Time". Right now, it constantly reads. So if you don't use the DAQmx Read, you will get a buffer overflow error. You need to put the DAQmx Start Task inside of the Start event case and then the Stop DAQmx Task inside of the Timeout event case when the measurement time is exceeded. You should also perform the read regardless inside of the Timeout event case.
I HATE telling the consumer loop to stop by destroying the queue. You can look data that way. Instead, send something an empty array. In the consumer, check for an empty array. If it is empty, stop. Destroy the queue only AFTER BOTH LOOPS HAVE COMPLETED.
You should also report your errors in some way. Simple Error Handler at the minimum.
Since you are using Continous Samples, don't limit the buffer. Leave the Samples per Channel unwired on the DAQmx Timing VI.
There are only two ways to tell somebody thanks: Kudos and Marked Solutions
Unofficial Forum Rules and Guidelines

Design Pattern for Tabbed Event Structure

Hi All,
Been working on a user interface using an event structure design pattern to respond to button presses.
The interface got quite large and I decided to organize the buttons into groups and put them on different tabs (to group the functionality).
However when I put a case statement around the event structure to seperate the code on the diagram too I was not able to switch tabs and the interface gets stuck.
Example attached.
I could put all the button events from different tabs into one event structure but thought this might get very large and unreadable. Is there a standard design pattern that addresses this problem?
Many thanks.
Mark.
Solved!
Go to Solution.
Attachments:
TabbedEventStructure.vi ‏13 KB

Medwar19 wrote:
Hi All,
Been working on a user interface using an event structure design pattern to respond to button presses.
The interface got quite large and I decided to organize the buttons into groups and put them on different tabs (to group the functionality).
However when I put a case statement around the event structure to seperate the code on the diagram too I was not able to switch tabs and the interface gets stuck.
Example attached.
I could put all the button events from different tabs into one event structure but thought this might get very large and unreadable. Is there a standard design pattern that addresses this problem?
Many thanks.
Mark.
Standard?
If you tabs equate to states then the top level get set up to call a sub-VI that is responcible for that state.
Then inside the sub-VI handling that state I use Dynamic Event registration to set up the events that I have to service while in that state.
This is what one of those sub-VI can look like.
Which makes it very clearn what we are handling in thois mode of operation.
Ton wrote a Nugget on Dynamic Event Registration that can be found here.
Ben
Ben Rayner
I am currently active on.. MainStream Preppers
Rayner's Ridge is under construction

Is this a good design pattern?

Hi,
I am working with a team to create an application that can acquire data from up to 24 thermcouple channels. We plan to create a wizard to assist the user in setting up the test (specifiying stop conditions, sample rate, channels to sample, output file, and email/text notifications). After the setup, if the user does not cancel out, then the program continues. We have defined parallel loops and their roles, trying to fit into a producer/consumer design pattern. I have attached the main file which shows the structure of the program. We are trying to keep it clean and modular. Before diving into development, we are finalizing a detailed design specification so that different people can work on each of the loops' subvis. I was hoping that people could review the block diagram and offer suggestions for improvement or validate that we have chosen a good method of implementation.
Notes about the block diagram
The plan is for data driven stop conditions to be detected by Event Notifier, handled by the Main UI event, and then the Data Collector is instructed to stop (as well as the Main UI loop). When Data Collector stops, it releases queues, the queue reference becomes invalid, and then the loops with the Dequeue will be stopped.
I am not sure if an additonal event structure should be in the DataCollector to specifically handle the Stop button on the UI because if it is in the Main UI event structure, the DataCollector would not be stopped immediately. This is important to stop quickly because the sample rate could be VERY long. I understand about using an event structure instead of WAIT, but didn't know if there is an efficient way leaving it down in the Main UI event structure....????
Data Collector is the producer, it currently only collects data and passes it on. If the program is going to contain a start collection button, or pause/resume, I need to figure out how to handle this with the Main UI event structure.
Event Notifier consumes data from the Data Collector. It looks at all data and should set a flag if a stop condition is met. The flag will be handled by the Main UI event structure to tell Data Collector to stop. The event notifier also sends a notification to the Email/Text Caller when thresholds have been exceeded warranting a reason for the user to be notified via text or email.
Email/Text Caller waits for notification and then calls a subVI to send an email or text with certain data and channel information. The subVI call is lengthy, so we decided to put this in its own loop instead of a case structure inside the Event Notifier.
Report Generator logs all collected data to a file. A second queue was used, but uses the same data as the first. Is this the correct implementation since two consumers need the same data?
Main UI - handles changing of sample rate, possibly the stop button, possibly start/pause/resume of data collection. Also handles when Event Notifier sets flag for a data driven stop condition, which then should provide a mechanism to stop the Data Collector.
A big question that i have is.... Should the Main UI event structure just be in the Data Collector so that the loop can be stopped immediately? All of the UI events (stop button, start/pause/resume data collection) directly effect the Data Collector. Should the data Collector have a state machine instead?
I feel that the 3 consumer loops (Event notifier, Email/Text Caller, and Report Generator) seem like a good implementation, but i am not confident about the Data Collector and Main UI loops. Should they be combined?
Solved!
Go to Solution.
Attachments:
updatedMain.vi ‏13 KB

A couple of thoughts:
Overall it looks workable- remember to name your queus and notifiers- then you can depop the BD and launch each loop in its own sub-vi without wiring all those queues between loops- Just obtain ref to named queue.
"Data collector" needs a supervisor to control how often the data is collected- I'm thinking a QSM with "init- collect- stop" states. A simple timed loop could produce the "time to acq ticks" under control of the sample rate control. This puts both the DAQ init and cleanup functions within the Data Collector's perview so it aids coehesion and solves the exit without waiting for the loop to iterate.
I'm not a fan of duplicating data- I assume analisys is pretty much just some comparisons- so it is pretty quick- Why not append summary data to the same queue? And make it a single element queue- the report loop can then inspect the queue and look for a summary data "index" - if its greater than the last index written append the darn data to the report? Preview queue works great for these situations.
Do not combine the UI handeler with anything else! Do you see the snag?- hint: here thar be spaghetti. keep the functions seperated for solid scalable and maintainable code.
One last thought: Its getting less popular to stop loops on the error out of a dequeue. First it returns a default element that can cause your listeners some headaches. Second it doesn't let you run cleanup code. Use a stop command <enum> built into the element data as an executive to determine when to stop. We'll try to get the examples updated to show current best practices.
Jeff

Complex architecture - design pattern?

I am trying to determine which architecture, or combination woudl best suite my application:
The core aspect is a client receiving data from a network at variable speeds - I have this working using a producer consumer loop (data) design pattern, that starts and stops by placing it inside a case structure with start button. I'd now like to combine this with an event driven user interface, which also uses the serial port to talk to external hardware every now and then. I have that working seperately with a producer consumer (event) design pattern. I'd like to combine them and include regular events (handshaking with the external hardware) to take place whilst the producer-consumer (data) reads from the network.
How can I mix the two design patterns?! I'm struggling to get my head 'round this.
Thanks

Here is an example of two Producer-Consumers in one vi. Notice the two queues. One is for commands to the event loops. The other is for the serial data loops. When certain data is received, you can send commands to the event loop (actually to Consumer loop, 2nd from top) to cause some action. So the user can initiate some action via event structure, and received data can also initiate some action.
This is complex and it might not do all you want. But its a start.
- tbob
Inventor of the WORM Global
Attachments:
DoubleProducerConsumer.vi ‏29 KB

New LabHSM Toolkit - Agile development of complex event-driven maintainable LabVIEW applications with active objects / actors based on a universal Hierarchical State Machine / statechart template.

Dear Fellow LabVIEW programmers:
Most of the systems you deal with are reactive. It means that their
primary function is constant interaction with their environment by
sending and receiving events. But most likely, they can have something
happening inside them too, even when they are not processing messages
received from outside. So, such systems have to continuosly react to
external and internal stimuli. Right? Moreover, most likely, they
consist of subsystems that are reactive too and, in turn, can have
their own "life", to an extent independent from other parts (with
which they still communicate, of course). Reactive (event-driven)
systems are more naturally modeled with active objects. So, why then
should we try to model and code them with GOOP and its passive
("dead"!) objects?
"Flat" State Machines have been known for decades to have severe
limitations. It's been more than 20 years since Dr. Harel invented
Hierarchical State Machines (statecharts) to fight those limitations.
Then why does NI still tout the same old good Moore FSM as the
ultimate tool for event-driven programming in LabVIEW in its $995
State Diagram KIt?
The LabHSM toolkit we are happy to present, makes it possible to
easily create and then maintain complex event-driven applications in
LabVIEW as a collection of HSM-driven active object VIs using a higher
level of abstraction and agile software development methodologies.
These active object VIs are created based on a universal Hierarchical
State Machine ( HSM or statechart ) template. So. all your code looks
similar regardless of its functionality!
We all love just jump to code, right? However, to be good boys, we
need to do design first. Then implement it in code. If the logic is
modified we need to redo the design first and then redo the code. When
using LabHSM where behavior information is abstracted into a separate
HSM data file editable with a supplied editor, there is no need for
coding separate from design any more. The modified behavior becomes
code automatically as soon as the HSM file is saved. Design is code!
The implementation basically follows Dr. Samek's Quantum Programming
paradigm. (see http://www.quantum-leaps.com). However, as already
mentioned, LabHSM stores the behavior information in a file separate
from the code itself. It also adds state dependent priorities to
events, a separate queue for public events/messages, and, of course,
some LabVIEW specific code like capturing front panel user events and
putting them into the private Events queue. Communication and
instantiation functions are also rather specific for LabVIEW.
It is available for UNLIMITED PERIOD trial. Please visit
http://www.labhsm.com for details and download. The site also contains
references which you may want to check to learn more about
hierarchical state machines and active object computing.
Since this is our debut we will appreciate any comments and
suggestions. Our contact information is available on our site, of
course.
Have a G'day!

Symtx is currently hiring the following position. Please contact me if interested.
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Symtx, HR
[email protected]
Symtx, the leading supplier of functional test equipment, hires the brightest & most talented engineering professionals to design & manufacture complex custom electronic systems for advanced technology leaders in the defense, aerospace, communications, medical, transportation & semiconductor industries. Symtx’ challenging & dynamic work environment seeks to fill openings with highly qualified electronic engineering design professionals.The ideal candidate will be responsible for defining the requirements, software design and code development, and integration of test control software for custom functional test systems. Candidate should be familiar with data acquisition concepts, instrument control, complex test, measurement and calibration algorithm development and definition and implementation of control interfaces to hardware. Prefer familiarity with instrument control via GPIB, VXI, MXI, RS-232 desirable. Requires BS/MSEE and 3 -7+ yrs of experience in one or several of the following test applications in a Windows NT/2000/XP environment using Labwindows CVI, TestStand, Labview, Visual Basic, C++ and knowledge of RF systems is a plus. Job responsibilities will include software design, development, integration, team leadership, and interfacing with customers( includes PDR’s & CDR’s).

Producer/Consumer Design Pattern with Classes

I'm starting a new project which involves acquiring data from various pieces of equipment using a GPIB port. I thought this would be a good time to start using Classes. I created a GPIB class which contains member data of: Address, Open State, Error; with member vis such as Set Address, Get Address, Open, Close...general actions that all GPIB devices need to do. I then created a child class for a specific instrument (Agilent N1912 Power Meter for this example) which inherits from the GPIB class but also adds member data such as Channel A power and Channel B power and the associated Member Functions to obtain the data from the hardware. This went fine and I created a Test vi for verfication utilizing a typical Event Structure architecture.
However, in other applications (without classes) I typically use the Producer/Consumer Design Pattern with Event Structure so that the main loop is not delayed by any hardware interaction. My queue data is a cluster of an "action" enum and a variant to pass data. Is it OK to use this pattern with classes? I created a vi and it works fine and attached is a png (of 1 case) of it.
Are there any problems doing it this way?
Jason

JTerosky wrote:
I'm starting a new project which involves acquiring data from various pieces of equipment using a GPIB port. I thought this would be a good time to start using Classes. I created a GPIB class which contains member data of: Address, Open State, Error; with member vis such as Set Address, Get Address, Open, Close...general actions that all GPIB devices need to do. I then created a child class for a specific instrument (Agilent N1912 Power Meter for this example) which inherits from the GPIB class but also adds member data such as Channel A power and Channel B power and the associated Member Functions to obtain the data from the hardware. This went fine and I created a Test vi for verfication utilizing a typical Event Structure architecture.
However, in other applications (without classes) I typically use the Producer/Consumer Design Pattern with Event Structure so that the main loop is not delayed by any hardware interaction. My queue data is a cluster of an "action" enum and a variant to pass data. Is it OK to use this pattern with classes? I created a vi and it works fine and attached is a png (of 1 case) of it.
Are there any problems doing it this way?
Including the error cluster as part of the private data is something I have never seen done and ... well I'll have to think about that one.
Ben
Ben Rayner
I am currently active on.. MainStream Preppers
Rayner's Ridge is under construction

The search for a perfect design pattern

The search for a perfect design pattern
I am searching tip to upgrade a labview program used. I am looking for the best approach to make
my program more robust and scalable.Software and hardware.
Today the system consists of :
GPS, weather station, ultrasound wind sensor, echosounder and a webcamera, all connected to a computer.
The computer OS is XP pro with Labview version 2009 installed. Except for the webcamera all peripherals have serial communication interface.
The webcamera interface is ethernet.
Device                           Data type                 Interval                    Interface
GPS                                NMEA string                1 second                   Serial rs232
Ultrasound wind sensor   NMEA string                1 second                   Serial rs232
Weather, temp,humid     NMEA string               35 seconds                 Serial rs232
Echosounder                  NMEA string               500ms-5 seconds       Serial rs232
                                                                      (depending of depth)
Webcamera                    Jpg                           Snapshot on demand Ethernet
The tasks to be solved
All the data have to be stored.
All the data have to be displayed in real time.
Weather history data must be buffered to be plotted in a graph (pop-up windows).
Absolute wind speed and direction have to be calculated.
Option to send navigation data attached to email at desired time intervals.
Image snapshot at desired time intervals.
The data wich streams into the serial ports are not synchronized and vary in intervals . Would it be a good idea to put all the serial peripherals into a serial device server. Is it possible to let the server place a event in my software, to trigger reading of the data, from the the server. Or how could that be done in other ways?
The perfect design pattern
In version one of the software I use 4 parallel loops to read data at the serial ports, inside these loops the data was separated and outputted to a datasocket server. I used datasocked read in other loops to present and store data. But it did not look good, and yes it was a mess, a really rat nest. I am looking for a better design pattern.
Could some kind of producer/ consumer pattern be the way to go?
Comments and tips are wanted.
TIK

Nice Project! Congratulations ;-)
I am not an expert for large applications. But your project looks nice and manageable. Here my straightforward ideas:
I would build a GUI, with menu bar, pop-up graph display thing.
THen low level tasks with each one handling a RS232 device and feeding queues or fireing notifications.
Maybe handle the snapshots in an event structure on the GUI blockdiagramm.
When GUI and producer tasks are running, close the gap with a middle layer which captures the data and actualizes GUI by reference. For instance the graph-thing-dialogue triggers a "flush queue" and displays the last 200 entries of the array of data.
I often run into issues when having defined too many userevent structures. I guess i am too stupid for that. Thats why i rather use queues (when i dont want to loose data) or notifiers (data is up-to-date). Nevertheless I like a "stop-all-tasks" user event...traditionally.
My team is actually developing a large application. We are using LabVIEW classes and objects.
So have fun!
Yours
RZM

Choice of design pattern for data acquisition system

Hello all
I have a trouble about selecting the suitable design pattern / architecture for a data acquisition system.
Here is the details of the desired system:
There is data acquisition hardware and I need to use it by observing parameters on User interface.
the data acquisiton period, channel list to scan should be chosen on User interface. Besides, there are many user interface interactions. e.g. if user selects a channel to add scanlist, then I need to enable and make visible some other parts on user interface.
When user completes the channel selection, then he will press the button to start data acquisition. Then I also need to show the scanned values on a graph in real time and log them in txt file.
I know that I cannot use producer consumer pattern here. because the data acquisition loop should wait for parameters to scan channels. and it works in a given period by user. so the user interface loop performs higher rate then consumer loop (data acquisition loop). it means queue will be bigger bigger. if I use notifier it will loss some data come from user interface.
is there any idea about that ? is there any suitable design pattern for this case ?
Thanks in advance
best regards
Veli BAYAR
Embedded Systems Software and Hardware Engineer
"You live in a graphical world. Why not program in one?"
Solved!
Go to Solution.

johnsold wrote:
Veli,
I recommend the Producer/Consumer model with some modifications.
You might need three loops. I cannot tell for sure from your brief description.
The User Interface loop responds to the user inputs for configuration and start/stop of acquisition. The parameters and commands are passed to the Data Acquisition loop via a queue. In this loop is a state machine which has Idle, Configuration, Acquisition, and Shutdown states (and perhaps others). The data is sent to the Processing loop via a different queue. The Processing loop performs any data processing, displays the data to the user, and saves it to file. A notifier can be used to send the Stop or shutdown command from the User Interface loop to the other loops. If the amount of processing is minimal and the file write times are not too long, the Processing loop functions might be able to occur in the Timeout case of the UI loop Event structure. This simplifies things somewhat but is not as flexible when changes need to be made.
I am not sure that a Design Pattern for this exact setup exists but it is basically a combination of the Producer/Consumer (Events) and Producer/Consumer (Data) Design Patterns.
Lynn
Check out this thread: http://forums.ni.com/t5/LabVIEW/Multiple-poll-case-structures-to-event-help/td-p/2551309
There are discussions there about a 3-loop architecture that may help you.
Jeff
Jeffrey Zola

Design Pattern / Best Practice Question

Hi,
I have been using Flex for a while now, but there is a
scenario which I still have not found a solution I'm entirely happy
with. I'm wondering if anyone else out there might have suggestions
on a design pattern or best practice.
Suppose I have a view which depends on model data which
resides in some back end systems. That model data may or may not
have been loaded (e.g. via a web service or remote object call) at
the time the view is displayed.
I don't know if the user will ever visit this part of the
application so I would prefer to defer retrieval of the data until
the user actually navigates to this view. Or I want to retrieve the
data each time the view is displayed because the data is dynamic
and could change between one presentation of the view and the next.
Because the data comes from several systems, I cannot simply
make one service call and display the view when it completes and
all the data is available. I need to call several services which
could complete in any order but I only want to display my view
after I know all of them have completed and all of the model data
is available. Otherwise, I can present the user an incomplete view
(e.g. some combo boxes are empty until the corresponding service
call to get the data completes).
The solution I like best so far is to dispatch a single event
(I am using Cairngorm) handled by a single command which acts as
the caller and responder for all of the services. This command then
remembers which responses it has received and dispatches another
event to navigate to the view once all the results have returned.
If the services being called are used in different
combinations on different screens, this results in proliferation of
events and commands. An event and command for each service and
additional events and commands to bundle the services and the
handling of their responses in the right combinations for each of
the views.
Another approach is to have some helper class listen for all
of the model changes and only display the view when the model
enters some state that is acceptable. It is sometimes difficult to
determine just by looking at the model whether it is in the right
state (e.g. how can I tell that a collection is the new collection
that should just have been requested versus an old one lingering
from a previous call). The logic required can get kind of
convoluted and brittle.
Basically, all of the solutions I've come up with so far seem
less than ideal and a little hackish. I keep thinking there is some
elegant solution out there that I am just missing ... but so far,
no luck finding it. Thoughts?
Thanks.
Bill

i think a service class is right - to coordinate your calls.
i would have 1 event per call (so you could listen to individual
responses if you wanted to).
then i would use a flag. if you want to check for staleness,
you would probably want two objects to map your service flag to
lastRequested and lastCompleted. when you check, check if it's
completed, and if it's not stale and that your lastRequested is
less than lastCompleted (meaning that you're not currently waiting,
i.e. you've returned since making a request). then make the request
and update your lastRequested.
here's a snippet of what i mean.
./paul
public static const SVC1_LOADED:int = 1;
public static const SVC2_LOADED:int = 2;
public static const SVC3_LOADED:int = 4;
public static const SVCALL_LOADED:int = 7;
private var completedFlag:int = 0;
then each call would have it's own callback.
private function onSvc1Complete( evt:Event):void {
completedFlag |= SVC1_LOADED;
lastCompleted[ SVC1_LOADED ] = getTimer();
dispatchEvent( new Event("svc1complete") );
checkDone();
private function checkDone():void{
if( completedFlag == SVCALL_LOADED )
dispatchEvent(new Event( "allLoaded" ));

Best architecture and design pattern to use

I am currently designing a moderately sized LabView application and cannot decide on the best architecture/design pattern or combinations thereof to implement.
The program basically polls an instrument connected to a serial port continuously at a 2-10Hz rate. When operator clicks a button to start a run, the polled data is then filtered, has math functions performed on the data, writes collected data to files, and produces reltime graphs and calculates point-by-point statistics. At the completion of a run, some additional data files are written. I pretty much know how to accomplish all these tasks.
What is also required is main-vi front panel interaction by the operator to access a database (via a C# dll with .Net) to query for specifications to apply in order to determine pass/fail status. Setup conditions also need to be changed from time to time by the operator and applied to the data in real time (ie- a measurement offset). I have prototyped the database portion successfully thus far.
For the main vi, I started off using the Top Level Application Using Events design pattern (Event structure within a while loop). Copious use of bundled clusters and shift registers keep the database data updated. I cannot figure out how to have the top level vi concurrently poll the serial device as outlined above. The Event structure is only active when defined control values change, and use of a timeout is no help since it prevent data from being collected while the user is accessing the database. All database and setup parameters must be applied to the data as it comes in from the serial port. Error trapping/recovery is a must.
I believe what I need is two parallel processes running under my main vi (one for database and setup, the other for polling and data processing/display, but what would be the preferred choice here?
Revert back to a polled loop in lieu of Events, use notifiers, occurrences, user-defined events, Producer-consumer loops?
It�s been about 3 years since I have had an application on this level (which used a state machine architecture), and a lot has changed in LabView (using 7.1 Prof Devel). I am currently having a mental block while trying to digest a lot of features I have never used before.
Suggestions Welcome!
Thanks.
~~~~~~~~~~~~~~~~~~~~~~~~~~
"It’s the questions that drive us.”
~~~~~~~~~~~~~~~~~~~~~~~~~~

I suggest a (3) state machine(s) architecture. The first is the user interface. Your main VI is a good start. Another state machine handles the instrument I/O (your serial polling). The third handles all data processing. The three loops run independently. Each has a wait to allow LV's scheduler to share CPU time among the loops. I like queues or functional globals for inter-loop communications.
This allows a lot of flexibility. Each portion can be tested alone. Each can have different timing and priority.
Spend some time thinking about what needs to be done and plan the structure carefully before you do any coding (except for little test programs to try new ideas).
I also like Conway and Watts "A Software Engineering Approach to LabVIEW", published
by Prentice Hall.
Lynn

Observer Design Pattern: Looking for redesign ABAP OO code example

Hello folks,
I am looking for an example for ABAP OO code that has been redesigned by applying the Observer Design Pattern. I would be very interested in both the code before as well as the code after the pattern is being applied.
Thanks in advance and kind regards, Alex

Observer can be implemented using the EVENTS.
I had recently implemented the observer at one of my client's place. I had screen with so many ALVs. One ALV was kind of editable and other were just showing the information of the current row as well as some total information. So, initially I started with the Main ALV and SUB(1 and 2) for other ALVs. Now, when I need to refresh my ALVs based on the main ALV data, I had to explicitly update the data of the each Sub ALV. The code was kind of static and requirement was not yet fixed.
Later on we need to add one more ALV on the same screen. It was easy to change the existing method where I was doing the explicit refresh of each ALV. But I thought of using the Events.
I created an event REFRESH_DETAILS for main ALV. so, when data gets changed (which I was catching by DATA_CHANGED event of ALV), I raise the event.
RAISE EVENT REFRESH_DETAILS
exporting new_data = it_Data.
In Sub ALVs, I created the event handler method to handle the event REFRESH_DETAILS of the main ALV.
methods: handle_refresh_details
for event REFRESH_DETAILS of ZCL_MAIN_ALV.
I also had to register the Handler.
SET HANDLER me->handle_refresh_details FOR ALL INSTANCES.
I'll soon write a post on my [ABAP Help blog|http://help-abap.zevolving.com/] with all the details.
Regards,
Naimesh Patel

Wpf: Event driven vs MVVM design pattern

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