I'm still learning Flutter and trying to understand best practices.
One of them seems to be that global variables are bad, and recommends using something like Provider instead.
In my case, I have a bunch of settings that are read from the back end when the app first starts. The variables in globals.dart are assigned values on startup, and then don't change after being assigned.
int maxLoginAttempts...
String welcomeMessage...
...
My confusion comes from the fact that most people who've asked this question (that I have found, sorry if I missed a post somewhere) are creating variables whose value may change as the application is used. I can understand something like Provider and ChangeNotifier would work well for that.
But in my case the values are assigned once - and don't change thereafter - they're only read.
So are globals still bad in my case?
In your case no provider/state management is needed if you are just assigning some value to global variable and and its value change does not require widget rebuild.
Related
I have added some variables in my simulation start page. Now I want to access them from the main class. However, with ((Simulation)getExperiment()).[...] the variables are no longer displayed. When I press option+space only one of about 150 variables is displayed. This one variable is not different from the other variables in the properties.
All variables are on public and I was able to access all of them a few weeks ago. Now not anymore, although I didn't change much in the model. I have already tried restarting. Did I accidentally remove some package or something? Can someone help me here please? I have only noticed the problem now as my model has runtime errors, presumably as the now unrecognized variables are not being initiated.
Pictures:
You should not access variables from experiments on Main. If you need access, you should pass them on as parameters into Main.
If you have hundreds of variables in the experiment, turn them into fields in a single Java class. Pass 1 instance of that Java class into Main as a parameter.
This way, you always have access, it is easy to add more variables, it is easy to change them and you do not need the awkward (and bad) type-casting you currently do
Roald has written an excellent guide for the Twincat Eventlogger.
https://roald87.github.io/twincat/2020/11/03/twincat-eventlogger-plc-part.html
https://roald87.github.io/twincat/2021/01/20/twincat-eventlogger-hmi-part.html
For us this is exactly what we want, there is however 1 thing I haven't figured out. How to get the sourcename of the alarm in multiple languages in the HMI. params::sourceName gives the path in the software (example: MAIN.fbConveyor1.Cylinder1) This path can be customized when initializing the alarm (as Roald has shown). This doesn't work in my case, since I would like to define a generic alarm (example: "Cilinder not retracted within maximum time") that is instantiated multiple times.
I was thinking of using the source as a way to show the operator where the alarm occurs. We use this way (path) already for saving machine settings among other things. The machines we build are installed all over the world, so multilanguage is a must.
Beckhoff does support multilanguage alarm names (when defined), but the source is not defined, but dynamically generated.
Anyone have an idea how this problem can be solved?
If I understand your question correctly, then being able to parameterize the event text with information of the source of the problem should help you out.
If you define the event text as Cylinder {0} has not retracted in time. then you can add the arguments of that text during runtime.
IF bRaiseAlarm THEN
bRaiseAlarm := FALSE;
fbAlarm.ipArguments.Clear().AddString('Alice');
fbAlarm.Raise(0);
END_IF
However, since this also stated in the articles you mentioned, I am unsure if this would solve your problem.
'Alice' in this example, can be hard to localize. The following options come to my mind.
The string can be based on an ENUM. Enums can have textlist support, so if you add your translations there, that should allow multilingual output. However... this does require a lot of setup, placing translations inside your code, and making sure the PLC application is aware of the language that the parameter should use.
Use tags to mark the source device, as tags can be language invariant. It is not the most user-friendly method, but it could work for you. It would become something like: "Cylinder 'AA.1123' did not retract in time.". 'AA.1123' as a tag would have to be stored inside your PLC code as a string. You will have to trust that your operator can relate the tag back to the actual source.
Hopefully, this helped, or else please help me understand the problem better.
The question is simple & hard to understand at the same time. Why do I need to mark all my variables in the Statefull Widget as private?
Now when I declare a variable I declare it like private.
bool _isOpened = true;
String _currentUserUID;
...
But why do I need this? I do not access these variables from other widgets. I do not override. I know my app logic & totally insured in the safety of my code in all widgets.
So why do I need private variables? Is there any impact of using private variables & methods? Maybe performance benefits?
The prefix _ makes the variable private within the .dart code file you are in. The IDE (or at least VSCode, that I use) will then be able to tell you if you are using the variable or not. It also tells me, as the developer, that I need look no further than the code file that I am working in for changes to this variable. All important information as far as I am concerned.
The same is true when making methods private with the _ prefix. This is also highly recommended.
However, imho, this question relates to the the use of private variables in general so here is a link to a more general question and a much better set of answers than mine https://softwareengineering.stackexchange.com/questions/143736/why-do-we-need-private-variables
What does = do here?
List<Segment> totalSegments = flight.departureFlight.segments;
Do both, totalSegments and flight.departureFlight.segments point to the same memory reference or totalSegments has the same data as flight.departureFlight.segments but points to a different memory location?
My understanding was that the latter should happen since dart is pass by value and not reference. However, a very annoying bug occurred when I added this line below that one:
totalSegments.addAll(flight.returnFlight.segments);
This above line actually modified the flight variable which in turn somehow modified the AsyncSnapshot from the StreamBuilder. Although, I wasn't using the variable anywhere else and not modifying other variables mentioned.
This all happened inside build function of a Stateless Widget. Maybe that has to do something with it.
I tried reading dart documentation for it, but either I couldn't find what I am looking for or the information is simply missing there. Read this too, but according to this, my use case shouldn't happen.
When it comes to objects as in your case, you are assigning a reference to an existing object (memory location) to a new variable. While acting upon that reference, you change the same object.
If this is not what you intend, check out answers related to (deep) copying of the objects
You were mistaken by the fact that Dart passes by value, and not by reference. Actually, it is exactly the opposite: (Almost) everything is always passed by reference (Which is usually a good thing!) Therefore, it is quite logical that because you edited totalSegments your departureflight.segments got edited too. It is a synonym. One of the ways to solve your problem would be:
List<Segment> totalSegments = List();
totalSegments.addAll(flight.departureFlight.segments.toList());
List<Segment> totalSegments = flight.departureFlight.segments;
This expression does the following.
Assigns the value of the expression flight.departureFlight.segments to variable totalSegments.
This and only this and nothing more.
There is no need to know what is really happening, because this is what happens.
P.S.
What value will be obtained as a result of executing the expression flight.departureFlight.segments is another question, because it depends on the implementation of the members of the operands of the expression flight.departureFlight.segments.
My rules in a drl file will return 1 or more String values back to the calling program.
Right now the way I do that is via global variables as below
global java.util.List<String> statuses;
The calling program will pass it an empty ArrayList() and within my rules, I'll add the String values into the List. Finally, my calling program will retrieve the statuses list that might contain zero or many String items in the list as below
session.getGlobal("statuses")
But in the drools user guide, it said that it's not recommended that rules should change values of the global variables.
Are global variables the best way to return values back to the calling program? If not, what's the best way please? I have to deal with concurrency in my web application as well so I'm looking for the best way to return values back to the calling program for concurrency as well.
Thanks
There is nothing wrong with changing the value of a global variable in the code within the consequence ("right hand side") of a rule. What you describe is one of standard use cases for globals.
What the author of the Drools user guide (please add the exact reference!) meant is that changes of a global that is used in the condition ("left hand side") of a rule are not considered in the (re-)evaluation of these conditions. Therefore, this is to be avoided.
As for concurrency: Use a properly synchronized List object as a global.