When a long-running process is being executed, it is a good practice to provide feedback to the user, for example, updating a progress bar.
Some FAQs for GUI libraries suggest something like this:
function long_running_progress()
do_some_work()
update_progress_bar()
while finish
do_some_work()
update_progress_bar()
end while
end function
Anyway, we know it is a best practice to separate business logic code from user interface code. The example above is mixing user interface code inside a business logic function.
What is a good technique to implement functions in the business logic layer whose progress could be easily tracked by an user interface without mixing layers?
Answers for any language or platform are welcome.
Provide a callback interface. The business logic will call its method every once in a while. The user layer will update the progress or whatever. If you want to allow cancellation – no problem, let the callback method have a return value which will indicate a need for cancellation. This will work regardless of number of threads.
If you used a MVC paradigm you could have the Model publish its current progress state as a property, the Controller could extract this every x seconds and then put it into the view. This assumes multi-threading though, which I'm not sure if you allow.
Publishing is a great way to go. It all depends on the platform how this is done. However, when it comes to the user experience there are a couple of things to consider as well:
Don't give the user a progress bar if you don't know how long the task is executing. What time is left? What does half-way mean? It's better to use hour-glass functionality (spinning wheels, bouncing progress bars, etc).
The only interesting thing to view progress on is time; what does Half-way in a process mean? You want to know if you got time for that cup of coffee. If you show other things you are probably displaying the workings of the system programming. Most users are not interested or just get confused.
Long running progress should always support the user with an escape, a way to cancel the request. You don't want to lock up the user for long time. Better still is to handle a long running request completely in the background and let the user get back when the result is back.
Related
I was attracted to Fyne (and hence Go) by a promise of thread safety. But now that I'm getting better at reading Go I'm seeing things that make be believe that the API as a whole is not thread safe and perhaps was never intended to be. So I'm trying to determine what "thread safe" means in Fyne.
I'm looking specifically at
func (l *Label) SetText(text string) {
l.Text = text
l.textProvider.SetText(text) // calls refresh
}
and noting that l.Text is also a string. Assignments in Go are not thread safe, so it seems obvious to me that if two threads fight over the text of a label and both call label.SetText at the same time, I can expect memory corruption.
"But you wouldn't do that", one might say. No, but I am worried about the case of someone editing the content of an Entry while an app thread decides it needs to replace all the Entry's text - this is entirely possible in my app because it supports simultaneous editing by multiple users over a network, so updates to all sorts of widgets come in asynchronously. (Note I don't care what happens if two people edit the same Entry at the same time; someone's changes will be lost and I don't care who's. But it must not result in memory corruption.) Note that one approach I could take would be to have the background thread create an entirely new Entry widget, which would then replace the one in the current Box. But is that thread safe?
It's not that I don't know how to serialize things with channels. But I was hoping that Fyne would eliminate the need for it (a blog post claims it does); and even using channels I can't convince myself that a user meddling with a widget in various ways while some background thread is altering it, hiding it, etc, isn't going to result in crashes. Maybe all that is serialized under the covers and is perfectly safe, but I don't want to find out the hard way that it isn't, because I'll have no way to fix it.
Fyne is clearly pretty new and seems to have tons of promise, but documentation seems light on details. Is more information available somewhere? Have people tried this successfully?
You have found some race conditions here. There are plans to improve, but the 1.2 release was required to get a new "BaseWidget" first - and that was only released a few weeks ago.
Setting fields directly is primarily for setup purposes and so not expected to be used in the way you illustrate. That said, we do want to support it. The base widget will soon introduce something akin to SetFieldsAndRefresh(func()) which will ensure the safety of the code passed and refresh the widget afterward.
There is indeed a race currently within Refresh(). The use of channels internally were designed to remove this - but there are some corners such as multiple goroutines calling it. This is the area that our new BaseWidget code can help with - as they can internally lock automatically. Using this approach will be thread safe with no changes to the developer in a future release.
The API so far has made it possible for developers to not worry about threading and work from any goroutines - we do need to work internally to make it safer - you are quite right. https://github.com/fyne-io/fyne/issues/506
I want to do some tests on our tcl tk application regarding the user interaction. As the application has parts similar to a CAD for which every mouse movement is relevant, I would like to do something like record all events of some user interactions. My goal would be to playback these events laterwards and on every program change to discover potential changes. Or even better to assure the GUI behaves always the same and produces always the same data.
I know, that I can generate some enter motion and button events, but this would not be the same like the thousands of events generated by a real user interaction. But it is very important for me to have exactly these thousands of events.
Is there any possibility to achieve this?
It's relatively easy to record events of particular types with bind — you'll find that <ButtonPress>, <ButtonRelease>, <Enter>, <Leave>, <FocusIn>, <FocusOut>, <KeyPress> and <KeyRelease> cover pretty much everything that you are interested in — and then play them back with event generate. (You need to record quite a bit of information about each event in order to regenerate it correctly, but the underlying model is that of X events with similar names.) Assuming you're not wanting to support inter-application cut-and-paste or drag-and-drop for the purposes of recording; those complicate things a lot. You'll likely have a lot of events; recording to an SQLite database might make a lot of sense.
However, you should think carefully about which parts of the application you want to record. Does it matter if the order of two buttons in the outer shell of the application outside the CAD-like area get swapped in order? For most users, provided you're clear about what the buttons do (through clear labels and icons) it isn't very important, but for replaying recorded events it can matter hugely. Instead, for the parts of the application that are simple buttons and edit fields, I'd not record the details of them but would instead just record when the buttons are clicked and the changes to the text content of entries and so on. In effect, it's capturing higher-level events, and that's much easier to replay correctly. It's only when the user is in that main CAD area that you need the full detail.
Also, beware of changes to font sizes and screen sizes/scaling. They can change how things are laid out and may happen because of system-level alterations outside the scope of your application.
We started out the way you describe: record all those thousands of motion events, etc. Including exact timings which are extremely important for a GUI application as well.
It quickly became appearent that those recordings became too hard to maintain. They are also overly brittle in light of UI changes. Another problem where the hardcoded time values. A switch to a more powerful machine (or a cpu under load) would break the execution.
The two biggest improvements we introduced
Event compression: recognize the high-level action the user wanted to perform (like selecting a menu item). The recorded activateItem command would then perform the necessary work (event emulation) on replay.
Synchronization functions: instead of relying on a particular timing commands like waitForObject wait for an object to come into existance and become ready for interaction.
It took several years for this to work fluently, however. Including a central Object Map repository, property and screenshot verifications, high-level test descriptions in BDD and others. Feel free to take a look a the Squish for Tk product that came out of this work.
A simple search for DoEvents brings up lots of results that lead, basically, to:
DoEvents is evil. Don't use it. Use threading instead.
The reasons generally cited are:
Re-entrancy issues
Poor performance
Usability issues (e.g. drag/drop over a disabled window)
But some notable Win32 functions such as TrackPopupMenu and DoDragDrop perform their own message processing to keep the UI responsive, just like DoEvents does.
And yet, none of these seem to come across these issues (performance, re-entrancy, etc.).
How do they do it? How do they avoid the problems cited with DoEvents? (Or do they?)
DoEvents() is dangerous. But I bet you do lots of dangerous things every day. Just yesterday I set off a few explosive devices (future readers: note the original post date relative to a certain American holiday). With care, we can sometimes account for the dangers. Of course, that means knowing and understanding what the dangers are:
Re-entry issues. There are actually two dangers here:
Part of the problem here has to do with the call stack. If you call .DoEvents() in a loop that itself handles messages that use DoEvents(), and so on, you're getting a pretty deep call stack. It's easy to over-use DoEvents() and accidentally fill up your call stack, resulting in a StackOverflow exception. If you're only using .DoEvents() in one or two places, you're probably okay. If it's the first tool you reach for whenever you have a long-running process, you can easily find yourself in trouble here. Even one use in the wrong place can make it possible for a user to force a stackoverflow exception (sometimes just by holding down the enter key), and that can be a security issue.
It is sometimes possible to find your same method on the call stack twice. If you didn't build the method with this in mind (hint: you probably didn't) then bad things can happen. If everything passed in to the method is a value type, and there is no dependance on things outside of the method, you might be fine. But otherwise, you need to think carefully about what happens if your entire method were to run again before control is returned to you at the point where .DoEvents() is called. What parameters or resources outside of your method might be modified that you did not expect? Does your method change any objects, where both instances on the stack might be acting on the same object?
Performance Issues. DoEvents() can give the illusion of multi-threading, but it's not real mutlithreading. This has at least three real dangers:
When you call DoEvents(), you are giving control on your existing thread back to the message pump. The message pump might in turn give control to something else, and that something else might take a while. The result is that your original operation could take much longer to finish than if it were in a thread by itself that never yields control, definitely longer than it needs.
Duplication of work. Since it's possible to find yourself running the same method twice, and we already know this method is expensive/long-running (or you wouldn't need DoEvents() in the first place), even if you accounted for all the external dependencies mentioned above so there are no adverse side effects, you may still end up duplicating a lot of work.
The other issue is the extreme version of the first: a potential to deadlock. If something else in your program depends on your process finishing, and will block until it does, and that thing is called by the message pump from DoEvents(), your app will get stuck and become unresponsive. This may sound far-fetched, but in practice it's surprisingly easy to do accidentally, and the crashes are very hard to find and debug later. This is at the root of some of the hung app situations you may have experienced on your own computer.
Usability Issues. These are side-effects that result from not properly accounting for the other dangers. There's nothing new here, as long as you looked in other places appropriately.
If you can be sure you accounted for all these things, then go ahead. But really, if DoEvents() is the first place you look to solve UI responsiveness/updating issues, you're probably not accounting for all of those issues correctly. If it's not the first place you look, there are enough other options that I would question how you made it to considering DoEvents() at all. Today, DoEvents() exists mainly for compatibility with older code that came into being before other credible options where available, and as a crutch for newer programmers who haven't yet gained enough experience for exposure to the other options.
The reality is that most of the time, at least in the .Net world, a BackgroundWorker component is nearly as easy, at least once you've done it once or twice, and it will do the job in a safe way. More recently, the async/await pattern or the use of a Task can be much more effective and safe, without needing to delve into full-blown multi-threaded code on your own.
Back in 16-bit Windows days, when every task shared a single thread, the only way to keep a program responsive within a tight loop was DoEvents. It is this non-modal usage that is discouraged in favor of threads. Here's a typical example:
' Process image
For y = 1 To height
For x = 1 to width
ProcessPixel x, y
End For
DoEvents ' <-- DON'T DO THIS -- just put the whole loop in another thread
End For
For modal things (like tracking a popup), it is likely to still be OK.
I may be wrong, but it seems to me that DoDragDrop and TrackPopupMenu are rather special cases, in that they take over the UI, so don't have the reentrancy problem (which I think is the main reason people describe DoEvents as "Evil").
Personally I don't think it's helpful to dismiss a feature as "Evil" - rather explain the pitfalls so that people can decide for themselves. In the case of DoEvents there are rare cases where it's still reasonable to use it, for example while a modal progress dialog is displayed, where the user can't interact with the rest of the UI so there is no re-entrancy issue.
Of course, if by "Evil" you mean "something you shouldn't use without fully understanding the pitfalls", then I agree that DoEvents is evil.
What do you hate most about the modern game loop? Can the game loop be improved or is there just a better alternative, such as an event-driven architecture?
It seems like this really ought to be a CW...
I'm taking a grad-level game engine programming course right now and they're sticking with the game loop approach. Granted, that doesn't mean it's the only/best solution but it's certainly logical. Using a loop allows you to ensure that all game systems get their turn to run without requesting their own timed interrupts or something else. Control can be centralized: in my current project, I have a GameManager class that, each frame, loops through the Update(float deltaTime) function for every registered object in turn. I don't have to debug an event system or set up timed signals, I just use a loop to call a series of functions. No muss, no fuss.
To answer your question of what do I hate most, the loop approach does logically lend itself to liberal use of inheritance and polymorphism which can bloat the size/complexity of your objects. If you're not careful, this can be a mild-to-horrible pitfall. If you are careful, it may not be a problem at all.
No matter there is any event in the game or not, game is supposed to draw itself and update it at a fixed rate, so I don't think dropping the game loop is possible. Still I would be wondered if anyone can think of any other alternative of game loop.
Usually the event driven architectures work best for games (only do something if the user wants something done). BUT, you'll still always need to have a thread constantly redrawing the world.
To be fully event based you could spawn an extra thread that does nothing but put a CPUTick event into your event queue every x milliseconds.
In JavaScript this is usually the more natural route, because you can so easily create an extra 'thread' that sends you the events with setInterval().
Or, if you already have a loop in the framework containing your game - like JS has in the browser, or python has with twisted - you can tell that looper to call you back at fixed intervals. e.g.:
function gameLoop() {
// update, draw...
}
window.setInterval(gameLoop, 1000/fps);
I'm not sure if this has been asked or not yet, but how much logic should you put in your UI classes?
When I started programming I used to put all my code behind events on the form which as everyone would know makes it an absolute pain in the butt to test and maintain. Overtime I have come to release how bad this practice is and have started breaking everything into classes.
Sometimes when refactoring I still have that feeling of "where should I put this stuff", but because most of the time the code I'm working on is in the UI layer, has no unit tests and will break in unimaginable places, I usually end up leaving it in the UI layer.
Are there any good rules about how much logic you put in your UI classes? What patterns should I be looking for so that I don't do this kind of thing in the future?
Just logic dealing with the UI.
Sometimes people try to put even that into the Business layer. For example, one might have in their BL:
if (totalAmount < 0)
color = "RED";
else
color = "BLACK";
And in the UI display totalAmount using color -- which is completely wrong. It should be:
if (totalAmount < 0)
isNegative = true;
else
isNegative = false;
And it should be completely up to the UI layer how totalAmount should be displayed when isNegative is true.
As little as possible...
The UI should only have logic related to presentation. My personal preference now is to have the UI/View
just raise events (with supporting data) to a PresenterClass stating that something has happened. Let the Presenter respond to the event.
have methods to render/display data to be presented
a minimal amount of client side validations to help the user get it right the first time... (preferably done in a declarative manner) screening off invalid inputs before it even reaches the presenter e.g. ensure that the text field value is within a-b range by setting the min and max properties.
http://martinfowler.com/eaaDev/uiArchs.html describes the evolution of UI design. An excerpt
When people talk about self-testing
code user-interfaces quickly raise
their head as a problem. Many people
find that testing GUIs to be somewhere
between tough and impossible. This is
largely because UIs are tightly
coupled into the overall UI
environment and difficult to tease
apart and test in pieces.
But there are occasions where this is
impossible, you miss important
interactions, there are threading
issues, and the tests are too slow to
run.
As a result there's been a steady
movement to design UIs in such a way
that minimizes the behavior in objects
that are awkward to test. Michael
Feathers crisply summed up this
approach in The Humble Dialog Box.
Gerard Meszaros generalized this
notion to idea of a Humble Object -
any object that is difficult to test
should have minimal behavior. That way
if we are unable to include it in our
test suites we minimize the chances of
an undetected failure.
The pattern you are looking for may be Model-view-controller, which basically separates the DB(model) from the GUI(view) and the logic(controller). Here's Jeff Atwood's take on this. I believe one should not be fanatical about any framework, language or pattern - While heavy numerical calculations probably should not sit in the GUI, it is fine to do some basic input validation and output formatting there.
I suggest UI shouldn't include any sort of business logic. Not even the validations. They all should be at business logic level. In this way you make your BLL independent of UI. You can easily convert you windows app to web app or web services and vice versa. You may use object frameworks like Csla to achieve this.
Input validations attached to control.
Like emails,age,date validators with text boxes
James is correct. As a rule of thumb, your business logic should not make any assumption regarding presentation.
What if you plan on displaying your results on various media? One of them could be a black and white printer. "RED" would not cut it.
When I create a model or even a controller, I try to convince myself that the user interface will be a bubble bath. Believe me, that dramatically reduces the amount of HTML in my code ;)
Always put the minimum amount of logic possible in whatever layer you are working.
By that I mean, if you are adding code to the UI layer, add the least amount of logic necessary for that layer to perform it's UI (only) operations.
Not only does doing that result in a good separation of layers...it also saves you from code bloat.
I have already written a 'compatible' answer to this question here. The rule is (according to me) that there should not be any logic in the UI except the UI logic and calls for standard procedures that will manage generic/specific cases.
In our situation, we came to a point where form's code is automatically generated out of the list of controls available on a form. Depending on the kind of control (bound text, bound boolean, bound number, bound combobox, unbound label, ...), we automatically generate a set of event procedures (such as beforeUpdate and afterUpdate for text controls, onClick for labels, etc) that launch generic code located out of the form.
This code can then either do generic things (test if the field value can be updated in the beforeUpdate event, order the recordset ascending/descending in the onClick event, etc) or specific treatments based on the form's and/or the control's name (making for example some work in a afterUpdate event, such as calculating the value of a totalAmount control out of the unitPrice and quantity values).
Our system is now fully automated, and form's production relies on two tables: Tbl_Form for a list of forms available in the app, and Tbl_Control for a list of controls available in our forms
Following the referenced answer and other posts in SO, some users have asked me to develop on my ideas. As the subject is quite complex, I finally decided to open a blog to talk about this UI logic. I have already started talking about UI interface, but it might take a few days (.. weeks!) until I can specifically reach the subject you're interested in.