I'm writing a Flutter app and planning to use GetX for state management (amongst other things). The common design pattern seems to be to use one dart file per view (screen) and for each view to have its own associated controller file.
However, what is the best approach when you have state that isn't just used within a single screen, but instead needs to be shared across screens? It seems wrong to import controllers from one view into the view or controller for another screen. Is there a better way?
Thanks
You can use GetxService: https://pub.flutter-io.cn/documentation/get_state_manager/latest/get_state_manager/GetxService-class.html
As it says: Unlike GetxController, which serves to control events on each of its pages, GetxService is not automatically disposed (nor can be removed with Get.delete()). It is ideal for situations where, once started, that service will remain in memory, such as Auth control for example. Only way to remove it is Get.reset().
Related
Is there any library / framework that could simplify Universal Windows Platform app development of the application that contains multiple Frames.
I mean, using MVVM Light or BezySoftware MVVM-Navigation the application is highly tied to the idea of navigating between different pages that are hosted by single frame.
The UI I try to develop consist of multiple content frames (main, left, right) which content varies. I need something that will let me navigate easily between different views (by placing these views into appropriate frame) and provide the same features I would have with BezySoftware MVVM-Navigation, so:
handling of the view model state persistence
the ability to activate / query deactivate view models
back button feature
Few different options:
Combination of a single navigation service injected into your view model AND user controls for areas that need to be repeated view to view (e.g. a tabs, status bars, etc). With this route, every time you create a new view you would paste in the common user controls that need to appear. You would also be able to expose bindable properties from said user controls.
Combination of ContentControl, DataTemplate, and DataTemplateSelector to load in either an entire view (Page) or fragments of XAML. As one person pointed out you cannot use DataType attribute, instead you use the DataTemplateSelector class to do the mapping for you. With this approach you can also use triggers to dynamically change the template (content) based on changes to properties on your view model and/or user interactions.
A mix between 1 and 2 above.
I might right the whole thing right here, but it too lengthy. I just recommend you see this article to get your answer.
MVVM patter in UWP
https://github.com/brianchance/MvvmCross-UserInteraction is a very nice plugin for showing cross platform Alerts!
But for this question, can we assume it can not use a UIAlertView (or some other top level MessageBox type call on other platforms) but needs to show a Message within a given subsection of the screen (i.e. on IPhone you would need to supply a UIView to the plugin which it will use to show the message within).
So, how would you set this up so the ViewModel knows what View to use as its display container?
As a specific example, if I wanted an Error Service, as so -
public interface IErrorPFService
{
void Show();
void Hide();
void SetErrors(List<Error> errors);
}
and I create a platform specific implementation for it.
If I inject this into my ViewModel so it can control Error Show/Hide/Set how do I tell it the UIView (or equivalent) that I want my Errors to show within?
Can I just expose the IErrorPFService field as a public property and do -
MyViewModel.ErrorPFService = new ErrorPFService(View);
in my ViewDidLoad ...
Or is this coupled incorrectly vs Mvvm Practice?
I would expect the ViewModel to subscribe itself to the ErrorService.
When receiving a message it would expose it in a collection(?) and the View would bind to that collection.
This way the View is unknown to the service and the ViewModel has the chance to influence the View contrary to your solution.
It would help if you could give an example for the scenario you are describing.
Sometimes, the way you visually want to display something might not be the best way, so if it's possible for you, you might find a different and simpler way, which spares you from having to find a solution regarding what you are describing.
Generally, I always do the best I can to avoid the idea of having to actually pass a 'view' or an abstraction of it, from the view-model to view. Also, cross-platform wise, things can work very different in terms of UI interaction. You can find yourself in a situation when things are complicated just because UI works differently than what you expected.
But let's try find another perspective:
At any given point, the view knows what data \ feature it's displaying. So when you are calling from the view-model an user interaction action (by a service, property change, event, etc) the view should 'expect' it.
For example, the platform specific user interaction implementation is able to get the currently displayed top-view and interact it in a platform specific manner or based a relationship. In your example, the message-box can be displayed in a specific sub-view of the top level view.
In advanced scenarios, I guess you could try to create a cross-platform approach for this, but you should try to put in balance all the abstraction you want to create just for that. Think about doing this as a plan ... Z. If possible. Again, giving an example might help.
We've been using the recommended GWT approach of building parts of our application in an MVP manner. The logic we use is based on Google's examples - the Presenter fetches/prepares data and sets it on the View, and the View contains a reference to the Presenter which it calls (e.g. in UiHandlers).
Some parts of the application which we built should be reused in other views. For example - a view which is sometimes the "main view" of a part of the application - can be used inside a pop-up in another part of the application (of course, the views/presenters are initialized differently in this other case, but basically it is the same thing).
What would be the correct approach to do stuff like this? I cannot seem to find a suitable one without resorting to ugly hacky stuff.
For example - if I put the presenter of the reused component inside the main view - it is easy to initialize the reused component, but it is ugly to receive a result back in the main presenter. This can be solved by passing a runnable or creating a custom handler or passing the parent presenter itself to the reused presenter.
All of these approaches don't seem right to me though and seem ugly.
Any ideas/experiences?
What you're describing is a view being able to be controlled by 2 distinct presenters. Abstracting those presenters behind a common API, in the form of an interface, should be enough.
You can also see it as a composite widget being used within two distinct views. The composite widget would then expose events and a public API that both views could wire to their specific presenters.
See Activites and Places,It can help you to desing and structure you app.
https://developers.google.com/web-toolkit/doc/latest/DevGuideMvpActivitiesAndPlaces
.
I want to implement navigating a tree structure like it is done in e.g. the WinRT file picker. I then want to be able to drop this behavior as part of any page.
My current attempt, is to try and register a secondary FrameAdapter/INavigationService in the container and use that for a frame that is different from the app root frame. So far, I could not get it to work.
My motivation behind that, is, that I do not want to reimplement sth. that the INavigationService already provides.
Basic structure:
ShellView that represents the general app layout (header, footer, navigation) and is currently an OneActive conductor.
Frame control (x:Name="ActiveItem") on the ShellView inside which the hierarchical navigation should occur
The chosen conductor has no relevance yet, since I'll probably have to nest the FrameControl inside another view later to really set up a MDI interface. I'll will want to have multiple screens that should be able to hierarchically navigate
I could not find a CM WP7 example of such a scenario
Can you help me out here?
My problems so far:
How do I access the container from a view code-behind without resorting to using the Application.Current. I figured, it is in the code-behind where I would want to setup the secondary FrameAdapter, since it is here that I have access to the FrameControl
How do I setup the INavigationService so that the initial loading by CM (populating the ActiveItem) is registered with it. There does not seem to be a navigation event for this initial display of the ActiveItem.
Many thanks in advance,
Tobias
PS: I have cross-posted to the Caliburn Micro discussions (Discussion over at CodePlex CM)
I have been using the MVVM Light Toolkit to help learn the MVVM pattern. However, I have not been able to solve the problem of usercontrols within controls scenario.
For example, in a Timesheet application, lets say we have a control called NewUnitOfWork. When it first loads, a panel with a ListBox with a list of projects is loaded as the Content of the NewUnitOfWork. The user clicks on one. A new panel is swapped in with a ListBox containing the possible tasks for that project. A task is selected and a new panel is loaded which will contain controls to input data for the chosen task of the chosen project.
So, we have the selected item in one usercontrol being passed to the other two user controls, which are, in turn swapped in as the Content of the NewUnitOfWork control (or window).
If each control has its own ViewModel, we need to pass the selected value from one ViewModel to the next etc.
I have got it working in a single user situation using global variables (via a "service"). However, there are concurrency issues with that and it is not a good solution. It's sub-par.
I have seen many times the suggestion on this forum to have on ViewModel as a member of another ViewModel. Whilst this solves the problem at hand, I believe it is a violation of the MVVM pattern. Another ViewModel is not UI-related functionality that the ViewModel shoule be directly.
So. Has anyone found a clean MVVM-complying way to do this sort of thing?
Cheers
Please always keep in mind that MVVM is just a pattern and it is designed to help you separate your UI and logic. Do not be afraid to “violate the pattern” if it helps to increase testability or maintainability of the application.
Having a master ViewModel with several child ViewModels is very handy if you have a complex UI. The main ViewModel may be responsible for handling the top level UI controls and for coordination of the child VMs, while other ViewModels are responsible for communication with the sub regions of your UI.
Moreover, if you have a really complex UI with the multiple nesting UI layers, you can implement an infrastructure to automatically cascade all the events from master to child VMs.
And of cause, you may try to use one of the more advanced MVVM frameworks. For example Catel implements pretty comprehensive model to resolve such situations with nested VMs.
I don't see a problem with ViewModels referencing other ViewModels (based on my experience with TreeViews). Have a look at any article about TreeView and MVVM. You will see that each node is a ViewModel, that references a collection of child nodes, which are ViewModels. Trying to do that without VM-VM references would be a nightmare.
Josh Smith
http://www.codeproject.com/KB/WPF/TreeViewWithViewModel.aspx
I have been using the following setup:
A 'master' VM with a 'collection' VM and a 'details' VM as nested properties.
The master VM is tied to a View that is used as a master-detail form. This master-detail View is composed from two other Views.
I find it a very neat setup because it allows me to put search criteria in the master View(Model) and keeps the other View(Model)s clean.
I can't see how this would break the pattern.