I am implementing MVP pattern in my GWT application. On each history token change, I am initialising presenter to get respective view. If user navigate to a single view GridView (say) 2 times then respective presenter GridViewPresenter (say) is initialized 2 times (according to MVP pattern. If we will not make presenter as singelton).
like
Presenter presenter = new GridViewPresenter();
In this case all events in this presenter will registerd and listen 2 times. How can we avoid this scenario without making our presenter singelton.
You'll have to track your presenters lifecycle (before switching to a new one, dispose the current one so it unregisters its handlers).
Alternatively, you could use the Places & Activities micro framework from GWT, that does just that: you activity has explicit start and onStop and onCancel (when started asynchronously).
GWT-Platform (an MVP framework for GWT) also has similar lifecycle.
Another alternative is to not have your presenter add handlers on its view: the view manages its event handlers, and notifies the presenter through a contract expressed as an interface (the presenter implementing that interface); and there can have only one presenter at a time for a given view (that way, the new presenter overwrites the previous one).
Managing your presenters lifecycle has the advantage that the presenters can be garbage collected as soon as they're no longer used, rather than waiting for a new presenter to take their place in the singleton/long-lived view.
Related
What I would like to do is unsubscribe from events at the moment the ViewModel is no longer needed. I tried implementing IDisposable but nobody calls Dispose(), not Xamarin.Forms nor Prism.Forms.
We have an app created with Xamarin.Forms. We use Prism.Forms to do MVVM. When navigating to a new page (push on stack) Prism.Forms wires the ViewModel to the Page. When navigating back (pop from stack) the ViewModel gets GarbageCollected after a while.
The problem is however that at a point in time we have a couple of the same type of ViewModels with subscriptions to events that are not bound to a View. When the events fire all these ViewModels start doing their thing. So I am looking for a way to unsubscribe at the moment the subscription is no longer needed.
Does anyone have a solution?
You can make sure the Dispose() is called in the OnDisappearing() event of the View, if you want to ensure that ViewModel is not present anymore in the memory than the view.
It is better if you care only about subscribe and unsubscribe of events, then to do it in the OnAppearing() and OnDisappearing(). In that case you will be sure of no event handlers been present on the viewmodel once view is not visible.
Implement IDestructible or INavigationAware
(or both as in BaseViewModelsample from Prism).
Depending on your object lifecycle :
Implement your disposal code in the Destroy method of IDestructible interface.
Implement your disappearing/appearing code in the OnNavigatedFrom/OnNavigatedTo methods on INavigationAware interface.
Bonus:
IDestructible can be implemented also by the View (and it will called accordingly by Prism when the view is destroyed).
Note:
While the solution above using OnAppearing/OnDisappearing works, it induces that ViewModel will depends on a call from View for managing its lifecycle (not clean). Moreover these methods don't exist on ContentView.
Just to make it clear: I know MvvmCross is very flexible about where and how view models can be created. My question is more about proper separation of concerns, to simplify design of complex cross-platform applications.
Consider we have an app with customer list and customer details. On iPad and Surface the list and details are shown on the same page, but on smaller devices customer list and details for a selected customer are split between separate pages. So we have a PCL with CustomerListViewModel and CustomerDetailsViewModel. Now how should we manage view model lifetime from within the portable class library?
I originally did it using code in CustomerListViewModel implementation that looks like this:
public ICommand SelectCustomerCommand
{
get { return new MvxCommand(SelectCustomer); }
}
public void SelectCustomer()
{
if (formFactor == FormFactor.Phone)
{
ShowViewModel<CustomerDetailsViewModel>(new CustomerDetailsViewModel.NavObject(this.SelectedCustomer));
}
else
{
this.CustomerDetails = new CustomerDetailsViewModel(this.SelectedCustomer);
}
}
What is essential here is that we either call ShowViewModel that in turns asks a presenter to construct a CustomerDetailsViewModel object and render it in a new page or explicitly create an instance of CustomerDetailsViewModel and bind it to CustomerDetails.
After having seen episodes 32 and 42 of N+1 MvvmCross video series I am not quire sure this is the right way to do it. Of course it works, but should a view model care about instantiation details of other view model?
My second thought was to refactor this code and put this logic into a presenter, so I can simply write in CustomerListViewModel implementation:
public void SelectCustomer()
{
ShowViewModel<CustomerDetailsViewModel>(new CustomerDetailsViewModel.NavObject(this.SelectedCustomer));
}
... and presenter will do the rest inside the code triggered by ShowViewModel call. However, in the episode 42 it's shown how to control view model lifetime right from the associated view:
protected override void OnNavigatedFrom(System.Windows.Navigation.NavigationEventArgs e)
{
base.OnNavigatedFrom(e);
VisibleViewModel.IsVisible(false);
if (e.NavigationMode == NavigationMode.Back)
KillableViewModel.KillMe();
}
But if a view model lifetime is controlled by a presenter, shouldn't we try to place KillMe call inside presenter's logic? I know so small piece of code doesn't make much difference but couldn't this be an advantage to put it in presenter's class and reduce code-behind?
Definitely the ViewModel should not handle anything in regards to view (screen).
One quick idea I have is use a custom presenter which is able to create views based on the ShowViewModel<> requests.
The custom presenter is a view responsibility so you can test for screen orientation.
http://slodge.blogspot.co.uk/2013/06/presenter-roundup.html
For the second part of this question:
But if a view model lifetime is controlled by a presenter, shouldn't we try to place KillMe call inside presenter's logic? I know so small piece of code doesn't make much difference but couldn't this be an advantage to put it in presenter's class and reduce code-behind?
Currently view model presentation is orchestrated by the presenter - it gets ViewModelRequest objects and decides what to do with them.
However, it doesn't normally create the ViewModels - instead:
the presenter normally creates/shows a View
then the View creates (or locates) a ViewModel as part of the OnCreate, ViewDidLoad or OnNavigatedTo handling.
And so I don't think a ViewModel's lifetime is normally "controlled by a presenter" - instead I think a ViewModel is a "Model for a View" - so it's lifetime is "controlled by its view".
For the occasions where you need shutdown/dispose/killMe logic in your ViewModel, if you wanted to move that logic back inside the presenter - or into some other object - then you definitely could do so if you wanted to - it's your app and the app is king.
However, in these cases I suspect you would need the presenter to get some kind of notification from the View - as often the presenter doesn't know when Views are removed (when a modal is dismissed, when a Back button is pressed, when Android clears up stack views to save memory, etc).
As another way of thinking about this, if the presenter were renamed as INavigationService then what roles do you want that INavigationService to own within your app?
I went through the link,
but there it was a bit difficult for me to understand the basic reason as to why
we shouldn't use view.getSaveButton().addClickHandler().... i.e to say why we should not have getter in our Display interface?? . What I understood from the video is that it is not a good practice to maintain views state information within the presentation, but sorry to say I didn't understood how is that gonna help us. ?
In that video its also mentioned that we should use setListeners() rather than addListeners(), that is to avoid more than one listener in the interface.
He says he is diverging from Rays understanding of MVP, with respect to not using HasClickHandlers() , please explain why ?
One of the reason why I couldnt understand the point could be that in that video there is lot of Google Wave specific code, It would had been easier for me to understand had he used a more generic and a more simpler code to make us understand his point.
Please pardon me if I have offended any one.
1) Presenter should not be depended from UI widgets - there is some reasons for it:
Separating of UI representation in Views and application logic in Presenter
Defined in interfaces behaviour of View and Presenter
Easy to write Unit-tests for presenter layer
Several views(desktop, mobile, e.t.c.) for one presenter
Simple set of interfaces named MVP allows us to separate this two layers:
// View interface
interface View extends AsWidget {
void setPresenter(Presenter presenter); // cross reference to presenter
interface Presenter { // presenter must extends from this interface
void onContactEditClick(); // callback from view handling in presenter without any UI code
}
}
// View realization (part)
public class ViewImpl extends Composite implements View {
#UiHandler('someButton') // or 'somelink', e.t.c.
void onContactEditClick(ClickEvent event) {
presenter.onContactEditClick();
}
}
Presenter is isolated from controls that firing events. It can be button, can be link, e.t.c. So it is possible to create several implementations of views(for desktop and mobile platforms for example) with a different set of UI controls in each view and use this views with a one presenter.
Google MVP
2) Listeners are related to UI controls. In my opinion, it is better to avoid usage of listeners between view and presenter via interface methods like in Point1 sample. In fact, Presenter interface inside of View interface is a better listener for all events from view.
3) The same. HasClickHanlers related to View layer and should be separated from presenter.
I've started a GWT project, I've decided to give the UiBinder a try. I'm having a difficult time laying an MVP pattern on top of UiBinder.
When I was using GWT-pure-java: I'd use Gin to inject my presenter with the corresponding view. It was pretty straight forward, and if I wanted to pass an id into the presenter, then I would simply pass an id into the presenter's constructor.
Not so straight forward with UiBinder. I'm almost certain that I'm missing something, because lots of people are claiming that UiBinder and MVP are a match made in heaven...so I'm hoping to get some solid responses on this query ;-)
What I've seen in several trivial GWT-UiBinder examples, is that the view is created by the binder then either:
The view constructs the presenter either in its constructor or via a #UIFactory method.
The corresponding presenter is passed to the view (via a setter, needless to say after the view is constructed).
With the first approach, how does one pass an id to the presenter if the presenter is being constructed in the view? Would you do view.getPresenter().setId(42);, and then the presenter would go to the server get some info and ask the view to display it...smells bad.
With the second approach, one would end up with a non-intuitive object graph in which it is not clear who is the consumer and who is the producer. Also, in situations where the view requires information from the presenter (almost all use-cases require this) what would one do:
//some code would create the presenter pass it the id and then call view.setPresenter
class MyView {
void setPresenter(MyPresenter p) {
this.presenter = p;
//OK now that i have my presenter how do I ask it to fetch data from the server.
//do i turn around and do: presenter.setView(this); and then the presenter takes
//over and uses the view to display the data?
}
}
This is equally smelly...Sorry for the long post, and thanks in advance...
You're right in that it does seem a bit unclean to have both a View reference its Presenter, and have the presenter reference the View.
The way I see it, and how the google dev pages on MVP outline there are two flavours of MVP:
Have a view that doesn't know about its presenter, and "wrap" it with a presenter. The view provides a sufficient API for the presenter to get/set all the data it cares about. Additionally the presenter must know about all event types the view can generate, in order to respond to user interaction. This is the method of MPV part 1.
The presenter once more knows about the view, but this time only in a get/set data capacity. The presenter does not concern itself with the UI events in the view. Instead, the presenter provides an API for the view to call/notify it when events happen in the form of "onSomethingHappened()" methods in our presenter. This allows us to capture all behaviour/logic in the presenter itself, and invoke it as necessary when something happens in the view. The view can then handle events at a very low level in any way suitable - whether using GWT events/widgets or raw DOM events with elements/HTML (UiBinder/GWT performance best practices). This is the method of MVP part 2.
I prefer option 2 as it allows the presenter to be purely focused on the necessary behaviour. The view can deal with widgets/html/event handling as necessary and simplify it to calls to "onSomething()" calls for the presenter. Those widget/event implementations might be simple or complex and optimized events. The presenter is unaffected (and unpolluted) by the detail, as it just gets notified. I feel this option is a cleaner separation of presentation and behaviour. Note it's also a 1 to 1 implementation of the Observer Pattern, so the interconnection between View and Presenter is necessary.
As for creation, I feel the Presenter is the stronger entity despite it playing the Observer role. I would create the necessary presenter and then pass it the view of concern. The presenter can then take control of the view, and pass the view a self reference.
As for your producer/consumer analogy I think the presenter is the consumer. The view produces UI events (user interaction) and the presenter responds by providing the necessary behaviour. That should be the only point of contact between the view and presenter - the view calls methods like "onSomethingHappened()" and the presenter does the work. The view would never tell the presenter "fetchData()" or anything like that.
I've just started using UiBinder + MVP recently myself, so this is just what I think. I hope it helps!
It has been a few days since I started this thread and read the responses. I've decided on my approach. And I'd thought I just mention it here.
Once again, thanks for the thoughtful responses...they were helpful.
As filip-fku pointed out presenters are the main entity, so I decided to continue to threat them as such during the life-cycle management of my mvp objects. In other words, the views do not instantiate the presenters. The presenters are instantiated by other presenters (at some point i'll delegate to Gin).
The builder of a presenter has access to its corresponding view, and injects the presenter (via constructor) with the view.
The views themselves are either instantiated by the builder or instantiated by UiBinder as part of a larger view. In the latter case where UiBinder instantiates the view the parent view has a getter. Here is an example of this case:
/*pojo for the parent ui-binder*/
public class Form implements FormPresenter.View {
#UiField PromptView namePrompt;
#override
PromptPresenter.View getNamePromptDisplay() {
return namePrompt; //introduced into this pojo via #UiField
}
//bunch of view code
}//end of the class
Then in the FormPresenter I do:
private void buildNamePrompt() {
new PromptPresenter(display.getNamePromptDisplay(), etc....);
}
I have essentially kept the mvp life-cycle similar to the pure java approach. Once I get some milage out of this, I'll refactor it with Gin.
Thanks again.
P.S. if you haven't seen the i/o presentation mentioned above, it is worth checking out.
We are building an app using the MVVM pattern, we have controllers that wire up all the views and viewmodels using DI. All examples of MVVM I've seen are really simplistic and have 1 view. How do/should viewmodels talk back to the controller? The controller knows about the models and views, should the viewmodel send events back to the controller? Where should a save happen? Model? Controller?
Could your ViewModel not take a dependency on an IController or some other interface, so they can talk back to it? I try to keep as much application logic out of the ViewModel as possible, as these classes can easily become bloated.
MyViewModel(IController controller)
{
this.controller = controller;
}
void Save()
{
this.controller.Save();
}
I do agree that the MVVM frameworks tend to be too simplistic with their samples. In particular, moving between views/screens in your application is something I would like to see more examples of. I create an IViewManager interface, to allow my ViewModels to request that we move to another view.
We use Controllers too but in our case they are responsible for the application workflow. The Controller knows the ViewModel and the Model but not the concrete View because this will be injected by the IoC Container.
If you are interested in an example that shows more than just one UI (modal dialog, wizard with conditional workflow) then you might have a look at:
WPF Application Framework (WAF) - http://waf.codeplex.com
In case of an application that has multiple modules and requires separation of concerns I would recommend using prism framework.
http://msdn.microsoft.com/en-us/library/gg406140.aspx
I use a similar setup to you. In my controller, where my DI and view injection goes down, I sometimes keep reference to the ViewModel (which hold the View). Some cases I may have an event on the VM which is handled by the controller. In other, extreme cases (like if the VM/V was created outside the controller, say in another VM), I may even use the EventAggregator (with a strong ref) to listen to events that may be fired on the VM. In that case, a stored ref to the VM is not needed.
How about using events wherein the controller subscribes to VM events or using a mediator pattern where in a mediator is injected in a VM.