I'm trying to use ReactiveUI with a redux style model.
I want to have a property that looks like
public string Code
{
get => _state.Code;
set => _state.Dispatch(new SetCodeMessage(value));
}
and have the _state determine what properties changed when the message gets processed.
I can't figure out how to setup the binding and subscription to make this happen.
Right now I don't even have the Dispatch method being called, but the view still displays whatever gets typed into the textbox this property is bound to. I tried adding RaisePropertyChanged to the setter which ends up having the getter called, but the UI still doesn't change.
First of all, what is the type of _state.Code?
Then, instead of inventing the wheel on your own (unless that's what you want to do), use something ready and tested, like redux.NET.
When using RxUI, try to keep in line with best practices, which would be to have properties be as simple as possible.
public string Code {
get => _code;
set => this.RaiseAndSetIfChanged(ref _code, value);
}
// in VM contructor
this.WhenAnyValue(x => x.Code).Do(_state.Dispatch); // you can hook this up in WhenActivated to dispose it when necessary
_state.CodeChanged // this is analog to computed properties from Redux, should be of type IObservable<string>
.Do(c => this.Code = c);
This way you can simply bind a textbox to ViewModel.Code and the rest will be handled.
One piece of advice, don't do that :)
The central store should only contain values that you want to share between multiple ViewModels. So, unless you want to update other control as user types, push the value to the global only when user clicks OK.
All properties that are bound to textboxes etc should rather be handled locally, in the ViewModel.
Related
My colleague and I want to create an application with some pages, where the user is allowed to change some settings. These settingspages do not have savebuttons. There is a main settingspage and two child settingspages. The settings should be persisted if the user navigates back from one of the three pages or the user makes some changes in one of the child pages.
The model is loaded in the OnAppearing() Method of the main settingspage. So we can not use OnDisappearing() Method of the childpages to save the model, because the sequence of these methods and Pushed() and Popped() of the navigationpage differs on each platform: forums.xamarin.com/discussion/21417/navigationpage-push-pop-event-sequence-different-across-platforms
Our solution at this time is to save the model in the setters of the properties of the childpages viewmodels. But that could not be best practise. So our question is: Are there any other solutions to solve this problem? What are the best practises for that problem?
Code:
public ReminderRepetitive ListViewSelectedItem
{
get { return _settingsEntity.ReminderRepetitive; }
set
{
if (!Equals(_settingsEntity.ReminderRepetitive, value))
{
_settingsEntity.ReminderRepetitive = value;
OnPropertyChanged("ListViewSelectedItem");
Save();
_navigation.PopAsync();
}
}
}
public void Save()
{
_businessLayer.UpdateSettings(_settingsEntity);
}
There are improvements to be made with your code.
You shouldn't do the navigation from a property - if you're going to be navigating away at least make that a method. That method should be async and you should await for the _navigation.PopAsync()
Even calling the Save() shouldn't be there. Consider what happens if an exception occurs while saving: duplicate keys, null fields etc are all possible. Then not only you won't Save but you won't PopAsync either and your ListViewSelectedItem = newItem will cause your selected item handler to blow up as well. And if you're going to have a property that also handles showing error messages you're definitely well inside MessyCode territory.
You can use the PropertyChanged on your model (ReminderRepetitive) and save immediately as soon as a property is changed.
It looks like you have a _businessLayer the whole business of saving or updating may belong there.
As a whole your question is pretty broad and can be treated as "opinion based". Next time try asking about specific parts of the code you're not sure about or cause errors - or use Code Review
I am using MVVM light and figured out since that the ViewModelLocator can be used to grab any view model and thus I can use it to grab values.
I been doing something like this
public class ViewModel1
{
public ViewModel1()
{
var vm2 = new ViewModelLocator().ViewModel2;
string name = vm2.Name;
}
}
This way if I need to go between views I can easily get other values. I am not sure if this would be best practice though(it seems so convenient makes me wonder if it is bad practice lol) as I know there is some messenger class thing and not sue if that is the way I should be doing it.
Edit
static ViewModelLocator()
{
ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
SimpleIoc.Default.Register<ViewModel1>();
SimpleIoc.Default.Register<ViewModel2>();
}
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance",
"CA1822:MarkMembersAsStatic",
Justification = "This non-static member is needed for data binding purposes.")]
public ViewModel1 ViewModel1
{
get
{
return ServiceLocator.Current.GetInstance<ViewModel1 >();
}
}
Edit
Here is a scenario that I am trying to solve.
I have a view that you add price and store name to. When you click on the textbox for store name you are transferred to another view. This view has a textbox that you type the store you are looking for, as you type a select list get populated with all the possible matches and information about that store.
The user then chooses the store they want. They are transferred back to the view where they "add the price", now the store name is filled in also.
If they hit "add" button it takes the price, the store name, and the barcode(this came from the view BEFORE "add price view") and sends to a server.
So as you can see I need data from different views.
I'm trying to understand what your scenario is. In the MVVMlight forum, you added the following context to this question:
"I have some data that needs to be passed to multiple screens and possibly back again."
For passing data between VMs, I would also - as Matt above - use the Messenger class of MVVMLight as long as it is "fire and forget". But it is the "possibly back again" comment that sounds tricky.
I can imagine some scenarios where this can be needed. Eg. a wizard interface. In such a case I would model the data that the wizard is responsible for collecting and then bind all Views to the same VM, representing that model object.
But that's just one case.
So maybe if you could provide a little more context, I would be happy to try and help.
Yes, you can do this, in as much as the code will work but there is a big potential issue you may run into in the future.
One of the strong arguments for using the MVVM pattern is that it makes it easier to write code that can be easily tested.
With you're above code you can't test ViewModel1 without also having ViewModelLocator and ViewModel2. May be that's not too much of a bad thing in and of itself but you've set a precedent that this type of strong coupling of classes is acceptable. What happens, in the future, when you
From a testing perspective you would probably benefit from being able to inject your dependencies. This means passing, to the constructor--typically, the external objects of information you need.
This could mean you have a constructor like this:
public ViewModel1(string vm2Name)
{
string name = vm2Name;
}
that you call like this:
var vm1 = new ViewModel1(ViewModelLocator.ViewModel2.name);
There are few other issues you may want to consider also.
You're also creating a new ViewModelLocator to access one of it's properties. You probably already have an instance of the locator defined at the application level. You're creating more work for yourself (and the processor) if you're newing up additional, unnecessary instances.
Do you really need a complete instance of ViewModel2 if all you need is the name? Avoid creating and passing more than you need to.
Update
If you capture the store in the first view/vm then why not pass that (ID &/or Name) to the second VM from the second view? The second VM can then send that to the server with the data captured in the second view.
Another approach may be to just use one viewmodel for both views. This may make your whole problem go away.
If you have properties in 1 view or view model that need to be accessed by a second (or additional) views or view models, I'd recommend creating a new class to store these shared properties and then injecting this class into each view model (or accessing it via the locator). See my answer here... Two views - one ViewModel
Here is some sample code still using the SimpleIoc
public ViewModelLocator()
{
ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
SimpleIoc.Default.Register<IMyClass, MyClass>();
}
public IMyClass MyClassInstance
{
get{ return ServiceLocator.Current.GetInstance<IMyClass>();}
}
Here is a review of SimpleIOC - how to use MVVMLight SimpleIoc?
However, as I mentioned in my comments, I changed to use the Autofac container so that my supporting/shared classes could be injected into multiple view models. This way I did not need to instantiate the Locator to access the shared class. I believe this is a cleaner solution.
This is how I registered MyClass and ViewModels with the Autofac container-
var builder = new ContainerBuilder();
var myClass = new MyClass();
builder.RegisterInstance(myClass);
builder.RegisterType<ViewModel1>();
builder.RegisterType<ViewModel2>();
_container = builder.Build();
ServiceLocator.SetLocatorProvider(() => new AutofacServiceLocator(_container));
Then each ViewModel (ViewModel1, ViewModel2) that require an instance of MyClass just add that as a constructor parameter as I linked initially.
MyClass will implement PropertyChanged as necessary for its properties.
Ok, my shot at an answer for your original question first is: Yes, I think it is bad to access one VM from another VM, at least in the way it is done in the code example of this question. For the same reasons that Matt is getting at - maintainability and testability. By "newing up" another ViewModelLocator in this way you hardcode a dependency into your view model.
So one way to avoid that is to consider Dependency Injection. This will make your dependencies explicit while keeping things testable. Another option is to use the Messenger class of MVVMLight that you also mention.
In order to write maintainable and testable code in the context of MVVM, ViewModels should be as loosely coupled as possible. This is where the Messenger of MVVMLight can help. Here's a quote from Laurent on what Messenger class was intended for:
I use it where decoupled communication must take place. Typically I use it between VM and view, and between VM and VM. Strictly speaking you can use it in multiple places, but I always recommend people to be careful with it. It is a powerful tool, but because of the very loose coupling, it is easy to lose the overview on what you are doing. Use it where it makes sense, but don't replace all your events and commands with messages.
So, to answer the more specific scenario you mention, where one view pops up another "store selection" view and the latter must set the current store when returning back to the first view, this is one way to do it (the "Messenger way"):
1) On the first view, use EventToCommand from MVVMLight on the TextBox in the first view to bind the desired event (eg. GotFocus) to a command exposed by the view model. Could be eg. named OpenStoreSelectorCommand.
2) The OpenStoreSelectorCommand uses the Messenger to send a message, requesting that the Store Selector dialog should be opened. The StoreSelectorView (the pop-up view) subscribes to this message (registers with the Messenger for that type of message) and opens the dialog.
3) When the view closes with a new store selected, it uses the Messenger once again to publish a message that the current store has changed. The main view model subscribes to this message and can take whatever action it needs when it receives the message. Eg. update a CurrentStore property, which is bound to a field on the main view.
You may argue that this is a lot of messaging back and forth, but it keeps the view models and views decoupled and does not require a lot code.
That's one approach. That may be "old style" as Matt is hinting, but it will work, and is better than creating hard dependencies.
A service-based approach:
For a more service-based approach take a look at this recent MSDN Magazine article, written by the inventor of MVVMLight. It gives code examples of both approaches: The Messenger approach and a DialogService-based approach. It does not, however, go into details on how you get values back from a dialog window.
That issue is tackled, without relying on the Messenger, in this article. Note the IModalDialogService interface:
public interface IModalDialogService
{
void ShowDialog<TViewModel>(IModalWindow view, TViewModel viewModel, Action<TViewModel> onDialogClose);
void ShowDialog<TDialogViewModel>(IModalWindow view, TDialogViewModel viewModel);
}
The first overload has an Action delegate parameter that is attached as the event handler for the Close event of the dialog. The parameter TViewModel for the delegate is set as the DataContext of the dialog window. The end result is that the view model that caused the dialog to be shown initially, can access the view model of the (updated) dialog when the dialog closes.
I hope that helps you further!
I'm working on a ASP.NET MVC system where you may click on a ajax link that will open a window (kendo window but it does not affect the situation) which a complex flow. To make this less of a nightmare to manage, I made a ViewModel (as I should) but this ViewModel is a complex object due to the complexity of the procedure.
There is anywhere from a single to 5 windows that asks various questions depending on a lot of conditions (including, but not limited to, what time you click the link, who you are, what schedule is attached to your account and, obviously, your previous answers in this flow).
The problem is that having a complex object, I cannot simply make #Html.HiddenFor(o=>o.XXX). So I proceeded to find an alternative and it led me with a single option, TempData. I'm really not a fan of dynamics and object types. I'd really like to have this View Model strongly typed.
What would be the best way to approach this?
Here is a case where using Session or TempData might make sense. Contrary to popular belief, you can make these somewhat strongly-typed. Not like a viewmodel, but you can avoid keychain messes by using extension methods.
For example, instead of doing something like this:
TempData["NestedVariable1"] = someObject;
...
var someObject = TempData["NestedVariable1"] as CustomType;
You can write extension methods to store these variables, and encapsulate the keys and casting in the extension methods.
public static class ComplexFlowExtensions
{
private static string Nv1Key = "temp_data_key";
public static void NestedVariable1(this TempData tempData, CustomType value)
{
// write the value to temp data
tempData[Nv1Key] = value;
}
public static CustomType NestedVariable1(this TempData tempData)
{
// read the value from temp data
return tempData[Nv1Key] as CustomType;
}
}
You can then read / write these values from either controllers or views like this:
TempData.NestedVariable1(someObject);
...
var someObject = TempData.NestedVariable1();
You could use the same pattern with Session as well. And instead of saving each individual scalar value in a separate variable, you should be able to store an entire nested object graph in the variable. Either that, or serialize it to JSON and store that, then deserialize when you get it back out. Either way, I think this beats a ton of hidden fields written out to your view's form.
I have started working out with Entity Framework (EF) for an MVC n-tier application. It would seem that very obvious that this being a web application (which is stateless), I would have to use detached object models. There is no ambiguity with doing an Add operation. However when doing an edit there are here are two ways
Fetch the original object in context, attach the updated object and
then save to database. Something like mentioned in answer to this
question
EF4 Context.ApplyCurrentValues does not update current values
Set individual modified properties explicitly using the IsModified property of individual fields of the object like
mentioned in this article
http://msdn.microsoft.com/en-us/data/jj592677.aspx
Method 1 has disadvantage of having to load object into memory from database each time an update needs to be performed.
Method 2 would require having to manually pass which fields to be set as IsModified to true from wherever the object an be updated. So for e.g. for each object, I may need to create a boolean collection object for each field of the object.
e.g.
SaveEntity(EntityClass e, EntityStateClass ec)
{
context.Entry(e).Property("Name").IsModified = ec.NameState;
context.SaveChanges();
}
class EntityStateClass{ public bool NameState;}
I would prefer method 2 simply for the sake of performance but I am hindered by the n-tier architecture and repository pattern I am using. My Repository interface restricts save method for any object to be
SaveEntity(EntityClass e);
So I cannot pass the "state" object. Context class is not available and should not be available outside DAL. So I cannot set property outside. Is there any "proper" way to achieve this ?
Note: Self-Tracking Entity is also out of question since I cannot send entities with state to client (the browser) since I am intent on keeping the html lightweight.
EDIT: After a lot of thinking, I am trying to use following mechanism to keep track of modified state for each field in my domain class
Declare a partial class for entity class.
For each field that is updateable, declare a boolean property like "IsModified_FieldName"
Set the "IsModified_FieldName" property when the field is set.
However for this I need Entity Framework to generate explicit properties for me instead of implicit properties that it auto-generates. Does EF provide an handle to do this ?
Here is sample code of what I am trying to achieve
//Save Method for class EntityClass.
SaveEntity(EntityClass e)
{
context.Entry(e).Property("Name").IsModified = e.IsModified_Name;
context.SaveChanges();
}
//EntityClass is class autogenerated by EF
public partial class EntityClass
{
//This is auto-generated property by EF
public string Name {get; set;}
/* This is what I would like EF to do
private string name;
public string Name
{
get {return Name;}
set {
name = value;
//this is what I would like to do
this.IsModified_Name = true;
};
}
*/
}
//This is another partial definition for EntityClass that I will provide
public partial class EntityClass
{
//This property will be set to true if "Name" is set
public bool IsModified_Name {get; set;}
}
PS: It seems the information I have provided is not sufficient and therefore there are no responses.
I am using DbContext (Database first model)
EF auto-generates the class files for me. So each time I update my database, the class files are regenerated.
To your concrete question: The entities are generated by a T4 template and it should be possible to modify this template (which is in text format) to generate the entities in a way you want to shape them.
But I have a few remarks about your concept:
In a web application data are usually changed by a user in a browser. To have a definite knowledge what really has been changed you need to track the changes in the browser (probably by some Javascript that sets flags in the data (a ViewModel for example) when a user edits a text box for instance).
If you don't track the changes in the browser what happens? The data get posted back to the server and you don't know at the server side (with MVC in a controller) which property has been changed. So, your only chance is to map all properties that has been posted back to your EntityClass and every property will be marked as Modified, no matter if the user really did a change or not. When you later call SaveChanges EF will write an UPDATE statement that involves all those properties and you have an unnecessary overhead that you you want to avoid.
So, what did you win by setting individual properties instead of setting the whole entity's state to Modified? In both cases you have marked all properties as Modified. Exceptions are partial changes of an entity, for example: You have a Customer entity that has a Name and City property and a view that only allows to edit the Name but not the City and a corresponding ViewModel that only contains a Name property. In this case your procedure would only mark the Name property of the Customer entity as Modified but not the City. You might save here a little bit because you don't save the City property value to the database. But you still save the Name even if it didn't change.
If you use solution 1 (ApplyCurrentValues) you have to load the entity first from the database, yes, but it would only mark the properties as Modified that really changed compared to their values in the database. If the user didn't change anything no UPDATE would be written at all.
Keep in mind that you are only at the beginning to implement your concept. There are other changes to the data that can happen in the browser than only scalar property changes, namely relationship changes. For example a user changes the relationship from an Order to a Customer or you have a view that has an Order and a collection of OrderItems and the user cannot only edit the Order header but also edit the OrderItems and remove and add new OrderItems. How do you want to recognize when the data come back from the browser to the server which collection item has been added and which has been removed - unless you track all those changes in the browser and send tracking information back to the server in addition to the actual data or unless you reload the Order and OrderItems from the database and merge the changes into the original entities from the database?
Personally I would vote for option 1 for these reasons:
You can use real POCOs that don't carry additional tracking information. (BTW: I have some doubt if you aren't reinventing the wheel by implementing your own tracking that EF change tracking proxies provide out of the box.)
You don't need to track changes in the browser which can become quite complex and will require Javascript in every Edit view to write change flags into hidden form fields or something.
You can use standard features of EF without having to implement your own tracking.
You are required to load entities from the database when you want to update an entity, that's true. But is this the real performance bottleneck in a web application where data have to run through the wire back and forth (and reflection (which isn't really known as to be fast) is involved by the model binder)? I have nothing said if your database is remote from the web server and connected by a 9600 baud modem. But otherwise, your plan is not only premature optimization, it is kind of premature architecture. You are starting to build a potentially complex architecture based on "it could be slow" to solve a performance problem that you actually don't know of whether it really exists.
I am developing an application that involves a type hierarchy and started by defining the models for each type via inheritance. When it comes to writing the corresponding controllers I am not sure how to approach the whole thing in a clean way. Should I write only one controller for the base type that is able to handle derived models or should there be one controller for each subtype? How should the view-controller bindings be set up to work with the different controllers?
You might want to check out SproutCore's new experimental polymorphism support: http://groups.google.com/group/sproutcore-dev/browse_thread/thread/b63483ab66333d15
Here's some information on defining sub-classes and overriding properties and methods:
http://wiki.sproutcore.com/w/page/12412971/Runtime-Objects.
From my (limited) use of Sproutcore, I've only been able to bind 1 view to 1 controller.
As such, if you are planning to use a single view (e.g. ListView) to display your data, then I think you will only be able to bind that view to 1 controller. This means the 1 base type that is able to handle derived models seems to be the way to go.
Typically you populate the content of ArrayController instances with the results of App.store.find calls. SC.Store#find can take an SC.Query instance, which typically looks like:
MyApp.myController.set('content') = MyApp.store.find(SC.Query.local(MyApp.MyModel));
This should return all instances of MyApp.MyModel, including any instances of MyApp.MyModel's subclasses.
The first argument to SC.Query.local can either be an SC.Record subclass or a string referring to the subclass. So if you've got some intermediary SC.Record subclasses, you might want to try using them there.
Controllers should just be proxies for objects, when dealing with single instances of your model. In other words, ObjectController can proxy anything. Here is what I mean in code:
You have two objects, Person and Student.
App.Person = SC.Object.extend({
// person stuff here
})
App.Student = App.Person.extend({
// student stuff here, you have have all Person things because you are extending person.
})
You then want to define controllers:
App.personController = SC.ObjectController.create({
contentBinding: 'App.path.to.person'
})
App.studentController = SC.ObjectController.create({
contentBinding: 'App.path.to.student'
})
note that you would only bind the controller's content to something if the person/student is a result of a selection, or some other flow where bindings fire. In other words, if you set the person manually (say from a statechart, as the result of an interaction), you would still define the controller but would do
App.personController.set('content', person);
You set up the controller differently depending on whether the Person is a 'top level' object in your app, or some intermediate object that gets selected. Also, you might only need one controller, you would only have a studentController and a personController if you were acting on a person and a student at the same time. Both are just ObjectControllers, and those can proxy anything.
Finally, in your view you would bind the relevant view element to the controller:
...
nameView: SC.LabelView.design({
layout: {/* props */},
valueBinding: SC.Binding.oneWay('App.personController.name')
})
...
note that the oneway binding is if the name is not going to be changed on the view, if the view can change the name, then just do a normal binding. Also note the path here. I am not binding to
'App.personController.content.name'
Since the personController proxies the object, you bind to the
'namespace.controller.property-on-object-controller-proxies'
If you are putting a lot of business logic in your controller, you are doing it wrong. Controllers should just be for proxying objects (at least ObjectControllers should be). Business logic should be on the models themselves, and decision making logic should be in statecharts.