I've been trying to find the correct approach for this problem I got, using Prism with Xamarin Forms:
I've a model class, Customer, that contains another class, Address as a property. In my view, I show fields from both objects. I would like to have a "save" button, that only gets enabled after you've made some changes to those models.
Now, the button is bound to a Command, with the corresponding CanSave() function, as is normal with DelegateCommands. I'm trying to find an approach where I can end up with a single IsDirty property on my view model, that gets to "true" after any changed to the underlying models.
The MVVM approach
First thing I thought was the "purist" mvvm approach. A "flat" view model, with properties for each visual element, implemented as a Prism BindableObject, where each getter/setter gets/sets values from/to the underlying model classes.
That failed though, since SetProperty<> has a ref parameter, where I can't use properties from my models.
The over-engineered approach [?]
Second thing I thought was that, if my inner models were observables themselves, I could listen for changes from all of them, throughout the tree. Which opens up a whole new world of issues. Do I register property change listeners in my View model ? Do I make inner models observables, and have the parents listen for change events on their children and propagate that ?
Won't that observable models approach quickly become event handler hell ?
The simplest thing
And last, the simplest thing possible. I have a flat observable ViewModel, that only reads/writes values to/from the actual inner hierarchical model upon read & save
What do you guys think ?
Maybe I didn't understand your question right, but I'm wondering why you limit yourself to such a small helper function like SetProperty. It has 4 Lines of code. All it does is checking for equality, setting a value and raising an event.
You could easily create another helper function like this.
MyBindableBase
protected virtual bool SetProperty<T>(Func<T> get, Action<T> set, T value, [CallerMemberName] string propertyName = null)
{
if (object.Equals(get(), value)) return false;
set(value);
OnPropertyChanged(propertyName);
return true;
}
Model
class Model
{
public string Property { get; set; }
}
ViewModel
class ViewModel : BindableBase
{
private Model Model { get; set; }
public string Property
{
get { return Model.Property; }
set { SetProperty(() => Model.Property, x => Model.Property = x, value); }
}
}
I think you can shorten the usage, if you introduce some naming rules for the mapping and/or use reflections.
Well, in the end I went for option 3, the simplest thing I could do.
I was leaning towards returning properties from my model, which would be easy, and using the nullable [?.] syntax it would be null-safe too, but I found that at times I'll have to wrap the actual model properties with something that is more UI-friendly, exposing more/different properties than my actual DB model classes.
So, I went for that, until some other complexity forces me to change my mind again :)
Thanks a lot #Sven-Michael Stübe and #adminSoftDK for the help
Related
I'm pushing myself to make the applications I write simpler, and I've taken some steps to do that, but I'm left with an interesting problem that doesn't at all feel like it would be unique to me. I'm wondering what I'm doing wrong.
I have a ViewModel that keeps a collection of model objects. The view is a ListView that displays all of the objects in the collection. The model objects have all the logic in them to manipulate them. Inside the ListView row for each item I have a button, and that button needs to be wired to call a method on the model object.
To get this to work I need to add a command binding, but to the parent window data context, that passes a parameter of the model object in the row, all so that model object can be used inside the ViewModel (the parent window data context) to call the method on the model object that's being passed in.
This seems really much more complex than it needs to be. I'm willing to throw out anything I've done already, there are no sacred cows, I just want this to be done in a simpler method that will be easy to look back on in a year and figure out what I was doing.
{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}},
Path=DataContext.MyCommand}
Create a presenter class in your ViewModel for the model objects and have a collection of those. You can then put the ICommand property on those instead and pass a reference to the method you want to call in the parent datacontext.
Perhaps something like the following:
public class ModelPresenter : INotifyPropertyChanged
{
private Model _model;
public ModelPresenter(Model model, Action<Model> parentAction)
{
_model = model
_action = parentAction;
}
public ICommand MyAction
{
get { return new RelayCommand(() => _parentAction(_model)); }
}
...
}
It also sounds like you might be binding to Properties of your model your view. You shouldn't do this as it can cause a memory leak if your models aren't implementing INotifyPropertyChanged (see: http://support.microsoft.com/kb/938416/en-us).
I am finding it difficult understanding how best to implement 'IView' interface properties which are not simple types, and was wondering how others approach this in a Model View Presenter application.
The articles i've read are really good but none of them seem to approach more complex Views where you have List<> properties which are of an interface type which represent a class in your domain model, i.e. IPerson, or IName etc.
I will try to outline a scenario as briefly as i possibly can.
Presume i have a requirement for a View to ultimately persist a list of names, each consisting of 3 properties 'Forename', 'Surname', and 'Title'.
Typically i will have a domain model with a class called 'Name' with the 3 properties. This domain model will implement an Interface (in a separate 'Interfaces' class Library) called 'IName'.
Now in the 'Views' namespace in my 'Interaces' library i have an interface called 'IViewNames'. This is the view interface which any view which wants to ultimately persist the list of names will implement.
How to define this 'IViewNames' interface puzzles me. If i give it a property like so:
public List<IName> Names {get;set;}
then my implementing concrete view will ultimately have a complex property 'Names' which will need a 'getter' which loops through the fields on the View, somehow instantiate an instance of 'IName', set its properties, add to a List, before returning the List. The 'setter' will be just as complex, receiving a list of 'INames' and iterating through them setting fields on the View.
I feel like this is breaking one of the major goals of the MVP approach, which is being able to thoroughly test the application code without any concrete View implemntations. After all, i could easily write a presenter which looks at the 'View.Names' property and sends it on to a Service Layer, or set the 'View.Names' property when receiving a list of 'Name' objects back from the Service Layer. I could easily write a lot of tests which ensure everything works, everything except from that COMPLEX property in the View.
So my question is, how do others approach IView properties which are not simple types, but are in fact types of your domain model? (well types of interfaces which represent your domain model, as i clearly dont want a reference from my Presentation Layer to my Domain Model layer).
I'm more than certain there is a known technique to achieving this in an elegant way, which adheres to the Model View Presenter goals, more than my example approach does.
Thanks in advance for any help people.
I have not worked much on the MVP design pattern but will surely try my hands on it.
Approach1 : DataBinding
In this case you can also create individual properties in IView and bind these properties in presenter to the model properties. This way, your view will not get complicated. The experience is fast and seamless as the values from UI can be directly used in model. Changing the property value in model will reflect in UI immedietly. You may have to use NotifyPropertyChange events for this.
Approach 2 : Complex Types
You can try creating List or Tuples to store these values and use the values in the presenter. You may have to use events or actions to reflect the value from model to view and vice versa.
Please let me know if it helped you. Thanks.
I have lifted this explanation from one of the articles I am writing on my website
Presenter to View Communication
There are two styles utilised for populating the View with data from the Presenter and Model that I have used. The only difference between them is how tightly coupled you mind your View being to the Model. For the example of this, we will have the following as our Model:
public class Person
{
public int ID { get; private set; }
public int Age { get; set; }
public String FirstName { get; set; }
public String LastName { get; set; }
Public Genders Gender { get; set; }
}
Method 1: Using the Model
Now our View code:
public interface IEmployeesView
{
void ClearList();
void PopulateList(IEnumerable<Person> people);
}
And finally the Presenter:
public class IEmployeesPresenter
{
public void Display()
{
_view.ClearList();
_view.PopulateList(_model.AllEmployees);
}
}
This method of population produces a link between the Model and the View; the Person object used as a parameter in PopulateList.
The advantage of this is that the concrete implementation of the IEmployeesView can decide on what to display in its list of people, picking from any or all of the properties on the Person.
Their are two disadvantages of this method. The first is that there is nothing stopping the View from calling methods on the Person, which makes it easy for lazy code to slip in. The second is that if the model were to change from a List<Person> to a List<Dog> for instance, not only would the Model and the Presenter need to change, but so the View would too.
Method 2: Using Generic Types
The other method population relies on using Tuple<...>, KeyValuePair<,> and custom classes and structs:
Now our View code:
public interface IEmployeesView
{
void ClearList();
void PopulateList(IEnumerable<Tuple<int, String> names);
}
And finally the Presenter:
public class IEmployeesPresenter
{
public void Display()
{
var names = _model.AllEmployees.Select(x => new Tuple<int, String>(x.ID, x.FirstName + " " + x.LastName));
_view.ClearList();
_view.PopulateList(names);
}
}
The advantages of this method of population is that the Model is free to change without needing to update the View, and the View has no decisions to make on what to display. It also prevents the View from calling any extra methods on the Person, as it does not have a reference to it.
The down sides to this method, are that you loose strong typing, and discoverability - It is quite obvious what a Person is but what a Tuple<int, String> is less obvious.
I have aggregated models like Customer:Order:Product.
As my View is bound to the BillingViewModel which has a Property Customers of type ObservableCollection
and ONE customer in this collection has a "list" of orders
named ObservableCollection
and ONE order in this collection has a "list" of products
named ObservableCollection
Well I need the ObservableCollection`s for databinding but should a domain model really have a ObservableCollection ? normally it has a
List or IEnumerable !
Is this bad habit or having side effects?
I append an explanation to the above what is right:
class Customer
{
int CustomerID {get;set;}
ObservableCollection<Order> { get;set;}
}
class BillingViewModel
{
ObservableCollection<Customer> _customers;
public BillingViewModel()
{
Customers= GetAggregatedCustomersOrdersProductsFromRepository();
}
public ObservableCollection<Customer> Customers
{
get{ return _customers;}
set
{
_customers = value;
this.RaisePropertyChanged("Customers");
}
}
}
I hope its more clear now! I have ObservableCollection in my ViewModel and Model!
From the examples I have read it appears that one practices is to take your Domain model Customer:Order:Product and rearrange it into MainViewModel:CustomerViewModel:OrderViewModel:ProductViewModel when it reaches the client side. This would allow you to mark any of the VMs dirty and save only when needed. It would also allow you to compose your View of many Views each driven by their own VM, so if later you decided to change the View from one large Screen into many Modals it would be fairly seamless. The reason for the MainViewModel, is more of a Controller then a ViewModel, its duty would be to get the Domain Model and break it apart into the VMs and also could be the Controller for how your Views will be displayed (Grouped or Modal), it could also contain Commands such as SaveAllDirty.
It depends on if those properties need the built in change notification. If you have some kind of logic that depends on doing something when those change state, then it's fine. If it's only there to support data binding and that class is not itself a ViewModel, then I think it's bad form.
This questions doesn't let me sleep as it's since one year I'm trying to find a solution but... still nothing happened in my mind. Probably you can help me, because I think this is a very common issue.
I've a n-layered application: presentation layer, business logic layer, model layer. Suppose for simplicity that my application contains, in the presentation layer, a form that allows a user to search for a customer. Now the user fills the filters through the UI and clicks a button. Something happens and the request arrives to presentation layer to a method like CustomerSearch(CustomerFilter myFilter). This business logic layer now keeps it simple: creates a query on the model and gets back results.
Now the question: how do you face the problem of loading data? I mean business logic layer doesn't know that that particular method will be invoked just by that form. So I think that it doesn't know if the requesting form needs just the Customer objects back or the Customer objects with the linked Order entities.
I try to explain better:
our form just wants to list Customers searching by surname. It has nothing to do with orders. So the business logic query will be something like:
(from c in ctx.CustomerSet
where c.Name.Contains(strQry) select c).ToList();
now this is working correctly. Two days later your boss asks you to add a form that let you search for customers like the other and you need to show the total count of orders created by each customer. Now I'd like to reuse that query and add the piece of logic that attach (includes) orders and gets back that.
How would you front this request?
Here is the best (I think) idea I had since now. I'd like to hear from you:
my CustomerSearch method in BLL doesn't create the query directly but passes through private extension methods that compose the ObjectQuery like:
private ObjectQuery<Customer> SearchCustomers(this ObjectQuery<Customer> qry, CustomerFilter myFilter)
and
private ObjectQuery<Customer> IncludeOrders(this ObjectQuery<Customer> qry)
but this doesn't convince me as it seems too complex.
Thanks,
Marco
Consider moving to DTO's for the interface between the presentation layer and the business layer, see for example:- http://msdn.microsoft.com/en-us/magazine/ee236638.aspx
Something like Automapper can relieve much of the pain associated with moving to DTOs and the move will make explicit what you can and cannot do with the results of a query, i.e. if it's on the DTO it's loaded, if it's not you need a different DTO.
Your current plan sounds a rather too tightly coupled between presentation layer and data layer.
I would agree with the comment from Hightechrider in reference to using DTOs, however you have a valid question with regard to business entities.
One possible solution (I'm using something along these lines on a project I'm developing) is to use DTOs that are read-only (at least from the presentation layer perspective. Your query/get operations would only return DTOs, this would give you the lazy loading capability.
You could setup your business layer to return an Editable object that wraps the DTO when an object/entity is updated/created. Your editable object could enforce any business rules and then when it was saved/passed to the business layer the DTO it wrapped (with the updated values) could be passed to the data layer.
public class Editable
{
//.......initialize this, other properties/methods....
public bool CanEdit<TRet>(Expression<Func<Dto, TRet>> property)
{
//do something to determine can edit
return true;
}
public bool Update<TRet>(Expression<Func<Dto, TRet>> property, TRet updatedValue)
{
if (CanEdit(property))
{
//set the value on the property of the DTO (somehow)
return true;
}
return false;
}
public Dto ValueOf { get; private set;}
}
This gives you the ability to enforce if the user can get editable objects from the business layer as well as allowing the business object to enforce if the user has permission to edit specific properties of an object. A common problem I run into with the domain I work in is that some users can edit all of the properties and others can not, while anyone can view the values of the properties. Additionally the presentation layer gains the ability to determine what to expose as editable to the user as dictated and enforced by the business layer.
Other thought I had is can't your Business Layer expose IQueryable or take standard expressions as arguments that you pass to your data layer. For example I have a page building query something like this:
public class PageData
{
public int PageNum;
public int TotalNumberPages;
public IEnumerable<Dto> DataSet;
}
public class BL
{
public PageData GetPagedData(int pageNum, int itemsPerPage, Expression<Func<Dto, bool>> whereClause)
{
var dataCt = dataContext.Dtos.Where(whereClause).Count();
var dataSet = dataContext.Dtos.Where(whereClause).Skip(pageNum * itemsPerPage).Take(itemsPerPage);
var ret = new PageData
{
//init this
};
return ret;
}
}
When setting a property on an entity object, it is saving the value to the database even if the value is exactly the same as it was before. Is there anyway to prevent this?
Example:
If I load a Movie object and the Title is "A", if I set the Title to "A" again and SaveChanges() I was hoping that I wouldn't see the UPDATE statement in SqlProfiler but I am. Is there anyway to stop this?
Yes, you can change this. Doing so isn't trivial, however, in the current version of the Entity Framework. It will become easier in the future.
The reason you're seeing this behavior is because of the default code generation for the entity model. Here is a representative example:
public global::System.Guid Id
{
get
{
return this._Id;
}
set
{
// always!
this.OnIdChanging(value);
this.ReportPropertyChanging("Id");
this._Id = global::System.Data.Objects.DataClasses
.StructuralObject.SetValidValue(value);
this.ReportPropertyChanged("Id");
this.OnIdChanged();
}
}
private global::System.Guid _Id;
partial void OnIdChanging(global::System.Guid value);
partial void OnIdChanged();
This default code generation is reasonable, because the Entity Framework doesn't know the semantics of how you intend to use the values. The types in the property may or may not be comparable, and even if they are, the framework can't know how you intend to use reference equality versus value equality in all cases. For certain value types like decimal, it's pretty clear, but in a general sense it's not obvious.
You, on the other hand, know your code, and can customize this some. The trouble is that this is generated code, so you can't just go in and edit it. You need to either take over the code generation, or make it unnecessary. So let's look at the three options.
Take over the code generation
The essential approach here is to create a T4 template which does the code behind, and that the default code generation from the Entity Framework. Here is one example. One advantage of this approach is that the Entity Framework will be moving to T4 generation in the next version, so your template will probably work well in future versions.
Eliminate code generation
The second approach would be to eliminate cogeneration altogether, and do your change tracking support manually, via IPOCO. Instead of changing how the code is generated, with this approach you don't do any code generation at all, and instead provide change tracking support to the Entity Framework by implementing several interfaces. See the linked post for more detail.
Wait
Another option is to live with the Entity Framework the way it is for the time being, and wait until the next release to get the behavior you desire. The next version of the Entity Framework will use T4 by default, so customizing the code generation will be very easy.
According to MSDN:
The state of an object is changed from
Unchanged to Modified whenever a
property setter is called. This occurs
even when the value being set is the
same as the current value. After the
AcceptAllChanges method is called, the
state is returned to Unchanged. By
default, AcceptAllChanges is called
during the SaveChanges operation.
Looks like you'll want to check the value of properties on your Entity objects before you update to prevent the UPDATE statement.
At a generic level, if your entities are implementing INotifyPropertyChanged, you don't want the PropertyChanged event firing if the value is the same. So each property looks like this :-
public decimal Value
{
get
{
return _value;
}
set
{
if (_value != value)
{
_value = value;
if (_propertyChanged != null) _propertyChanged(this, new PropertyChangedEventArgs("Value"));
}
}
}
Hope that's relevant to Entity Framework.
One thing you can do is just wrap the property yourself using a partial class file, and then use your property instead of the first one:
public sealed partial class MyEFType {
public string MyWrappedProperty {
get {
return MyProperty;
}
set {
if (value == MyProperty)
return;
MyProperty = value;
}
}
}
It wouldn't be very practical to do this to every property, but if you have a need to detect that a particular property has actually changed and not just been written to, something like this could work.