Does anyone know if BindableBase is still a viable or should we stick with INotifyChanged events? It seems like BindableBase has lost its luster quickly. Thanks for any info you can provide.
INotifyPropertyChanged
The ViewModel should implement the INotifyPropertyChanged interface and should raise it whenever the propertychanges
public class MyViewModel : INotifyPropertyChanged
{
private string _firstName;
public event PropertyChangedEventHandler PropertyChanged;
public string FirstName
{
get { return _firstName; }
set
{
if (_firstName == value)
return;
_firstName = value;
PropertyChanged(this, new PropertyChangedEventArgs("FirstName"));
}
}
}
}
Problem is with ICommand interface as most of the code is duplicated also since it passes string it becomes error prone.
Whereas Bindablebase is an abstract class that implements INotifyPropertyChanged interface and provide SetProperty<T>.You can reduce the set method to just one line also ref parameter allows you to update its value. The BindableBase code below comes from INotifyPropertyChanged, The .NET 4.5 Way - Revisited
public class MyViewModel : BindableBase
{
private string _firstName;
private string _lastName;
public string FirstName
{
get { return _firstName; }
set { SetProperty(ref _firstName, value); }
}
}
//Inside Bindable Base
public abstract class BindableBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected bool SetProperty<T>(ref T storage, T value, [CallerMemberName] String propertyName = null)
{
if (Equals(storage, value))
{
return false;
}
storage = value;
this.OnPropertyChanged(propertyName);
return true;
}
protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChangedEventHandler eventHandler = this.PropertyChanged;
if (eventHandler != null)
{
eventHandler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
It is not a choice between these two.
BindableBase implements INotifyPropertyChanged.
So if you use BindableBase you will be using INotifyPropertyChanged.
INotifyPropertyChanged is more or less mandatory when implementing MVVM using DataBinding.
Whether to use BindableBase or an other implementation depends on preference and use of Prism.
To expand on Rohit's answer, if you are using .NET 4.6 you can take advantage of the Null-conditional operator and simplify the OnPropertyChanged method in the following way:
protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
INotifyPropertyChanged, The .NET 4.6 Way explains it in more detail.
Related
I'm binding an activity indicator to a property called IsLoading to show that the page is busy (e.g. processing API calls). I need to implement this in all my MAUI app pages, so my question, how can I re-use this code? The property is implemented as follows:
public event PropertyChangedEventHandler PropertyChanged;
private bool isLoading;
public bool IsLoading
{
get => isLoading;
set
{
isLoading = value;
OnPropertyChanged();
}
}
private void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
You probably want to put this in a base class that you inherit from. For example.
Create a class/file BaseViewModel.cs, of course the name can be whatever you want. It might look like this:
public class BaseViewModel
{
public event PropertyChangedEventHandler PropertyChanged;
private bool isLoading;
public bool IsLoading
{
get => isLoading;
set
{
isLoading = value;
OnPropertyChanged();
}
}
private void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
Now, whenever you create a new view model, you need to inherit from this. Let's say that you create a EditPersonViewModel, that would look like this:
public class EditPersonViewModel : BaseViewModel // This is where you inherit from the BaseViewModel
{
private bool isSaved;
public bool IsSaved
{
get => isSaved;
set
{
isSaved = value;
OnPropertyChanged();
}
}
}
Now you have access to all the things that are also in BaseViewModel. So you can set IsLoading to true or false, or you can implement a new property, IsSaved in my above example, and you can still call OnPropertyChanged to make the UI aware of the value change.
Here's what I have now:
class MyClass
{
public string status;
private void DoSomething()
{
// do something and make change to this.status;
}
}
class MyClass2
{
public List<MyClass> MyClassLst;
private void DetectChangeInList()
{
// if the status property of an item in this.MyClassLst changed, remove this item from list
}
}
I have a List<MyClass>, and each of the MyClass will do some work and change the property status. I want to detect if any of the MyClass has its status changed and remove this item from MyClassLst.
I read about something on event but not very clearly about how to make it work.
If you need to be notified about changes to individual properties of each MyClass instance, it's not something that can magically happen.
Your MyClass will have to be responsible for firing an event whenever something changes (usually the PropertyChanged event, i.e. the INotifyPropertyChanged interface), and the other class will have to attach a handler to each item in the list to get notified.
C#6 has a couple of syntactic improvements which simplify this a bit, but you still have lots of work to do for each property:
public class Model : INotifyPropertyChanged
{
// this is the event which gets fired
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
// you need to raise the event in each property's setter
private _someValue;
public string SomeValue
{
get { return _someValue; }
set { if (value != _someValue) { _someValue = value; OnPropertyChanged(); } }
}
private _anotherVal;
public string AnotherValue
{
get { return _anotherVal; }
set { if (value != _anotherVal) { _anotherVal = value; OnPropertyChanged(); } }
}
}
In your case, it would be:
public class MyClass : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
// Never use public fields!
// Status should be a property which gets and sets
// the actual private backing field (_status).
private _status;
public string Status
{
get { return _status; }
set { if (value != _status) { _status = value; OnPropertyChanged(); } }
}
}
You will also most likely want to change List<MyClass> into your own implementation of ICollection<T> which will attach and detach these handlers as you add or remove items. It is usually done by deriving from Collection<T> and overriding relevant methods. If you are not comfortable with that, a slightly simpler approach might be to make the list private and expose Add/Remove and similar methods where you will attach/detach to the PropertyChanged event.
What is the best way to bind a property to a control so that when the property value is changed, the control's bound property changes with it.
So if I have a property FirstName which I want to bind to a textbox's txtFirstName text value. So if I change FirstName to value "Stack" then the property txtFirstName.Text also changes to value "Stack".
I know this may sound a stupid question but I'll appreciate the help.
You must implement INotifyPropertyChanged And add binding to textbox.
I will provide C# code snippet. Hope it helps
class Sample : INotifyPropertyChanged
{
private string firstName;
public string FirstName
{
get { return firstName; }
set
{
firstName = value;
InvokePropertyChanged(new PropertyChangedEventArgs("FirstName"));
}
}
#region Implementation of INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
public void InvokePropertyChanged(PropertyChangedEventArgs e)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, e);
}
#endregion
}
Usage :
Sample sourceObject = new Sample();
textbox.DataBindings.Add("Text",sourceObject,"FirstName");
sourceObject.FirstName = "Stack";
A simplified version of the accepted answer that does NOT require you to type names of properties manually in every property setter like OnPropertyChanged("some-property-name"). Instead you just call OnPropertyChanged() without parameters:
You need .NET 4.5 minimum.
CallerMemberName is in the System.Runtime.CompilerServices namespace
public class Sample : INotifyPropertyChanged
{
private string _propString;
private int _propInt;
//======================================
// Actual implementation
//======================================
public event PropertyChangedEventHandler PropertyChanged;
[NotifyPropertyChangedInvocator]
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
//======================================
// END: actual implementation
//======================================
public string PropString
{
get { return _propString; }
set
{
// do not trigger change event if values are the same
if (Equals(value, _propString)) return;
_propString = value;
//===================
// Usage in the Source
//===================
OnPropertyChanged();
}
}
public int PropInt
{
get { return _propInt; }
set
{
// do not allow negative numbers, but always trigger a change event
_propInt = value < 0 ? 0 : value;
OnPropertyChanged();
}
}
}
Usage stays the same:
var source = new Sample();
textbox.DataBindings.Add("Text", source, "PropString");
source.PropString = "Some new string";
Hope this helps someone.
I am implementing a cart in Xamarin.Forms. In my cart page there is a ListView with data. Each of the cell contains a button to select the count of item and amount. In the cart view there is a grand total label.
My problem is the grand total is not updating while the number picker changes. The calculation method is called upon item adding view cell. I know that i need to implement INotifyProperty for this, but I'm unsure of how to do it.
I have a base view model which inherits INotifyProperty that contains an event.
public class BaseViewModel : INotifyPropertyChanged
{
private double _price;
public double Price
{
get
{
return _price;
}
set
{
_price = value;
OnPropertyChanged("Price");}
}
protected virtual void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
View model
public BaseViewModel()
{
App.Instance.ViewModel = this;
TempList = TempList ?? new ObservableCollection<cm_items>();
this.Title = AppResources.AppResource.Cart_menu_title;
this.Price = CartCell.price;
}
As a design methodology, its better to implement MVVM as a subclass and implement it to your ViewModel.
Sample Implementation:
public class ObservableProperty : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
var handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
I also strongly suggest implementing ICommand as a Dictionary structure like:
public abstract class ViewModelBase : ObservableProperty
{
public Dictionary<string,ICommand> Commands { get; protected set; }
public ViewModelBase()
{
Commands = new Dictionary<string,ICommand>();
}
}
So all todo in your ViewModel is just inherit the ViewModelBase class and use it
class LoginViewModel : ViewModelBase
{
#region fields
string userName;
string password;
#endregion
#region properties
public string UserName
{
get {return userName;}
set
{
userName = value;
OnPropertyChanged("UserName");
}
}
public string Password
{
get{return password;}
set
{
password = value;
OnPropertyChanged("Password");
}
}
#endregion
#region ctor
public LoginViewModel()
{
//Add Commands
Commands.Add("Login", new Command(CmdLogin));
}
#endregion
#region UI methods
private void CmdLogin()
{
// do your login jobs here
}
#endregion
}
Finally: Xaml Usage:
<Entry Placeholder="Username" Text="{Binding UserName}"/>
<Entry Placeholder="Password" Text="{Binding Password}" IsPassword="True"/>
<Button Text="Login" Command="{Binding Commands[Login]}"/>
For example try this view model:
public abstract class BaseViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected bool SetPropertyValue<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
{
if (value == null ? field != null : !value.Equals(field))
{
field = value;
var handler = this.PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
return true;
}
return false;
}
}
and in inherited classes use it like this:
private int myProperty;
public int MyProperty
{
get { return this.myProperty; }
set { this.SetPropertyValue(ref this.myProperty, value); }
}
When I started Xamarin coding, the MVVM was a bit confusing until I discovered that the PropertyChangedEvent on the ViewModel fired off a signal to the View (ContentPage), and updated the Label/textbox/etc.
For those looking for the 'latest and greatest'... Here's some revised code:
private void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
and on your property Setter:
public string SomeProperty
{
get { return _somProperty; }
set
{
_someProperty= value;
OnPropertyChanged();
}
}
}
Nice? No? Saves having to pass the property name each time!
I use NotifyPropertyWeaverMsBuildTask to handle NotifyPropertyChanged for automatic properties. I know OnPropertyChanged() method rise when Property value is changed. But when this method is called value of property is changed and old value is lost. Is there any way to get old value?
tanx.
If you want to use the old value inside the OnPropertyChanged then write it like this
public void OnPropertyChanged(string propertyName, object before, object after)
Then if your code looks like this
public class Person : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public string Name { get; set; }
public void OnPropertyChanged(string propertyName, object before, object after)
{
// do something with before/after
var propertyChanged = PropertyChanged;
if (propertyChanged != null)
{
propertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
This will be injected
public class Person : INotifyPropertyChanged
{
private string name;
public event PropertyChangedEventHandler PropertyChanged;
public string Name
{
get { return name; }
set
{
object before = Name;
name = value;
OnPropertyChanged("Name", before, Name);
}
}
public void OnPropertyChanged(string propertyName, object before, object after)
{
// do something with before/after
var propertyChanged = PropertyChanged;
if (propertyChanged != null)
{
propertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
More information is available here https://github.com/SimonCropp/NotifyPropertyWeaver/wiki/BeforeAfter
Does this meet your requirements?
INotifyPropertyChanged doesn't provide a means to capture the previous value of a property; you'd have to implement your own.