I have WPF application and my work method play my files in different threads
This is my Global variable that update my UI:
public static int _totalFilesSent;
Now because i am implement INotifyPropertyChanged in my model i have also this:
public static int TotalFilesSent
{
get { return _totalFilesSent; }
set
{
_totalFilesSent = value;
OnStaticlPropertyChanged("TotalFilesSent");
}
}
(i didn't add the event function because this is not relevant here).
So every time i am update my Global variable this way:
Interlocked.Increment(ref _totalFilesSent );
Now because i need to update my UI with my INotifyPropertyChanged event i need to use TotalFilesSent instead of _totalFilesSent but in this way i got this compilation error:
A property, indexer or dynamic member access may not be passed as an
out or ref parameter.
What does it mean and how can i solved it ?
You may easily raise the StaticPropertyChanged event after calling Interlocked.Increment:
private static int _totalFilesSent;
public static int TotalFilesSent
{
get { return _totalFilesSent; }
}
public static void IncrementTotalFilesSent()
{
Interlocked.Increment(ref _totalFilesSent);
OnStaticPropertyChanged("TotalFilesSent");
}
Related
Static method should not contain a state. What does 'state' means here ?
I have read that static method do not need to be instantiated, and do not use instance variables. So when can I use static methods? I have read that static methods are bad? Should I include it when coding?
State means storing some information, static methods are loaded when a class is loaded so there is no need of instance to call the static methods, you can call this methods using name of class, it's depend on condition when to use static methods. you can use static methods as single component of product just pass your parameters and get your work done.
As an answer here's an example:
public class SomeUtilityClass {
private static boolean state = false;
public static void callMeTwiceImBad() throws Exception {
if (state) {
throw new Exception("I remember my state from previous call!");
}
state = true;
}
public static int sum(int a, int b) {
return a + b;
}
}
By themselves they are neither bad nor good, they are just static.
I have the following Global class file:
Global.cs
public static class Global
{
private static string _globalVar = "";
public static string GlobalVar
{
get { return _globalVar; }
set { _globalVar = value; }
}
}
I set the new value of string GlobarVar in Form1.cs as '1234'.
Form1.cs
public Form1()
{
InitializeComponent();
Global.GlobalVar = "1234";
}
I tried to display the value to Form2.cs using the message box
public Form2()
{
InitializeComponent();
MessageBox.Show(Global.GlobalVar); // displays blank values
}
Am I missing something?
Four options:
You're not constructing Form1 before you construct Form2
Something else is setting Global.GlobalVar back to null or an empty string
Your forms are in different app domains, so they'll have entirely separate Global types
You're running the application twice; static variables don't live on across different processes
It's hard to tell which of these is the case, but personally I'd try to avoid using global state to start with. It's a pain for testability and reasoning about how your program works.
Try your property page (file Global.cs) like these:
public class Global
{
private static string _globalVar;
public string GlobalVar
{
get { return _globalVar; }
set { _globalVar = value; }
}
}
Class C implements INotifyPropertyChanged.
Assume the C has Length, Width and Area propreties, where Area = Length * Width. A change in either might cause a change in area. All three are bound, i.e. the UI expects all three to notify of changes in their values.
When either Length or Width change, their setters call NotifyPropertyChanged.
How should I treat the calculated Area property? Currently the pattern I can think of is detecting in NotifyPropertyChanged whether the changed property is either Length or Width and, if such is the case, initiate an addional PropertyChanged notification for Area. This, however, requires that I maintain inside NotifyPropertyChanged the dependencies graph, which I feel is an anti-pattern.
So, my question is: How should I code dependency properties that depend on other dependency properties?
edit: People here suggested that Length and Width also call NotifyPropertyChanged for Area. Again, I think this is an anti-pattern. A property (IMHO) shouldn't be aware of who depends on it, as shouldn't NotifyPropertyChanged. Only the property should be aware of who it depends on.
This issue kept on bugging me, so I re-opened it.
First, I'd like to appologize for anyone taking my "anti-pattern" comment personally. The solutions offered here were, indeed, how-it's-done in WPF. However, still, IMHO they're bad practices caused, deficiencies in ther framework.
My claim is that the information hiding guide dictates that when B depeneds on A, A should not be aware of B. For exmaple, when B derives from A, A should not have code saying: "If my runtime type is really a B, then do this and that". Simiarily, when B uses A, A should not have code saying: "If the object calling this method is a B, then ..."
So it follows that if property B depends on property A, A shouldn't be the one who's responsible to alert B directly.
Conversely, maintaining (as I currently do) the dependencies graph inside NotifyPropertyChanged is also an anti-pattern. That method should be lightweight and do what it name states, not maintain dependency relationships between properties.
So, I think the solution needed is through aspect oriented programming: Peroperty B should use an "I-depend-on(Property A)" attribute, and some code-rewriter should create the dependency graph and modify NotifyPropertyChanged transparently.
Today, I'm a single programmer working on a single product, so I can't justify dvelving with this any more, but this, I feel, is the correct solution.
Here is an article describing how to create a custom attribute that automatically calls PropertyChanged for properties depending on another property: http://www.redmountainsw.com/wordpress/2012/01/17/a-nicer-way-to-handle-dependent-values-on-propertychanged/
The code will look like this:
[DependsOn("A")]
[DependsOn("B")]
public int Total
{
get { return A + B; }
}
public int A
{
get { return m_A; }
set { m_A = value; RaisePropertyChanged("A"); }
}
public int B
{
get { return m_B: }
set { m_B = value; RaisePropertyChanged("B"); }
}
I haven't tried it myself but I like the idea
When the Length or Width properties are changed you fire PropertyChanged for Area in addition to firing it for either Length or Width.
Here is a very simple implementation based on backing fields and the method OnPropertyChanged to fire the PropertyChanged event:
public Double Length {
get { return this.length; }
set {
this.length = value;
OnPropertyChanged("Length");
OnPropertyChanged("Area");
}
}
public Double Width {
get { return this.width; }
set {
this.width = value;
OnPropertyChanged("Width");
OnPropertyChanged("Area");
}
}
public Double Area {
get { return this.length*this.width; }
}
Doing it like this is certainly not an anti-pattern. That is exactly the pattern for doing it. You as the implementer of the class knows that when Length is changed then Area is also changed and you encode it by raising the appropriate event.
Then you should raise twice, in Length and Width property setters. One for the actual property and one for the Area property.
for example:
private int _width;
public int Width
{
get { return _width; }
set
{
if (_width == value) return;
_width = value;
NotifyPropertyChanged("Width");
NotifyPropertyChanged("Area");
}
}
People here suggested that Length and Width also call
NotifyPropertyChanged for Area. Again, I think this is an
anti-pattern. A property (IMHO) shouldn't be aware of who depends on
it, as shouldn't NotifyPropertyChanged. Only the property should be
aware of who it depends on.
This is not an anti-pattern. Actually, your data encapsulated inside this class, so this class knows when and what changed. You shouldn't know outside of this class that Area depends on Width and Length. So the most logical place to notify listeners about Area is the Width and Length setter.
A property (IMHO) shouldn't be aware of who depends on it, as
shouldn't NotifyPropertyChanged.
It does not break encapsulation, because you are in the same class, in the same data structure.
An extra information is that knockout.js (a javascript mvvm library) has a concept which accessing this problem: Computed Observables. So I believe this is absolutely acceptable.
Here is a possible implementation of an attribute:
public class DependentPropertiesAttribute : Attribute
{
private readonly string[] properties;
public DependentPropertiesAttribute(params string[] dp)
{
properties = dp;
}
public string[] Properties
{
get
{
return properties;
}
}
}
Then in the Base View Model, we handle the mechanism of calling property dependencies:
public class ViewModelBase : INotifyPropertyChanged
{
public ViewModelBase()
{
DetectPropertiesDependencies();
}
private readonly Dictionary<string, List<string>> _dependencies = new Dictionary<string, List<string>>();
private void DetectPropertiesDependencies()
{
var propertyInfoWithDependencies = GetType().GetProperties().Where(
prop => Attribute.IsDefined(prop, typeof(DependentPropertiesAttribute))).ToArray();
foreach (PropertyInfo propertyInfo in propertyInfoWithDependencies)
{
var ca = propertyInfo.GetCustomAttributes(false).OfType<DependentPropertiesAttribute>().Single();
if (ca.Properties != null)
{
foreach (string prop in ca.Properties)
{
if (!_dependencies.ContainsKey(prop))
{
_dependencies.Add(prop, new List<string>());
}
_dependencies[prop].Add(propertyInfo.Name);
}
}
}
}
protected void OnPropertyChanged(params Expression<Func<object>>[] expressions)
{
expressions.Select(expr => ReflectionHelper.GetPropertyName(expr)).ToList().ForEach(p => {
RaisePropertyChanged(p);
RaiseDependentProperties(p, new List<string>() { p });
});
}
public event PropertyChangedEventHandler PropertyChanged = delegate { };
protected virtual void RaisePropertyChanged(string propertyName)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
protected void RaiseDependentProperties(string propertyName, List<string> calledProperties = null)
{
if (!_dependencies.Any() || !_dependencies.ContainsKey(propertyName))
return;
if (calledProperties == null)
calledProperties = new List<string>();
List<string> dependentProperties = _dependencies[propertyName];
foreach (var dependentProperty in dependentProperties)
{
if (!calledProperties.Contains(dependentProperty))
{
RaisePropertyChanged(dependentProperty);
RaiseDependentProperties(dependentProperty, calledProperties);
}
}
}
}
Finally we define dependencies in our ViewModel
[DependentProperties("Prop1", "Prop2")]
public bool SomeCalculatedProperty
{
get
{
return Prop1 + Prop2;
}
}
Whats the best way to save variables like userid that is stored and reachable from different pages in WP7.
There's the querystring method, but can be kind of a pain to implement.
When navigating, pass the parameter like a HTTP querystring.
Then, on the otherside, check if the key exists, and extract the value. The downside of this is if you need to do more than 1, you need to type it in yourself, and it only supports strings.
So to pass an integer, you'd need to convert it. (And to pass a complex object, you need to take all the pieces you need to recompile it on the other side)
NavigationService.Navigate(new Uri("/PanoramaPage1.xaml?selected=item2", UriKind.Relative));
protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
string selected = String.Empty;
//check to see if the selected parameter was passed.
if (NavigationContext.QueryString.ContainsKey("selected"))
{
//get the selected parameter off the query string from MainPage.
selected = NavigationContext.QueryString["selected"];
}
//did the querystring indicate we should go to item2 instead of item1?
if (selected == "item2")
{
//item2 is the second item, but 0 indexed.
myPanorama.DefaultItem = myPanorama.Items[1];
}
base.OnNavigatedTo(e);
}
Here's a sample app that uses a querystring.
http://dl.dropbox.com/u/129101/Panorama_querystring.zip
A easier (and better) idea is to define a variable globally, or use a static class. In App.xaml.cs, define
using System.Collections.Generic;
public static Dictionary<string,object> PageContext = new Dictionary<string,object>;
Then, on the first page, simply do
MyComplexObject obj;
int four = 4;
...
App.PageContext.Add("mycomplexobj",obj);
App.PageContext.Add("four",four);
Then, on the new page, simply do
MyComplexObj obj = App.PageContext["mycomplexobj"] as MyComplexObj;
int four = (int)App.PageContext["four"];
To be safe, you should probably check if the object exists:
if (App.PageContext.ContainsKey("four"))
int four = (int)App.PageContext["four"];
You may use an App level variable (defined in App.xaml.cs) and access it from anywhere within your app. If you want to persist, shove it into Isolated Storage and read it on App launch/activate. There are helpers available to JSon serialize/deserialize your reads/writes from the Isolated Storage.
Check out Jeff's post (here) on tips to use Isolated Storage.
Hope this helps!
Well "best" is always subjective, however, I think an application service is a good candidate for this sort of thing:-
public interface IPhoneApplicationService : IApplicationService
{
string Name {get; set;}
object Deactivating();
void Activating(object state);
}
public class AuthenticationService : IPhoneApplicationService
{
public static AuthenticationService Current {get; private set; }
public void StartService(ApplicationServiceContext context)
{
Current = this;
}
public void StopService()
{
Current = null;
}
public string Name {get; set;}
public object Deactivating()
{
// Return an serialisable object such as a Dictionary if necessary.
return UserID;
}
public void Activating(object state)
{
UserID = (int)state;
}
public int UserID { get; private set; }
public void Logon(string username, string password)
{
// Code here that eventually assigns to UserID.
}
}
You place an instance of this in your App.xaml:-
<Application.ApplicationLifetimeObjects>
<!--Required object that handles lifetime events for the application-->
<shell:PhoneApplicationService
Launching="Application_Launching" Closing="Application_Closing"
Activated="Application_Activated" Deactivated="Application_Deactivated"/>
<local:AuthenticationService Name="AuthServ" />
</Application.ApplicationLifetimeObjects>
Now you do need to tweak the App.xaml.cs:-
private void Application_Activated(object sender, ActivatedEventArgs e)
{
var state = PhoneApplicationService.Current.State;
foreach (var service in ApplicationLifetimeObjects.OfType<IPhoneApplicationService>())
{
if (state.ContainsKey(service.Name))
{
service.Activating(state[service.Name]);
}
}
}
private void Application_Deactivated(object sender, DeactivatedEventArgs e)
{
var state = PhoneApplicationService.Current.State;
foreach (var service in ApplicationLifetimeObjects.OfType<IPhoneApplicationService>())
{
if (state.ContainsKey(service.Name))
{
state[service.Name] = service.Deactivating();
}
else
{
state.Add(service.Name, service.Deactivating());
}
}
}
You can now access you UserID anywhere in your app with:-
AuthenticationService.Current.UserID
This general pattern can be used to maintain seperation of key application wide services (you don't load a whole bunch of incohesive properties into your App class). It also provides the hooks for maintaining state between activations which is essential.
I have a method Translate extension which searches for a translation. Normally translations are loaded in Window constructor (I tried in App.Setup too). No if i run the application all the translations are displayed correctly, but when opening a user control all translations are gone.
So the question is where do I put my initialization code so it would be executed before VS initializes design window
it should be default constructor
Either the class constructor (or code called from it) or some static member initialized by a static constructor.
Option 1:
public partial class MyUserControl : UserControl
{
int thisWillWork = 1;
int thisWillAlsoWork;
public MyUserControl()
{
thisWillAlsoWork = 1;
InitializeComponents();
}
Option 2:
public class SomeOtherClass
{
public static int YouCanUseThis = 1;
public static int AndThisAlso;
static SomeOtherClass()
{
AndThisAlso = 1;
}
}