The problem I'm having is while using Linq2Sql with inheritance after declaring a new instance of the inherited class the discriminator property is still set to its initial value, not the correct value for the sub-type. It gets the correct value after attaching it to a context and calling SubmitChanges(). There are times where I want to declare a new object of the inherited type and call methods on the base class with the base class knowing inherited type it is working with and the most logical choice would be to use the discriminator property.
Is there a way to force the setting of the discriminator property? I don't want to go to all my sub-classes and implement the OnCreated() partial method for something the context already knows how to do.
I did come up with a slightly better workaround than putting code in the OnCreated() method of each inheriting class and figured I'd leave it here in case anyone stumbles here.
In the OnCreated() of the base class I added code that looked similar to this:
partial void OnCreated()
{
if (this is BaseClass1)
{
this.[DiscriminatorProperty] = DiscriminatorValueForBaseClass1;
}
else if(this is BaseClass2)
{
this.[DiscriminatorProperty] = DiscriminatorValueForBaseClass2;
}
}
It is still duplicating the functionality that the context already knows how to do but at least I'm not implementing the OnCreated() in every base class. I also don't like the fact that if a new class is added or a discriminator value changes you have to update it in the DBML and in the OnCreated(). For this reason I'd still like a way for the context to assign the value, in fact it should be doing this when the inherited class is created.
Related
I would like to check if a mixin has been applied to a custom element, but I don't think I can use 'instanceof', since a mixin is not properly a base class (I tried, of course).
I would need to enforce that an element added to a collection can be only of a kind with a particular class mixin applied...
Any suggestions?
Not sure I understand you question correctly.
I assume you want to check something like MyCustomElement has already apply MyMixin or not?
You can check from the instance
let instance = new MyCustomElement()
console.log(instance instanceof MyMixin)
This will only work when MyMixin is a class not a factory function. If you follow documentation you need to change it.
Another way, you can declare some static function in MyMixin. Then you can call from MyCustomElement to check it.
I have this:
public class Base {
public function whoAmI() {
trace("base");
}
}
public class Extended extends Base {
public function Extended() {
this.whoAmI() //prints extended
super.whoAmI() //prints base
var test = super;
test.whoAmI() //prints extended
}
public override function whoAmI() {
trace("extended");
}
}
The problem is when I do var test = super, it seems like this is assigned to test instead of super.
Is it possible to do the assignment so that test.whoAmI() prints "base"?
Edit: In the comments it is being said that using super in the way I propose would break overriding. I don't think that's the case. The way I am thinking of it, super could be used the same way as this. I understand that is not the way super is implemented, but using it that way would not break overriding as people are claiming. So for example the same way this is possible:
var test = this;
test.whoAmI();
This should be possible:
var test = super;
super.whoAmI();
It is obviously the choice of the language implementer to not do things this way, and I don't understand the reason why. It doesn't break things, but I guess it does make them more complicated.
I am not suggesting type-casting this to the super class. Obviously that wouldn't work.
You are thinking of "this" and "super" as 2 different instances, 2 different things but they in fact point to the same object (obviously) so at the end it's always "this". Using super is just a special keyword that allows the instance to point to the overrided definitions up the inheritance chain, it does not point to a different object. So "super" does correctly its job, it points to the instance and allow you each time you use it to access overrided definitions and that's it. There's of course no point on trying to store that keyword in a variable since in that case it just return correctly the instance it points to which is always "this".
It's simply a case of misunderstood inheritance principle and I've seen it before, super is mistaken for some kind of instance wrapper up the inheriatnce chain around the object "this" while it's in fact and always the same object.
No, this is not possible.
If this were possible, then overriding methods wouldn't be possible!
For example, take this function...
public function test(a:Object):void {
trace(a.toString());
}
You'd only get [object Object] back if your idea was how things worked.
Ok I understand what you mean your question is more about language definition and specification.
Look at this exemple in c# that explain how you can manage more precisely overriding in c# :
http://www.dotnet-tricks.com/Tutorial/csharp/U33Y020413-Understanding-virtual,-override-and-new-keyword-in-C
But
let's explain a litlle how it's work.
when you extend a class, it's like if you create an object composed of all the object in the inheritance tree so if B extends A and C extends B you have two objects like this:
(B+A) and (C+B+A) with hierarchy between each other B->A and C->B->A. Super is just a way to ascend in the hierachy.
When you cast a C in A for example. In memory you always have an object (C+B+A) but interpreted as A. When you override you just say that a method in child has an higher priority than in parent.
You can try downcasting this manually to any of your class's predecessors. The pointer will still be equal to this but the methods called will use the class table of the class used to downcast.
public class Extended extends Base {
public function Extended() {
this.whoAmI() //prints extended
super.whoAmI() //prints base
var test:Base = this;
test.whoAmI() //should print base
}
public override function whoAmI() {
trace("extended");
}
}
Should your Base extend something, which methods are known or the superclass is dynamic, and there is code that adds methods to prototype of a class, you might use such a downcast to call a superclass's method that might not be there at compile time, but make sure you first call hasOwnProperty in case of a dynamic class to determine whether a method or property exists.
I cannot seem to find any simple examples of this.
I have a WPF UI that I wish to display a view as a child control within another view. The MvxWpfView inherits from UserControl so it should be possible, however I cannot seem to work out how to do the binding.
I get a BindingExpression path error, as it cannot find ChildView property in my ParentViewModel.
So how do I bind a view to control content?
Firstly it's possible that you just need to add the BViewModel you want displayed on AView as a property on ViewModelA
E.g.
public class AViewModel: MvxViewModel
{
public BViewModel ChildViewModel
{
get;set;//With appropriate property changed notifiers etc.
}
}
Then inside AView you just add a BView, and you can set the datacontext of BView as follows:
<UserControl DataContext="{Binding ChildViewModel}"/>
However, if you want something more flexible (and you want the presentation handled differently for different platforms) then you will need to use a Custom Presenter
Inside your setup.cs you override CreateViewPresenter:
protected override IMvxWpfViewPresenter CreateViewPresenter(Frame rootFrame)
{
return new CustomPresenter(contentControl);
}
Now create the class CustomPresenter you need to inherit from an existing presenter. You can choose between the one it's probably using already SimpleWpfPresenter or you might want to go back a bit more to basics and use the abstract implementation
The job of the presenter is to take the viewmodel you have asked it to present, and display it "somehow". Normally that mean identify a matching view, and bind the two together.
In your case what you want to do is take an existing view, and bind a part of it to the second view mode.
This shows how I have done this in WinRT - but the idea is very similar!
public override void Show(MvxViewModelRequest request)
{
if (request.ViewModelType == typeof (AddRoomViewModel))
{
var loader = Mvx.Resolve<IMvxViewModelLoader>();
var vm = loader.LoadViewModel(request, new MvxBundle());
if (_rootFrame.SourcePageType == typeof (HomeView))
{
HomeView view = _rootFrame.Content as HomeView;
view.ShowAddRoom(vm);
}
}
else
{
base.Show(request);
}
}
So what I'm doing is I'm saying if you want me to present ViewModel AddRoom, and I have a reference to the HomeView then I'm going to just pass the ViewModel straight to the view.
Inside HomeView I simply set the data context, and do any view logic I may need to do (such as making something visible now)
internal void ShowAddRoom(Cirrious.MvvmCross.ViewModels.IMvxViewModel vm)
{
AddRoomView.DataContext = vm;
}
Hopefully that makes sense! It's well worth putting a breakpoint in the show method of the presenters so you get a feel how they work - they are really simple when you get your head around them, and very powerful.
I'm currently working on a project where I have a ton of classes inheriting from other classes which inherit from other classes and so on. It's probably more complex than it should be, but I am a sucker for abstraction.
Anyway, at times I need to change a getter/setter from being public to private. I suppose it's not really a need, but a desire to cut off things that are preset in child classes, but still need to be publicly accessible in the parent classes.
So an example would be:
Class Base {
public function set label( value:String ):void{};
}
Class A extends Base {}
Class B extends A {
public function B() {
super();
this.label = "stuff";
}
override public function set label( value:String ):void {
//this setter should not be publicly available since the label should not be possible to change in this class
}
}
Currently, I am doing one of two things in these cases:
override the setter to do nothing or set it to the default value so that it can still update/render/whatever
throw an error saying it is unavailable in that class
I've done some searching and everything seems to point to this being impossible, but I've never found it explicitly stated that it is impossible. So is it possible to change the access modifier on an inherited property/function?
It is not possible, and it really should not be, because it leads to confusing and unpredictable class hierarchies. For starters, if you did something like that, you would break the Liskov Substitution Principle: A super class should at all times be replaceable by its derived classes. Changing the API would clearly prevent that - and thus possibly lead to runtime errors and/or inexplicable glitches, if another programmer accidentally exchanged types.
If the classes you are modeling have different behavior in such a way that would make you "hide" an otherwise public API method, you should probably not use inheritance for this - or perhaps in a different way. From what you are describing, I would guess that in a larger part of your hierarchy, you should probably be using composition instead of inheritance, anyway.
It is not possible for the very reason in the comments by Marty Wallace. But it's not an uncommon thing to do.
However in the alternative you used, The property owner is the base class & hence it should always know of anything that the derived class does with it's own properties.
Instead of your hack I would thus prefer something like this :
public class Base {
protected var _isLabelUsable:Boolean = true;
public function set label( value:String ):void {
if (!_isLabelUsable)
throw new Error("Access of undefined property label.");
// Set Label here
}
}
public class A extends Base {
}
public class B extends A {
public function B() {
super();
_isLabelUsable = false;
}
}
These are all valid points, but...
There are cases where they are all void.
Given a base class that comes from an external source. Like, say, mx:Panel.
It has the property 'titleIcon:Class'
The derived class inherits all properties and functions. But people using it shall never set the titleIcon directly, because part of the derived class' functionality depends on the availability of an icon name being known. It provides a property iconName:String. Setting it will also set the titleIcon.
Now how to prevent people from still setting the icon directly? The UI is offering the old property for AS3 and MXML, and the compiler will (of course) not complain.
If titleIcon is a setter/getter pair (in this case, it is), and not final, then the derived class can override the setter and throw an error, while the iconName setter will assign the icon class to super.titleIcon.
However, this is clumsy and will not work for final functions or variables.
If there were a way to at least tell the UI to not offer the property anymore or show a warning...
I always created additional property to MovieCLips using the syntax
myMC.myProperty
without any sort of declaration... But i can use this method only with MovieClips.. What about if i want to add a property to a button or any different type of object? I need to extend the class? Do you can me suggest how? Many thanks
You can add property to movieclips in runtime because MovieClip is dynamic class. If the class is not dynamic, you should extend it to create methods and properties.
Read about dynamic classes.
I tend to create custom classes for nearly everything.
I would extend the relevant class and set up a private var for your new property. You can then pass in the value to the constructor or add a getter/setter method to call externally.
private function _myProperty:int;
public function get myProperty():int
{
return _myProperty;
}
public function set myProperty(newVal:int):void
{
_myProperty = newVal;
}
Getter/setter methods add a few lines of code that may seem unnecessary but on big projects when you find a property is being set and you don't know why, you can put a break point in your set myProperty
Subclass is main solution.
Next works only with mx components (flex sdk 3).
Most components have data : Object property that you can freely use to store data.
Monkey patching sometimes is the only way to go. It allows you to add custom properties to flex sdk classes. I don't think you should use it in your case. But I used it to change core logic that is locked by private keyword in flex sdk.
Hope that helps.