In my Windows Phone App there's a simple hierarchical model consisting of a class containing a collection of other domain objects.
In my xaml i have declared an ItemsContainer control that renders the items in the above mentioned collection as simple rectangles.
Now, at the VM level i have a structure that resembles my model with a parent VM having a collection of children VMs. Each child-VM encapsulates its own model.
Whenever the user taps the view bound to a child-VM a method of the parent-model object should be invoked taking the relevant child-model as parameter. This will in turn change some internal state that will be reflected (possibly) on all the child-views (not just the tapped one).
SO... given that i'm using the MVVM Light framework my current implementation is as follows:
Child-VM exposes a command
The command Execute method will use the messenger to notify the parent-VM of the tap event. The message (GenericMessage class) content will be the domain object encapsulated by the VM
The parent-VM executes the method of the parent-model using the message content as parameter
If the operation succeeds the parent-VM sends a new message to inform child-VMs of this fact. Once again the message content is the model object used as parameter in the method that was just invoked
Child-VMs raise a couple of PropertyChanged events that, finally, will update the bound views
It works but i fill it's a bit cumbersome. The thing that bugs me the most is the fact that when a child-view is tapped the associated VM will broadcast its encapsulated model object. Do you feel that there would be a better way of implementing such a system?
Thanks in advance for your precious help
Could you not just put the command on the parent viewmodel and pass the child viewmodel as the command parameter?
The parent view model can then just call methods on the child viewmodels to update them. I'm not sure I see the need for all these messages?
Related
I've been running into endless problems attempting to use Windsor with Web API and injecting HttpRequestMessage into downstream dependencies of a controller. Since I've tried all the matching answers on Stackoverflow, I'd like to ask the question in a different way:
In Castle Windsor, how can I resolve a component instance while supplying a value for a downstream dependency? That is, the supplied value is required by a component that is required by the component being resolved.
For context, I'm trying to inject HttpRequestMessage so that I can use it to resolve the request context (primarily to resolve an absolute URL).
Edit I'd also like to point out that I don't currently have a dependency on Web Host / System.Web and I'd rather not change that.
A proper approach is to
Create IMyDesiredRouteParameterProvider
Implement it. Get the current request inside it and get the url
Register it and inject it in the desired dependent class via constructor.
I made myself such an implementation and I can say that this way it works fine. You can make Web.Infrastructure assembly and put the implementation there. Or put both the interface and the implementation there if you are going to reference it from another web module.
using System;
using System.Web;
namespace RouteParameterProvider
{
interface IMyRouteParameterProvider
{
string GetRouteParameter();
}
public class ControllerActionMethodRouteParameterProvider : IMyRouteParameterProvider
{
public string GetRouteParameter()
{
string Parameter = HttpContext.Current.Request.RequestContext.RouteData.Values["controller"] as string;
if (string.IsNullOrEmpty(Parameter))
{
throw new InvalidOperationException();
}
return Parameter;
}
}
}
You can get every possible thing that the Request Context contains from :
HttpContext.Current.Request.RequestContext
And it will be better if you rethink your design decision :
I need HttpRequestMessage to be regstered prior to creating each
instance of SomethingController so that it will be available down at
the LinkGenerator layer.
Containers are to be initialized at runtime and then used to resolve.
I need HttpRequestMessage to be regstered prior to creating each
instance of SomethingController so that it will be available down at
the LinkGenerator layer.
It sounds like you want to register an item with the container at runtime, post-startup. In general, this is not a good practice--registration should be a discrete event that happens when the app is fired up, and the container's state should not be changed during runtime.
Dependency Injection is about resolving service components, not runtime state--state is generally passed via methods (method injection). In this case it sounds like your LinkGenerator component needs access to the ambient state of the request.
I'm not that familiar with HttpRequestMessage, but this answer seems to show that it is possible to retreive it from HttpContext.Current. You could make this a method on your LinkGenerator class, or wrap this call in a separate component that gets injected into LinkGenerator (HttpRequestMessageProvider?). The latter would be my preferred method, as it allows LinkGenerator to be more testable.
Given the lack of a clean way of doing this and Web API not providing information as to the hosted endpoint beyond per-request context objects, I ended up injecting the base url from configuration.
Is this library by Mark Seemann the answer? In the description he writes explicitly :
This approach enables the use of Dependency Injection (DI) because the
request can be injected into the services which require it.
Then gives an example :
// Inside an ApiController
var uri = this.Url.GetLink(a=> a.GetById(1337));
By which you can then pass the URL down the road in the service that you have injected in the controller.
UPDATE :
Mark Seemann wrote about the same exact problem here:
"Because HttpRequestMessage provides the context you may need to
compose dependency graphs, the best extensibility point is the
extensibility point which provides an HttpRequestMessage every time a
graph should be composed. This extensibility point is the
IHttpControllerActivator interface:..."
This way you can pass request context information to a component deep in the object graph by getting from the HttpRequestMessage and passing it to the DI container.
Just take a look at the interface of IHttpControllerActivator.
The WEB API framework gets the IHttpControllerActivator through DependencyResolver. You probably already replaced it by your CastleWindsorDependencyResolver. Now you have to implement and register your HttpControllerActivator and register it.
When the WEB API framework gets IHttpControllerActivator from DependencyResolver (your Castle Windsor DR) and calls IHttpControllerActivator.Create() it will pass you the HttpRequestMessage. You can get your info from there and pass it to the your CastleDR before you call Resolve(typeof(MyController)) which will resolve the whole object graph - that means you will have MyHttpContextInfo to inject in your XYZComponent deep in the resolution stack.
This way tou are passing the arguments in the last possible moment but it is still possible. In Castle Windsor I make such passing of arguments though CreationContext.AdditionalArguments["myArgument"];.
I have gone through David Nolen's basic Om tutorial but I am still a bit confused about the difference between application state and component local state. When cursors are referred to is it referring to one or both of these too?
As I understand it:
Application state is the "global" state that all components in the component tree can access, through cursors. This is the state that your application is in and basically what is being rendered by Om. So, for example, if you are writing a chat program, the application state would contain a list of users in the conversation and all of the messages that have been sent, or whatever.
Component local state is state that is local to a single component and cannot be seen outside of this component. It is set either by passing {:init-state } to build, or by implementing IInitState and returning a map from init-state - or both (in this case, they are conj'd together). David Nolen recommends that local state should only be used for transient state, such as if the mouse is currently pressed in a drag/drop component and that all other state should be application state. That is, if you have a tab widget, the currently selected tab should be set in application state (not local state!), but if the tab is being dragged to a new location, the current position and mouse state would be (temporarily - until the drag operation is complete) be stored in component local state. Things like core.async channels can also be stored in local state (though I've also stored them (and seen others do the same) in shared state and additional data - see below for details on both)
Cursors only apply to application state and are like windows into it so that components far down the tree can access only the data that they actually need to access.
Application state is always accessed through a cursor (app in the tutorial) and modifying application state is done through a cursor - both om/update! and om/transact! take a cursor as their first argument. You can also set the application state atom directly with reset! and swap!, but David recommends against this as by doing that you lose out on some of Om's more advanced features (like being notified of change deltas).
Local state can be received through IRenderState or by accessing it directly with om/get-state. You can set local state with om/set-state! and om/upate-state!. All three of these take a component backing object (owner in the tutorial).
There is also a third type of state in Om: shared state. Shared state is passed to om/root using the {:shared ...} option and can be accessed from any component in the tree under that root, using om/get-shared. The difference between this and application state is that application state is narrowed down through cursor paths - that is, sub components may not have access to the entire application state - while shared state is always accessible. Also, modifying application state causes the component to be re-rendered while shared state does not trigger renders.
As an aside, there is actually a fourth type too - you can pass additional data to components through build using the {:opts ...} option. This is data that lives outside of the Om/react lifecycle - that is, its immutable data that you can access from a component, but the component does not manage it in any way. This seems to be most useful for configuration data.
First off, I've done a lot of research on the web (including this site) and have found lots of conflicting information on how the model and controller communicate in an MVC pattern. Here is my specific question (I'm using AS3), but it's a general MVC question...
I have two main components... a list of recipes and a form that displays a selected recipe. The form has an edit state that allows you to edit the recipe and then save or cancel the changes. What is the best way (using MVC principles) to handle changes made to a recipe? So far, I have the save button trigger an event which is captured by the controller.
Should I have the save button (the view) pass an object with the current state of the fields along with the event (some logic in view)? Should I allow the controller to hold access to the view and have the controller figure out what's in the fields on its own (added coupling)? Should events be made every time a field in the form is changed and the controller keeps track of the state of each field (lots of events)? Or is their another way? Note: I don't want to bind the fields to the model because I only want the data to save if the save button is clicked.
Any help would be greatly appreciated. Thanks!
Should I have the save button (the view) pass an object with the
current state of the fields along with the event (some logic in view)?
yes, this is not 'logic in the view', it does not decide anything, simply reporting an action and its current state
Should I allow the controller to hold access to the view and have the
controller figure out what's in the fields on its own (added
coupling)?
no, this would become very messy, pass a VO with the event
Should events be made every time a field in the form is changed and
the controller keeps track of the state of each field (lots of
events)?
is an option, but this basically is the same as hitting the save button, the trigger is different (TextField.onChange), but you can dispatch the same event (setup is form = view, dispatches one general event with a VO, not an event for each field)
Or is their another way?
MVC flow w/ events:
onClick save btn: a RecipeEvent.SAVE is dispatched (from the view), with a VO (value object) containing the Recipe data (e.g. RecipeVO)
the controller catches this, and as the controller is where the logic resides, it decides what to do with it: update the RecipesModel (either directly by calling a method on the model, or by a custom event e.g. RecipeModelEvent.SAVE)
the model stores the data, and dispatches the RecipeEvent.UPDATE event (with the RecipeVO)
the views updated itself accordingly (check if the RecipeVO.ID is same, update data representation e.g. title…)
optionally, a controller could save the data to a back-end/remote database
As for the event listeners: views listen to the model, controller listen to the views.
About de-coupling:
use interfaces (by using an IModel you can swap out the model easily by another implementation, as you register the event listeners against the interface instead of the actual implementation)
Obviously, all this results in a lot of registering/removing event listeners, and keeping reference to the models/views/controllers to be able to register to the appropriate instance. An alternative is to use a framework as RobotLegs, as it makes a central event-bus available + easy/automatic cleanup of event listeners in the mediator class of a view.
I think this is really a database question. what I would do is create a stored procedure that first checks to see if the recipe already exists. If yes then update it. If not then add a new recipe. (you'll have to bind your entity to a stored procedure. other MVC frameworks can do this. I don't know about actionscript)
if that's not an option then I guess you would have to cache the original form in a helper class in the controller and then compare it to what the user is trying to save. And have the controller decide whether to update the recipe.
I think it's much cleaner to use the first way, but I've never used actionscript so...
Colin Moock's lecture on MVC in ActionScript is quite old, but still one of the best explications: http://www.moock.org/lectures/mvc/
Your model should populate your view, and your view should send input events to the controller, which should decide what to do with the input. As g10 says, wait until the save button is clicked and then pass an object with the modified fields up to the controller for processing. The controller can then decide whether or not to accept it, and whether to update an existing model object or create a new one.
What exactly will this addActionListener Do.....we we call button.addActionListener(this) what will happen
It basically adds this (the current object) to a list of objects that will be notified when the component has an action performed on it, such as a button being pressed.
It's a way of registering your interest in what is happening to the component and is useful in that you don't have to keep polling a component to check its status.
Your object (or class, really) simply implements the interface methods for listening (such as actionPerformed) and that method will be called for each event that happens.
The Java tutorials have a large variety of different articles on the various listeners that you're likely to use.
I have a tree control in my GUI (with naturally lots of GUI/platform specific functions to handle the nodes).
I have a data model with its own complex set of nodes, children, properties etc..
I want the tree to display a representation of the model, be able to send messages to the nodes inside the model and be told to redraw itself when the model changes.
But I don't want the GUI code to need to know the details of the model's data types and I don't want to pollute the model by linking it to the GUI classes.
I can't get my head around how the controller is supposed to do this and what functions it should provide?
(this is in C++ but that shouldn't matter)
GUI "controls" don't quite fit neatly into a model-view-controller pattern because they typically have their own internal model rather than accepting a reference to one. If the control is structured that way, you'll need an adapter class that "data-binds" the control's internal model to the underlying data model.
This can accomplish something similar to what model-view-controller would, except that the adapter class plays the role both of a view hookup component (updating the GUI from the data model) and a controller (interpreting GUI events into model actions).
Qt provides a group of classes for model-view programming. You can hook a tree view to a filesystem model, for example, and neither directly know anything about each other (except a pointer to the model in the view).
Your requirements are:
Tree displays representation of model
Nodes in tree can send messages to nodes inside model
Tree redraws itself based on model changes
I don't know exactly what kind of data you're working with here, but a hierarchical model is a fairly simple thing. I'll take it as a given you know how to iterate hierarchical data and populate a tree view.
Your controller should have member function(s) for sending messages to the model. The parameters should be a model element and the message you want to send. This way, the UI is completely unaware of how the message gets to the element, but can get the messages through.
The last requirement is more tricky, and depends on a few things (e.g., the lifetime of the controller, application architecture, etc.) I'm going to assume the controller lives as long as the tree view does. If that's the case, then your controller should provide a way to set a callback on model changes. Then, when the controller changes the model, it can callback to the UI without being aware of the UI.
i think your troubles start with an unfortunate choice of words. the 'control' thingies doesn't have anything to do with the 'controller' in MVC. that's why some GUI libraries use other names (widgets is a common one).
your 'tree control' is the view, not the controller. it should be tied to the GUI, both for display, and to get GUI events and turn them into 'tree events'.
the controller gets those 'tree events' and does the necessary modifications to the model. that's where you couple the 'action' with the 'response'.
First Solution: You can implement a "Subject observer" pattern between model and view, with model as the subject and view as the observer. Whenever there is a change in the state of model, it can fire a event to all the observers those are registered, they can update themselves.
Second Solution: Introduce a controller, which registers with model as observer. Upon receiving a event for update from Model, it can update the view. Even you can decouple view from controller using one more subject observer pattern between controller and view
Third Solution: Use MVP pattern. Model view Presenter. This pattern used whenever there is no much computation in controller i.e., job of the controller is just to update its corresponding view. Here controller becomes Presenter.
You need a controller that sits outside the display widget but has the state of the tree (in MFc there are CTreeView/CTreeCtrl classes - there is a similiar separation in Qt) the tree controller handles all the data storage and calls redraws on the tree widget.
Changes in the tree widget get sent to the tree controller - so this controller needs to know about the gui functions.
The model will need set/get functions for all the relevant parameters for the nodes. But these can return simple types so aren't dependent on the gui.
Updating the view form the model requires sending a message, if you don't want the model to know about your gui messaging the best you can do is register a callback function (a void pointer to a function) from the tree controller - and call this to do an update.
This update function can then query the model for the changes.