I am trying to implement this Command Pattern on my .NET MVC 3 application, specifically for saving edits to a Thing. I am undecided on how to proceed. Before I get to the actual question, here is the simplified code:
public class ThingController
{
private readonly ICommandHandler<EditThingCommand> handler;
public ThingController(ICommandHandler<EditThingCommand> handler)
{
this.handler = handler;
}
public ActionMethod EditThing(int id)
{
...build EditThingViewModel and return with View...
}
[HttpPost]
public ActionMethod EditThing(int id, EditThingViewModel vm)
{
var command = new EditThingCommand
{
...not sure yet...
};
this.handler.Handle(command);
...redirect somewhere...
}
}
My EditThingViewModel is wholly disconnected from my domain, which consists of POCO classes. It seems like my EditThingCommand should look like this:
public class EditThingCommand
{
Thing ModifiedThing;
}
However, building ModifiedThing would then still be happening in my controller. That's the majority of the work in this case. By the time ModifiedThing is built (and the "old" timestamp applied to it for optimistic concurrency checking), all that's left is for command to call Update on my data context.
Clearly there is value in being able to easily decorate it with other commands, but I'd also like to be able to move the construction of ModifiedThing outside of my controller. (Perhaps this question is really just about that.) EditThingCommand is in my domain and doesn't have a reference to EditThingViewModel, so it can't go there. Does it make sense to have another command in my presentation layer for mapping my viewmodel to my poco entity?
I created an EditThingPostCommand outside of my domain, which takes the EditThingViewModel as a parameter. The EditThingPostCommandHandler is responsible for creating the EditThingCommand and calling its handler.
It works, but I'm not going to assume that's the best answer to my question. Arguably most of what the EditThingPostCommandHandler is doing could be done in a custom AutoMapper configuration, which would still serve the purpose of cleaning up the controller action method.
After several months of using this pattern on other projects, it is apparent to me that the commands on this particular project were simply too general and therefore too complex, requiring too much setup. It would have been better to create, for example, an EditThingTitleCommand and a MoveThingPiecesCommand and so on, and call them from their own ActionMethods.
In other words, when using the command pattern, don't just use the commands as replacements for typical CRUD operations. With more specificity comes more benefit.
Related
I am building a WebAPI service for an application and i am stuck at a cross roads, i understand the SOLID principles and wanting to implement my WebAPI services following these principles, however i have spent the last couple of days struggling with an implementation and would like some advice from the community.
The Single Responsibility principle states that every class should have responsibility over a single part of the functionality provided by the software (see Solid Principles)
I have a two ApiControllers at the moment but there will be many more, and the two currently have Get, Post and Put functionality, but each controller will also require a Search function and my question is should the search methods be moved into a separate ApiController, called SearchController for example.
For example:
I have a user controller
public class UserController : ApiController {
Get - returns a User
Post and Put - Add and update a User
}
and i have a patient controller along the same lines as above, should i have my search methods in each controller of should i have a SearchController, eg.
public class SearchController : ApiController {
SearchUsers() - returns ICollection<User>
SearchPatients() - returns ICollection<Patient>
}
I realise there is no definitive answer and would just like some advice on the best practice
If you go insane with SRP and don't find the proper level of responsibility you end up with a zillion classes doing nothing.
I view the User controller as being responsible for mediating between the User resource api and how that concept is implemented inside of the application. In that case it makes sense to have all api operations in a single controller.
That said we follow Command/Query separation so our controllers delegate to commands or queries to do the actual work. That way the only reason our controller should change is if we want to change the API we provide.
I would introduce search as an action on the respective controller and if you want to track all controllers that have a search action, put an interface on it.
public interface ISearchable
{
IHttpActionResult Search(string q);
}
public class UsersController : ApiController, ISearchable
{
[Route("api/Users/Search")]
[HttpGet]
public IHttpActionResult Search([FromUri] string q)
{
var userRepo = new UserRepo();
//this search method can be as complex as needed
//search initials, middle names, phone numbers, whatever you need.
var results = userRepo.Search(q);
return this.Ok(results);
}
}
Usage:
https://domain.com/api/Users/Search?q=smith
This keeps it very clear to your clients since they don't have to know about a specific SearchController that would basically be a bunch of RPC's. If you think about what search is, it's basically a verb that isn't a standard http verb, so we attach it to the route as an action. We are still acting upon the Users collection similar to a GET or POST against the collection. Since we don't have SEARCH as a valid HTTP method, an action is the next best thing.
In reference to SOLID. This keeps your controller as simple a controller. It's job (IMO) is to control the request and route that to logic. In this case, it took the route, mapped it to a UserRepo and executed the Search method. Searching is not the job of the controller, you have another class doing that heavy lifting. The controller simply handles what logic should be executed given the request. Even the data returned is deferred until later in the pipeline via serializers so SOLID should hold true on the controller.
I will explain with an example. My GWT project has a Company module, which lets a user add, edit, delete, select and list companies.
Of these, the add, edit and delete operations lands back the user on the CompanyList page.
Thus, having three different events - CompanyAddedEvent, CompanyUpdatedEvent and CompanyDeletedEvent, and their respective event handlers - seems overkill to me, as there is absolutely not difference in their function.
Is it OK to let a single event manage the three operations?
One alternative I think is to use some event like CompanyListInvokedEvent. However, somewhere I think its not appropriate, is the event actually is not the list being invoked, but a company being added/updated/deleted.
If it had been only a single module, I would have get the task done with three separate events. But other 10 such modules are facing this dilemma. It means 10x3 = 30 event classes along with their 30 respective handlers. The number is large enough for me to reconsider.
What would be a good solution to this?
UPDATE -
#ColinAlworth's answer made me realize that I could easily use Generics instead of my stupid solution. The following code represents an event EntityUpdatedEvent, which would be raised whenever an entity is updated.
Event handler class -
public class EntityUpdatedEvent<T> extends GwtEvent<EntityUpdatedEventHandler<T>>{
private Type<EntityUpdatedEventHandler<T>> type;
private final String statusMessage;
public EntityUpdatedEvent(Type<EntityUpdatedEventHandler<T>> type, String statusMessage) {
this.statusMessage = statusMessage;
this.type = type;
}
public String getStatusMessage() {
return this.statusMessage;
}
#Override
public com.google.gwt.event.shared.GwtEvent.Type<EntityUpdatedEventHandler<T>> getAssociatedType() {
return this.type;
}
#Override
protected void dispatch(EntityUpdatedEventHandler<T> handler) {
handler.onEventRaised(this);
}
}
Event handler interface -
public interface EntityUpdatedEventHandler<T> extends EventHandler {
void onEventRaised(EntityUpdatedEvent<T> event);
}
Adding the handler to event bus -
eventBus.addHandler(CompanyEventHandlerTypes.CompanyUpdated, new EntityUpdatedEventHandler<Company>() {
#Override
public void onEventRaised(EntityUpdatedEvent<Company> event) {
History.newItem(CompanyToken.CompanyList.name());
Presenter presenter = new CompanyListPresenter(serviceBundle, eventBus, new CompanyListView(), event.getStatusMessage());
presenter.go(container);
}
});
Likewise, I have two other Added and Deleted generic events, thus eliminating entire redundancy from my event-related codebase.
Are there any suggestions on this solution?
P.S. > This discussion provides more insight on this problem.
To answer this question, let me first pose another way of thinking about this same kind of problem - instead of events, we'll just use methods.
In my tiered application, two modules communicate via an interface (notice that these methods are all void, so they are rather like events - the caller doesn't expect an answer back):
package com.acme.project;
public interface CompanyServiceInteface {
public void addCompany(CompanyDto company) throws AcmeBusinessLogicException;
public void updateCompany(CompanyDto company) throws AcmeBusinessLogicException;
public void deleteCompany(CompanyDto company) throws AcmeBusinessLogicException;
}
This seems like overkill to me - why not just reduce the size of this API to one method, and add an enum argument to simplify this. This way, when I build an alternative implementation or need to mock this in my unit tests, I just have one method to build instead of three. This gets to be clearly overkill when I make the rest of my application - why not just ObjectServiceInterface.modify(Object someDto, OperationEnum invocation); to work for all 10 modules?
One answer is that you might want want to drastically modify the implementation of one but not the others - now that you've reduced this to just one method, all of this belongs inside that switch case. Another is that once simplified in this way, the inclination often to further simplify - perhaps to combine create and update into just one method. Once this is done, all callsites must make sure to fulfill all possible details of that method's contract instead of just the one specific one.
If the receivers of those events are simple and will remain so, there may be no good reason to not just have a single ModelModifiedEvent that clearly is generic enough for all possible use cases - perhaps just wrapping the ID to request that all client modules refresh their view of that object. If a future use case arises where only one kind of event is important, now the event must change, as must all sites that cause the event to be created so that they properly populate this new field.
Java shops typically don't use Java because it is the prettiest language, or because it is the easiest language to write or find developers for, but because it is relatively easy to maintain and refactor. When designing an API, it is important to consider future needs, but also to think about what it will take to modify the current API - your IDE almost certainly has a shortcut key to find all invocations of a particular method or constructor, allowing you to easily find all places where that is used and update them. So consider what other use cases you expect, and how easily the rest of the codebase can be udpated.
Finally, don't forget about generics - for my example above, I would probably make a DtoServiceInterface to simplify matters, so that I just declare the one interface with three methods, and implement it and refer to it as needed. In the same way, you can make one set of three GwtEvent types (with *Handler interfaces and possibly Has*Handlers as well), but keep them generic for all possible types. Consider com.google.gwt.event.logical.shared.SelectionEvent<T> as an example here - in your case you would probably want to make the model object type a parameter so that handlers can check which type of event they are dealing with (remember that generics are erased in Java), or source from one EventBus for each model type.
OK, i am creating a web application. I am using MVC3. I have ViewModel for every view and also I have Data Model that supports viewModel and does the actuall CRUD operation in my sql table, while my viewModel validates and alters data as necessary.
Here is a question. The way I have been coding is
public class MyClassViewModel
{
public string member1{get;set;}
public int member2{get;set;}
public static GetAllMembers(MyClassViewModel obj, out string strErrMsg)
{
// code goes here, read operation
}
public static UpdateMyClass(MyClassViewModel obj, out string strErrMsg)
{
// code goes here, write operation.
}
}
Both My ViewModel and DataModels have been coded this way, My Controller on HttpPost just do something like this..
MyClassViewModel.UpdateMember(obj,out strErrMsg)
Since its mvc on every post it creates on a strongly typed view, a new object of my ViewModel, hence non static members are different and are not shared between sessions.
I am using Linq and therefore on each static method of my DataModel i use
var db = new MyApplicationDataContext()
to get my linq classes and work on them. This solves my open/close reader problems.
My question is, are there any issues regarding concurrency with this design? I know I might have problems in changing my data layer if it is not designed via interface, but I highly doubt that is necessary based on my application.
You are essentially using the factory design pattern. That's OK pattern to use for this; each static method has its own context, and that's OK too. You always have to worry about concurrency; however, the LINQ to SQL architecture has concurrency detection built in and throws a ChangeConflictException when a concurrent change has been made in reaction to this.
I would also highly recommend that you are disposing your contexts properly at the end of each static method call, because you could leave open connections to the database which can cause performance problems.
Also, another problem you may run into is interlinking data; you cannot link two objects together by reference that were created from different contexts. And in LINQ to SQL, there is no way to explicitly detach an object...
I have a page that has a status text label and a panel doing some DB query. since the query can take some time I am loading it using
add(new AjaxLazyLoadPanel("resultPanel")
{
#Override
public Component getLazyLoadComponent(String id) {
return new SearchResultPanel(id);
}
};
which works very well.
My question is how to update the status label which is outside of the resultPanel, to show the number of search results?
I was thinking along the lines of addComonent(target) but I don't have a target? am i off the track?
Well the SearchResultPanel might look like this:
public class SearchResultPanel extends Panel implements IHeaderContributor{
public SearchResultPanel(String id){
super(id);
....
}
public void renderHead(IHeaderResponse response){
response.renderOnDomReadyJavascript("alert('hello');");
}
}
Now when it is loaded it should throw out that javascript. Another way answered on stackoverflow previously (though I do not like it) is to use an AjaxSelfUpdatingTimerBehavior, which unless the javascript should be ran more then once I do not like, and still it is less elegant in my opinion.
Look here for their answer: Wicket: reload AjaxLazyLoadPanel automatically
You can always obtain the current request target using RequestCycle.get().getRequestTarget(), provided that there is an active request cycle, so in theory you could do that from your lazy-loaded component constructor, check if it is an Ajax target, and add the component if it is.
Another solution is to look at the source code of AjaxLazyLoadPanel and create your own component based on it. (It's really simple but as you can see if you look at the code, there's no way you can make it expose the request target. This isn't a very OO thing to do, but as all the important functionality is wrapped in the constructor, you have very little choice..
I would avoid having to tamper with Javascript, unless there's really no other way.
Happened to come across this post and I have something to add as well.
The AjaxLazyLoadPanel now has an overridable method called onComponentLoaded(Component, AjaxRequestTarget) which could also solve your problem.
I'm writing my code in Haxe. This is quite irrelevant to the question though, as long as you keep in mind that it's a high level language and compareable with Java, ActionScript, JavaScript, C#, etc. (I'm using pseudocode here).
I'm going to work on a big project and am busy preparing now. For this question I'll create a small scenario though: a simple application which has a Main class (this one is executed when the application launches) and a LoginScreen class (this is basically a class that loads a login screen so that the user can login).
Typically I guess this would look like the following:
Main constructor:
loginScreen = new LoginScreen()
loginScreen.load();
LoginScreen load():
niceBackground = loader.loadBitmap("somebg.png");
someButton = new gui.customButton();
someButton.onClick = buttonIsPressed;
LoginScreen buttonIsPressed():
socketConnection = new network.SocketConnection();
socketConnection.connect(host, ip);
socketConnection.write("login#auth#username#password");
socketConnection.onData = gotAuthConfirmation;
LoginScreen gotAuthConfirmation(response):
if response == "success" {
//login success.. continue
}
This simple scenario adds the following dependencies and downsides to our classes:
Main will not load without LoginScreen
LoginScreen will not load without the custom loader class
LoginScreen will not load without our custom button class
LoginScreen will not load without our custom SocketConnection class
SocketConnection (which will have to be accessed by a lot of different classes in the future) has been set inside LoginScreen now, which is actually quite irrelevant from it, apart from the fact that the LoginScreen requires a socket connection for the first time
To solve these problems, I have been suggested to do "Event-Driven-Programming", or loose coupling. As far as I understand, this basically means that one has to make classes independent from each other and then bind them together in separate binders.
So question 1: is my view on it true or false? Does one have to use binders?
I heard Aspect Oriented Programming could help here. Unfortunately Haxe does not support this configuration.
However, I do have access to an event library which basically allows me to create a signaller (public var loginPressedSignaller = new Signaller()), to fire a signaller (loginPressedSignaller.fire()) and to listen to a signalller (someClass.loginPressedSignaller.bind(doSomethingWhenLoginPressed)).
So, with little further investigation I figured this would change my previous setup to:
Main:
public var appLaunchedSignaller = new Signaller();
Main constructor:
appLaunchedSignaller.fire();
LoginScreen:
public var loginPressedSignaller = new Signaller();
LoginScreen load():
niceBackground = !!! Question 2: how do we use Event Driven Programming to load our background here, while not being dependent on the custom loader class !!!
someButton = !!! same as for niceBackground, but for the customButton class !!!
someButton.onClick = buttonIsPressed;
LoginScreen buttonIsPressed():
loginPressedSignaller.fire(username, pass);
LoginScreenAuthenticator:
public var loginSuccessSignaller = new Signaller();
public var loginFailSignaller = new Signaller();
LoginScreenAuthenticator auth(username, pass):
socketConnection = !!! how do we use a socket connection here, if we cannot call a custom socket connection class !!!
socketConnection.write("login#auth#username#password");
This code is not finished yet, eg. I still have to listen for the server response, but you probably understand where I am getting stuck.
Question 2: Does this new structure make any sense? how should I solve the problems above mentioned in the !!! delimiters?
Then I heard about binders. So maybe I need to create a binder for each class, to connect everything together. Something like this:
MainBinder:
feature = new Main();
LoginScreenBinder:
feature = new LoginScreen();
MainBinder.feature.appLaunchedSignaller.bind(feature.load);
niceBackgroundLoader = loader.loadBitmap;
someButtonClass = gui.customButton();
etc... hopefully you understand what I mean. This post is getting a bit long so I have to wrap it up a bit.
Question 3: does this make any sense? Doesn't this make things unnecessarily complex?
Also, in the above "Binders" I only had to use classes which are instantiated once, eg. a login screen. What if there are multiple instances of a class, eg. a Player Class in a game of chess.
well, concerning the how, I would point out my 5 commandments to you. :)
For this question only 3 are really important:
single responsibility (SRP)
interface segregation (ISP)
dependency inversion (DIP)
Starting off with SRP, you have to ask yourself the question: "What is the responsibility of class X?".
The login screen is responsible for presenting an interface to the user to fill in and submit his login data. Thus
it makes sense for it to depend on the button class, because it needs the button.
it makes no sense it does all the networking etc.
First of all, you let's abstract the login service:
interface ILoginService {
function login(user:String, pwd:String, onDone:LoginResult->Void):Void;
//Rather than using signalers and what-not, I'll just rely on haXe's support for functional style,
//which renders these cumbersome idioms from more classic languages quite obsolete.
}
enum Result<T> {//this is a generic enum to return results from basically any kind of actions, that may fail
Fail(error:Int, reason:String);
Success(user:T);
}
typedef LoginResult = Result<IUser>;//IUser basically represent an authenticated user
From the point of view of the Main class, the login screen looks like this:
interface ILoginInterface {
function show(inputHandler:String->String->Void):Void;
function hide():Void;
function error(reason:String):Void;
}
performing login:
var server:ILoginService = ... //where ever it comes from. I will say a word about that later
var login:ILoginInterface = ... //same thing as with the service
login.show(function (user, pwd):Void {
server.login(user, pwd, function (result) {
switch (result) {
case Fail(_, reason):
login.error(reason);
case Success(user):
login.hide();
//proceed with the resulting user
}
});
});//for the sake of conciseness I used an anonymous function but usually, you'd put a method here of course
Now ILoginService looks a little titchy. But to be honest, it does all it needs to do. Now it can effectively be implemented by a class Server, that encapsulates all networking in a single class, having a method for each of the N calls your actual server provides, but first of all, ISP suggests, that many client specific interfaces are better than one general purpose interface. For the same reason ILoginInterface is really kept to its bare minimum.
No matter, how these two are actually implemented, you will not need to change Main (unless of course the interface changes). This is DIP being applied. Main doesn't depend on the concrete implementation, only on a very concise abstraction.
Now let's have some implementations:
class LoginScreen implements ILoginInterface {
public function show(inputHandler:String->String->Void):Void {
//render the UI on the screen
//wait for the button to be clicked
//when done, call inputHandler with the input values from the respective fields
}
public function hide():Void {
//hide UI
}
public function error(reason:String):Void {
//display error message
}
public static function getInstance():LoginScreen {
//classical singleton instantiation
}
}
class Server implements ILoginService {
function new(host:String, port:Int) {
//init connection here for example
}
public static function getInstance():Server {
//classical singleton instantiation
}
public function login(user:String, pwd:String, onDone:LoginResult->Void) {
//issue login over the connection
//invoke the handler with the retrieved result
}
//... possibly other methods here, that are used by other classes
}
Ok, that was pretty straight forward, I suppose. But just for the fun of it, let's do something really idiotic:
class MailLogin implements ILoginInterface {
public function new(mail:String) {
//save address
}
public function show(inputHandler:String->String->Void):Void {
//print some sort of "waiting for authentication"-notification on screen
//send an email to the given address: "please respond with username:password"
//keep polling you mail server for a response, parse it and invoke the input handler
}
public function hide():Void {
//remove the "waiting for authentication"-notification
//send an email to the given address: "login successful"
}
public function error(reason:String):Void {
//send an email to the given address: "login failed. reason: [reason] please retry."
}
}
As pedestrian as this authentication may be, from the point of view of the Main class,
this doesn't change anything and thus will work just as well.
A more likely scenario is actually, that your login service is on another server (possibly an HTTP server), that makes the authentication, and in case of success creates a session on the actual app server. Design-wise, this could be reflected in two separate classes.
Now, let's talk about the "..." I left in Main. Well, I'm lazy, so I can tell you, in my code you are likely to see
var server:ILoginService = Server.getInstance();
var login:ILoginInterface = LoginScreen.getInstance();
Of course, this is far from being the clean way to do it. The truth is, it's the easiest way to go and the dependency is limited to one occurrence, that can later be removed through dependency injection.
Just as a simple example for an IoC-Container in haXe:
class Injector {
static var providers = new Hash < Void->Dynamic > ;
public static function setProvider<T>(type:Class<T>, provider:Void->T):Void {
var name = Type.getClassName(type);
if (providers.exists(name))
throw "duplicate provider for " + name;
else
providers.set(name, provider);
}
public static function get<T>(type:Class<T>):T {
var name = Type.getClassName(type);
return
if (providers.exists(name))
providers.get(name);
else
throw "no provider for " + name;
}
}
elegant usage (with using keyword):
using Injector;
//wherever you would like to wire it up:
ILoginService.setProvider(Server.getInstance);
ILoginInterface.setProvider(LoginScreen.getInstance);
//and in Main:
var server = ILoginService.get();
var login = ILoginInterface.get();
This way, you practically have no coupling between the individual classes.
As to the question how to pass events between the button and the login screen:
this is just a matter of taste and implementation.
The point of event driven programming is that both the source and the observer are only coupled in the sense,
that the source must be sending some sort of notification and the target must be able to handle it.
someButton.onClick = handler; basically does exactly that, but it's just so elegant and concise you don't make a fuzz about it.
someButton.onClick(handler); probably is a little better, since you can have multiple handlers, although this is rarely required of UI components. But in the end, if you want signalers, go with signalers.
Now when it comes to AOP, it is not the right approach in this situation. It's not a clever hack to wire up components between one another, but about dealing with cross-cutting concerns, such as adding a log, a history or even things as a persistence layer across a multitude of modules.
In general, try not to modularize or split the little parts of your application.
It is ok to have some spaghetti in your codebase, as long as
the spaghetti segments are well encapsulated
the spaghetti segments are small enough to be understood or otherwise refactored/rewritten in a reasonable amount of time, without breaking the app (which point no. 1 should guarantee)
Try rather to split the whole application into autonomous parts, which interact through concise interfaces. If a part grows too big, refactor it just the same way.
edit:
In response to Tom's questions:
that's a matter of taste. in some frameworks people go as far as using external configuration files, but that makes little sense with haXe, since you need to instruct the compiler to force compilation of the dependencies you inject at runtime. Setting up the dependency in your code, in a central file, is just as much work and far simpler. For more structure, you can split the app into "modules", each module having a loader class responsible for registering the implementations it provides. In your main file, you load the modules.
That depends. I tend to declare them in the package of the class depending on them and later on refactor them to an extra package in case they prove to be needed elsewhere. By using anonymous types, you can also completely decouple things, but you'll have a slight performance hit on platforms as flash9.
I wouldn't abstract the button and then inject an implementation through IoC, but feel free to do so. I would explicitely create it, because in the end, it's just a button. It has a style, a caption, screen position and size and fires click events. I think, this is unnecessary modularization, as pointed out above.
Stick to SRP. If you do, no class will grow unneccessarily big. The role of the Main class is to initialize the app. When done, it should pass control to a login controller, and when that controller acquires a user object, it can pass it on to the main controller of the actual app and so forth. I suggest you read a bit about behavioral patterns to get some ideas.
greetz
back2dos
First of all, I'm not familiar with Haxe at all. However, I would answer that what is described here sounds remarkably similar to how I've learned to do things in .NET, so it sounds to me like this is good practice.
In .NET, you have an "Event" that fires when a user clicks a button to do something (like logon) and then a method executes to "handle" the event.
There will always be code that describes what method is executed in one class when an event in another class is fired. It is not unnecessarily complex, it is necessarily complex. In the Visual Studio IDE, much of this code is hidden in "designer" files, so I don't see it on a regular basis, but if your IDE doesn't have this functionality, you've got to write the code yourself.
As for how this works with your custom loader class, I hope someone here can provide you an answer.