All the GWT / MVP examples I've learned about seem too simplistic to give a clear view what best practises are regarding displaying and handling slightly more complex model objects.
For example, most examples are something like a presenter that attaches a click handler to a few TextBoxes on the view...if save is clicked, the presenter's save() is called which simply gets the updated values, and we're done, MVP style. That's not so realistic though.
For example, let's say we have:
PresenterX
- gets a 'model' object, let's say any object with an unknown number of various primitives or whatever
ViewX
- should show the model object in a table, and/or allows it to be edited/re-saved
...so that sounds very, very basic. But, we don't know the amount of fields in the model object that we will need to display. So that might relate to a dynamic number of rows/columns. Probably no problem for a table...but how should the presenter give this to the view's table? As the model object that the view understands, or break it down into a bunch of Lists...that the view essentially still has to understand.
Also, certain fields might be editable, of which are unknown until we get the model object (something in the model determines what fields are editable, say) -- so who should be responsible for figuring out what is editable or not? Probably the presenter, but how do we refelct that in the view, the MVP way?
Lastly, let's say there's a 'save' button on the view...who's job is it to figure out all the rows in the table which were changed?
Seems to me that the view either needs to understand the model more, or the presenter needs to really understand the view more -- neither of which are nice MVP :( ... Or perhaps there should be some intermediary object.
I know there are some nicer/newer helpful ways for this kind of stuff (Editors/RequestFactory, etc.), but I'm looking for suggestions on the above scenarious.
As I understand it, MVP is a line with M-P-V points. So P interacts with both, but M and V only with P.
Also, one of design objectives of MVP is to have a testable P and M, which means that V must be replaceable with a mock version. So, V should not expose any implementation-dependent interfaces (e.g. HasClickHandlers instead of Button).
Now, if V should show generic table, you should create generic methods: addColumn(..) to define columns and addRow(..) to add data. The new CellTable is pretty flexible and supports adding columns dynamically.
About changes - V should notify, P should act. Also, there are the new Editors, which IMHO do not fit nicely into MVP, but are supposed to be easy to use.
Related
I'd like to define an enum (or something) to give names to possible values of seletedIndex and define the order of destinations in, say, a Flutter NavigationBar in such a way that I only have to go to one place to change the order of the destinations.
The examples I've seen so far always require reordering be done in two places: one in the list of destinations given to the NavigationBar that have an implicit, invisible, unnamed index, and the other is a switch or a subscripting based on selectedIndex to establish the selected page. This seems like a bug magnet. Is there any way to set it up so I only define the order of the destinations in one place and have the list of destinations bound to the correct pages?
It seems to me that NavigationDestination should have a property to handle this binding or a "displayOrder" property, but I don't see this. If there isn't a better way, I guess I could write a function to construct the destination list based on an object (like an enum) that binds the numeric indicies to the corresponding page view, but this seems like a lot of machinery to create for a routine navbar task, especially in this modern OO context -- weird that selecting navbar pages would need a switch in the switchless programming world. Yet the example implementations have switches and two separate areas of the code need to be maintained to do one thing. But, what do I know; obligatory newbie disclaimer follows...
I think my question above was poorly phrased, based on ignorance and seeing some naive examples that suggested page navigation was generally done with an integer like selectedIndex. Once I learned about the Navigator class, and other navigation and List related builders, I saw the OO techniques Flutter provides for putting together hierarchical menus and such without the use of switch statements and integer indicies. So my answer to somebody like me who asks "do I really need to re-invent the navigation wheel" is to learn about Navigation Basics first.
I'm new with GWT and recently I've added Actvivities, Places and ActivityMappers to my code. I have one ActivityManager-ActivityMapper per each region. Regions like the header or the menu dont't change a lot so I have to write a lot of boilerplate code in the ActivityMapper for load the same Presenter every time but with a different constructor. For every possible Place I have to write another constructor for the Presenter in order to take the instance given by the ActivityMapper. There's any way to do that easier? Moreover, I'm not happy with the idea of creating a new Presenter every time that we move to a new place(even if you are going to load the same Presenter). In fact I have a big problem with that, cause my activities never die and they keep receiving events.
When an ActivityMapper returns the exact same Activity instance (reference equality, i.e. ==, not equals()) as previously, then the activity is not restarted, and the region is not touched. This is a deliberate optimization for those cases of regions that don't change often (e.g. headers or menus, or a master region in a master-detail setup). This is also the reason for the CachingActivityMapper (and FilteredActivityMapper, specifically designed to be used with the CachingActivityMapper in a master-detail setup)
In your case, it seems like you're imposing a rule to yourself that makes it hard for you to take advantage of this optimization: you're passing the current place to your activity's constructor, for no apparent reason.
(note: names were changed to protect the guilty)
So, let's say I am doing modifications in VisualCruft 8 (long since discontinued by the vendor), and applying those modifications to ERP software from "Company A" (long since bought out by "Company S", who then discontinued the software a few years later due to VisualCruft being discontinued itself).
One of the modifications I frequently need to do is to add, change, or remove fields to a form. This is the bread-and-butter of most ERP software and shows up several times a year. The layout of our ERP package, we'll call it "HotPockets Version 3", divides the forms into "class libraries" that in turn are just buckets to hold the form objects proper; and of course, there are "control" objects embedded inside the "forms".
With each install of HotPockets 3 you get the system version of the form, and a child version in a separate class library that is just a re-class of the parent, so that the forms, controls, business logic, etc. are all present in the child. The intent, as stated by the original ("Company A") vendor, is to allow you to make changes to the child that override the parent, while allowing said vendor to release patches/changes/whatever into the parent. In theory this looks great. In practice, there are issues. Specifically, how does one remove or change a control on a form, when the control itself is defined in the parent, and the parent is not to be changed? The intent is to replace a control with a different control that has different behavior, although in some cases I have also had to remove a control altogether. So, you could also ask this as "How do I remove an inherited object from a child without altering the parent"?
There are some known solutions to this, but I am asking the question because I want to see if there is a better answer than what I will provide. To avoid duplicate answers, I will list the currently known solutions, but keep in mind, I am looking for what is not listed here.
Improper Solution 1
While I am not supposed to modify the parent, I do have access to the parent code. I can simply note all of the properties and methods for the control in question, delete the control from the parent, then go back into the child and re-implement all of the properties and methods. For drastic changes, such as replacing a single text box with a grid of something, this is doable. It is, however, completely "against the rules" and the VAR that provides support for this product kinda frowns on it (note: the VAR is NOT "Company A" or "Company S"). Do keep in mind that not only is the original vendor defunct but the product and the language are defunct as well.
Improper Solution 2
"Hide" the component on the parent form, then (if incorporating a new component) add the new changes to the child. This sounds great in practice, until you realize that all of the old code hooks into the parent, which is now...invisible. Which means renaming the parent's version of the component, and going through all of the "port the properties and methods" listed in Solution 1. In which case...why am I doing this again?
Improper Solution 3
For those instances where I want a control to go away, I can hide the control in the child by setting the visibility property. This does not remove or alter any existing code, and everything continues to function. However, the control and the database field bound to it are now "tied up" and unavailable unless I look at something like Solution 1 or Solution 2, because any attempt to mess with the control will possibly result in side effects during calls, and/or replacing data underneath the control programmatically makes it impossible for the user to observe what is happening. And this only addresses the control visually going away - it does not address replacing it.
Extremely Improper Solution 4
Damn the torpedos, delete the control from the parent, full steam ahead!
Obviously, this is a very bad decision...
You'll note that all of the solutions involve touching the parent, something that I am not supposed to do - which brings us full circle to the question.
Other Solutions Not Available
Replace HotPockets Version 3 with something else. This is not in the realm of my decision making.
Rewrite whatever I need for HotPockets Version 3 with (insert favorite pet language here). While I would not mind doing this - I do have my own pet languages like most folks - it's not a possibility because VisualCruft pretty much works with just itself. Trust me on this, it's about 10x more effort than it is worth to write an extension in VisualWidget 3.14159 or HeyDoYouSmellCoffee or even ILikeSnakes, figure out some kind of cross-language calling, integrate it, test it, get IT management to sign off on it, get the VAR to also sign off on it, than it is to just stick with the native tools in VisualCruft.
Have the VAR do it. Don't take it the wrong way, the VAR does a good job providing ongoing support and has provided lots of useful modifications. But there are cost-factors that play into this, and it's more cost-effective if the changes were made by me, rather than the VAR. This has nothing to do with them at all.
First let me say that from my point of view it is not so easy to quickly get what your question is.
With each install of HotPockets 3 you get the system version of the
form, and a child version in a separate class library that is just a
re-class of the parent, so that the forms, controls, business logic,
etc. are all present in the child. The intent, as stated by the
original ("Company A") vendor, is to allow you to make changes to the
child that override the parent, while allowing said vendor to release
patches/changes/whatever into the parent. In theory this looks great.
In practice, there are issues.
>In theory this looks great.
No it does not! Inheritance is the strongest relationship two entities (aka classes) can have. That is why it has been proposed by Gamma in 1994 and it is widely agreed that:
Favor delegation over inheritance!
Delegation is far more decoupled then inheritance. In your case e.g. you could create a key-value store for your customers custom form fields.
However from your question it is not clear to me if such a change is within your field of responsibility.
But this is the source of all the "evil" you are describing.
Further on my impression is within your question you mix up a parent-child-relationship within the class structure at design time and a parent-child-structure of a tree-like class-composition of the objects-structure at run time.
Think of it this way: Your classes are File and Folder, and Folder can contain Files... By this you can create arbitrary tree like structures at runtime. This is called the "Composition Design Pattern". If you are not aware of it you will get lots of information on the net.
Finally within your bounty you are proposing
that this is a limit of the OOP paradigm
.
IMHO it is not - it is a serious flaw in the design due to a clear misuse of inheritance.
So back to your question:
How do I remove an inherited object from a child without altering the parent?
At design time or at run time? (You can not alter a inheritance at run time)
Remove the inheritance completely at design time.
The question "How do I remove an inherited member of a child" in and of itself is incorrect. I actually know what you mean in this context. However, your wording for the question is basically asking something impossible. If you are creating a parent-child, is-a, inheritance chain between two types, you are assuming one is a more specialized form of the other. If you ever think that human may someday evolve so much that it would have nothing in common with "animals", you should never inherit "human" from "animals". Let's remember that inheritance is as coupled as it gets between classes.
I think Martin is correct in stating that this is not really a limitation in the OOP paradigm. Surely "Hot Pocket" designers knew what type of modifications their users should be able to do when they used inheritance for the child classes. They definitely wanted to "allow" certain changes and "disallow" certain other changes. For example they wanted you to be able to change the behaviour of a control but not be able to remove it.
Having said that, if you do not agree and believe that adding or removing controls is definitely something "Hot Pocket" designers had in mind, then they should have also provided you with a means of doing just that.
All in all, there are three possible scenarios:
-A "Hot Pocket" designers actually give you a way to achieving
this(which obviously wouldn't have led to your question here).
-B "Hot Pocket" designers messed up. They didn't have a clue a user may need
to delete a control in a child class.
-C "Hot Pocket" designers knew
what they were doing. They don't want you to be able to add or delete a control
to the child forms.
In Case A), we already have a solution. Just as the "Hot Pocket" people.
In case B), as it is their fault they can't frown on you having to change the parent because otherwise you have no "good" choice.
In case C), you either submit to the limitation or as in your case, you are forced to change the parent although that is not supported. Of course, that would cause frowns and come with its caveats because you are going against the design.
This is getting longer that I intended, I finish with maybe one more solution that you have not added.
Given the fact that you have no way of "deleting" an inherited member, maybe a better approach than changing the parent would be actually adding a new parent for your special case. If you need to remove a control, in a new copy of the parent make your changes and have the child inherit from the new parent class. By doing this you are actually extending the framework rather than changing it. Maybe this would cause less frowns?
I started writing some code for a 2D game, created a class "objets" trying to keep it as generic as possible. I have a few methods and attributes that are common to every kind of element (buldings, ppl, interface buttons, etc) like (w, h, x, y ...you know) but most of them only make sense when applied to and specific type of item.
So I would have to inherit a new class for every type of actor in the game?
Just wondering if this is a common practice, or maybe i should manage it in a different way.
Thanks in advance.
If you're introducing behaviour then subclass, however if the difference is attribute based then don't e.g.
Animal (has .colour and .makeSound) -> Dog (has .eatOwnPoop) -> RedDog (no, too specific, covered by colour)
Notice how I had ".makeSound" in Animal. I could have put .bark in dog, but then I'd have to put .meow in cat etc. The subclass can simply override and provide a concrete sound.
However, you can use interfaces to better cross-cut your code, but that's quite a lengthy topic and probably overkill for your needs (although it could help any unit testing you do).
It sounds like you are over-using inheritance. It is certainly a red flag when you simultaneously say "common attributes like ..." and "...only make sense when applied to a specific type." Also, it is a red flag that domain objects such as building share a common base class with an interface object like button. Finally, it is quite unusual to define your own objet (object?) class from which every class in your system derives. It's not inconceivable, but in combination with your other comments, it sounds like you've started down an unproductive path.
You might want to refer to a good tutorial on object-oriented design and analysis such as "Head First OOA&D"
You do not HAVE to do anything. Generally, it is useful to use derived classes if they exhibit some kind of commonality but become more specialised in nature requiring specific functionality at each level of inheritance. It is also good to use if you want to have polymorphic behaviour. You have asked a very open ended question but basically do not feel that you HAVE to use inheritance as not every problem requires it and indeed some people overuse inheritance, introducing it in places where it really is not needed. All in all, I would really recommend that if you haven't already that you read a good book on object oriented design as this will then get you to think about your code from a different perspective and greatly improve the way you view software and design it. It may sound like a cop out but this kind of question is very hard to answer without knowing all details of what you are doing.
I'm developing an app based on a TabBarController.
I have 2 views that do some similar actions.
My question is, should I make 2 different classes or should I use only one class with some "if" statements asking if is a class is one or the other. I need maximum performance on this.
The 2 views load a MKMapView, so I need to know if it is better to load just one object that does the entire thing, or two objects that do similar things.
Thanks!
In Object Orientation it's important to bear in mind the difference between a class and instances of that class.
If you ever find yourself thinking of writing some code in a class that says "What class am I? If I'm class X, do thing A; otherwise do thing B" -- don't! It's a classic problem begging for a nice object oriented solution. There are two common solutions in this kind of situation:
1) Write a single class that at instantiation time gets passed in some vital information that it then uses later. Then another part of your code is making instances of this class configured in the correct way -- e.g. in your problem, two instances of this class get made, each with a different bit of info (map location perhaps?) passed into the init method
2) Write a superclass that has two subclasses that specialise the general bahaviour of the superclass. So most of your logic and code goes in the superclass - suppose it's called MapDisplayViewController - but then you extend this class with two subclasses called (for example) MapDisplayViewControllerA and MapDisplayViewControllerB that override one or more methods in important, different ways to differentiate them.
For your problem it sounds like approach 1) would be good.
Having code which says "What class am I?" is often a good example of a 'code smell' -- in other words, a sign that something could be designed much better.
I would say load two objects. iOS will automatically unload your currently not displayed views if it needs more resources anyway. (Assuming you implemented viewDidLoad and viewDidUnload properly, of course).
In addition, if in case your view needs to initialize/load a lot of data when tab is switched, and the common flow involves the user switching from one tab to another frequently, the app may appear to lag during frequent loading, if you use only 1 object. No one likes long and frequent loading times.
Just my opinion though, based on the information your original post provides. A lot of additional factors can still come into play.