I have an Activity that contains 3 RecyclerViews. I need populate RecyclerViews with data from remote repository (3 different requests). Can I use multiple ViewModels in the Activity, or is there any better solution (best practice).
According to the open/closed principle, you should create three different ViewModels. The complexity isn't increased that much, and you are gaining the ability to move one ViewModel (or just reuse it) with the corresponding RecyclerView to the another Activity very easily.
Of course, sometimes breaking rules makes sense - for example if you know, there is no chance, that RecyclerView will be reused or moved to another screen, and then you can go for simpler solution with one ViewModel.
The same situation if the ViewModel (even with the 3 lists) is likely to stay always very simple (just three LiveData fields, just a few lines of code to populate them), you can break this rule.
However violation of O/CP is not a good practice - it's just a conscious breaking of rule.
In this case I would recommend to use one view model which populates three different LiveData objects. This way the UI can get updated whenever one of your three requests gets a response. For details how to use a RecyclerView with LiveData take a look into the Google Example.
I think having multiple viewmodels per activity only increases complexity and I do not see any value in doing that.
I got two recyclerview in a fragment. I think that use two ViewModels would be better. Cause different recyclerviews got their own data request, and state handling especially connections error.
In this case separate into different ViewModels would not increase the the complexity, but I think it well fit the rule of decupling
Even simpler, you can have one ViewModel, that uses one service class, which in turn uses the three repositories to get the data. For example:
XActivity --> XViewModel --> XService --> {Arepository, Brepository, Crepository}
I am working on an Android test project composed of 3 main parts, each of which developed following the MVP pattern.
These parts are nested into each other and I would like to know if the strategy I am following is the correct/best one or not
Structure:
Book: ViewPager containing different Pages
Page: Custom LinearLayout containing multiple Items
Item: Custom View (in this example a simple button)
Each part uses a MVP structure (so for example for Book I made BookPresenter, BookView and BookModel, and the same for Page and Item)
As a user case I would like to keep track of how many times the user clicks the button and for each time change the Page background to a random color, and when the user reaches the 10th click tell the BookPresenter to go to the second page.
To do so I set up things so
BookView creates the BookPresenter, which in turns creates each PageView.
Each PageView creates the PagePresenter which in turns creates the ItemView (which ultimately creates the ItemPresenter)
In all this, the BookPresenter has a reference to the PagePresenter, and the PagePresenter has a reference to the ItemPresenter, so when some actions need to take place they can communicate to the child or the parent presenter in the structure
Now the question:
Is this the right way to set up a system with nested MVPs?
Because if I then want to have a PageView but instead of in a Book I need to put it in a Newspaper (other class with some alternative behaviour to Book) I still would need to recreate the whole chain of dependencies with Presenters and all the rest...
How does a “Child-Presenter” communicate with its “Parent-Presenter”? They don't (directly, not via EventBus)
From my point of view such Parent-Child relations are code smells because they introduce a direct coupling between both Parent and Child, which leads to code that is hard to read, hard to maintain, where changing requirement affects a lot of components (hence it’s a virtually impossible task in large systems) and last but not least introduces shared state that is hard to predict and even harder to reproduce and debug.
So far so good, but somehow the information must flow from Presenter A to Presenter B: How does a Presenter communicate with another Presenter? They don’t! What would a Presenter have to tell another Presenter? Event X has happened? Presenters don’t have to talk to each other, they just observe the same Model (or the same part of the business logic to be precise). That’s how they get notified about changes: from the underlying layer.
Whenever an Event X happens (i.e. a user clicked on a button in View 1), the Presenter lets that information sink down to the business logic. Since the other Presenters are observing the same business logic, they get notified by the business logic that something has changed (model has been updated).
Source: http://hannesdorfmann.com/android/mosby3-mvi-4
So let's apply this on your example.
You should have something like a Readable (if you want to make an abstraction over Book implements Readable and NewsPaper implements Readable). With Readable.getPageCount() you get the number of pages for ViewPager. With Readable.getCurrentPage() you get the current Page. Then you need some kind of Observer Pattern to be notified whenever the page is changed. You also need a Listener / Observer pattern when the user clicks on a button in ItemView. Such a click would be an Event X from the graphic above. On click on the button you let the information flow down through Presenter to your business logic which changes the Readable object. This change will then notify observers of this Readable object like PagePresenter that then updates the PageView to set the background color of the page.
So the key is: communicate via Business Logic, instead of some View to View or Presenter to Presenter communication.
Hope that helps.
I am working on android application and some team members suggested using one activity and one parent layout for the whole application and each time we need a new screen we just inflate a new layout in the parent layout and destroy the old one.
I on the other hand think that using fragments would be the way to go.
Do you think one way is better than the other?
I would be grateful if you can think of solid arguments to support your claim.
Thank you
UPDATE:
We are using MVC in our application. They want to create a new class for the view while i opted for using the fragment as my view. As it stands now the application has one activity and one layout. To change the view we are calling the same layout,removing its child views and inflating the new view .I dont see how that would be better then just using the fragments as views
I think most Android developers will tell you to use one or more Activities or Fragments. But why?
Basically, because one Activity which is responsible for each and every View in your app would be something like a "god object". This is considered an antipattern, I suppose because it may fast become a maintenance nightmare.
The Android pattern of one Activity and several Fragments on the other hand is following the Single Responsibility Principle, so your code is easier to maintain. All the more so because Android-specific things like saving state on configuration change are much easier to implement if you are able to use the built-in methods.
I am planning an app and trying to explore all the possible development options/methods I have available. One thing I'm struggling to get my head around is Fragments. I see a lot of people praising them as you can "resuse" fragments throughout the app but I can't think of an example situation.
From some example apps I have looked at (all been tabular layouts) the code for each fragment has one layout, so why not have a seperate activity instead?
I am hoping to implement a tabular layout in my app. If anyone can give me an example of a fragment being reused within an app I hope it will give me a better understanding of the benefits.
"Reuse" is overrated. Of course - you can put this same fragment (with this same features) in different places of your application - let's say that you can use a fragment in different, horizontal and vertical layouts (as you probably saw in Google's tutorial).
But at the end using fragments simplifies your project - for example - you can switch fragments inside one activity and get benefits of much easier navigation and in app communication.
Using fragments gives you one more thing - flexibility. It's much easier to move some view from one place to another, or just remove from application. All that because fragment encapsulates logic and usually a view, still offering power of externally managed lifecycle.
(Thanks for comment from Richard Le Mesurier)
Fragment is not a View neither a ViewGroup. It is not a visual element at all. Fragment inherits directly from Object.
One should think of a Fragment as a unity of a reusable code, reusable in various Activities (the Activities consist of visible elements).
Thus if you can think of any code you can reuse through several Activities (even the same Activity with different layout) and that code somehow depends on Activity lifecycle, then you probably should make this code a Fragment.
This question already has answers here:
What is the benefit of using Fragments in Android, rather than Views?
(6 answers)
Closed 10 years ago.
What is the advantage to using Fragments over using custom Views that are reused in different layouts?
In the original blog post introducing fragments, Dianne Hackborn says that
[Fragments] make it easier for developers to write applications that can scale
across a variety of screen sizes, beyond the facilities already
available in the platform.
and she goes on to explain Fragments in the context of making a tablet layout for an app that combines the UI of two activities from the phone version of the same app.
But it seems that the same reuse could be achieved using custom Views. The main different between Fragments and Views seems to be that they have differing lifecycles...
The Fragment lifecycle is:
onAttach(), onCreate(), onCreateView(), onActivityCreated(), onStart(), onResume(), onPause(), onStop(), onDestroyView(), onDestroy(), onDetatch().
The View lifecycle is:
ctor, onFinishInflate(), onAttachedToWindow(), onMeasure(), onLayout(), onDetatchedFromWindow()
I'd like to hear from developers with experience writing large apps about what benefits (if any) they've seen in using Fragments vs custom Views to divide up the UI into reusable pieces.
The main reason is that fragments are more reusable than custom views.
Sometimes you can't create a fully encapsulated UI component relying on views alone. This is because there are things you would want to put into your view but can't because only an Activity can handle them, thus forcing tight coupling between an Activity and a View.
Here is one such example. Lets say you want to create a reusable UI component that, among many things, want to capture a photo and do something with it. Traditionally you would fire an intent that starts the camera and returns with the captured image.
Notice that your custom UI component can't fully encapsulate this functionality because it will have to rely on hosting Activity's startActivityForResult because views don't accept activity results (they can indirectly fire an intent through context).
Now if you wanted to reuse your custom UI component in different activities you would be repeating the code for Activity.startActivityForResult.
Fragment on the other hand cleanly solve this problem.
Similarly your fragment can contribute items to your options menu, something traditionally only an Activity could do. Again this could be important if the state of your custom view dictates what goes in the menu.
A fragment is way more than just a view. In fact it can even be totally without a view. It can have all sorts of stuff in it including AsyncTasks, various Listeners, file and database access and so on and so on.
Think of it as a small activity, but you can have multiple of them on the screen and work with them all including communicating with each other while they are visible.
E.g. you could have a list of shopping cart displayed in one fragment and the currently selected cart in detail in another fragment. You then e.g. change the quantity of an item in the detail view and the list view could be notified about it and update the total price in the list view. You can totally orchestrate interactions like that nicely while e.g. still having only one of them visible on a smaller screen device.
I have refactored a large business app (>15 activities) from activities to fragments to get good tablet support and I would never start a new app without fragments.
Update Feb 2016: While the above still holds true, there are complexities with fragments that caused many people to entirely avoid using them. Newer patterns such as usage of MVC approaches and more powerful views provide alternatives. As they say .. YMMV.
Some description:
Imagine Activity as a plate that hold one big cake.
Fragment would be a container that slices the same cake into pieces.
Each slice contains it own logics (listeners, etc).
And in total they are almost no different with the one big cake.
The benefit:
When you plate can't hold a big cake. (Screen is small) You can easily use a a few plates (Activity) to hold each of them WITHOUT the need to move your logics into the new activity.
Better re-usability. I have some instances where I could reuse a fragment entirely in another App. You might claim that a custom view could does that too. But refer to point 1, I could reuse it with just few lines of layout changes but for a custom view, it have to find a way to plug it into both layout and code.
It is, in some sense, a more OO ways of organising your UI logics in Android programming. When you have a feature (A new partition on the screen for example), you create a new Fragment class, with minor modification to existing activity class. However if you are programming only with activity, you will need to add logics and make big modification on tested class.
Just my 2 cents. :)
The lifecycle methods are probably your biggest hint. If you think about it, they correlate closely to the activity lifecycle (with some hooks into the activity and views). In fact, in the article you linked, Hackborn says:
In some ways you can think of a Fragment as a mini-Activity
As with many things in software design/development, there are a multitude of ways to do things. There are many different places you could put your code. Yes, you could probably put a lot into a view, but keeping different concerns separated in different classes is a good thing though. The classic pattern of this is MVC and it applies in this scenario. You don't want to bake in too much controller logic into your view. It's better to keep it in controller-like classes which are the activity and now the fragment. This is why the fragment's lifecycle is more like the activity's than the view's--it was made to facilitate this kind of organization.
I touched Fragments once and found them not very useful (see this post). From what I have read, A Fragment is really a fancy word for an Object with access to Activity Context. I like to ignore Fragments in my work, and just create these Objects myself. I have created very large, very demanding apps by passing an Activity to constructors, instead of Context. One major benefit, however, for using Fragments is that they are supported by the View layout system - so you can easily add them to Android xml (if you use it for your layouts).
Custom views are much more work than just using fragments in place of your activities. if you decide to use Activities and custom Views, you have to create your custom view, and then you have to implement the same activity lifecycle methods in your activity (a very similar lifecycle is used for fragments).
Using Fragments also allows you to separate components into their own classes (Fragments), rather than having too much logic in a single Activity. Let me ground that with an example:
Say you're implementing a magazine reader application. using fragments, you could create a fragment: ArticleList, which displays a list of articles, and another fragment: ArticleDisplay, which handles the logic for displaying content. you can then specify how these fragments should interact using the fragments tools, so that on a handset, you can use the full screen real-estate for ArticleDisplay, while on a tablet, you can display the fragments side by side.
If you were to attempt this with an Activity/custom view, you'd have the logic for both Fragments in your monolithic Activity, you'd have to write a custom view, and you'd have to debug this unwieldy monster.
Fragments are, in general, a more sophisticated and powerful way to write your applications. They can do everything an Activity can do, and more. If you don't need the extra functionality, the defaults will probably get you where you need to go, and with less work.