I am still trying to understand how a "clean" BLOC design would be built. I am confused as to what goes in repositories, what goes in BLOC.
Let's say I have an app which fetches its settings initially from a server, and stores them locally.
If I have the following:
repositories:
local_storage: knows where to read/write settings
server_apis: knows how to interact with the server's APIs
BLOCs:
app_init_status: conditions the display, potentially lets the user enter a user account etc...
Where would be the logic which, upon init, reads the local_storage, and figures out if a server API call is needed (and launches it automatically)?
Would it go in the BLOC, even though it is not directly linked to displaying data, or would it be a third 'app_config' repository, talking directly to local_storage and server_apis ?
When we call the server APIs and get our config data back, who would relay that data back to local_storage to store it?
I do not believe this is an opinion based question, as it is a pretty basic use case used to illustrate how BLOC is used.
Thanks!
it's depend on situation, but most cases you can use local storage repository inside server_api repository and then use server_api repository inside bloc
Related
I've been trying to use Reso Coder's Flutter adaptation of Uncle Bob's Clean Architecture.
My app connects to an API, and most requests (other than logging in) require an authentication token.
Furthermore, upon logging in, user profile information (like a name and profile picture) is received.
I need a way to save this data upon login, and use it in both future API requests and my app's UI.
As I'm new to Uncle Bob's Clean Architecture, I'm not quite sure where this data belongs. Here are the ideas I've come up with, all involving storing the data in a User object:
Store the User in the repository layer, in an authentication feature directory. Other repository-level methods can pass it to the appropriate datasource methods.
This seems to make the most sense; other repository-level methods that call other API calls can use the stored User easily, passing it to methods in the data source layer.
If this is the way to go, I'm not quite sure how other features (that use the API) would access the User - is it okay to have a repository depend on another, and pass the authentication repository to the new feature repository?
Store the User in the repository layer, in an authentication feature directory. Other (non-login) usecases can depend on both this repository and on one relevant to their own feature, passing the User to their repository methods.
This is also breaking the vertical feature barrier, but it may be cleaner then idea 1.
For both these ideas, here's what my repository looks like:
abstract class AuthenticationRepository {
/// The current user.
User get currentUser;
/// True if logged in.
bool get loggedIn;
/// Logs in, saving the [User].
Future<void> login(AuthenticationParams params);
/// Logs out, disposing of the [User].
Future<void> logout();
/// Same as [logout], but logs out of all devices.
Future<void> logoutAll();
/// Retrieves stored login credentials.
Future<AuthenticationParams> retrieveStoredCredentials();
}
Are these ideas "valid", and are there any better ways of doing it?
I see another option to tackle the problem. The solution I want to talk about comes from the domain-driven design and is an event based approach.
In DDD you have the concept of a bounded context. A business object (uncle bob's entity) can have different meanings in different bounded contexts. Take a look at your user business object. The data and methods that some use case uses is often differnt to the data and methods that other use cases use. That's why you have differnt user objects in differnt bounded contexts. They are a kind of perspective that each use case has on the same business object.
If a business object is modified in one bounded context it can emit a business event. Another feature can listen to those events. The event mechanism can either be a simple observer pattern or if you need to distribute your application features via microservices a message queue. In case you use a simple observer patter the event emitter and event handler can run within the same data source transaction. But they can also run in differnt ones. It depends on your needs.
So when the sign-up use case registers a new user it emits a UserSignedUpEvent. Other features can now listen to this event. The event carries the information of the user, like the email, the name, the profile image and other infomration that the user provided during sign-up. Other features can now save the piece of data they need to their own data source. It can be the same as the sign-up use case uses (just other tables or another schema). But it is also possible that is a completely differnt data source, maybe another kind of data source like a nosql db. The part I wrote above about transactions is of course more difficult if you have different data sources.
The main point is that each feature has it's own data and manages it. It might be a copy of the whole user infromation, but in a lot of cases it is only a subset.
The event-based approach can give you perfect modularization. But as it is always when something looks great, it comes at a cost. You have to duplicate some part or even all data. When you think of a microservice architecture and some features are in different microservices it means that the duplication increases the availability of the service. The service can operate even the main service that manages the data is down, because a local copy exists. But now you have to deal with consistency issues - eventual consistency.
At this point I like to stop and guide you to other sources for details:
Chapter 8: Domain Events, Implementing Domain-Driven Desing, Vaughn Vernon
The many meanings of event-driven architectures, Martin Fowler
I am a rather fresh Flutter programmer so please excuse any flaws in the questions below…
I am struggling with a structural/ architecture dilemma. Here is the background:
App rationale:
my app allows its users to check little jobs available in their area and if they find time and are in a proper location to execute the job for a remuneration,
the app uses standard REST API (not Firebase) so that the server cannot be relied on sending status change notifications to trigger re-fetching of data,
the critical elements are (1) up-to date list of jobs for a given address - other user may have already taken on a job in an address (timed refresh of list e.g. every 5 mins), and (2) the app needs to keep track of the user’s location and accordingly ask the server for jobs if the user relocates by more than 2km in less than the refresh time,
The challenge:
I guess that on the basic level the app should have the following providers: (1) auth – providing the authToken, (2) geolocation – regularly checking user’s location, (3) jobList - for particular location (fetches high level job descriptions and addresses(, (4) jobDetails – fetches exact instructions for carrying a particular job,
as you can see: (2) geolocation and (3) jobList – need to refresh programmatically (at interval or on some change of geolocation), while (1) auth, (4) jobDetails are triggered by the user.
The Big Question ;) is … what is the proper architecture for the above type of app? More specifically:
should I use services for connecting to the server API and these would in turn be used by the providers?
how to ensure programmatic refetch of jobList on timer and relocation event from geolocation?
how to continually listen to location changes to detect a relocation but not overwhelming the app with processing?
should I store the (quickly outdating) jobLIst data just in its object class or should I use settings provider or a local db or maybe there is an easy way of storing the latest JSON response not to have to build the settings provider or db mapping?
in all my call to Auth api I need to provide the deviceId - how to make it available accross the app - this is pretty static but is needed in authentication so should checking it be a part of the auth provider?
If you could comment on the above or suggest a source of relevant examples I would be really grateful.
Thanks and cheers!
Here are my thoughts:
how to continually listen to location changes to detect a relocation but not overwhelming the app with processing?
You can rely on third party to do this for you. Such as: geolocator. With this, you can specify the amount of distance the user must have moved before the package notifies you of the change in user location.
should I store the (quickly outdating) jobLIst data just in its object class or ...
Since it is likely for a job listing app to use this data often and in various places, I would prefer to use db. It would be helpful in the long run too, if you plan to have some sort of analytics done on the mobile end or to gather any insights.
in all my call to Auth api I need to provide the deviceId - how to make it available accross the app ...
When you app is initialized, you could fetch the deviceID and store it in shared_preferences. Then in auth api, you could just retrieve it before making the API call.
should I use services for connecting to the server API and these would in turn be used by the providers?
As for geo location, geolocator can update you about the change in location and you could make an API call based on that.
However, if you plan to have a timer based approach to refresh your job listing, then you must realize that your users are likely to face issues arising from your inconsistent data. If you have plans to tackle it, then this implementation here might help. But I strongly feel that server supporting push notifications or maybe a web socket approach would be ideal here.
I’m building a new web application which needs to work seamlessly even when there is no internet connection. I’ve selected Angular and am building a PWA as it comes with built-in functionality to make the application work offline. So far, I have the service worker working perfectly and driven by the manifest file, this very nicely caches the static content and I’ve set it to cache a bunch of API requests which I want to use whilst the application is offline.
In addition to this, I’ve used localStorage to store attempts to invoke put, post and delete API requests when the user is offline. Once the internet connection is re-established, the requests stored in localStorage are sent to the server.
This far in my proof of concept, the user can access content whilst offline, edit data and the data gets synced with the server once the user’s internet connection is re-established. This is where my quandary begins though. There is API request data cached automatically by the service worker as defined in the manifest file, and there is a separate store of data for data edits whilst offline. This leads to a situation where the user edits some data, saves the data, refreshes the page and the data is served by the service worker cached API.
Is there a built in mechanism to update API data cached automatically by the service worker? I don’t fancy trying to unpick this manually as it seems hacky and I can’t imagine it’ll be future proof as service workers evolve.
If there isn’t a standard way to achieve what I need to do, is it common for developers to take full control of offline data by storing it all in IndexedDB/localStorage manually? i.e. I could invoke API requests and write some code which caches the results in a structured format in IndexedDB to form an offline database, then writes back to the offline database whenever the user edits some data, and uploads any data edits when the user is back online. I don’t envisage any technical problems with doing this, it just seems like a lot of effort to achieve something which I was hoping to be standard functionality.
I’m fairly new to developing with Angular, but have many years of other development experience. So please forgive me if I’m asking obvious questions, I just can’t seem to find a good article on best practices for working with data storage with service workers.
Thanks
I have a project where my users can edit local data when they are offline and I use Cloud Firestore to have a local database cached available. If I understood you correctly, this would be exactly your requirement.
The benefit of this solution is that with just one line of code, you get not only a local db, but also all the changes made offline are automatically synchronised with the server once the client gets online again.
firebase.firestore().enablePersistence()
.catch(function(err) {
// log the error
});
// Subsequent queries will use persistence, if it was enabled successfully
If using this NoSQL database is an option for you I would go with it, otherwise you need to implement the local updates by yourself as there is not a built in solution for that.
Context
Greetings,
One day I randomly found RethinkDB and I was really fascinated by the whole real-time changes thing. In order to learn how to use this tool I quickly spinned up a container running RethinkDB and i started making a small project. I wanted to make something very simple therefore i thought about creating a service in which speakers can create room and the audience can ask questions. Other users can upvote questions in order to let the speaker know which one are the best. Obviously this project has a lot of realtime needs that i believe are best satisfied by using RethinkDB.
Design
I wanted to use a vary specific set of tools for this. The backend would be made in Laravel Lumen, the frontend in Vue.JS and the database of course would be RethinkDB.
The problem
RethinkDB as it seems is not designed to be exposed to the end user directly despite the fact that no security concern exists.
Assuming that the user only needs to see the questions and the upvoted in real time, no write permissions are needed and if a user changed the room ID nothing bad will happen since the rooms are all publicly accessible.
Therefore something is needed in order to await data updates and push it through a socket to the client (socket.io for example or pusher).
Given the fact that the backend is written in PHP i cannot tell Lumen to stay awake and wait for data updates. From what i have seen from the online tutorials a secondary system should be used that should listen for changes and then push them. (lets say a node.js service for example)
This is understandable however i strongly believe that this way of transferring the data to the user is inefficient and it defeats the purpose of RethinkDB.
If I have to send the action from the client's computer (user asks a question), save it to the database, have a script that listens for changes, then push the changes to socket.io and finally have the client (vue.js) act when a new event arrives, what is the point of having a real-time database in the first place?
I could avoid all this headache simply by having the Lumen app push the event directly to socket.io and user any other database system instead.
I really cant understand the point of all this. I am not experienced with no-sql databases by any means but i really want to experiment with them.
Thank you.
This is understandable however i strongly believe that this way of transferring the data to the user is inefficient and it defeats the purpose of RethinkDB.
RethinkDB has no built in mechanism to transfer data to end-users. It has no access control (in the conventional sense) as well. The common way, like you said, is to spin up one / multiple node instance(s) running socket.io. On each instance you can listen on your RethinkDB change streams and use socket.io's broadcast functionality. This would be a common way, but as RethinkDB's streams are pretty optimized, you could also open a change stream for every incoming socket.io connection.
I am writing an app for iOS that uses data provided by a web service. I am using core data for local storage and persistence of the data, so that some core set of the data is available to the user if the web is not reachable.
In building this app, I've been reading lots of posts about core data. While there seems to be lots out there on the mechanics of doing this, I've seen less on the general principles/patterns for this.
I am wondering if there are some good references out there for a recommended interaction model.
For example, the user will be able to create new objects on the app. Lets say the user creates a new employee object, the user will typically create it, update it and then save it. I've seen recommendations that updates each of these steps to the server --> when the user creates it, when the user makes changes to the fields. And if the user cancels at the end, a delete is sent to the server. Another different recommendation for the same operation is to keep everything locally, and only send the complete update to the server when the user saves.
This example aside, I am curious if there are some general recommendations/patterns on how to handle CRUD operations and ensure they are sync'd between the webserver and coredata.
Thanks much.
I think the best approach in the case you mention is to store data only locally until the point the user commits the adding of the new record. Sending every field edit to the server is somewhat excessive.
A general idiom of iPhone apps is that there isn't such a thing as "Save". The user generally will expect things to be committed at some sensible point, but it isn't presented to the user as saving per se.
So, for example, imagine you have a UI that lets the user edit some sort of record that will be saved to local core data and also be sent to the server. At the point the user exits the UI for creating a new record, they will perhaps hit a button called "Done" (N.B. not usually called "Save"). At the point they hit "Done", you'll want to kick off a core data write and also start a push to the remote server. The server pus h won't necessarily hog the UI or make them wait till it completes -- it's nicer to allow them to continue using the app -- but it is happening. If the update push to server failed, you might want to signal it to the user or do something appropriate.
A good question to ask yourself when planning the granularity of writes to core data and/or a remote server is: what would happen if the app crashed out, or the phone ran out of power, at any particular spots in the app? How much loss of data could possibly occur? Good apps lower the risk of data loss and can re-launch in a very similar state to what they were previously in after being exited for whatever reason.
Be prepared to tear your hair out quite a bit. I've been working on this, and the problem is that the Core Data samples are quite simple. The minute you move to a complex model and you try to use the NSFetchedResultsController and its delegate, you bump into all sorts of problems with using multiple contexts.
I use one to populate data from your webservice in a background "block", and a second for the tableview to use - you'll most likely end up using a tableview for a master list and a detail view.
Brush up on using blocks in Cocoa if you want to keep your app responsive whilst receiving or sending data to/from a server.
You might want to read about 'transactions' - which is basically the grouping of multiple actions/changes as a single atomic action/change. This helps avoid partial saves that might result in inconsistent data on server.
Ultimately, this is a very big topic - especially if server data is shared across multiple clients. At the simplest, you would want to decide on basic policies. Does last save win? Is there some notion of remotely held locks on objects in server data store? How is conflict resolved, when two clients are, say, editing the same property of the same object?
With respect to how things are done on the iPhone, I would agree with occulus that "Done" provides a natural point for persisting changes to server (in a separate thread).