Upload data in the background depending on the connectivity - xamarin.android

I am having problems with understanding how to approach this problem as I am really new to xamarin and android both.
My problem is the following: I need to develop an android app, which takes photos of things and uploads them to a rest server. The confusing part is that the users have really poor connection, so I need to check the internet connection constantly and try to upload the photos when the internet is there. This check needs to be done in the background so the user wont notice any lag.
When the user clicks on "save" the app should save the photo and metadata to a local database or file (json or sqlite) and upload them when the internet is there.
I have tried many different approaches but failed due to lack of understanding how android threading and services work. (Bound service, Foregroundservice, SQLite)
As I dont have much time to research and test all the posibilities, I am asking you guys: How can I do this ?
Thanks for your understanding.

There is one nuget plugin Xam.Plugin.Connectivity for check internet continuously, and you can manage your code in event of this plugin.
For background, you need to create one background thread and you can call it from ConnectivityChanged event.
CrossConnectivity.Current.ConnectivityChanged += (object sender, Plugin.Connectivity.Abstractions.ConnectivityChangedEventArgs e) =>
{
if(e.IsConnected){
//your code here for fetching data when internet connected.
//Create task for background process
Task.Run(() =>
{
//your code for background
}).ConfigureAwait(false);
}
}

Related

Firestore local cache is stale when returning to mobile app - can I force a re-sync?

I'm watching a cloud firestore list for changes using query.onSnapshop in a react-native-firestore app, currently testing on iOS.
While my app is in the foreground, I can make data changes elsewhere (eg. in my companion web app) and the mobile app immediately updates as expected. Usually, if I make changes while the app is closed or offline, they get picked up no problem once it is re-opened or comes online again. Happy days.
However, sometimes, when the app is in the background (not closed, just some other apps have been used in the meantime), I'll make a change elsewhere (eg. add/delete a record which meets the query's criteria), then when I come back to the app, the list does not change - eg. it contains deleted records, or doesn't contain the new ones. Nothing I do on the app can change this - it remains out-of-sync, even if I make local changes, like editing one of the records (even a deleted one). Changing network conditions also does nothing (eg. switching airplane mode off/on again).
The only way the list will get back in sync is if I make another change elsewhere, while the app is still in the foreground, or if I force-close the app and re-open it again.
The issue seems to occur when connecting to both the emulator, and the actual firestore.
I don't think I'm doing anything fancy. Basically following the examples in the documentation:
import React, { useEffect, useState } from 'react';
import firestore from '#react-native-firebase/firestore';
const MyAssignments = (props) => {
const [records, setRecords] = useState([])
useEffect(() => {
const onSnapshot = (snapshot) => {
console.log(snapshot) // this IS triggered but data is stale
setRecords(snapshot)
}
return firestore()
.collection('assignments')
.where('assignedTo', 'array-contains', props.userId)
.onSnapshot(onSnapshot, console.error);
}, [props.userId]);
// render the list
return ...
}
I'm not sure if this is a general firestore issue, a react-native-firebase issue, an issue with the underlying firebase ios SDK, or just my own misunderstanding?
In either case, is there a way to force the local cache to re-sync programatically, ideally when the app regains focus? Or has anyone solved a similar issue or have any ideas what to try next?
Edit 1: Note the code example above is slightly simplified for readability, as parts are spread across a few files and typed with typescript. In reality, I'm using crashlytics.recordError(e) for error handling in production, but console logging, as above, in development.
Edit 2: To debug, I've tried the following:
Switch on debug logging:
import firebase from '#react-native-firebase/app';
firebase.firestore.setLogLevel('debug');
However, this gave no extra logs in my javascript console.
I found I could view native device logs by following this guide and then filtering for Firebase, like so: idevicesyslog --match Firebase
This still shows very few logs, so I don't think debug logging is switched on properly. However, it does log this error every time I foreground the app:
<Notice>: 8.9.1 - [Firebase/Firestore][I-FST000001] WatchStream (10c244d58) Stream error: 'Unavailable: Network connectivity changed'
This error happens every time though. Even when the onSnapshot successfully picks up changes

Can anyone recommend the solution to solve ionic background issue in IOS

The app is able to send the HTTP POST successfully as long as it's in the foreground, but as soon as it goes to the background, the HTTP POST appears to stop. This is only an issue for iOS.
I tried the cordova-plugin-background-mode plugin but i don't want the audio part. is there any working example code using angular ng.IHttpService.
public GetEmployeeList(): ng.IPromise<Mobile.Models.Employee[]> {
return this.$http.get("~/api/Employee/EmployeeList")
.then((response: ng.IHttpPromiseCallbackArg<Mobile.Models.Employee[]>): Mobile.Models.Employee[] => {
return response.data;
});
}
if you just wanna keep your app alive when its in the background, surely the audio part is the easiest way.and to sove your particular issue, you need know how the "Background Model" works. https://upload-images.jianshu.io/upload_images/1501971-91e6c1164e13c3ee.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1000
Being curious why dont you wanna use the normal way to salve the problem since there are so many tracks to use with the background model.

Can I use NSURLSessionUploadTask for offline syncing tasks?

I need something similar to Facebook's offline post capabilities. Basically I want users to create content locally on the device regardless of connection state, and whenever internet becomes available it should POST/PUT to the server.
I've searched the internet for a solution and I found that NSURLSessionUploadTask can be used for POST-ing in the background. But I couldn't figure out if the following scenarios are supported:
Will my task remain in the background queue when the user is offline and will the operating system try to execute items in the queue upon reconnecting with a network?
What happens if the application is force-closed by the user or crashes?
What happens if the operation fails?
First of all, background NSURLSession allows file upload only. If that is ok for you:
The task will be in the queue until it receives a server answer.
If your app is force-closed, the task will still be executing. When the request is done, your app will be launched in non-interactive background state and receive application:handleEventsForBackgroundURLSession:completionHandler:. After you process the signal and call the completion handler or 30 second timeout, the app will be closed.
I the operation fails, you will receive URLSession:task:didCompleteWithError:
There is a good tutorial on background NSURLSessions. I suggest you to read all 4 parts of this great article.
If file upload is not an option for you, i suggest you to save information into local database and then wait for internet is reachable. (a good approach here is use of Reachability library, Alamofire allows to do that too). When internet becomes available, simply call your http requests with saved data.
We were running into connectivity issues with our internal apps, so we wrote a Swift framework that allows any network operations to be enqueued and sent whenever the device has access to the internet -
https://cocoapods.org/pods/OfflineRequestManager. You'll still have to handle the network request itself within the object conforming to OfflineRequest, but it sounds like a good fit for your use case.
The simplest use case would look something like the following, though most actual cases (saving to disk, specific request data, etc.) will have a few more hoops to jump through:
import OfflineRequestManager
class SimpleRequest: OfflineRequest {
func perform(completion: #escaping (Error?) -> Void) {
doMyNetworkRequest(withCompletion: { response, error in
handleResponse(response)
completion(error)
})
}
}
///////
OfflineRequestManager.defaultManager(queueRequest: SimpleRequest())

iCloud + Core Data: First import and user's feeling of loss of data

I've implemented an iPhone application that has around 50k users. Switching from iOS7 to iOS8 a lot of these users have experienced a terrible feeling when they thought that they data get lost.
I've implemented the first-import behaviour that I thought was the one suggested by Apple
1) Users launch the App
2) iCloud, automatically, starts synching data previously stored on iCloud
3) At some point user get notified that data from iCloud is ready thanks to NSPersistentStoreUbiquitousTransitionTypeInitialImportCompleted
The problem is with 3) At some point:
Users that have to sync a lot of data need minutes to get the synch completed and in the meanwhile they think that their data is lost.
I really don't know how to let my users know that they have to wait to see their data synched, because I don't know when this operation starts.
I'm thinking about a possible solution:
During the first launch of the App, asking to the user if he wants to use iCloud. If he chooses to use it, building the database with iCloud options, so I know exactly that the synch is starting here (I suppose...)
I'm really not sure about how to implement this behaviour since I've always seen Core Data settings into the AppDelegate but to achieve this behaviour I suppose I need to move all the CoreData settings in a Controller.
What do you think about this solution? how are you working around this problem in you Apps?
Your idea is right, at least it is that what we do. But leave it in the appDelegate.
Differentiate between with iCloud and without iCloud when doing the "addPersistentStoreWithType". If you do it with iCloud options, it will directly start to build the local store which is a kind of a placeholder ( I'm sure you know that, but just to make my thoughts clear). As soon as this is done, the sync starts from iCloud. So this is the starting point I understood you were looking for.
You can watch that process using the notifications by NSPersistentStoreCoordinatorStoresDidChangeNotification and inform you user accordingly triggered by that notification.
If you look at "Reacting to iCloud Events" in the docs https://developer.apple.com/library/ios/documentation/DataManagement/Conceptual/UsingCoreDataWithiCloudPG/UsingSQLiteStoragewithiCloud/UsingSQLiteStoragewithiCloud.html#//apple_ref/doc/uid/TP40013491-CH3-SW5 there is a detailed desc.
To summarize, the event you're describing is part of the account transitions process. An account transition occurs when one of the following four events is triggered:
Initial import
the iCloud account used did change
iCloud is disabled
your application's data is deleted
During this event, Core Data will post the NSPersistentStoreCoordinatorStoresWillChangeNotification and NSPersistentStoreCoordinatorStoresDidChangeNotification notifications to let you know that an account transition is happening. The transition type we're interested in is NSPersistentStoreUbiquitousTransitionTypeInitialImportCompleted.
For information, I've moved all Core Data related code to my own Manager for simplicity and use it with a singleton design pattern. While setting up the singleton, I register the Manager for all relevant notifications (NSPersistentStoreDidImportUbiquitousContentChangesNotification, NSPersistentStoreCoordinatorStoresWillChangeNotification, NSPersistentStoreCoordinatorStoresDidChangeNotification, NSPersistentStoreCoordinatorWillRemoveStoreNotification).
I store several informations in my settings (NSUserDefaults or anything) like the last iCloud state (enabled, disabled, unknown), if the initial import is done or not, etc.
What I end up doing was having a prompt (UIAlertController or anything) to get a confirmation if the user wants to use iCloud or not. I have a displayICloudDialogAndForce:completion: method to do that and only do that if my iCloud state setting is unknown or I use the force parameter.
Then, after the user input, I call a setupCoreDataWithICloud: method, the iCloud boolean parameter depending on the user choice. I would then setup my Core Data stack, on the cloud or not according to the iCloud parameter.
If I'm setting up using iCloud, I would check my settings for the value of an iCloud imported key (boolean). If the value is NO, then I'm presenting a new modal to warn the user about the incoming import that could take some time.
I've registered my manager for different notifications and specially NSPersistentStoreCoordinatorStoresDidChangeNotification. In my storeDidChange: callback, I'm checking the transition type and if it's NSPersistentStoreUbiquitousTransitionTypeInitialImportCompleted, I'm changing the content of my modal to show the user that the import was successful and removing it a few seconds later, saving in my settings that the initial import is done.
- (void)storeDidChange:(NSNotification *)notification
{
NSPersistentStoreUbiquitousTransitionType transitionType = [notification.userInfo[NSPersistentStoreUbiquitousTransitionTypeKey] integerValue];
if (transitionType == NSPersistentStoreUbiquitousTransitionTypeInitialImportCompleted) {
[settings setDefaults:#(YES) forKey:kSettingsICloudImportedKey];
[ICloudModal dismissWithSuccess];
// ...
}
// Do other relevant things...
}

Make http call on iOS while offline

Perhaps I have been reading the wrong stuff, but one thing that all of the literatures that I have been reading seem to agree on is that: iOS does not allow background threads to run for longer than ten minutes. That seems to violate one of the greatest principles of app development: the internet should be invisible to your users. So here is a scenario.
A user is going through a tunnel or flying on an airplane, which causes no or unreliable network. At that instant, the user pulls out my email app, composes an email, and hits the send button.
Question: How do I the developer make sure that the email is sent when network becomes available? Of course I am using email as a general example, but in reality I am dealing with a very much simple http situation where my app needs to send a POST to my server.
Side Note: on android, I use Path’s priority job queue, which allows me to set it and forget it (i.e. as soon as there is network it sends my email).
another Side Note: I have been trying to use NSOperationQueue with AFNetworking, but does not do it.
What you want to achieve can be done using a background NSURLSession. While AFNetworking is based on NSURLSession I’m not quite sure if it can be used with a background session that runs while your app doesn’t. But you don’t really need this, NSURLSession is quite easy to use as is.
As a first step you need to create a session configuration for the background session:
let config = URLSessionConfiguration.background(withIdentifier: "de.5sw.test")
config.isDiscretionary = true
config.waitsForConnectivity = true
The isDiscretionary property allows the system to decide when to perform the data transfer. waitsForConnectivity (available since iOS 11) makes the system wait if there is no internet connection instead of failing immediately.
With that configuration object you can create your URL session. The important part is to specify a delegate as the closure-based callbacks get lost when the app is terminated.
let session = URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue.main)
To perform your upload you ask the session to create an upload task and then resume it. For the upload task you first create your URLRequest that specifies the URL and all needed headers. The actual data you want to upload needs to be written to a file. If you provide it as a Data or stream object it cannot be uploaded after your app terminates.
let task = session.uploadTask(with: request, fromFile: fileUrl)
task.resume()
To get notified of success or failure of your upload you need to implement the URLSessionDataDelegate method urlSession(_:task:didCompleteWithError:). If error is nil the transfer was successful.
The final piece that is missing is to handle the events that happened while your app was not running. To do this you implement the method application(_:handleEventsForBackgroundURLSession:completionHandler:) in your app delegate. When the system decides that you need to handles some events for background transfers it launches your app in the background and calls this method.
In there you need first store the completion handler and then recreate your URLSession with the same configuration you used before. This then calls it’s delegate for the events you need to handle as usual. Once it is done with the events it calls the delegate method urlSessionDidFinishEvents(forBackgroundURLSession:). From there you need to call the completion handler that was passed to your app delegate.
The session configuration provides some more options:
timeoutIntervalForResource: How long the system should try to perform your upload. Default is 7 days.
sessionSendsLaunchEvents: If false the app will not be launched to handle events. They will be handled when the user opens the app manually. Defaults is true.
Here is a small sample project that shows how everything fits together: https://github.com/5sw/BackgroundUploadDemo
Your app needs to store the data internally and then you either need something which will cause the app to run in the background (but you shouldn't necessarily add something specially if you don't already have a reason to be doing it) or to wait until the user next brings the app to the foreground - then you can check for a network connection and make the call.
Note that e-mail is very different to a POST, because you can pass an e-mail off to the system mail app to send for you but you can't do exactly the same thing with a POST.
Consider looking also at NSURLSessionUploadTask if you can use it.
In three words: you don't.
And that's actually a good thing. I certainly do not want to have to think and speculate about my last 20 apps, if they are still running in the background, using memory and battery and bandwidth. Furthermore, they would be killed if more memory is needed. How would the user be able to predict if it completed its task successfully? He can't, and need to open the app anyhow to check.
As for the email example, I'd go with showing the email as "pending" (i.e. not sent), until it transferred correctly. Make it obvious to the user that he has to come back later to fulfill the job.
While every developer thinks that his app has an extremely good reason for backgrounding, reality is, for the user in 99% it's just a pain. Can you say "task manager"? ;-)
I wrote a pod that does pretty much this - https://cocoapods.org/pods/OfflineRequestManager. You'd have to do some work listening to delegate callbacks if you want to monitor whether the request is in a pending or completed/failed state, but we've been using it to ensure that requests go out in poor or no connectivity scenarios.
The simplest use case would look something like the following, though most actual cases (saving to disk, specific request data, etc.) will have a few more hoops to jump through:
import OfflineRequestManager
class SimpleRequest: OfflineRequest {
func perform(completion: #escaping (Error?) -> Void) {
doMyNetworkRequest(withCompletion: { response, error in
handleResponse(response)
completion(error)
})
}
}
///////
OfflineRequestManager.defaultManager(queueRequest: SimpleRequest())

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