Develop Reference

How do I run code whenever the user opens a particular screen?

I have the code below that I thought would run whenever the given screen is reached, i.e. whenever the user goes to this screen. This screen creates some temporary files for the user. I don't need them after the user leaves the screen, so I wanted to flush them everytime the user reaches this screen. However, the line with await cleanupTempAudioFiles(); doesn't seem to be doing its job.
#override
void initState() {
super.initState();
initialize();
}
void initialize() async {
uid = auth.currentUser;
filesInProgressFileDirString = systemTempDir.path + '/App/AppAudioFiles/FilesInProgress/';
fileInProgressFileDir = await Directory(fileInProgressFileDirString).create(recursive: true);
myRecorder = await FlutterSoundRecorder().openRecorder();
myPlayer = await FlutterSoundPlayer().openPlayer();
controller = AnimationController(
vsync: this,
duration: const Duration(seconds: 10),
)..addListener(() {
setState(() {});
});
controller?.reset();
await cleanupTempAudioFiles(); //This code seems to not be running every time the screen is opened
setState(() {
sendableFileExists = 0;
});
}
Future<void> cleanupTempAudioFiles() async {
final dir = Directory(filesInProgressFileDir.path);
final List<FileSystemEntity> files = await dir.list().toList();
files.forEach((file) async {
if (file.path == filesInProgressFileDir.path + currentAppFilename) {
await file.delete();
}
if (file.path == filesInProgressFileDir.path + currentAppFilename + 'High.mp3') {
await file.delete();
}
if (file.path == filesInProgressFileDir.path + currentAppFilename + 'Low.mp3') {
await file.delete();
}
});
}

The initState() function is called when your object is put into the widget tree. This is not the same as every time it is displayed. Navigator.pop() for example will show the screen without re-inserting the widget into the tree. see https://api.flutter.dev/flutter/widgets/State/initState.html
To run every time the user sees a widget you should put the code into the build function or consider #overriding dispose(), didChangeDependencies() or
didUpdateWidget() instead to get the right part of the widget lifecycle.

Somehow catchError of http request does not work using cordova

So yesterday I was developing some sort of offline functionality. Therefore, I added an ApiService that returns Observables.
Currently, I fetch my access_token for jwt-Authentication and then use this token to generate Headers for my API-Request. After a successful request, I save the result to my storage. This works fine. Now here is the problem I want to check for an unsuccessful request (e.g. servers are down, app is offline) and then return my stored result from storage. But I can't get it to work.
Here is my code:
getJobs(): Observable<any> {
this.auth.checkToken()
return from(this.storage.get(ACCESS_TOKEN)).pipe(
switchMap(token => {
let options = this.auth.addToken(token)
return this.http.get(API_URL + "jobs", options)
}),
map(res => {
if (res) {
this.storage.set(JOBS, res)
return res
} else {
return from(this.storage.get(JOBS))
}
}),
catchError(() => {
return from(this.storage.get(JOBS))
})
)
}
Further investigations have shown that after the server or the app is offline neither the map() nor the catchError() functions were executed.
UPDATE:
The solution provided by DJ House is correct. My Code works perfectly in my browser but if I build my app with ionic cordova build android it gets stuck after the this.http.get(...) So it's clearly and issue with cordova
SOLUTION:
Wow! Something magical happened! I've found out that the catchError method gets called BUT after almost 2 Minutes, which is way to slow... So I will implement a timeout.
Thanks
flixoflax

The main issue that you may be facing is you are using the map incorrectly. Map acts upon a normal value (usually, its not an observable) and returns a new value. map() should always return the same type of value. In your map() you are either return the response (which I am assuming is of type Jobs) OR you are return an Observable<Jobs>. This will cause your subscribers to need verbose logic to handle that.
It looks like you are trying to use that map() to set your local storage with the returned jobs from your api. I would recommend using tap() since you aren't trying to change the value you are returning.
function getJobs(): Observable<any> {
this.auth.checkToken()
return from(this.storage.get(ACCESS_TOKEN)).pipe(
switchMap(token => {
let options = this.auth.addToken(token)
return this.http.get(API_URL + "jobs", options)
}),
// change to tap()
tap(res => {
if (res) {
this.storage.set(JOBS, res)
}
}),
catchError(() => {
return from(this.storage.get(JOBS))
})
)
}
If the switchMap throws an error, the tap will be skipped. That will ensure you only set storage if you recieve a value from the API. If you always want to set the storage (even if the API threw an error) then move the tap() to be after the catchError().

Can you please try moving the catchError operator as first operator inside pipe method. This is to ensure that you catch error as soon as you recieve it from observable. Please change it like below:
getJobs(): Observable<any> {
this.auth.checkToken()
return from(this.storage.get(ACCESS_TOKEN)).pipe(
switchMap(token => {
let options = this.auth.addToken(token)
return this.http.get(API_URL + "jobs", options)
}),
catchError(() => {
return from(this.storage.get(JOBS))
})
map(res => {
if (res) {
this.storage.set(JOBS, res)
return res
} else {
return from(this.storage.get(JOBS))
}
}),
)
}

How to wait for firebase data fetch finishes and get values not a promise in react-native?

I have data in firebase data that looks like the following:
The code for getting customers data:
getCustomersOnQeueu = async () => {
let customers = this.customersRef.orderByChild("ticket").once('value')
return customers
}
Code for rendering data:
renderCustomers = () => {
let customersViews = []
this.getCustomersOnQeueu().then((customers) => {
let customersTickets = customers.val()
console.log(customersTickets)
let sortedKeys = Object.keys(customersTickets).sort(function(a, b){
return customersTickets[b].ticket - customersTickets[a].ticket
})
console.log(sortedKeys)
for(i=0; i<sortedKeys.length; i++) {
let key = sortedKeys[i]
console.log(customersTickets[key]["customer"])
customersViews.push(<View>
<Text>{customersTickets[key["customer"]}</Text>
</View>)
}
})
return (<View>
<Text>Available Customers: </Text>
{customersViews}
</View>)
}
render() {
return (
<View>
{this.renderCustomers()}
</View>
)
}
Now after data being fetched and sorted I can see the following in console:
I have a problem that this line of code is never executed:
customersViews.push(<View>
<Text>{customersTickets[key["customer"]}</Text>
</View>)
I am guessing that it might be because customersViews array is initialized after rendering is done and not before, how can I wait for data fetching and sorting to finish then render the data?

When you are attempting to get your firebase response you're not actually waiting for it. The code below does not wait for it to be executed.
getCustomersOnQeueu = async () => {
let customers = this.customersRef.orderByChild("ticket").once('value')
return customers
}
To WAIT for it to be executed use AWAIT:
getCustomersOnQeueu = async () => {
let customers = await this.customersRef.orderByChild("ticket").once('value')
return customers
}

setting state via this.setState() always rerenders a component. If you want to rerender a vraible's latest value, put it in the state. Putting customerViews in state and updating it via this.setState() might solve your problem here.

Not sure what you are trying to render. a list of names?
It seems that your line you are talking about is not working because customersTickets.push cant push react element to the array.
You can even try it in your developer console
let array = []
arr.push(test)
and the result is "Uncaught SyntaxError"

Background AsyncTask Takes to long to fullfill

I'm developing an Android App (Android 6.0 and above) and I have performance issues with large XML-Files. (about 17 mB)
When the app is started the required XML-document is loaded from the private storage and a List is returned (takes about 2-3 sek.) and filled in an custom adapter -> this works perfectly fine.
But the user can start a synchronization manually inside the app (e.g.: data
was updated on server)
Therefor I've implemented a background download-service so that the UI stays responsive during the download.
The downloaded data is stored inside the private folder again.
problem:
The background download works perfectly and my UI stays responive until I start reading the information from the new XML-File.
I don't get any result back -> even after 3 min there is no return value from the function although I use the same function for reading the XML like I do at the beginning -> GetKontaktliste()
public class MainActivity : AppCompatActivity
{
alert.SetPositiveButton("Synchronisieren", async (senderAlert, args) =>
{ new DownloadTask(this).Execute("");});
}
public class DownloadTask : AsyncTask
{
protected override Java.Lang.Object DoInBackground(params Java.Lang.Object[] #params)
{
App_Tools lAppTools = new App_Tools();
ThreadPool.QueueUserWorkItem(async state =>
{
//Download Function -> received XML-Data is saved in local storage
bool lKontakte = await lAppTools.DownloadKontakte(_context);
bool lVorgaenge = await lAppTools.DownloadVorgaenge(_context);
bool lVorgaengeImport = await lAppTools.ImportVorgaenge(_context);
if (lKontakte == true)
{
//Problem is that i dont get any results back here
List<KontaktItem> lResult = new List<KontaktItem>();
LinqAbfragen lLinq = new LinqAbfragen();
lLinq.GetKontaktliste();
}
});
return true;
}
}
public class LinqAbfragen
{
//Read Contacts
public List<KontaktItem> GetKontaktliste()
{
List<KontaktItem> lResult = new List<KontaktItem>();
//Read from IEnumerable<XElement>
var KontakteAsXElement = ReadXmlAsXElement("Kontakte.xml", "Kontakt");
lResult = (from kontakt in KontakteAsXElement
select new KontaktItem
{
AdressNr = kontakt.Element("AdressNr").Value,
Vorname = kontakt.Element("Vorname1").Value,
Nachname = kontakt.Element("Name1").Value,
xmlData = (string)kontakt.ToString()
}
).ToList();
return lResult;
}
}

Not able to update Picker.SelectedIndex

I have an odd issue I can't explain the reason for it - maybe someone here can shed some light on it
I have a ticket scanning app in Xamarin Forms currently testing it on android
the interface allows you to:
type an order number and click the check order Button
use the camera scanner to scan which automatically triggers check order
use the barcode scanner to scan which automatically triggers check order
after the check order validation, user has to select the number of tickets from a drop down list and press confrim entry button
what I'm trying to do, is if the seats available on that ticket is just 1 - then automatically trigger confirm entry button functionality
problem that I have is that - some of my logic depends on setting the drop down index in code - for some reason it doesn't update - as seen in the debugger shot here
and this is the second tme I've noticed this today, earlier it was a var I was trying to assign a string and it kept coming up as null - eventually I replaced that code
is this a bug in xamarin ?
code has been simplified:
async void OnCheckOrderButtonClicked(object sender, EventArgs e)
{
await ValidateOrderEntry();
}
private async void scanCameraButton_Clicked(object sender, EventArgs e)
{
messageLabel.Text = string.Empty;
var options = new ZXing.Mobile.MobileBarcodeScanningOptions();
options.PossibleFormats = new List<ZXing.BarcodeFormat>() {
ZXing.BarcodeFormat.QR_CODE,ZXing.BarcodeFormat.EAN_8, ZXing.BarcodeFormat.EAN_13
};
var scanPage = new ZXingScannerPage(options);
scanPage.OnScanResult += (result) =>
{
//stop scan
scanPage.IsScanning = false;
Device.BeginInvokeOnMainThread(async () =>
{
//pop the page and get the result
await Navigation.PopAsync();
orderNoEntry.Text = result.Text;
//automatically trigger update
await ValidateOrderEntry();
});
};
await Navigation.PushAsync(scanPage);
}
private async Task ValidateOrderEntry()
{
//...other code....
checkInPicker.Items.Clear();
if (availablTickets == 1)
{
checkInPickerStack.IsVisible = true;
checkInPicker.SelectedIndex = 0;
messageLabel.Text = "Ticket OK! - " + orderNoEntry.Text;
messageLabel.TextColor = Color.Green;
//select the only element
checkInPicker.SelectedIndex = 0;
await PostDoorEntry();
}
//...other code....
}
private async Task PostDoorEntry()
{
int entryCount = checkInPicker.SelectedIndex + 1;
//... more code...
//...post api code..
}

Maybe I'm overlooking something, but you clear all the items a few lines above the one you are pointing out. That means there are no items in your Picker and thus you can't set the SelectedIndex to anything other than -1, simply because there are no items.