I was learning to use Contacts in flutter app, and while understanding and writing code, i saw this expression - _contacts?.length ?? 0,
I am not able to understand what does this mean, what is the use of question marks here?
Here is the full code -
import 'package:flutter/material.dart';
import 'package:contacts_service/contacts_service.dart';
class ContactsPage extends StatefulWidget {
#override
_ContactsPageState createState() => _ContactsPageState();
}
class _ContactsPageState extends State<ContactsPage> {
Iterable<Contact> _contacts;
#override
void initState() {
getContacts();
super.initState();
}
Future<void> getContacts() async {
//Make sure we already have permissions for contacts when we get to this
//page, so we can just retrieve it
final Iterable<Contact> contacts = await ContactsService.getContacts();
setState(() {
_contacts = contacts;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: (Text('Contacts')),
),
body: _contacts != null
//Build a list view of all contacts, displaying their avatar and
// display name
? ListView.builder(
itemCount: _contacts?.length ?? 0,
itemBuilder: (BuildContext context, int index) {
Contact contact = _contacts?.elementAt(index);
return ListTile(
contentPadding:
const EdgeInsets.symmetric(vertical: 2, horizontal: 18),
leading: (contact.avatar != null && contact.avatar.isNotEmpty)
? CircleAvatar(
backgroundImage: MemoryImage(contact.avatar),
)
: CircleAvatar(
child: Text(contact.initials()),
backgroundColor: Theme.of(context).accentColor,
),
title: Text(contact.displayName ?? ''),
//This can be further expanded to showing contacts detail
// onPressed().
);
},
)
: Center(child: const CircularProgressIndicator()),
);
}
}
Here is the link to the page - How to access contacts in flutter
It first checks the _contacts.
If it is not null, it checks the _contacts.length.
If the _contacts.length is not null, it sets its value, otherwise it sets zero.
_contacts?.length ?? 0
first ? means if contacts is null, don't try to call length and just return null because calling .length on null will throw an error. So basically you're avoiding a null pointer error.
the second ?? means if the contacts or length is null (i.e., the value before ??), return 0.
The Dart Language Tour - Operators Section is a good reference for Dart's operator.
It means that if _contacts array is null, you will get 0 for length.
Related
I'm having this super annoying issue of being unable to grab and display a table from my server hosted on PhpmyAdmin. (I've managed to grab the data and have it printed in the console, but now that I'm trying to display it in a table I can't seem to get it working)
I've tried nulling my variables but I'm not really sure what the main culprit for this error is. Any help would be greatly appreciated.
Image of Error
data.dart File
class dataListing extends StatefulWidget {
const dataListing({Key? key}) : super(key: key);
#override
State<dataListing> createState() => _dataListingState();
}
class _dataListingState extends State<dataListing> {
#override
Widget build(BuildContext context) {
return Container();
}
}
class listingData{
String? ListingID, listingName, listingDescription, address, suburbName, phoneNumber, openingHours, Email, Website;
listingData({
this.ListingID,
this.listingName,
this.listingDescription,
this.address,
this.suburbName,
this.phoneNumber,
this.openingHours,
this.Email,
this.Website,
});
//constructor
List<listingData> datalist = [];
factory listingData.fromJSON(Map<String, dynamic> json){
return listingData(
ListingID: json["ListingID"],
listingName: json["listingName"],
listingDescription: json["listingDescription"],
address: json["address"],
suburbName: json["suburbName"],
phoneNumber: json["phoneNumber"],
openingHours: json["openingHours"],
Email: json["Email"],
Website: json["Website"],
);
}
}
Directory.dart file
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'package:app/pages/data.dart';
class directoryPage extends StatefulWidget {
#override
State<directoryPage> createState() => _directoryPageState();
}
class _directoryPageState extends State<directoryPage> {
// List serviceListing = [];
//
// getAllListing()async{
// String url = "URL HERE";
// var response = await http.get(Uri.parse(url));
// if (response.statusCode == 200){
// setState (() {
// serviceListing = json.decode(response.body);
// });
// print (serviceListing);
// return serviceListing;
// }
// }
bool error = false, dataloaded = false;
var data;
String dataurl = "URL HERE";
#override
void initState (){
loaddata();
super.initState();
// getAllListing();
}
void loaddata() {
Future.delayed(Duration.zero,() async {
var res = await http.post(Uri.parse(dataurl));
if (res.statusCode == 200) {
setState(() {
data = json.decode(res.body);
dataloaded = true;
});
}
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Directory'),
centerTitle: true,
elevation: 0,
backgroundColor: Color(0xFFA30B32),
//WSU Appbar Icon
leading: Padding(
padding: const EdgeInsets.all(8.0),
child: Image.asset("assets/wsulogo.png", scale: 8.0),
),
),
body: Container(
padding: EdgeInsets.all(15),
child:dataloaded?datalist():
Center(
child:CircularProgressIndicator()
),
)
);
}
Widget datalist(){
if(data["error"]) {
return Text(data["errmsg"]);
}else{
List<listingData> datalist = List<listingData>.from(data["data"].map((i){
return listingData.fromJSON(i);
})
);
return Table( //if data is loaded then show table
border: TableBorder.all(width:1, color:Colors.black45),
children: datalist.map((listingdata){
return TableRow( //return table row in every loop
children: [
//table cells inside table row
TableCell(child: Padding(
padding: EdgeInsets.all(5),
child:Text(listingdata.ListingID!)
)
),
TableCell(child: Padding(
padding: EdgeInsets.all(5),
child:Text(listingdata.listingName!)
)
),
TableCell(child: Padding(
padding: EdgeInsets.all(5),
child:Text(listingdata.listingDescription!)
)
),
TableCell(child: Padding(
padding: EdgeInsets.all(5),
child:Text(listingdata.address!)
)
),
]
);
}).toList(),
);
}
}
}
Looks like the issue was actually unrelated to the dart side of things, the php code wasn't properly structuring the data. Cannot have underscores or spaces.
Correct-> $json["dballlisting"] = array (); (I renamed it to just "data" later)
Incorrect->$json["db_all_listing"] = array ();
The error seems to be originating from this line, the data['data'] is null which is expected to be an Array.
List<listingData> datalist = List<listingData>.from(data["data"].map((i){
return listingData.fromJSON(i);
})
You need to investigate your API call to make sure why it is happening. If the null value is expected then you need to add safeguards in your code to make sure it won't break when it encounter such scenarios. You can add null safety checks for that one way to do it would be to
List<listingData> datalist = List<listingData>.from((data["data"] ?? []).map((i){
return listingData.fromJSON(i);
})
so I'm currently working on an application that has a listview on the first screen (implemented on main.dart).
The listview fetches it's data from internet (async).
The problem is that, the listview does not get updated when the data is changed.
(I can implement this functionality simply by designing a 'reload' button and pressing it every time I want the new data. But that's not what I want right now).
In other words, how can I update the listview automatically?
EDIT1: ADDING SOME CODE
code might be messy; see the description at the end.
class RssFeed extends StatelessWidget {
String title;
String pubDate;
RssFeed(this.title, this.pubDate);
#override
Widget build(BuildContext context) {
return Scaffold(
body: Column(
children: [
Align(
alignment: Alignment.topRight,
child: Text(title),
),
Text(pubDate)
],
),
);
}
}
class FeedsList extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return _FeedsListState();
}
}
class _FeedsListState extends State<FeedsList> {
List<Widget> list1 = new List<Widget>();
#override
void initState() {
super.initState();
ls();
}
Future ls() async {
list1.clear();
list.clear();
sites1.clear();
RSS_reader rss_reader = new RSS_reader();
for (var i in saver.list.items) {
sites1.add(
site(siteAdress: i.siteAdress, siteDescription: i.siteDescription));
}
var res = await rss_reader.Get_items(sites1);
for (var val in res) {
list.add(InkWell(
onTap: () => _launchURL(val.item.link),
child: Container(
height: 50,
color: Colors.amber[100],
child: Center(
child: new RssFeed(val.item.title, val.item.pubDate.toString()),
),
)));
}
print(list.length);
setState(() {
list1 = list;
});
}
Widget build(BuildContext context) {
return Scaffold(
body: ListView.builder(
itemCount: list1.length,
itemBuilder: (BuildContext context, int i) {
return list1[i];
}));
}
}
DESCRIPTION:
As you can guess, this is a RSS reader.
So, I have a class RSSFeed; which makes one of the tiles of Listview.
then in the FeedsList class (stateful widget), I make the listview.
I have a class called RSS_reader and a method Get_items, which gets a bunch of sites as input and puts those sites' newest feeds in a list ('res' in the above code).
Then, I put the items in a list of 'Container's and then build the listview.
Then, in the main function, I create a container like below:
Container(
height: 500,
width: 580,
child: FeedsList(),
)
and there appears the problem; the FeedsList class does not get updated automatically. although if I put a button and navigate to FeedsList class through that button, the list is refreshed and OK.
Thanks for reading and help.
If you just want to fetch data once from your external source use a FutureBuilder, if you want to fetch data multiple times take a look to StreamBuilder. Both widgets will have the behavior you are looking for, with no refresh button.
Simple example of how to use a FutureBuilder:
Future<List<String>> _fetchData() {
return // fetch data from source
}
#override
Widget build(BuildContext context) {
return FutureBuilder(
future: _fetchData,
builder: (BuildContext context, AsyncSnapshot<List<String>> snapshot) {
if (snapshot.hasData && snapshot.data != null) {
// This widget will be built when data is fetched
const List<String> list = snapshot.data;
return ListView(
children: list.map(
(element) => ListTile(
title: Text(element),
),
).asList(),
);
} else {
// This widget will be built while you are waiting for your data to be fetched
return Container(
child: Center(
child: Text("Loading data..."),
),
);
}
},
);
}
You have to stream data and ListView will update automatically.
In the button that you say you can re call your ls() functions, your list should update on tap button
sample:
return Scaffold(
body: ListView.builder(
itemCount: list1.length,
itemBuilder: (BuildContext context, int i) {
return list1[i];
},
),
floatingActionButton: FloatingActionButton(
child: Icon(Icons.refresh),
onPressed: () => ls(),
),
);
This problem is about how to keep the favorited item in the List, even after I re-open the app.
I want to keep the favorited item in doaList into favDoa, even after I close my app and re-open it. I've seen about the shared_preference package in flutter to store data, but i confused how can i implement it into my app. Here is my code :
import 'package:flutter/material.dart';
import 'package:json_test/class/doa.dart';
import 'package:json_test/page/DoaPage.dart';
class MainPage extends StatefulWidget {
#override
_MainPageState createState() => _MainPageState();
}
class _MainPageState extends State<MainPage> {
List<Doa> doaList;
List<Doa> favDoa;
bool _isInit = true;
Future<void> fetchDoa(BuildContext context) async {
final jsonstring =
await DefaultAssetBundle.of(context).loadString('assets/doa.json');
doaList = doaFromJson(jsonstring);
_isInit = false;
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("JSON Data test"),
),
body: Container(
child: FutureBuilder(
future: _isInit ? fetchDoa(context) : Future(null),
builder: (context, _) {
if (doaList.isNotEmpty) {
return ListView.builder(
itemCount: doaList.length,
itemBuilder: (BuildContext context, int index) {
Doa doa = doaList[index];
return Card(
margin: EdgeInsets.all(8),
child: ListTile(
title: Text(doa.judul),
onTap: () {
Navigator.of(context).push(MaterialPageRoute(
builder: (BuildContext context) =>
DoaPage(
doa: doa,
)));
},
trailing: IconButton(
icon: Icon(
doa.fav
? Icons.favorite
: Icons.favorite_border,
color: doa.fav ? Colors.red : null,
),
onPressed: () => setState(() {
doa.fav = !doa.fav;
}),
)));
},
);
}
return CircularProgressIndicator();
})));
}
}
when I click the favorite icon in the list, it will be marked true in the "doa.fav". How can I implement the shared_preference package in my code to keep the doa.fav's data? Thank you so much for your answer :)
Try this:
Create a list of integer to store the Doa Ids, say you name it favoriteList
Each time you click the favorite button, add the Doa's id to favoriteList. Also save it to shared_preferences. It only support list of string, so you need to convert it first, something like:
List<String> stringFavoriteIds =
favoriteList.map((e) => e.toString()).toList();
SharedPrefs().favoriteIds = stringFavoriteIds ;
Next, each time you open the app, load SharedPrefs().favoriteIds to favoriteList
Compare the Doa Ids in favoriteList to your list of doa to mark Doa.fav to true for matching Ids.
I'm building a selectable checkbox contact list in flutter but if a contact has only an email and no number, an error is thrown. I want to create a loop to add a number of '99999' to the contacts phone number if they don't have one. Please can someone guide me with an explanation of what I should change to make this work? I have had a go, but I am quite new to flutter so I'm not completely certain on syntax etc...
Here is the part of the code that I am trying to put the function into.
setMissingNo()async {
Iterable<Contact> contactsToLoop = (await ContactsService.getContacts()).toList();
contactsToLoop.forEach((Contact) { contactsToLoop = []..add(Item.fromMap({'label': 'work', 'value': 99999})); });
}
//fetch contacts from setMissingNo
getAllContacts() async{
Iterable<Contact> _contacts = (await ContactsService.getContacts()).toList();
setState(() {
contacts = _contacts;
}
);
}
Here is my whole code
import 'package:flutter/material.dart';
// TODO: make it ask for permissions otherwise the app crashes
import 'package:contacts_service/contacts_service.dart';
class Home extends StatefulWidget {
#override
_HomeState createState() => _HomeState();
}
class _HomeState extends State<Home> {
List<Contact> contacts = [];
List<Contact> contactsFiltered = [];
TextEditingController searchController = new TextEditingController();
#override
void initState() {
super.initState();
getAllContacts();
searchController.addListener(() => filterContacts());
}
//remove the +'s and spaces from a phone number before its searched
String flattenPhoneNumber(String phoneStr) {
return phoneStr.replaceAllMapped(RegExp(r'^(\+)|\D'), (Match m) {
return m[0] == "+" ? "+" : "";
});
}
//loop and set all contacts without numbers to 99999, pass new list to getAllContacts
setMissingNo()async {
Iterable<Contact> contactsToLoop = (await ContactsService.getContacts()).toList();
contactsToLoop.forEach((Contact) { contactsToLoop = []..add(Item.fromMap({'label': 'work', 'value': 99999})); });
}
//fetch contacts from setMissingNo
getAllContacts() async{
Iterable<Contact> _contacts = (await ContactsService.getContacts()).toList();
setState(() {
contacts = _contacts;
}
);
}
//filtering contacts function to match search term
filterContacts() {
List<Contact> _contacts = [];
_contacts.addAll(contacts);
if (searchController.text.isNotEmpty) {
_contacts.retainWhere((contact) {
String searchTerm = searchController.text.toLowerCase();
String searchTermFlatten = flattenPhoneNumber(searchTerm);
String contactName = contact.displayName.toLowerCase();
bool nameMatches = contactName.contains(searchTerm);
if (nameMatches == true) {
return true;
}
if (searchTermFlatten.isEmpty) {
return false;
}
var phone = contact.phones.firstWhere((phn) {
String phnFlattened = flattenPhoneNumber(phn.value);
return phnFlattened.contains(searchTermFlatten);
}, orElse: () => null);
return phone != null;
});
setState(() {
contactsFiltered = _contacts;
});
}
}
final selectedContacts = Set<Contact>();
#override
Widget build(BuildContext context) {
bool isSearching = searchController.text.isNotEmpty;
return Scaffold(
body: SafeArea(
child: Column(
children: <Widget>[
AppBar(
title: Text('Create Group'),
),
Container(
child: TextField(
controller: searchController,
decoration: InputDecoration(
labelText: 'Search Contacts',
border: OutlineInputBorder(
borderSide: new BorderSide(
color: Theme.of(context).primaryColor
)
),
prefixIcon: Icon(
Icons.search,
color: Theme.of(context).primaryColor
)
),
),
),
Expanded( child: ListView.builder(
shrinkWrap: true,
itemCount: isSearching == true ? contactsFiltered.length : contacts.length,
itemBuilder: (context, index) {
Contact contact = isSearching == true ? contactsFiltered[index] : contacts[index];
//TODO: make it so when you clear your search, all items appear again & when you search words it works
return CheckboxListTile(
title: Text(contact.displayName),
subtitle: Text(
contact.phones.elementAt(0).value
),
value: selectedContacts.contains(contact),
onChanged: (bool value) {
if (value) {
selectedContacts.add(contact);
} else {
selectedContacts.remove(contact);
}
setState((){});
// TODO: add in function to add contact ID to a list
});
},
),
/*new Expanded(
child: Align(
alignment: Alignment.bottomLeft,
child: BottomNavigationBar(
currentIndex: _currentIndex,
items: const <BottomNavigationBarItem>[
//TODO: create new contact functionality to add someone by name + email
BottomNavigationBarItem(
icon: Icon(Icons.add),
title: Text('Add Contact'),
),
BottomNavigationBarItem(
icon: Icon(Icons.create),
title: Text('Create Group'),
),
],
onTap: (index) {
setState(() {
_currentIndex = index;
});
}
)
)
)*/
)
],
)
),
);
}
}
Updating all contacts listed on the device might take a long time depending on the size of the device's contact list. Add that the task is being done on the UI thread. You may want to consider using Isolate for the task to move away the load from the UI thread. If you can also provide the errors that you're getting, it'll help us get a picture of the issue.
Another thing is, the way you're approaching the issue might be impractical. Is it really necessary to write a placeholder phone number to the contacts? The issue likely stems from trying to fetch the number from a contact but the Object returns null. Perhaps you may want to consider only updating the phone number when the contact is selected.
I am able to fetch and print the data in json format in console and I am able to display the entire body of data in list format in the flutter app. But I can't figure out how to display only a specific key and its value in list. Its a list of maps. I have removed the consumer key and secret from this code.
import 'dart:async';
import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
void main(){
runApp(MaterialApp(
home: CustomHome(),
));
}
class CustomHome extends StatefulWidget {
#override
_CustomHomeState createState() => _CustomHomeState();
}
class _CustomHomeState extends State<CustomHome> {
List data;
Future<String> getData() async{
var response = await http.get('https://jbaat.com/wp-json/wc/v3/products/?consumer_key=&consumer_secret=');
setState(() {
var converted = jsonDecode(response.body);
data = converted;
});
}
#override
void initState() {
// TODO: implement initState
super.initState();
this.getData();
}
#override
Widget build(BuildContext context) {
print(data);
return Scaffold(
body: ListView.builder(
itemCount: data == null ? 0 : data.length,
itemBuilder: (BuildContext context, int index) {
return Container(
child: Card(
child: Padding(
padding: const EdgeInsets.all(8.0),
child: Text('$data'),
),
),
);
}),
);
}
}
Below is the response
[{id: 493, name: Bas 5 Min White Half Sleeve T-Shirt, slug: bas-5-min-white-half-sleeve-t-shirt, permalink: https://www.jbaat.com/product/bas-5-min-white-half-sleeve-t-shirt/, date_created: 2019-12-14T23:39:08, date_created_gmt: 2019-12-14T18:09:08, date_modified: 2019-12-14T23:48:01, date_modified_gmt: 2019-12-14T18:18:01, type: variable, status: publish, featured: false, catalog_visibility: visible, description: , short_description: , sku: , price: 500.00, regular_price: , sale_price: , date_on_sale_from: null, date_on_sale_from_gmt: null, date_on_sale_to: null, date_on_sale_to_gmt: null, price_html: <span class="woocommerce-Price-amount amount"><span class="woocommerce-Price-currencySymbol">₹</span>500.00</span>, on_sale: false, purchasable: true, total_sales: 0, virtual: false, downloadable: false, downloads: [], download_limit: -1, download_expiry: -1, external_url: , button_text: , tax_status: taxable, tax_class: , manage_stock: false, stock_quantity: null, stock_status: instock, backorders: no,
Jbaat, I'd recommend creating a model for your response data and use the values accordingly from each item's instance. There are few online converters available which converts your json response to Dart models, here is one - https://javiercbk.github.io/json_to_dart/. Below is a quick example of what it would look like based on your response data,
class Items {
final List<Items> items;
Items({this.items});
factory Items.fromJson(List<dynamic> json) {
List<Items> itemList = json.map((i) => Items.fromJson(i)).toList();
return Items(
items: itemList
);
}
}
class Item {
final int id;
final String name;
......
Item({this.id, this.name,.....});
factory Item.fromJson(Map<String, dynamic> json) {
return Item(
id: json['id'],
name: json['name']
......
);
}
}
And then your getData() would be,
Future<Items> getData() async{
var response = await http.get('https://jbaat.com/wp-json/wc/v3/products/?consumer_key=&consumer_secret=');
return Items.fromJson(json.decode(response.body)).items;
}
You should now have Item list which can be used to get specific item info. You should also use FutureBuilder to call your getData() instead of calling it in initState to make sure data is available before building widgets like so,
#override
Widget build(BuildContext context) {
return FutureBuilder(
future: getData(),
builder: (context, snapshot) {
if (snapshot.hasData) {
return Scaffold(
body: ListView.builder(
itemCount: snapshot.data.length,
itemBuilder: (BuildContext context, int index) {
Item item = snapshot.data[index]; //Your item
return Container(
child: Card(
child: Padding(
padding: const EdgeInsets.all(8.0),
child: Text(item.name),
),
),
);
}),
);
} else {
return Center(child: CircularProgressIndicator());
}
});
}
Hope this helps. Good luck!
Change the type for data to
List<Map<String,dynamic>>
A possible implementation for your use case:
Map y;
var keytobesearched='name';
List<Map<String,dynamic>> x= [{'id': 493, 'name': 'Bas 5 Min White Half Sleeve T-Shirt', 'slug': 'bas-5-min-white-half-sleeve-t-shirt'}];
x.forEach((Map<String,dynamic> ele){
if(ele.containsKey(keytobesearched))
y=Map.from(ele);
// display/alter y to your liking
});
If you would want a complete plug and play Woocommerce Sdk that handles authentication, products, customer, shipping etc for you, you can use the Woo Commerce SDK library for flutter at https://pub.dev/packages/woocommerce
Woocommerce myWoocommerce = WooCommerce(baseUrl: yourBaseUrl, consumerKey: yourConsumerKey, consumerSecret: consumerSecret);
Then simply get your lists eg:
WooProduct products = await myWocommerce.getProducts(); // returns the ist of products.
for (var product in products){ print(product.name)};