As below code showed, I got the list(type: Flow<List<T>>),
how can I use it to update lazyColumn's item, so when the room data changed, lazyColumn updates items accordingly?
Many thanks.
#Composable
fun SomeContent(context: Context) {
// get the view model ref
val viewModel: SomeViewModel =
viewModel(factory = SomeViewModelFactory(Db.getInstance(context)))
// get the list from room, it is a Flow list
val list = viewModel.sumDao.getAllRows()
LazyColumn(
) {
// I need to use the list in below, but got errors, how to do?
items(list.size) {
SomeListItem(list[it])
}
}
}
You must collect your FlowList as follows
val list = viewModel.sumDao.getAllRows().collectAsState(initial = emptyList())
Instead of items, use itemsIndexed
itemsIndexed(list) {idx, row -> SomeListItem(row)}
Let me know if the above works.
if using vn1gam's solution, might need to do
itemsIndexed(list.value) {idx, row -> SomeListItem(row)}
Related
Background: it is a Linked List (ordering task for delivery), which involves different shopping carts (Node). i need to write a function to check if the carts are sorted according to the totalprice in the Class ShoppingCart.
My questions are:
Compiler does not have error message, but my function is false. May I know how or where should i change the code?
Any articles about debugging are recommendable for the beginners?
Thanks for the kindness.
Code as below:
1.
class OrderProcessing {
var first: OrderNode? = null
// Node for linked List
data class OrderNode(val order: Order, var next: OrderNode?)
....
fun isSorted(): Boolean {
var run = first
var sort = false
while (run != null) {
if(run.order.shoppingCart.totalPrice > run.next!!.order.shoppingCart.totalPrice)
run = run.next
sort = true
}
return sort
}
2.
Class ShoppingCart () {
private val shoppingList = mutableListOf<Pair<Product, Int>>
// totalprice = sum of item price in each cart
val totalprice : double
... }
3.
data class Order(
val shoppingCart: ShoppingCart,
val address: Address
)
4.
fun main(){
// while loop until "exit", Menu with functions to add goods to cart, clear cart, show shopping list
I think in your case you need to start debugging your code and step by step check your program flow.
You can read this article - https://www.jetbrains.com/help/idea/debugging-code.html, it's about java but anyway it's a good starting point.
I have implemented group observable collection like below.
Grouped Property
private ObservableCollection<Grouping<String, Request>> _groupedList = null;
public ObservableCollection<Grouping<String, Request>> GroupedList {
get {
return _groupedList;
}
set {
_groupedList = value;
RaisePropertyChanged(() => GroupedList);
}
}
Creating List
var list = new List<Request>();
var grouped = from Model in list
group Model by Model.Done into Group
select new Grouping<string, Request>(Group.Key, Group);
GroupedList = new ObservableCollection<Grouping<string, TModel>>(grouped);
Now i need to update one item in the list without reloading full list for performance.
i did tried like this , mylist.FirstOrDefault(i => i.Id== mymodel.Id); Not worked for me.
I need to pick that particular item and edit and update into the list again using linq or something but i stuck here for group observable collection no efficient details to do this., anybody having idea about this help please.
And finally i get updated single item, but i need to do that without
GroupedList = new ObservableCollection<Grouping<string, TModel>>(grouped);
Because everytime it create new list and bind into my view.Thats again big performance though.
Thanks in advance.
What I understand from your question is that you want to push an updated group without overwriting the entire ObservableCollection.
To do that:
var targetGroup = GroupedList.FirstOrDefault(i => i.Id == mymodel.Id);
var targetIndex = GroupedList.IndexOf(targetGroup);
var modifiedGroup = ... //Do whatever you want to do
GroupedList[targetIndex] = modifiedGroup;
This will trigger a 'replace' operation of the target grouping.
var result=(from refridgerators in context.A
group refridgerators by new { refridgerators.Id, refridgerators.Name } into gr
select new DAO<String>
{
Key = gr.Key.Name,
Count = gr.Count()
}).OrderByDescending(k => k.Count).Take(numberOfRecords).ToList();
This is my linq to sql query this is working perfectly fine.
this shows top 5 records (sorted by their count) if i pass numberOfRecords =5.
now my problem is i don`t want to modify query. so what should i do in above query to show all records. This is in relation with my requirement i want to use same query to show all refridgerators and Top 5 , top 10 refridgerators.
I am not sure if it is possible using LINQ. but i guess there must be something related to this.
I would simply don't add the Take to the query but to the code where you consume this query.
So for example:
public static IEnumerable<DAO> GetRefrigerators()
{
var query = from refridgerators in context.A
group refridgerators by new { refridgerators.Id, refridgerators.Name } into gr
select new DAO<String>
{
Key = gr.Key.Name,
Count = gr.Count()
};
return query.OrderByDescending(k => k.Count);
}
Now you can either use:
var refrigerators = GetRefrigerators().Take(5).ToList();
or
var refrigerators = GetRefrigerators().Take(10).ToList();
or
var refrigerators = GetRefrigerators().ToList();
I'd make numberOfRecords a int? and if the value is null you should not call Take(numberOfRecords)
var result = (from refridgerators in context.A
group refridgerators by new { refridgerators.Id, refridgerators.Name } into gr
select new DAO<String>
{
Key = gr.Key.Name,
Count = gr.Count()
}).OrderByDescending(k => k.Count);
if(numberOfRecords.HasValue)
result = result.Take(numberOfRecords.Value);
return result.ToList();
I know it changes your query a little bit but I believe it is pretty acceptable, adding a numberOfRecords of a super high value adds an overheard to your query which isn't useful in your case.
So this post is very old, I know, but my solution is not provided (maybe someone wants to achieve the same after 2 years, like me):
You could create your own extension-method that also works with linq 2 sql. Example:
public static class LinqExtensions
{
public static IEnumerable<TResult> TakeIfNotNull<TResult>(this IEnumerable<TResult> source, int? count)
{
return !count.HasValue ? source : source.Take(count.Value);
}
}
this takes an int? instead of int and returns either the source-enumerable if count is null else it returns the result of default Take-method.
So you could write like:
.OrderByDescending(k => k.Count).TakeIfNotNull(numberOfRecords).ToList();
if numberOfCounts is null, it'll take all records.
As the right solution has already been posted by Tim, still I want your attention to the simpler solution, if suits to your requirement.
Add one more if condition at the top of this query.
if(allRecordRequired)
{
numberOfRecords = 2000000;
}
Leave your query as it is.
Pass a huge number to your method.A number that is equal (at least) or bigger than the items count.That should give you all records.
I have the following code -
public void LoadAllContacts()
{
var db = new ContextDB();
var contacts = db.LocalContacts.ToList();
grdItems.DataSource = contacts.OrderBy(x => x.Areas.OrderBy(y => y.Name));
grdItems.DataBind();
}
I'm trying to sort the list of the contacts according to the area name that is contained within each contact. When I tried the above, I get "At least one object must implement IComparable.". Is there an easy way instead of writing a custom IComparer?
Thanks!
try this:
public void LoadAllContacts()
{
var db = new ContextDB();
var contacts = db.LocalContacts.ToList();
grdItems.DataSource = contacts.OrderBy(x => x.Areas.OrderBy(y => y.Name).First().Name);
grdItems.DataBind();
}
this will order the contacts by the first area name, after ordering the areas by name.
Hope this helps :)
Edit: fixed error in code. (.First().Name)
I was in a discussion with #AbdouMoumen but in the end I thought I'd provide my own answer :-)
His answer works, but there two performance issues in this code (both in the answer as in the original question).
First, the code loads ALL contacts in the db. This may or may not be a problem, but in general I would recommend NOT to do this. Many modern controls support paging/filtering out of the box, so you'd be better off supplying an not-yet-evaluated IQueryable<T> instead of List<T>. If however you need everything in memory, you should delay the ToList to the last possible moment.
Second, in AbdouMoumen's answer, there is a so-called 'SELECT N+1' problem. Entity Framework will by default use lazy loading to fetch additional properties. I.e. the Areas property will not be fetched from the database until it's accessed. In this case this will happen in the controls 'for loop', while it's ordering the result set by name.
Open up SQL Server Profiler to see what I mean: you will see a SELECT statement for all the contacts, and an additional SELECT statement for each contact that fetches the Areas for that contact.
A much better solution would be the following:
public void LoadAllContacts()
{
using (var db = new ContextDB())
{
// note: no ToList() yet, just defining the query
var contactsQuery = db.LocalContacts
.OrderBy(x => x.Areas
.OrderBy(y => y.Name)
.First().Name);
// fetch all the contacts, correctly ordered in the DB
grdItems.DataSource = contactsQuery.ToList();
grdItems.DataBind();
}
}
Is it one to one relation (Contact->Area)?
if yeah then try the following :
public partial class Contact
{
public string AreaName
{
get
{
if (this.Area != null)
return this.Area.Name;
return string.Empty;
}
}
}
then
grdItems.DataSource = contacts.OrderBy(x => x.AreaName);
I have a couple of tables with similar relationship structure to the standard Order, OrderLine tables.
When creating a data context, it gives the Order class an OrderLines property that should be populated with OrderLine objects for that particular Order object.
Sure, by default it will delay load the stuff in the OrderLine property but that should be fairly transparent right?
Ok, here is the problem I have: I'm getting an empty list when I go MyOrder.OrderLines but when I go myDataContext.OrderLines.Where(line => line.OrderId == 1) I get the right list.
public void B()
{
var dbContext = new Adis.CA.Repository.Database.CaDataContext(
"<connectionString>");
dbContext.Connection.Open();
dbContext.Transaction = dbContext.Connection.BeginTransaction();
try
{
//!!!Edit: Imortant to note that the order with orderID=1 already exists
//!!!in the database
//just add some new order lines to make sure there are some
var NewOrderLines = new List<OrderLines>()
{
new OrderLine() { OrderID=1, LineID=300 },
new OrderLine() { OrderID=1, LineID=301 },
new OrderLine() { OrderID=1, LineID=302 },
new OrderLine() { OrderID=1, LineID=303 }
};
dbContext.OrderLines.InsertAllOnSubmit(NewOrderLines);
dbContext.SubmitChanges();
//this will give me the 4 rows I just inserted
var orderLinesDirect = dbContext.OrderLines
.Where(orderLine => orderLine.OrderID == 1);
var order = dbContext.Orders.Where(order => order.OrderID == 1);
//this will be an empty list
var orderLinesThroughOrder = order.OrderLines;
}
catch (System.Data.SqlClient.SqlException e)
{
dbContext.Transaction.Rollback();
throw;
}
finally
{
dbContext.Transaction.Rollback();
dbContext.Dispose();
dbContext = null;
}
}
So as far as I can see, I'm not doing anything particularly strange but I would think that orderLinesDirect and orderLinesThroughOrder would give me the same result set.
Can anyone tell me why it doesn't?
You're just adding OrderLines; not any actual Orders. So the Where on dbContext.Orders returns an empty list.
How you can still find the property OrderLines on order I don't understand, so I may be goofing up here.
[Edit]
Could you update the example to show actual types, especially of the order variable? Imo, it shoud be an IQueryable<Order>, but it's strange that you can .OrderLines into that. Try adding a First() or FirstOrDefault() after the Where.