SwiftUI List performane

We've been seeing a similar issue, although 30-45 seconds is a crazy amount of time (our delays were on the order of a second or two, which is still far too slow). You might want to profile in instruments to see just what is taking all the time, or even just pause the debugger and grab a stack to see if there's anything suspicious there.

One workaround we use is to just render the entire unfiltered list, and individually hide any rows that should be filtered. E.g., instead of:

List(filtered, id: \.self) { (item: Name) in
    NameListRow(item: item)
}

You would do:

List(names, id: \.self) { (item: Name) in 
    item.gender == self.selectedGender ? NameListRow(item: item) : nil
}

Obviously this isn't a good general-purpose solution to filtering (since you wouldn't want to do it with millions of source rows where the filter would return only a handful), but if you are just switching between two sets of hundreds or thousands of records, rendering the whole thing and selectively hiding individual rows had made it much smoother for us.

I'd guess the time is being spent scanning the view hierarchy to determine which row views have been moved, added, or deleted. This may step a ways down the graph, so if that's the case it may be worth looking for ways to short-circuit that.

One option is to explicitly set an identifier on the row view using the

.id()

modifier. You should be able to pass the item's

NSManagedObjectID

for that. Another way to do the same thing (if

List

is written how I'd expect and is setting an identifier implicitly from its input) is to use

\.objectID

as the identifier in the

List

constructor.

Another way might be to have your

NameListRow

view conform to

Equatable

, either implicitly or explicitly using

item.objectID

to determine equality. Once you have that then you can use

NameListRow(item: item).equatable()

to return an 'equatable' version of your view, i.e. one which will use a single equality call to determine if it needs to be updated—and presumably to tell if it should be removed from or added to a list.

On top of this, it may be worth looking at defining your

@FetchRequest

property using the initializer that takes an

NSFetchRequest

directly. You might need to define your own initializer (and initialize

self._names = FetchRequest(...)

as a result) but that would potentially free you up to modify the predicate on the fetch request when the gender toggle changes, rather than doing your own filtering. I'm not sure how much difference that would make, to be honest, but there may be benefits from pushing as much as possible out to CoreData to handle.

By rendering the entire list and individually hide any rows that should be filtered out improves the performance to milliseconds, which is great. However, by returning nil I end up with an empty row, taking up space. And that is not what I want. Any solution to that ? ( I writing it exactly as your example)

Unfortunately, using \.objectID as identifier does not improve performance at all.

List(filtered, id: \.objectID) { (item: Name) in  
    NameListRow(item: item)  
}

I have tried to create a fetch request that change the predicate when toggle gender, but that does not improve performance either. It's not the fetch request or the filtering that takes time. It is the rendering, scanning the view hierarchy for changes as you mention.

Did the explicit

.id(item.objectID)

or

.equatable()

calls make any change at all? The latter is intended to make view hierarchy recursion unnecessary, so with that in place it theoretically would only need to visit the nodes for the list rows themselves and nothing below.

I have the exact same problem. It seems that SwiftUI is not very smart, it compares the entire list, not only the visible rows. And what is worst, it processes each row, so if you have complex rows showing more data from relationships, is even slower.

I decided NOT doing long list with SwiftUI for now, but now I have another problems because my app uses a synchronizer for updating the coredata and when something is modified in the background, does not trigger the UI update, not even using @FetchRequest

SwiftUI is still BETA IMHO

Don't know if this is what you meant, but none of them worked:

.id(item.objectID)

List(filtered, id: \.objectID) { (item: Name) in
    NameListRow(item: item)
        .id(item.objectID)
}

.equatable() (not sure if I'm using it correctly)

List(filtered, id: \.objectID) { (item: Name) in
    NameListRow(item: item).equatable()
}

struct NameListRow: View, Equatable {
    
    static func == (lhs: NameListRow, rhs: NameListRow) -> Bool {
        lhs.item.objectID == rhs.item.objectID
    }

    ...

}

This really worked with great performance. Maybe not what I would expected from the framework, but until SwiftUI become more mature I think this would be an accepted answer.

Final solution that worked great:

struct NameList: View {
      
    @ObservedObject fileprivate var global = GlobalSettings()
      
    @FetchRequest(
        entity: Name.entity(),
        sortDescriptors: [NSSortDescriptor(key: "name", ascending: true)]
    ) var names: FetchedResults<Name>
      
    @State var selectedGender = Defaults.gender
      
    @State var filtered = [Name]()
    
    var body: some View {
          
        NavigationView {
              
            VStack {
                  
                Picker("Gender", selection: $global.gender) {
  
                    Text(Gender.female.rawValue.uppercased())
                        .tag(Gender.female)
  
                    Text(Gender.male.rawValue.uppercased())
                        .tag(Gender.male)
  
                    Text(Gender.unisex.rawValue.uppercased())
                        .tag(Gender.unisex)
  
                }
                .pickerStyle(SegmentedPickerStyle())
                .padding()
                  
                List( filtered, id: \.objectID) { (item: Name) in
                   NameListRow(item: item)
                }
            }
              
        }
          
        .onReceive(Defaults.publisher(for: \.gender)) { (gender) in
            self.selectedGender = gender
            self.filterNames()
        }
        
        .onAppear {
            self.filterNames()
        }
          
    }
    
    fileprivate func filterNames() {
        self.filtered = []
        DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(100)) {
            self.filtered = self.names.filter { $0.gender == self.selectedGender }
        }
    }
      
}

Happy to help, I didn’t have this problem yet, but I may in the future. I am sure Apple will improve SwiftUI in fix such special cases. Until then. We at least found a simple solution, even if it may not be perfect as you can see the list being empty for a second before it takes the new array.

Something that might make it a little more swifty is something in the publisher chain that would set an 'empty' value when a change appears, then publish the real value 100 milliseconds later—sort of like an old car where you had to double-clutch each time you changed gear. You might build that as a modifier on the publisher that you can just attach it to the Defaults publisher:

.onReceive(Defaults.publisher(for: \.gender).doubleClutched(.milliseconds(100))) { gender in
    self.selectedGender = gender
    self.filterNames()
}

I haven't explored Combine yet, so I can't suggest an implementation off the cuff like I can with SwiftUI, but from what I've read so far it seems like it should be possible to insert something like this into the chain of a repeating (is that the term they use?) publisher.

Genius

Is the time delay (e.g. 100 milliseconds) really necessary? Would just re-setting the "real" value of the array in the next run loop cycle be sufficient?

This helped me!

https://www.youtube.com/watch?v=h0SgafWwoh8

I have the similar problem.

I have a simple core data (one of the attributes called "finished" (Bool) ), and I use the data to make a List ("ListView").

When click the items on the List, it goes to a Detail List ("DetailView").

On the Detail List, I have a button to SHOW the status of "finished", and UPDATE its record in core data. I can use it to change and update the record successfully.

When I click the toggle button, at backend, the record's updated.

However, the main problem is it cannot reflect the changes in Detail List. Can I REFRESH / RELOAD the Detail List? How to do it? What's the best way to do this task?

Many thanks!