@ObservedObject: มันคืออะใร, การสังเกตวัตถุและการอัพเดท View

What is @ObservedObject in SwiftUI

@ObservedObject is a property wrapper that subscribes a View to ObservableObject changes. When an object marked with @ObservedObject changes any of its properties declared with @Published, SwiftUI automatically redraws the View. @ObservedObject does not create the object — it only establishes a connection between an existing ObservableObject instance and the View that should react to its changes.

The key difference between @ObservedObject and @StateObject is ownership. @ObservedObject assumes the object is created and stored somewhere higher in the View hierarchy. The child View receives a reference to this object through the initializer and simply observes it. If the child View is recreated, it receives the same reference from the parent — data is not lost.

According to Apple Developer Documentation — SwiftUI (2025), @ObservedObject is available starting with iOS 13, making it the only option for observing ObservableObject in projects supporting older iOS versions. In iOS 14+, @StateObject is preferred for creating objects, but @ObservedObject remains relevant for passing existing objects.

How @ObservedObject Works

The @ObservedObject mechanism is based on the ObservableObject protocol from the Combine framework. Each class conforming to ObservableObject automatically gets an objectWillChange publisher that sends a signal before any @Published property changes. SwiftUI subscribes to this publisher through @ObservedObject and upon receiving the signal marks the View as needing a redraw.

class TaskViewModel: ObservableObject {
    @Published var tasks: [Task] = []
    @Published var isLoading = false
    
    func loadTasks() async {
        isLoading = true
        // fetch data
        isLoading = false
    }
}

struct TaskListView: View {
    @ObservedObject var viewModel: TaskViewModel
    
    var body: some View {
        List(viewModel.tasks) { task in
            Text(task.title)
        }
        .task { await viewModel.loadTasks() }
    }
}

When the parent View passes viewModel to TaskListView through the initializer, SwiftUI creates a connection between the object and the View. When the tasks array or isLoading flag changes, SwiftUI redraws TaskListView. The object itself remains unchanged — it is stored in the parent View through @StateObject.

@ObservedObject vs @StateObject: When to Use What

The difference between @ObservedObject and @StateObject is the difference between an observer and an owner. @StateObject creates the object and manages its lifecycle. @ObservedObject only observes an object that was created and stored elsewhere. The choice between them is determined by the View’s responsibility for the data.

The main rule: if the View creates the object — @StateObject. If the View receives the object — @ObservedObject. Violating this rule by using @ObservedObject to create an object leads to data loss when the View is rebuilt. Violating it by using @StateObject to receive an object creates a duplicate instance independent from the parent.

@ObservedObject Usage Examples

A typical usage scenario for @ObservedObject is a task list where the root View creates a view model and each list cell receives it through @ObservedObject. Each cell can call view model methods, and changes are automatically reflected across the entire list since all cells observe the same object.

struct TaskRow: View {
    @ObservedObject var viewModel: TaskViewModel
    let task: Task
    
    var body: some View {
        HStack {
            Text(task.title)
            Spacer()
            Button("Done") {
                viewModel.completeTask(task)
            }
        }
    }
}

struct TaskListContainer: View {
    @StateObject var viewModel = TaskViewModel()
    
    var body: some View {
        List(viewModel.tasks) { task in
            TaskRow(viewModel: viewModel, task: task)
        }
    }
}

In this example, TaskListContainer creates viewModel through @StateObject, and each TaskRow receives it through @ObservedObject. When the user presses “Done” in any row, viewModel.completeTask changes a published property, and all Views observing this object update automatically.

Common Mistakes with @ObservedObject

The most common mistake is using @ObservedObject to create an object inside a View. When the View is rebuilt (e.g., when state changes), SwiftUI creates a new ObservableObject instance, leading to the loss of all accumulated data. This mistake is especially painful in NavigationStack, where a user might fill out a form and lose data when navigating back.

Mistake: @ObservedObject instead of @StateObject

// ❌ Data loss: @ObservedObject does not retain object
struct FormView: View {
    @ObservedObject var formVM = FormViewModel()
    // New formVM created on each View rebuild!
}

// ✅ Correct: @StateObject retains the object
struct FormView: View {
    @StateObject var formVM = FormViewModel()
    // Object created once per View lifetime
}

Mistake: passing @StateObject where @ObservedObject is needed

If a child View declares the same ObservableObject through @StateObject, it creates an independent copy. Changes in the parent object will not be visible in the child, and vice versa. Always use @ObservedObject for child Views that receive the object externally.

@ObservedObject Alternatives in SwiftUI

In modern SwiftUI there are several alternatives to @ObservedObject, each with its own advantages. The choice depends on the application architecture, iOS version, and the specific use case.

• @EnvironmentObject — allows getting an object from the SwiftUI environment without explicitly passing it through the initializer. Convenient for objects needed by many screens, but requires explicit injection through .environmentObject().
• @State + @Binding — for simple value types, ObservableObject is not needed. Use @State for storage and @Binding for passing to child Views.
• @AppStorage — for UserDefaults values that should automatically sync with the View.
• @SceneStorage — for preserving temporary state between scene restarts (e.g., scroll position in a list).

The choice between @ObservedObject and @EnvironmentObject is a matter of style and architecture. @ObservedObject explicitly shows View dependencies through the initializer, making the code more predictable. @EnvironmentObject is convenient for deep hierarchies but hides dependencies, which can complicate debugging.