Order of .task vs .onAppear modifiers

[ginnheimerCoder]

I came across a statement in the session „Task Cancellation“ in the task module that I want to mention.

At the end of the section „Using tasks in SwiftUI“ it says:

Another benefit is that the task will start before the view even appears. That’s quicker than our previous example, where we used the onAppear modifier.

When I compare the order of print statements in a simple SwiftUI view, it seems to be the opposite:

struct TaskModifierView: View {
    var body: some View {
        Text("Hello, World!")
            .task {
                print("running task")
            }
            .onAppear {
                print("onAppear")
            }
            
    }
}

When I run the SwiftUI preview in the console, I get the following output:

onAppear
running task

So to me it seems like .onAppear runs before the .task modifier.
Am I missing something or misunderstanding the statement in the course?

[AvdLee]

I understand your confusion! I'll have to rewrite the lesson. The statement is still true, but it's about scheduling.

This would be the rewrite:

Another benefit is that the task gets scheduled on the cooperative pool as early as possible—sooner than if you were to start it inside an onAppear closure. However, the exact execution time still depends on the underlying task executor. As a result, it might appear that regular code within onAppear runs earlier. This isn’t a contradiction—it simply reflects the timing decisions made by the executor.

Does that clarify it better?

[ginnheimerCoder]

sounds better, thanks.

[ginnheimerCoder]

Is there a way to demonstrate this? Because if code in .onAppear runs earlier, how can we prove or demonstrate the earlier task scheduling? And what is the benefit of scheduling earlier when it will be executed later?

[AvdLee]

You could demonstrate this as follows:

.task(priority: .high, {
    print("1")
})
.onAppear {
    Task(priority: .high) { print("2") }
}

The difference with your earlier example is that we're now comparing two tasks, so both are being scheduled on the concurrency cooperative pool. Explicitly setting the priority ensures these don't affect scheduling either. This will consistently result in:

1
2

[AvdLee]

This is now updated in the Task Cancellation lesson