问题
I started with a clean project and added 5 buttons and 5 spacers in a VStack and all is good. When I add the 6th spacer at the bottom, the code suddenly won't compile with the error: "Ambiguous reference to member 'buildBlock()'".
What is causing this error? Is this a bug related to SwiftUI? Or is it a feature? It's not the first time I notice that VStack or HStack is limited in the number of entries, is there some documentation around this?
Not exactly confidence inspiring, should I switch back to UIKit?
回答1:
SwiftUI uses ViewBuilder to construct the views that make up many SwiftUI views, like VStack, HStack, List, etc. If you take a look at the ViewBuilder documentation, you'll see that the buildBlock function has many copies, each with a different amount of views as arguments. The function with the most amount of views only takes in 10 views which is why you are seeing the limitation that you observed. A way to work around this is by using Groups:
VStack {
Group {
Text("Placeholder 0")
Text("Placeholder 1")
Text("Placeholder 2")
Text("Placeholder 3")
Text("Placeholder 4")
Text("Placeholder 5")
Text("Placeholder 6")
Text("Placeholder 7")
Text("Placeholder 8")
Text("Placeholder 9")
}
Group {
Text("Other Placeholder 10")
Text("Other Placeholder 11")
Text("Other Placeholder 12")
Text("Other Placeholder 13")
Text("Other Placeholder 14")
Text("Other Placeholder 15")
Text("Other Placeholder 16")
Text("Other Placeholder 17")
Text("Other Placeholder 18")
Text("Other Placeholder 19")
}
}
Although if you want 20 views that are really similar to each other, it is encouraged to use something like a ForEach to avoid making your views too bloated. The above workaround should only be used if the >10 views are truly unique. Even then, a more SwiftUI-y method would be to split up these views into more smaller views:
VStack {
SingleDigitPlaceholders()
TeensPlaceholders()
}
struct SingleDigitPlaceholders: View {
var body: some View {
ForEach(0..<10) { i in
Text("Placeholder \(i)")
}
}
}
struct TeensPlaceholders: View {
var body: some View {
ForEach(10..<20) { i in
Text("Other Placeholder \(i)")
}
}
}
Of course, in this specific example, you can just have the two ForEachs in the original view, but in more complex cases, the point still stands. For example, in a form with many elements (e.g. in a job application form: first name, last name, address, phone number text fields, education dropdown menus, date fields, etc.) you can still split up one view into smaller components (in the job application example - a personal information view, an educational information view, etc.).
回答2:
You can have at most 10 children in your VStack (and ZStack, HStack, and so forth). This is strictly related to their implementation and to the implementation of the @ViewBuilder closures in general. Look at the interfaces here below (you can find them through xCode, I slightly simplified them in order to be more readable):
public struct ViewBuilder {
/// Builds an empty view from an block containing no statements, `{ }`.
public static func buildBlock() -> EmptyView
/// Passes a single view written as a child view (e..g, `{ Text("Hello") }`) through
/// unmodified.
public static func buildBlock<Content>(_ content: Content) -> Content where Content : View
}
extension ViewBuilder {
public static func buildBlock<C0, C1>(_ c0: C0, _ c1: C1) -> TupleView<(C0, C1)> where C0 : View, C1 : View
}
extension ViewBuilder {
public static func buildBlock<C0, C1, C2>(_ c0: C0, _ c1: C1, _ c2: C2) -> TupleView<(C0, C1, C2)> where C0 : View, C1 : View, C2 : View
}
extension ViewBuilder {
public static func buildBlock<C0, C1, C2, C3>(_ c0: C0, _ c1: C1, _ c2: C2, _ c3: C3) -> TupleView<(C0, C1, C2, C3)> where C0 : View, C1 : View, C2 : View, C3 : View
}
extension ViewBuilder {
public static func buildBlock<C0, C1, C2, C3, C4>(_ c0: C0, _ c1: C1, _ c2: C2, _ c3: C3, _ c4: C4) -> TupleView<(C0, C1, C2, C3, C4)> where C0 : View, C1 : View, C2 : View, C3 : View, C4 : View
}
extension ViewBuilder {
public static func buildBlock<C0, C1, C2, C3, C4, C5>(_ c0: C0, _ c1: C1, _ c2: C2, _ c3: C3, _ c4: C4, _ c5: C5) -> TupleView<(C0, C1, C2, C3, C4, C5)> where C0 : View, C1 : View, C2 : View, C3 : View, C4 : View, C5 : View
}
extension ViewBuilder {
public static func buildBlock<C0, C1, C2, C3, C4, C5, C6>(_ c0: C0, _ c1: C1, _ c2: C2, _ c3: C3, _ c4: C4, _ c5: C5, _ c6: C6) -> TupleView<(C0, C1, C2, C3, C4, C5, C6)> where C0 : View, C1 : View, C2 : View, C3 : View, C4 : View, C5 : View, C6 : View
}
extension ViewBuilder {
public static func buildBlock<C0, C1, C2, C3, C4, C5, C6, C7>(_ c0: C0, _ c1: C1, _ c2: C2, _ c3: C3, _ c4: C4, _ c5: C5, _ c6: C6, _ c7: C7) -> TupleView<(C0, C1, C2, C3, C4, C5, C6, C7)> where C0 : View, C1 : View, C2 : View, C3 : View, C4 : View, C5 : View, C6 : View, C7 : View
}
extension ViewBuilder {
public static func buildBlock<C0, C1, C2, C3, C4, C5, C6, C7, C8>(_ c0: C0, _ c1: C1, _ c2: C2, _ c3: C3, _ c4: C4, _ c5: C5, _ c6: C6, _ c7: C7, _ c8: C8) -> TupleView<(C0, C1, C2, C3, C4, C5, C6, C7, C8)> where C0 : View, C1 : View, C2 : View, C3 : View, C4 : View, C5 : View, C6 : View, C7 : View, C8 : View
}
extension ViewBuilder {
public static func buildBlock<C0, C1, C2, C3, C4, C5, C6, C7, C8, C9>(_ c0: C0, _ c1: C1, _ c2: C2, _ c3: C3, _ c4: C4, _ c5: C5, _ c6: C6, _ c7: C7, _ c8: C8, _ c9: C9) -> TupleView<(C0, C1, C2, C3, C4, C5, C6, C7, C8, C9)> where C0 : View, C1 : View, C2 : View, C3 : View, C4 : View, C5 : View, C6 : View, C7 : View, C8 : View, C9 : View
}
As you can see you can build those kind of views with at most 10 children. Each buildBlock method takes an exact number of views as parameters. This is because @ViewBuilder closures, at least at the moment, don't support variadic arguments.
A workaround can be:
struct ContentView: View {
var body: some View {
VStack {
Group {
//10 views here
}
Group {
//10 views here
}
}
}
}
来源：https://stackoverflow.com/questions/58397964/are-there-maximum-limits-to-vstack