diff --git a/docs/swift/DesignDoc.md b/docs/swift/DesignDoc.md deleted file mode 100644 index 364a4d3f1b..0000000000 --- a/docs/swift/DesignDoc.md +++ /dev/null @@ -1,674 +0,0 @@ -# Protocol Buffers in Swift - -## Objective - -This document describes the user-facing API and internal implementation of -proto2 and proto3 messages in Apple’s Swift programming language. - -One of the key goals of protobufs is to provide idiomatic APIs for each -language. In that vein, **interoperability with Objective-C is a non-goal of -this proposal.** Protobuf users who need to pass messages between Objective-C -and Swift code in the same application should use the existing Objective-C proto -library. The goal of the effort described here is to provide an API for protobuf -messages that uses features specific to Swift—optional types, algebraic -enumerated types, value types, and so forth—in a natural way that will delight, -rather than surprise, users of the language. - -## Naming - -* By convention, both typical protobuf message names and Swift structs/classes - are `UpperCamelCase`, so for most messages, the name of a message can be the - same as the name of its generated type. (However, see the discussion below - about prefixes under [Packages](#packages).) - -* Enum cases in protobufs typically are `UPPERCASE_WITH_UNDERSCORES`, whereas - in Swift they are `lowerCamelCase` (as of the Swift 3 API design - guidelines). We will transform the names to match Swift convention, using - a whitelist similar to the Objective-C compiler plugin to handle commonly - used acronyms. - -* Typical fields in proto messages are `lowercase_with_underscores`, while in - Swift they are `lowerCamelCase`. We will transform the names to match - Swift convention by removing the underscores and uppercasing the subsequent - letter. - -## Swift reserved words - -Swift has a large set of reserved words—some always reserved and some -contextually reserved (that is, they can be used as identifiers in contexts -where they would not be confused). As of Swift 2.2, the set of always-reserved -words is: - -``` -_, #available, #column, #else, #elseif, #endif, #file, #function, #if, #line, -#selector, as, associatedtype, break, case, catch, class, continue, default, -defer, deinit, do, dynamicType, else, enum, extension, fallthrough, false, for, -func, guard, if, import, in, init, inout, internal, is, let, nil, operator, -private, protocol, public, repeat, rethrows, return, self, Self, static, -struct, subscript, super, switch, throw, throws, true, try, typealias, var, -where, while -``` - -The set of contextually reserved words is: - -``` -associativity, convenience, dynamic, didSet, final, get, infix, indirect, -lazy, left, mutating, none, nonmutating, optional, override, postfix, -precedence, prefix, Protocol, required, right, set, Type, unowned, weak, -willSet -``` - -It is possible to use any reserved word as an identifier by escaping it with -backticks (for example, ``let `class` = 5``). Other name-mangling schemes would -require us to transform the names themselves (for example, by appending an -underscore), which requires us to then ensure that the new name does not collide -with something else in the same namespace. - -While the backtick feature may not be widely known by all Swift developers, a -small amount of user education can address this and it seems like the best -approach. We can unconditionally surround all property names with backticks to -simplify generation. - -Some remapping will still be required, though, to avoid collisions between -generated properties and the names of methods and properties defined in the base -protocol/implementation of messages. - -# Features of Protocol Buffers - -This section describes how the features of the protocol buffer syntaxes (proto2 -and proto3) map to features in Swift—what the code generated from a proto will -look like, and how it will be implemented in the underlying library. - -## Packages - -Modules are the main form of namespacing in Swift, but they are not declared -using syntactic constructs like namespaces in C++ or packages in Java. Instead, -they are tied to build targets in Xcode (or, in the future with open-source -Swift, declarations in a Swift Package Manager manifest). They also do not -easily support nesting submodules (Clang module maps support this, but pure -Swift does not yet provide a way to define submodules). - -We will generate types with fully-qualified underscore-delimited names. For -example, a message `Baz` in package `foo.bar` would generate a struct named -`Foo_Bar_Baz`. For each fully-qualified proto message, there will be exactly one -unique type symbol emitted in the generated binary. - -Users are likely to balk at the ugliness of underscore-delimited names for every -generated type. To improve upon this situation, we will add a new string file -level option, `swift_package_typealias`, that can be added to `.proto` files. -When present, this will cause `typealias`es to be added to the generated Swift -messages that replace the package name prefix with the provided string. For -example, the following `.proto` file: - -```protobuf -option swift_package_typealias = "FBP"; -package foo.bar; - -message Baz { - // Message fields -} -``` - -would generate the following Swift source: - -```swift -public struct Foo_Bar_Baz { - // Message fields and other methods -} - -typealias FBPBaz = Foo_Bar_Baz -``` - -It should be noted that this type alias is recorded in the generated -`.swiftmodule` so that code importing the module can refer to it, but it does -not cause a new symbol to be generated in the compiled binary (i.e., we do not -risk compiled size bloat by adding `typealias`es for every type). - -Other strategies to handle packages that were considered and rejected can be -found in [Appendix A](#appendix-a-rejected-strategies-to-handle-packages). - -## Messages - -Proto messages are natural value types and we will generate messages as structs -instead of classes. Users will benefit from Swift’s built-in behavior with -regard to mutability. We will define a `ProtoMessage` protocol that defines the -common methods and properties for all messages (such as serialization) and also -lets users treat messages polymorphically. Any shared method implementations -that do not differ between individual messages can be implemented in a protocol -extension. - -The backing storage itself for fields of a message will be managed by a -`ProtoFieldStorage` type that uses an internal dictionary keyed by field number, -and whose values are the value of the field with that number (up-cast to Swift’s -`Any` type). This class will provide type-safe getters and setters so that -generated messages can manipulate this storage, and core serialization logic -will live here as well. Furthermore, factoring the storage out into a separate -type, rather than inlining the fields as stored properties in the message -itself, lets us implement copy-on-write efficiently to support passing around -large messages. (Furthermore, because the messages themselves are value types, -inlining fields is not possible if the fields are submessages of the same type, -or a type that eventually includes a submessage of the same type.) - -### Required fields (proto2 only) - -Required fields in proto2 messages seem like they could be naturally represented -by non-optional properties in Swift, but this presents some problems/concerns. - -Serialization APIs permit partial serialization, which allows required fields to -remain unset. Furthermore, other language APIs still provide `has*` and `clear*` -methods for required fields, and knowing whether a property has a value when the -message is in memory is still useful. - -For example, an e-mail draft message may have the “to” address required on the -wire, but when the user constructs it in memory, it doesn’t make sense to force -a value until they provide one. We only want to force a value to be present when -the message is serialized to the wire. Using non-optional properties prevents -this use case, and makes client usage awkward because the user would be forced -to select a sentinel or placeholder value for any required fields at the time -the message was created. - -### Default values - -In proto2, fields can have a default value specified that may be a value other -than the default value for its corresponding language type (for example, a -default value of 5 instead of 0 for an integer). When reading a field that is -not explicitly set, the user expects to get that value. This makes Swift -optionals (i.e., `Foo?`) unsuitable for fields in general. Unfortunately, we -cannot implement our own “enhanced optional” type without severely complicating -usage (Swift’s use of type inference and its lack of implicit conversions would -require manual unwrapping of every property value). - -Instead, we can use **implicitly unwrapped optionals.** For example, a property -generated for a field of type `int32` would have Swift type `Int32!`. These -properties would behave with the following characteristics, which mirror the -nil-resettable properties used elsewhere in Apple’s SDKs (for example, -`UIView.tintColor`): - -* Assigning a non-nil value to a property sets the field to that value. -* Assigning nil to a property clears the field (its internal representation is - nilled out). -* Reading the value of a property returns its value if it is set, or returns - its default value if it is not set. Reading a property never returns nil. - -The final point in the list above implies that the optional cannot be checked to -determine if the field is set to a value other than its default: it will never -be nil. Instead, we must provide `has*` methods for each field to allow the user -to check this. These methods will be public in proto2. In proto3, these methods -will be private (if generated at all), since the user can test the returned -value against the zero value for that type. - -### Autocreation of nested messages - -For convenience, dotting into an unset field representing a nested message will -return an instance of that message with default values. As in the Objective-C -implementation, this does not actually cause the field to be set until the -returned message is mutated. Fortunately, thanks to the way mutability of value -types is implemented in Swift, the language automatically handles the -reassignment-on-mutation for us. A static singleton instance containing default -values can be associated with each message that can be returned when reading, so -copies are only made by the Swift runtime when mutation occurs. For example, -given the following proto: - -```protobuf -message Node { - Node child = 1; - string value = 2 [default = "foo"]; -} -``` - -The following Swift code would act as commented, where setting deeply nested -properties causes the copies and mutations to occur as the assignment statement -is unwound: - -```swift -var node = Node() - -let s = node.child.child.value -// 1. node.child returns the "default Node". -// 2. Reading .child on the result of (1) returns the same default Node. -// 3. Reading .value on the result of (2) returns the default value "foo". - -node.child.child.value = "bar" -// 4. Setting .value on the default Node causes a copy to be made and sets -// the property on that copy. Subsequently, the language updates the -// value of "node.child.child" to point to that copy. -// 5. Updating "node.child.child" in (4) requires another copy, because -// "node.child" was also the instance of the default node. The copy is -// assigned back to "node.child". -// 6. Setting "node.child" in (5) is a simple value reassignment, since -// "node" is a mutable var. -``` - -In other words, the generated messages do not internally have to manage parental -relationships to backfill the appropriate properties on mutation. Swift provides -this for free. - -## Scalar value fields - -Proto scalar value fields will map to Swift types in the following way: - -.proto Type | Swift Type ------------ | ------------------- -`double` | `Double` -`float` | `Float` -`int32` | `Int32` -`int64` | `Int64` -`uint32` | `UInt32` -`uint64` | `UInt64` -`sint32` | `Int32` -`sint64` | `Int64` -`fixed32` | `UInt32` -`fixed64` | `UInt64` -`sfixed32` | `Int32` -`sfixed64` | `Int64` -`bool` | `Bool` -`string` | `String` -`bytes` | `Foundation.NSData` - -The proto spec defines a number of integral types that map to the same Swift -type; for example, `intXX`, `sintXX`, and `sfixedXX` are all signed integers, -and `uintXX` and `fixedXX` are both unsigned integers. No other language -implementation distinguishes these further, so we do not do so either. The -rationale is that the various types only serve to distinguish how the value is -**encoded on the wire**; once loaded in memory, the user is not concerned about -these variations. - -Swift’s lack of implicit conversions among types will make it slightly annoying -to use these types in a context expecting an `Int`, or vice-versa, but since -this is a data-interchange format with explicitly-sized fields, we should not -hide that information from the user. Users will have to explicitly write -`Int(message.myField)`, for example. - -## Embedded message fields - -Embedded message fields can be represented using an optional variable of the -generated message type. Thus, the message - -```protobuf -message Foo { - Bar bar = 1; -} -``` - -would be represented in Swift as - -```swift -public struct Foo: ProtoMessage { - public var bar: Bar! { - get { ... } - set { ... } - } -} -``` - -If the user explicitly sets `bar` to nil, or if it was never set when read from -the wire, retrieving the value of `bar` would return a default, statically -allocated instance of `Bar` containing default values for its fields. This -achieves the desired behavior for default values in the same way that scalar -fields are designed, and also allows users to deep-drill into complex object -graphs to get or set fields without checking for nil at each step. - -## Enum fields - -The design and implementation of enum fields will differ somewhat drastically -depending on whether the message being generated is a proto2 or proto3 message. - -### proto2 enums - -For proto2, we do not need to be concerned about unknown enum values, so we can -use the simple raw-value enum syntax provided by Swift. So the following enum in -proto2: - -```protobuf -enum ContentType { - TEXT = 0; - IMAGE = 1; -} -``` - -would become this Swift enum: - -```swift -public enum ContentType: Int32, NilLiteralConvertible { - case text = 0 - case image = 1 - - public init(nilLiteral: ()) { - self = .text - } -} -``` - -See below for the discussion about `NilLiteralConvertible`. - -### proto3 enums - -For proto3, we need to be able to preserve unknown enum values that may come -across the wire so that they can be written back if unmodified. We can -accomplish this in Swift by using a case with an associated value for unknowns. -So the following enum in proto3: - -```protobuf -enum ContentType { - TEXT = 0; - IMAGE = 1; -} -``` - -would become this Swift enum: - -```swift -public enum ContentType: RawRepresentable, NilLiteralConvertible { - case text - case image - case UNKNOWN_VALUE(Int32) - - public typealias RawValue = Int32 - - public init(nilLiteral: ()) { - self = .text - } - - public init(rawValue: RawValue) { - switch rawValue { - case 0: self = .text - case 1: self = .image - default: self = .UNKNOWN_VALUE(rawValue) - } - - public var rawValue: RawValue { - switch self { - case .text: return 0 - case .image: return 1 - case .UNKNOWN_VALUE(let value): return value - } - } -} -``` - -Note that the use of a parameterized case prevents us from inheriting from the -raw `Int32` type; Swift does not allow an enum with a raw type to have cases -with arguments. Instead, we must implement the raw value initializer and -computed property manually. The `UNKNOWN_VALUE` case is explicitly chosen to be -"ugly" so that it stands out and does not conflict with other possible case -names. - -Using this approach, proto3 consumers must always have a default case or handle -the `.UNKNOWN_VALUE` case to satisfy case exhaustion in a switch statement; the -Swift compiler considers it an error if switch statements are not exhaustive. - -### NilLiteralConvertible conformance - -This is required to clean up the usage of enum-typed properties in switch -statements. Unlike other field types, enum properties cannot be -implicitly-unwrapped optionals without requiring that uses in switch statements -be explicitly unwrapped. For example, if we consider a message with the enum -above, this usage will fail to compile: - -```swift -// Without NilLiteralConvertible conformance on ContentType -public struct SomeMessage: ProtoMessage { - public var contentType: ContentType! { ... } -} - -// ERROR: no case named text or image -switch someMessage.contentType { - case .text: { ... } - case .image: { ... } -} -``` - -Even though our implementation guarantees that `contentType` will never be nil, -if it is an optional type, its cases would be `some` and `none`, not the cases -of the underlying enum type. In order to use it in this context, the user must -write `someMessage.contentType!` in their switch statement. - -Making the enum itself `NilLiteralConvertible` permits us to make the property -non-optional, so the user can still set it to nil to clear it (i.e., reset it to -its default value), while eliminating the need to explicitly unwrap it in a -switch statement. - -```swift -// With NilLiteralConvertible conformance on ContentType -public struct SomeMessage: ProtoMessage { - // Note that the property type is no longer optional - public var contentType: ContentType { ... } -} - -// OK: Compiles and runs as expected -switch someMessage.contentType { - case .text: { ... } - case .image: { ... } -} - -// The enum can be reset to its default value this way -someMessage.contentType = nil -``` - -One minor oddity with this approach is that nil will be auto-converted to the -default value of the enum in any context, not just field assignment. In other -words, this is valid: - -```swift -func foo(contentType: ContentType) { ... } -foo(nil) // Inside foo, contentType == .text -``` - -That being said, the advantage of being able to simultaneously support -nil-resettability and switch-without-unwrapping outweighs this side effect, -especially if appropriately documented. It is our hope that a new form of -resettable properties will be added to Swift that eliminates this inconsistency. -Some community members have already drafted or sent proposals for review that -would benefit our designs: - -* [SE-0030: Property Behaviors] - (https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md) -* [Drafted: Resettable Properties] - (https://github.com/patters/swift-evolution/blob/master/proposals/0000-resettable-properties.md) - -### Enum aliases - -The `allow_alias` option in protobuf slightly complicates the use of Swift enums -to represent that type, because raw values of cases in an enum must be unique. -Swift lets us define static variables in an enum that alias actual cases. For -example, the following protobuf enum: - -```protobuf -enum Foo { - option allow_alias = true; - BAR = 0; - BAZ = 0; -} -``` - -will be represented in Swift as: - -```swift -public enum Foo: Int32, NilLiteralConvertible { - case bar = 0 - static public let baz = bar - - // ... etc. -} - -// Can still use .baz shorthand to reference the alias in contexts -// where the type is inferred -``` - -That is, we use the first name as the actual case and use static variables for -the other aliases. One drawback to this approach is that the static aliases -cannot be used as cases in a switch statement (the compiler emits the error -*“Enum case ‘baz’ not found in type ‘Foo’”*). However, in our own code bases, -there are only a few places where enum aliases are not mere renamings of an -older value, but they also don’t appear to be the type of value that one would -expect to switch on (for example, a group of named constants representing -metrics rather than a set of options), so this restriction is not significant. - -This strategy also implies that changing the name of an enum and adding the old -name as an alias below the new name will be a breaking change in the generated -Swift code. - -## Oneof types - -The `oneof` feature represents a “variant/union” data type that maps nicely to -Swift enums with associated values (algebraic types). These fields can also be -accessed independently though, and, specifically in the case of proto2, it’s -reasonable to expect access to default values when accessing a field that is not -explicitly set. - -Taking all this into account, we can represent a `oneof` in Swift with two sets -of constructs: - -* Properties in the message that correspond to the `oneof` fields. -* A nested enum named after the `oneof` and which provides the corresponding - field values as case arguments. - -This approach fulfills the needs of proto consumers by providing a -Swift-idiomatic way of simultaneously checking which field is set and accessing -its value, providing individual properties to access the default values -(important for proto2), and safely allows a field to be moved into a `oneof` -without breaking clients. - -Consider the following proto: - -```protobuf -message MyMessage { - oneof record { - string name = 1 [default = "unnamed"]; - int32 id_number = 2 [default = 0]; - } -} -``` - -In Swift, we would generate an enum, a property for that enum, and properties -for the fields themselves: - -```swift -public struct MyMessage: ProtoMessage { - public enum Record: NilLiteralConvertible { - case name(String) - case idNumber(Int32) - case NOT_SET - - public init(nilLiteral: ()) { self = .NOT_SET } - } - - // This is the "Swifty" way of accessing the value - public var record: Record { ... } - - // Direct access to the underlying fields - public var name: String! { ... } - public var idNumber: Int32! { ... } -} -``` - -This makes both usage patterns possible: - -```swift -// Usage 1: Case-based dispatch -switch message.record { - case .name(let name): - // Do something with name if it was explicitly set - case .idNumber(let id): - // Do something with id_number if it was explicitly set - case .NOT_SET: - // Do something if it’s not set -} - -// Usage 2: Direct access for default value fallback -// Sets the label text to the name if it was explicitly set, or to -// "unnamed" (the default value for the field) if id_number was set -// instead -let myLabel = UILabel() -myLabel.text = message.name -``` - -As with proto enums, the generated `oneof` enum conforms to -`NilLiteralConvertible` to avoid switch statement issues. Setting the property -to nil will clear it (i.e., reset it to `NOT_SET`). - -## Unknown Fields (proto2 only) - -To be written. - -## Extensions (proto2 only) - -To be written. - -## Reflection and Descriptors - -We will not include reflection or descriptors in the first version of the Swift -library. The use cases for reflection on mobile are not as strong and the static -data to represent the descriptors would add bloat when we wish to keep the code -size small. - -In the future, we will investigate whether they can be included as extensions -which might be able to be excluded from a build and/or automatically dead -stripped by the compiler if they are not used. - -## Appendix A: Rejected strategies to handle packages - -### Each package is its own Swift module - -Each proto package could be declared as its own Swift module, replacing dots -with underscores (e.g., package `foo.bar` becomes module `Foo_Bar`). Then, users -would simply import modules containing whatever proto modules they want to use -and refer to the generated types by their short names. - -**This solution is simply not possible, however.** Swift modules cannot -circularly reference each other, but there is no restriction against proto -packages doing so. Circular imports are forbidden (e.g., `foo.proto` importing -`bar.proto` importing `foo.proto`), but nothing prevents package `foo` from -using a type in package `bar` which uses a different type in package `foo`, as -long as there is no import cycle. If these packages were generated as Swift -modules, then `Foo` would contain an `import Bar` statement and `Bar` would -contain an `import Foo` statement, and there is no way to compile this. - -### Ad hoc namespacing with structs - -We can “fake” namespaces in Swift by declaring empty structs with private -initializers. Since modules are constructed based on compiler arguments, not by -syntactic constructs, and because there is no pure Swift way to define -submodules (even though Clang module maps support this), there is no -source-drive way to group generated code into namespaces aside from this -approach. - -Types can be added to those intermediate package structs using Swift extensions. -For example, a message `Baz` in package `foo.bar` could be represented in Swift -as follows: - -```swift -public struct Foo { - private init() {} -} - -public extension Foo { - public struct Bar { - private init() {} - } -} - -public extension Foo.Bar { - public struct Baz { - // Message fields and other methods - } -} - -let baz = Foo.Bar.Baz() -``` - -Each of these constructs would actually be defined in a separate file; Swift -lets us keep them separate and add multiple structs to a single “namespace” -through extensions. - -Unfortunately, these intermediate structs generate symbols of their own -(metatype information in the data segment). This becomes problematic if multiple -build targets contain Swift sources generated from different messages in the -same package. At link time, these symbols would collide, resulting in multiple -definition errors. - -This approach also has the disadvantage that there is no automatic “short” way -to refer to the generated messages at the deepest nesting levels; since this use -of structs is a hack around the lack of namespaces, there is no equivalent to -import (Java) or using (C++) to simplify this. Users would have to declare type -aliases to make this cleaner, or we would have to generate them for users.