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Playground QuickLook API Revamp

Introduction

The standard library currently includes API which allows a type to customize its description in Xcode playgrounds and Swift Playgrounds. This API takes the form of the PlaygroundQuickLook enum which enumerates types which are supported for quick looks, and the CustomPlaygroundQuickLookable protocol which allows a type to return a custom PlaygroundQuickLook value for an instance.

This is brittle, and to avoid dependency inversions, many of the cases are typed as taking Any instead of a more appropriate type. This proposal suggests that we deprecate PlaygroundQuickLook and CustomPlaygroundQuickLookable in Swift 4.1 so they can be removed entirely in Swift 5, preventing them from being included in the standard library's stable ABI. To maintain compatibility with older playgrounds, the deprecated symbols will be present in a temporary compatibility shim library which will be automatically imported in playground contexts. (This will represent an intentional source break for projects, packages, and other non-playground Swift code which use PlaygroundQuickLook or CustomPlaygroundQuickLookable when they switch to the Swift 5.0 compiler, even in the compatibility modes.)

Since it is still useful to allow types to provide alternate descriptions for playgrounds, we propose to add a new protocol to the PlaygroundSupport framework which allows types to do just that. (PlaygroundSupport is a framework delivered by the swift-xcode-playground-support project which provides API specific to working in the playgrounds environment). The new CustomPlaygroundDisplayConvertible protocol would allow instances to return an alternate object or value (as an Any) which would serve as their description. The PlaygroundLogger framework, also part of swift-xcode-playground-support, will be updated to understand this protocol.

Swift-evolution thread: Discussion thread topic for that proposal

Motivation

The PlaygroundQuickLook enum which currently exists in the standard library is substandard:

public enum PlaygroundQuickLook {
  case text(String)
  case int(Int64)
  case uInt(UInt64)
  case float(Float32)
  case double(Float64)
  case image(Any)
  case sound(Any)
  case color(Any)
  case bezierPath(Any)
  case attributedString(Any)
  case rectangle(Float64, Float64, Float64, Float64)
  case point(Float64, Float64)
  case size(Float64, Float64)
  case bool(Bool)
  case range(Int64, Int64)
  case view(Any)
  case sprite(Any)
  case url(String)
  case _raw([UInt8], String)
}

The names of these enum cases do not necessarily match current Swift naming conventions (e.g. uInt), and many cases are typed as Any to avoid dependency inversions between the standard library and higher-level frameworks like Foundation and AppKit or UIKit. It also contains cases which the PlaygroundLogger framework does not understand (e.g. sound), and this listing of cases introduces revlock between PlaygroundLogger and the standard library that makes it challenging to introduce support for new types of quick looks.

Values of this enum are provided to the PlaygroundLogger framework by types via conformances to the CustomPlaygroundQuickLookable protocol:

public protocol CustomPlaygroundQuickLookable {
  var customPlaygroundQuickLook: PlaygroundQuickLook { get }
}

This protocol itself is not problematic, but if PlaygroundQuickLook is being removed, then it needs to be removed as well. Additionally, there is a companion underscored protocol which should be removed as well:

public protocol _DefaultCustomPlaygroundQuickLookable {
  var _defaultCustomPlaygroundQuickLook: PlaygroundQuickLook { get }
}

Proposed solution

To solve this issue, we propose the following changes:

  • Introduce a new CustomPlaygroundDisplayConvertible protocol in PlaygroundSupport in Swift 4.1 to allow types to provide an alternate description for playground logging
  • Deprecate PlaygroundQuickLook and CustomPlaygroundQuickLookable in Swift 4.1, suggesting users use CustomPlaygroundDisplayConvertible instead
  • Remove PlaygroundQuickLook and CustomPlaygroundQuickLookable from the standard library in Swift 5.0
  • Provide an automatically-imported shim library for the playgrounds context to provide the deprecated instances of PlaygroundQuickLook and CustomPlaygroundQuickLookable for pre-Swift 5 playgrounds

Detailed design

To provide a more flexible API, we propose deprecating and ultimately removing the PlaygroundQuickLook enum and CustomPlaygroundQuickLookable protocol in favor of a simpler design. Instead, we propose introducing a protocol which just provides the ability to return an Any that serves as a stand-in for the instance being logged:

/// A type that supplies a custom description for playground logging.
///
/// All types have a default description for playgrounds. This protocol
/// allows types to provide custom descriptions which are then logged in
/// place of the original instance.
///
/// Playground logging can generate, at a minimum, a structured description
/// of any type. Playground logging is also capable of generating a richer,
/// more specialized description of core types -- for instance, the contents
/// of a `String` are logged, as are the components of an `NSColor` or
/// `UIColor`.
///
/// The current playground logging implementation logs specialized
/// descriptions of at least the following types:
///
/// - `String` and `NSString`
/// - `Int` and `UInt` (including the sized variants)
/// - `Float` and `Double`
/// - `Bool`
/// - `Date` and `NSDate`
/// - `NSAttributedString`
/// - `NSNumber`
/// - `NSRange`
/// - `URL` and `NSURL`
/// - `CGPoint`, `CGSize`, and `CGRect`
/// - `NSColor`, `UIColor`, `CGColor`, and `CIColor`
/// - `NSImage`, `UIImage`, `CGImage`, and `CIImage`
/// - `NSBezierPath` and `UIBezierPath`
/// - `NSView` and `UIView`
///
/// Playground logging may also be able to support specialized descriptions
/// of other types.
///
/// Implementors of `CustomPlaygroundDisplayConvertible` may return a value of
/// one of the above types to also receive a specialized log description.
/// Implementors may also return any other type, and playground logging will
/// generated structured logging for the returned value.
public protocol CustomPlaygroundDisplayConvertible {
  /// Returns the custom playground description for this instance.
  ///
  /// If this type has value semantics, the instance returned should be
  /// unaffected by subsequent mutations if possible.
  var playgroundDescription: Any { get }
}

Additionally, instead of placing this protocol in the standard library, we propose placing this protocol in the PlaygroundSupport framework, as it is only of interest in the playgrounds environment. Should demand warrant it, a future proposal could suggest lowering this protocol into the standard library.

If this proposal is accepted, then code like the following:

extension MyStruct: CustomPlaygroundQuickLookable {
  var customPlaygroundQuickLook: PlaygroundQuickLook {
    return .text("A description of this MyStruct instance")
  }
}

would be replaced with something like the following:

extension MyStruct: CustomPlaygroundDisplayConvertible {
  var playgroundDescription: Any {
    return "A description of this MyStruct instance"
  }
}

This proposal also allows types which wish to be represented structurally (like an array or dictionary) to return a type which is logged structurally instead of requiring an implementation of the CustomReflectable protocol:

extension MyStruct: CustomPlaygroundDisplayConvertible {
  var playgroundDescription: Any {
    return [1, 2, 3]
  }
}

This is an enhancement over the existing CustomPlaygroundQuickLookable protocol, which only supported returning opaque, quick lookable values for playground logging.

Implementations of CustomPlaygroundDisplayConvertible may potentially chain from one to another. For instance, with:

extension MyStruct: CustomPlaygroundDisplayConvertible {
  var playgroundDescription: Any {
    return "MyStruct description for playgrounds"
  }
}

extension MyOtherStruct: CustomPlaygroundDisplayConvertible {
  var playgroundDescription: Any {
    return MyStruct()
  }
}

Playground logging for MyOtherStruct would generate the string "MyStruct description for playgrounds" rather than the structural view of MyStruct. It is legal, however, for playground logging implementations to cap chaining to a reasonable limit to guard against infinite recursion.

Source compatibility

This proposal is explicitly suggesting that we make a source-breaking change in Swift 5 to remove PlaygroundQuickLook, CustomPlaygroundQuickLookable, and _DefaultCustomPlaygroundQuickLookable. Looking at a GitHub search, there are fewer than 900 references to CustomPlaygroundQuickLookable in Swift source code; from a cursory glance, many of these are duplicates, from forks of the Swift repo itself (i.e. the definition of CustomPlaygroundQuickLookable in the standard library), or are clearly implemented using pre-Swift 3 names of the enum cases in PlaygroundQuickLook. (As a point of comparison, there are over 185,000 references to CustomStringConvertible in Swift code on GitHub, and over 145,000 references to CustomDebugStringConvertible, so CustomPlaygroundQuickLookable is clearly used many orders of magnitude less than those protocols.) Furthermore, it does not appear that any projects currently in the source compatibility suite use these types.

However, to mitigate the impact of this change, we propose to provide a limited source compatibility shim for the playgrounds context. This will be delivered as part of the swift-xcode-playground-support project as a library containing the deprecated PlaygroundQuickLook and CustomPlaygroundQuickLookable protocols. This library would be imported automatically in playgrounds. This source compatibility shim would not be available outside of playgrounds, so any projects, packages, or other Swift code would be intentionally broken by this change when upgrading to the Swift 5.0 compiler, even when compiling in a compatibility mode.

Due to the limited usage of these protocols, and the potential challenge in migration, this proposal does not include any proposed migrator changes to support the replacement of CustomPlaygroundQuickLookable with CustomPlaygroundDisplayConvertible. Instead, we intend for Swift 4.1 to be a deprecation period for these APIs, allowing any code bases which implement CustomPlaygroundQuickLookable to manually switch to the new protocol. While this migration may not be trivial programatically, it should -- in most cases -- be fairly trivial for someone to hand-migrate to CustomPlaygroundDisplayConvertible. During the deprecation period, the PlaygroundLogger framework will continue to honor implementations of CustomPlaygroundQuickLookable, though it will prefer implementations of CustomPlaygroundDisplayConvertible if both are present on a given type.

Effect on ABI stability

This proposal affects ABI stability as it removes an enum and a pair of protocols from the standard library. Since this proposal proposes adding CustomPlaygroundDisplayConvertible to PlaygroundSupport instead of the standard library, there is no impact of ABI stability from the new protocol, as PlaygroundSupport does not need to maintain a stable ABI, as its clients -- playgrounds -- are always recompiled from source.

Since playgrounds are always compiled from source, the temporary shim library does not represent a new ABI guarantee, and it may be removed if the compiler drops support for the Swift 3 and 4 compatibility modes in a future Swift release.

Removing PlaygroundQuickLook from the standard library also potentially allows us to remove a handful of runtime entry points which were included to support the PlaygroundQuickLook(reflecting:) API.

Effect on API resilience

This proposal does not impact API resilience.

Alternatives considered

Do nothing

One valid alternative to this proposal is to do nothing: we could continue to live with the existing enum and protocol. As noted above, these are fairly poor, and do not serve the needs of playgrounds particularly well. Since this is our last chance to remove them prior to ABI stability, we believe that doing nothing is not an acceptable alternative.

Provide type-specific protocols

Another alternative we considered was to provide type-specific protocols for providing playground descriptions. We would introduce new protocols like CustomNSColorConvertible, CustomNSAttributedStringConvertible, etc. which would allow types to provide descriptions as each of the opaquely-loggable types supported by PlaygroundLogger.

This alternative was rejected as it would balloon the API surface for playgrounds, and it also would not provide a good way to select a preferred description. (That is, what would PlaygroundLogger select as the description of an instance if it implemented both CustomNSColorConvertible and CustomNSAttributedStringConvertible?)

Implement CustomPlaygroundDisplayConvertible in the standard library

As an alternative to implementing CustomPlaygroundDisplayConvertible in PlaygroundSupport, we could implement it in the standard library. This would make it available in all contexts (i.e. in projects and packages, not just in playgrounds), but this protocol is not particularly useful outside of the playground context, so this proposal elects not to place CustomPlaygroundDisplayConvertible in the standard library.

Additionally, it should be a source-compatible change to move this protocol to the standard library in a future Swift version should that be desirable. Since playgrounds are always compiled from source, the fact that this would be an ABI change for PlaygroundSupport does not matter, and a compatibility typealias could be provided in PlaygroundSupport to maintain compatibility with code which explicitly qualified the name of the CustomPlaygroundDisplayConvertible protocol.

Have CustomPlaygroundDisplayConvertible return something other than Any

One minor alternative considered was to have CustomPlaygroundDisplayConvertible return a value with a more specific type than Any. For example:

protocol CustomPlaygroundDisplayConvertible {
  var playgroundDescription: CustomPlaygroundDisplayConvertible { get }
}

or:

protocol PlaygroundDescription {}

protocol CustomPlaygroundDisplayConvertible {
  var playgroundDescription: PlaygroundDescription { get }
}

In both cases, core types which the playground logger supports would conform to the appropriate protocol such that they could be returned from implementations of playgroundDescription.

The benefit to this approach is that it is more self-documenting than the approach proposed in this document, as a user can look up all of the types which conform to a particular protocol to know what the playground logger understands. However, this approach has a number of pitfalls, largely because it's intentional that the proposal uses Any instead of a more-constrained protocol. It should be possible to return anything as the stand-in for an instance, including values without opaque playground quick look views, so that it's easier to construct an alternate structured view of a type (without having to override the more complex CustomReflectable protocol). Furthermore, by making the API in the library use a general type like Any, this proposal prevents revlock from occurring between IDEs and the libraries, as the IDE's playground logger can implement support for opaque logging of new types without requiring library changes. (And IDEs can opt to support a subset of types if they prefer, whereas if the libraries promised support an IDE would effectively be compelled to provide it.)

Have CustomPlaygroundDisplayConvertible return an Any? instead of an Any

One alternative considered was to have CustomPlaygroundDisplayConvertible return an Any? instead of an Any. This would permit individual instances to opt-out of a custom playground description by returning nil instead of a concrete value or object.

Although that capability is no longer present, in most cases implementors of CustomPlaygroundDisplayConvertible may return a custom description which closely mirrors their default description. One big exception to this are classes which are considered core types, such as NSView and UIView, as one level of subclass may wish to customize its description while deeper level may wish to use the default description (which is currently a rendered image of the view). This proposal does not permit that; the second-level subclass must return a custom description one way or another, and due to the chaining nature of CustomPlaygroundDisplayConvertible implementations, it cannot return self and have that reliably indicate to the playground logger implementation that that means "don't use a custom description".

This issue seems to be limited enough that it should not tarnish the API design as a whole. Returning Any and not Any? is easier to understand, so this proposal opts to do that. Should this be a larger issue than anticipated, a future proposal could introduce a struct like DefaultPlaygroundDescription<T> which the playground logger would understand to mean "don't check for a CustomPlaygroundDisplayConvertible conformance on the wrapped value".

Alternate Names for CustomPlaygroundDisplayConvertible

Finally, as this introduces a new protocol, there are other possible names:

  • CustomPlaygroundRepresentable
  • CustomPlaygroundConvertible
  • CustomPlaygroundPreviewConvertible
  • CustomPlaygroundQuickLookConvertible
  • CustomPlaygroundValuePresentationConvertible
  • CustomPlaygroundPresentationConvertible

CustomPlaygroundRepresentable was rejected as it does not match the naming convention established by CustomStringConvertible/CustomDebugStringConvertible. CustomPlaygroundConvertible was rejected as not being specific enough -- types conforming to this protocol are not themselves convertible to playgrounds, but are instead custom convertible for playground display. CustomPlaygroundPreviewConvertible is very similar to CustomPlaygroundDisplayConvertible, but implies more about the presentation than is appropriate as a playground environment is free to display it any way it wants, not just as a "preview". CustomPlaygroundQuickLookConvertible was rejected as it potentially invokes the to-be-removed PlaygroundQuickLook enum. CustomPlaygroundValuePresentationConvertible and CustomPlaygroundPresentationConvertible were rejected as too long of names for the protocol.