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[bgen] Add support for binding constructors in protocols. Fixes #14039.…
… (#20583) Add support for binding constructors in protocols. Given the api definition: ```cs [Protocol] public interface Protocol { [Abstract] [Export ("init")] IntPtr Constructor (); [Export ("initWithValue:")] IntPtr Constructor (IntPtr value); [BindAs ("Create")] [Export ("initWithPlanet:")] IntPtr Constructor (); } ``` we're binding it like this: ```cs [Protocol ("Protocol")] public interface IProtocol : INativeObject { [Export ("init")] public static T CreateInstance<T> () where T: NSObject, IProtocol { /* default implementation */ } [Export ("initWithValue:")] public static T CreateInstance<T> () where T: NSObject, IProtocol { /* default implementation */ } [Export ("initWithPlanet:")] public static T Create<T> () where T: NSObject, IProtocol { /* default implementation */ } } ``` Also add documentation and tests. Fixes #14039. --------- Co-authored-by: Manuel de la Pena <[email protected]> Co-authored-by: Alex Soto <[email protected]>
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| # Objective-C protocols | ||
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| This document describes how we bind Objective-C protocols in C#, and in | ||
| particular improvements we've done in .NET 9. | ||
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| ## Can Objective-C protocols be modeled as C# interfaces? | ||
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| Objective-C protocols are quite similar to C# interfaces, except when they're | ||
| not, and that makes binding them somewhat complicated. | ||
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| ### Optional/required members | ||
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| Objective-C protocols can have both optional and required members. It's always | ||
| been possible to represent required members in a C# interface (any interface | ||
| member would be required), but optional members were not possible until C# | ||
| added support for default interface members in C# 8. | ||
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| We represent optional members in two ways: | ||
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| * As an extension method on the interface (useful when calling the optional member). | ||
| * As an IDE feature that would show any optional members from an interface by | ||
| typing 'override ...' in the text editor (useful when implementing an optional member). | ||
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| This has a few drawbacks: | ||
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| * There are no extension properties, so optional properties would have to be | ||
| bound as a pair of GetProperty/SetProperty methods. | ||
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| * The IDE feature was obscure, few people knew about it, it broke on pretty | ||
| much every major release of Visual Studio for Mac, and it was never | ||
| implemented for Visual Studio on Windows. This made it quite hard to | ||
| implement optional members in a managed class extending an Objective-C | ||
| protocol, since developers would have to figure out the correct Export | ||
| attribute as well as the signature (which is quite complicated for more | ||
| complex signatures, especially if blocks are involved). | ||
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| ### Changing requiredness | ||
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| It's entirely possible to change a member from being required to being optional | ||
| in Objective-C. Technically it's also a breaking change to do the opposite (make | ||
| an optional member required), but Apple does it all the time. | ||
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| We've handled this by just not updating the binding until we're able to do | ||
| breaking changes (which happens very rarely). | ||
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| ### Static members | ||
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| Objective-C protocols can have static members. C# didn't allow for static | ||
| members in interfaces until C# 11, so until recently there hasn't been any | ||
| good way to bind static protocol members on a protocol. | ||
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| Our workaround is to manually inline every static member in all classes that | ||
| implemented a given protocol. | ||
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| ### Initializers | ||
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| Objective-C protocols can have initializers (constructors). C# still doesn't | ||
| allow for constructors in interfaces. | ||
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| In the past we haven't bound any protocol initializer at all, we've completely | ||
| ignored them. | ||
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| ## Binding in C# | ||
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| ### Initializers | ||
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| Given the following API definition: | ||
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| ```cs | ||
| [Protocol] | ||
| public interface Protocol { | ||
| [Abstract] | ||
| [Export ("init")] | ||
| IntPtr Constructor (); | ||
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| [Export ("initWithValue:")] | ||
| IntPtr Constructor (IntPtr value); | ||
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| [Bind ("Create")] | ||
| [Export ("initWithPlanet:")] | ||
| IntPtr Constructor (); | ||
| } | ||
| ``` | ||
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| we're binding it like this: | ||
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| ```cs | ||
| [Protocol ("Protocol")] | ||
| public interface IProtocol : INativeObject { | ||
| [Export ("init")] | ||
| public static T CreateInstance<T> () where T: NSObject, IProtocol { /* default implementation */ } | ||
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| [Export ("initWithValue:")] | ||
| public static T CreateInstance<T> () where T: NSObject, IProtocol { /* default implementation */ } | ||
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| [Export ("initWithPlanet:")] | ||
| public static T Create<T> () where T: NSObject, IProtocol { /* default implementation */ } | ||
| } | ||
| ``` | ||
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| In other words: we bind initializers as a static C# factory method that takes | ||
| a generic type argument specifying the type to instantiate. | ||
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| Notes: | ||
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| 1. Constructors are currently not inlined in any implementing classes, like | ||
| other members are. This is something we could look into if there's enough | ||
| interest. | ||
| 2. If a managed class implements a protocol with a constructor, the class has | ||
| to implement the constructor manually using the `[Export]` attribute in | ||
| order to conform to the protocol: | ||
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| ```cs | ||
| [Protocol] | ||
| interface IMyProtocol { | ||
| [Export ("initWithValue:")] | ||
| IntPtr Constructor (string value); | ||
| } | ||
| ``` | ||
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| ```cs | ||
| class MyClass : NSObject, IMyProtocol { | ||
| public string Value { get; private set; } | ||
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| [Export ("initWithValue:")] | ||
| public MyClass (string value) | ||
| { | ||
| this.Value = value; | ||
| } | ||
| } | ||
| ``` |
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