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Swift OpenAPI Generator Aims at Streamlining HTTP Client/Server Communication

Apple has introduced a new open source package, the Swift OpenAPI Generator, aimed at generating the code required to handle client/server communication through an HTTP API based on its OpenAPI document.

Swift OpenAPI Generator generates type-safe representations of each operation's input and output as well as the required network calls to deal with sending requests and processing responses on the client side and server-side stubs to delegate request processing to handlers.

Both the client and the server code are based on a generated APIProtocol type that contains one method for each OpenAPI operation. For example, for a simple GreetingService supporting HTTP GET requests at the /greet endpoint, the APIProtocol would contain a getGreeting method. Along with the protocol definition, a Client type implementing it is also generated for use on the client side. Server-side, the package generates a registerHandlers method belonging to the APIProtocol to register one handler for each operation in the protocol.

The generated code does not cover authentication, logging, or retrying. This kind of logic is usually too strictly associated with the business logic to allow for a general abstraction. Anyway, developers can implement those features in a middleware that conforms to the ClientMiddleware or ServerMiddleware protocols to be reusable in other projects based on Swift OpenAPI Generator.

The code Swift OpenAPI Generator generates is not tied to a specific HTTP framework but relies on a generic ClientTransport or ServerTransport type, which any compatible HTTP framework should implement to be usable with the generator. Currently, the Swift OpenAPI Generator can be used with a few existing transport frameworks, including URLSession from iOS own Foundation framework, HTTPClient from AsyncHTTPClient, Vapor, and Hummingbird.

All protocols and types used in the Swift OpenAPI Generator are defined in its companion project Swift OpenAPI Runtime, which is relied upon by generated client and server code.

The generator can be run in two ways: either as a Swift Package Manager plugin, integrated in the build process, or manually through a CLI. In the first case, the plugin is controlled by a YAML configuration file named openapi-generator-config.yaml that must exist in the target source directory along with the OpenAPI document in JSON or YAML format. Using this configuration file, you can specify whether to generate only the client code, the server code, or both in the same target. The CLI supports the same kind of configurability through command line options, as shown in the example below:

swift run swift-openapi-generator \
    --mode types --mode client \
    --output-directory path/to/desired/output-dir \

The Swift OpenAPI Generator is yet in its early stages and while it supports most of the commonly used features of OpenAPI, Apple says, it still lacks a number of desired features that are in the works.

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