Constructing and loading dynamic libraries at runtime in Swift

Why ought to we make a plugin system?

Within the modules and hooks article I used to be writing about how modules (plugins) can work collectively by utilizing varied invocation factors and hooks. The one downside with that method is that you could’t actually activate or off modules on-the-fly, since we normally construct our apps in a static approach.

A great plugin system ought to allow us to alter the conduct of our code at runtime. WordPress plugins are extraordinarily profitable, as a result of you possibly can add additional performance to the CMS with out recompiling or altering the core. Exterior the Apple ecosystem, there’s a enormous world that would benefit from this idea. Sure, I’m speaking about Swift on the server and backend functions.

My concept right here is to construct an open-source modular CMS that may be quick, protected and extensible via plugins. Fortuitously now we have now this wonderful type-safe programming language that we will use. Swift is quick and dependable, it’s the good selection for constructing backend apps on the long run. ✅

On this article I want to present you a easy methods to construct a dynamic plugin system. The entire idea is predicated on Lopdo‘s GitHub repositories, he did fairly an incredible job implementing it. Thanks very a lot for exhibiting me easy methods to use dlopen and different comparable capabilities. 🙏

The magic of dynamic linking

Handmade iOS frameworks are normally bundled with the applying itself, you possibly can study just about every little thing a couple of framework if you recognize some command line instruments. This time we’re solely going to concentrate on static and dynamic linking. By default Swift package deal dependencies are linked statically into your software, however you possibly can change this if you happen to outline a dynamic library product.

First we’re going to create a shared plugin interface containing the plugin API as a protocol.

import PackageDescription

let package deal = Bundle(
    title: "PluginInterface",
    merchandise: [
        .library(name: "PluginInterface", type: .dynamic, targets: ["PluginInterface"]),
    ],
    targets: [
        .target(name: "PluginInterface", dependencies: []),
    ]
)

This dynamic PluginInterface package deal can produce a .dylib or .so file, quickly there shall be a .dll model as nicely, based mostly on the working system. All of the code bundled into this dynamic library could be shared between different functions. Let’s make a easy protocol.

public protocol PluginInterface {

    func foo() -> String
}

Since we’re going to load the plugin dynamically we are going to want one thing like a builder to assemble the specified object. We are able to use a brand new summary class for this function.

open class PluginBuilder {
    
    public init() {}

    open func construct() -> PluginInterface {
        fatalError("You must override this methodology.")
    }
}

That is our dynamic plugin interface library, be happy to push this to a distant repository.

Constructing a dynamic plugin

For the sake of simplicity we’ll construct a module referred to as PluginA, that is the manifest file:

import PackageDescription

let package deal = Bundle(
    title: "PluginA",
    merchandise: [
        .library(name: "PluginA", type: .dynamic, targets: ["PluginA"]),
    ],
    dependencies: [
        .package(url: "path/to/the/PluginInterface/repository", from: "1.0.0"),
    ],
    targets: [
        .target(name: "PluginA", dependencies: [
            .product(name: "PluginInterface", package: "PluginInterface")
        ]),
    ]
)

The plugin implementation will in fact implement the PluginInterface protocol. You’ll be able to lengthen this protocol based mostly in your wants, you may as well use different frameworks as dependencies.

import PluginInterface

struct PluginA: PluginInterface {

    func foo() -> String {
        return "A"
    }
}

We have now to subclass the PluginBuilder class and return our plugin implementation. We’re going to use the @_cdecl attributed create perform to entry our plugin builder from the core app. This Swift attribute tells the compiler to save lots of our perform below the “createPlugin” image title.

import PluginInterface

@_cdecl("createPlugin")
public func createPlugin() -> UnsafeMutableRawPointer {
    return Unmanaged.passRetained(PluginABuilder()).toOpaque()
}

ultimate class PluginABuilder: PluginBuilder {

    override func construct() -> PluginInterface {
        PluginA()
    }
}

We are able to construct the plugin utilizing the command line, simply run swift construct within the undertaking folder. Now you’ll find the dylib file below the binary path, be happy to run swift construct --show-bin-path, this may output the required folder. We are going to want each .dylib information for later use.

Loading the plugin at runtime

The core software can even use the plugin interface as a dependency.

import PackageDescription

let package deal = Bundle(
    title: "CoreApp",
    dependencies: [
        .package(url: "path/to/the/PluginInterface/repository", from: "1.0.0"),
    ],
    targets: [
        .target(name: "CoreApp", dependencies: [
            .product(name: "PluginInterface", package: "PluginInterface")
        ]),
    ]
)

That is an executable goal, so we will place the loading logic to the foremost.swift file.

import Basis
import PluginInterface

typealias InitFunction = @conference(c) () -> UnsafeMutableRawPointer

func plugin(at path: String) -> PluginInterface {
    let openRes = dlopen(path, RTLD_NOW|RTLD_LOCAL)
    if openRes != nil {
        defer {
            dlclose(openRes)
        }

        let symbolName = "createPlugin"
        let sym = dlsym(openRes, symbolName)

        if sym != nil {
            let f: InitFunction = unsafeBitCast(sym, to: InitFunction.self)
            let pluginPointer = f()
            let builder = Unmanaged<PluginBuilder>.fromOpaque(pluginPointer).takeRetainedValue()
            return builder.construct()
        }
        else {
            fatalError("error loading lib: image (symbolName) not discovered, path: (path)")
        }
    }
    else {
        if let err = dlerror() {
            fatalError("error opening lib: (String(format: "%s", err)), path: (path)")
        }
        else {
            fatalError("error opening lib: unknown error, path: (path)")
        }
    }
}

let myPlugin = plugin(at: "path/to/my/plugin/libPluginA.dylib")
let a = myPlugin.foo()
print(a)

We are able to use the dlopen perform to open the dynamic library file, then we are attempting to get the createPlugin image utilizing the dlsym methodology. If we have now a pointer we nonetheless must forged that into a sound PluginBuilder object, then we will name the construct methodology and return the plugin interface.

Working the app

Now if you happen to attempt to run this software utilizing Xcode you will get a warning like this:

Class _TtC15PluginInterface13PluginBuilder is carried out in each…
One of many two shall be used. Which one is undefined.

That is associated to an outdated bug, however happily that’s already resolved. This time Xcode is the dangerous man, since it’s making an attempt to hyperlink every little thing as a static dependency. Now if you happen to construct the applying via the command line (swift construct) and place the next information in the identical folder:

• CoreApp
• libPluginA.dylib
• libPluginInterface.dylib

You’ll be able to run the applying ./CoreApp wihtout additional points. The app will print out A with out the warning message, for the reason that Swift package deal supervisor is recognizing that you just want to hyperlink the libPluginInterface framework as a dynamic framework, so it will not be embedded into the applying binary. In fact you must arrange the suitable plugin path within the core software.