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 numerous invocation factors and hooks. The one downside with that method is you can’t actually activate or off modules on-the-fly, since we normally construct our apps in a static approach.

An excellent 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 may add further performance to the CMS with out recompiling or altering the core. Outdoors the Apple ecosystem, there’s a large world that would reap the benefits of this idea. Sure, I’m speaking about Swift on the server and backend purposes.

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

On this article I wish to present you a tips on how to construct a dynamic plugin system. The entire idea relies on Lopdo‘s GitHub repositories, he did fairly a tremendous job implementing it. Thanks very a lot for displaying me tips on how to use dlopen and different related capabilities. 🙏

The magic of dynamic linking

Handmade iOS frameworks are normally bundled with the applying itself, you may study just about the whole lot a couple of framework if some command line instruments. This time we’re solely going to give attention to static and dynamic linking. By default Swift package deal dependencies are linked statically into your software, however you may 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 will probably 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 purposes. Let’s make a easy protocol.

public protocol PluginInterface {

    func foo() -> String
}

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

open class PluginBuilder {
    
    public init() {}

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

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

Constructing a dynamic plugin

For the sake of simplicity we’ll construct a module known 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 after all implement the PluginInterface protocol. You’ll be able to lengthen this protocol based mostly in your wants, it’s also possible to use different frameworks as dependencies.

import PluginInterface

struct PluginA: PluginInterface {

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

We’ve got 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 avoid wasting our perform beneath the “createPlugin” image title.

import PluginInterface

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

closing 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 venture folder. Now you could find the dylib file beneath the binary path, be at liberty to run swift construct --show-bin-path, it will output the required folder. We’ll want each .dylib recordsdata for later use.

Loading the plugin at runtime

The core software may also 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 essential.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 technique. If now we have a pointer we nonetheless must forged that into a sound PluginBuilder object, then we will name the construct technique 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 will probably be used. Which one is undefined.

That is associated to an previous bug, however thankfully that’s already resolved. This time Xcode is the dangerous man, since it’s attempting to hyperlink the whole lot as a static dependency. Now if you happen to construct the applying by the command line (swift construct) and place the next recordsdata in the identical folder:

• CoreApp
• libPluginA.dylib
• libPluginInterface.dylib

You’ll be able to run the applying ./CoreApp with out additional points. The app will print out A with out the warning message, because the Swift package deal supervisor is recognizing that you just wish to hyperlink the libPluginInterface framework as a dynamic framework, so it will not be embedded into the applying binary. After all you need to arrange the correct plugin path within the core software.