Dynamic libraries and code replacements in Swift

Dynamic library packages

I’ve already printed an article about constructing static and dynamic libraries utilizing the Swift compiler, if you do not know what’s a dynamic library or you might be merely a bit extra about how the Swift compiler works, you must positively check out that submit first.

This time we’ll focus a bit extra on using the Swift Package deal Supervisor to create our dynamic library merchandise. The setup goes to be similar to the one I’ve created within the loading dynamic libraries at runtime article. First we’ll create a shared library utilizing SPM.

import PackageDescription

let package deal = Package deal(
    identify: "TextUI",
    merchandise: [
        .library(name: "TextUI", type: .dynamic, targets: ["TextUI"]),
    ],
    dependencies: [
        
    ],
    targets: [
        .target(name: "TextUI", swiftSettings: [
            .unsafeFlags(["-emit-module", "-emit-library"])
        ]),
    ]
)

The package deal manifest is sort of easy, though there are a number of particular issues that we had so as to add. The very very first thing is that we outlined the product kind as a dynamic library. This may make sure that the best .dylib (or .so / .dll) binary will probably be created once you construct the goal. 🎯

The second factor is that we would prefer to emit our Swift module information alongside the library, we are able to inform this to the compiler by some unsafe flags. Do not be afraid, these are literally not so harmful to make use of, these flags will probably be straight handed to the Swift compiler, however that is it.

Now the supply code for our TextUI library goes to be quite simple.

public struct TextUI {

    public static dynamic func construct() -> String {
        "Hiya, World!"
    }
}

It is only a struct with one static operate that returns a String worth. Fairly easy, besides one factor: the dynamic key phrase. By including the dynamic modifier to a operate (or technique) you inform the compiler that it ought to use dynamic dispatch to “resolve” the implementation when calling it.

We’ll reap the benefits of the dynamic dispatch in a while, however earlier than we might transfer onto that half, we have now to construct our dynamic library and make it accessible for others to make use of. 🔨

Should you run swift construct (or run the undertaking through Xcode) it will construct all of the required information and place them beneath the correct construct folder. You can too print the construct folder by operating the swift construct -c launch --show-bin-path (-c launch is for launch builds, we’ll construct the library utilizing the discharge configuration for apparent causes… we’re releasing them). Should you checklist the contents of the output listing, you must discover the next information there:

• TextUI.swiftdoc
• TextUI.swiftmodule
• TextUI.swiftsourceinfo
• libTextUI.dylib
• libTextUI.dylib.dSYM

So, what can we do with this construct folder and the output information? We’ll want them beneath a location the place the construct instruments can entry the associated information, for the sake of simplicity we’ll put every thing into the /usr/native/lib folder utilizing a Makefile.

PRODUCT_NAME := "TextUI"
DEST_DIR := "/usr/native/lib/"
BUILD_DIR := $(shell swift construct -c launch --show-bin-path)

set up: clear
    @swift construct -c launch
    @set up "$(BUILD_DIR)/lib$(PRODUCT_NAME).dylib" $(DEST_DIR)
    @cp -R "$(BUILD_DIR)/lib$(PRODUCT_NAME).dylib.dSYM" $(DEST_DIR)
    @set up "$(BUILD_DIR)/$(PRODUCT_NAME).swiftdoc" $(DEST_DIR)
    @set up "$(BUILD_DIR)/$(PRODUCT_NAME).swiftmodule" $(DEST_DIR)
    @set up "$(BUILD_DIR)/$(PRODUCT_NAME).swiftsourceinfo" $(DEST_DIR)
    @rm ./lib$(PRODUCT_NAME).dylib
    @rm -r ./lib$(PRODUCT_NAME).dylib.dSYM

uninstall: clear
    
    @rm $(DEST_DIR)lib$(PRODUCT_NAME).dylib
    @rm -r $(DEST_DIR)lib$(PRODUCT_NAME).dylib.dSYM
    @rm $(DEST_DIR)$(PRODUCT_NAME).swiftdoc
    @rm $(DEST_DIR)$(PRODUCT_NAME).swiftmodule
    @rm $(DEST_DIR)$(PRODUCT_NAME).swiftsourceinfo

clear:
    @swift package deal clear

Now in the event you run make or make set up all of the required information will probably be positioned beneath the best location. Our dynamic library package deal is now prepared to make use of. The one query is how will we devour this shared binary library utilizing one other Swift Package deal goal? 🤔

Linking in opposition to shared libraries

We’ll construct a model new executable software known as TextApp utilizing the Swift Package deal Supervisor. This package deal will use our beforehand created and put in shared dynamic library.

import PackageDescription

let package deal = Package deal(
    identify: "TextApp",
    targets: [
        .target(name: "TextApp", swiftSettings: [
            .unsafeFlags(["-L", "/usr/local/lib/"]),
            .unsafeFlags(["-I", "/usr/local/lib/"]),
            .unsafeFlags(["-lTextUI"]),
        ], linkerSettings: [
            .unsafeFlags(["-L", "/usr/local/lib/"]),
            .unsafeFlags(["-I", "/usr/local/lib/"]),
            .unsafeFlags(["-lTextUI"]),
        ]),
    ]
)

The trick is that we are able to add some flags to the Swift compiler and the linker, in order that they’ll know that we have ready some particular library and header (modulemap) information beneath the /usr/native/lib/ folder. We would additionally prefer to hyperlink the TextUI framework with our software, with the intention to do that we have now to move the identify of the module as a flag. I’ve already defined these flags (-L, -I, -l) in my earlier posts so I suppose you are accustomed to them, if not please learn the linked articles. 🤓

import TextUI

print(TextUI.construct())

Our fundamental.swift file is fairly easy, we simply print the results of the construct technique, the default implementation ought to return the well-known “Hiya, World!” textual content.

Are you prepared to interchange the construct operate utilizing native technique swizzling in Swift?

Dynamic technique substitute

After publishing my unique plugin system associated article, I’ve received an electronic mail from one in every of my readers. To start with thanks for letting me know concerning the @_dynamicReplacement attribute Corey. 🙏

The factor is that Swift helps dynamic technique swizzling out of the field, though it’s by a personal attribute (begins with an underscore), which suggests it’s not prepared for public use but (yeah… similar to @_exported, @_functionBuilder and the others), however finally will probably be finalized.

You possibly can learn the unique dynamic technique substitute pitch on the Swift boards, there’s additionally this nice little snippet that incorporates a minimal showcase concerning the @_dynamicReplacement attribute.

Lengthy story quick, you need to use this attribute to override a customized dynamic technique with your personal implementation (even when it comes from a dynamically loaded library). In our case we have already ready a dynamic construct technique, so if we attempt we are able to override that the next snippet.

import TextUI

extension TextUI {

    @_dynamicReplacement(for: construct())
    static func _customBuild() -> String {
        "It simply works."
    }
}

print(TextUI.construct()) 

Should you alter the fundamental.swift file and run the undertaking you must see that even we’re calling the construct technique, it’ll be dispatched dynamically and our _customBuild() technique will probably be known as beneath the hood, therefore the brand new return worth.

It really works like a allure, however can we make this much more dynamic? Is it attainable to construct yet one more dynamic library and cargo that at runtime, then exchange the unique construct implementation with the dynamically loaded lib code? The reply is sure, let me present you ways to do that. 🤩

import PackageDescription

let package deal = Package deal(
    identify: "TextView",
    merchandise: [
        .library(name: "TextView", type: .dynamic, targets: ["TextView"]),
    ],
    targets: [
        .target(name: "TextView", swiftSettings: [
            .unsafeFlags(["-L", "/usr/local/lib/"]),
            .unsafeFlags(["-I", "/usr/local/lib/"]),
            .unsafeFlags(["-lTextUI"]),
        ], linkerSettings: [
            .unsafeFlags(["-L", "/usr/local/lib/"]),
            .unsafeFlags(["-I", "/usr/local/lib/"]),
            .unsafeFlags(["-lTextUI"]),
        ]),
    ]
)

Similar SPM sample, we have simply created a dynamic library and we have used the TextUI as a shared library so we are able to place our TextUI extension into this library as a substitute of the TextApp goal.

To this point we have created 3 separated Swift packages shared the TextUI module between the TextApp and the TextView packages as a pre-built dynamic library (utilizing unsafe construct flags). Now we’ll lengthen the TextUI struct inside our TextView package deal and construct it as a dynamic library.

import TextUI

extension TextUI {

    @_dynamicReplacement(for: construct())
    static func _customBuild() -> String {
        "It simply works."
    }
}

We are able to use an analogous makefile (to the earlier one) or just run the swift construct -c launch command and duplicate the libTextView.dylib file from the construct listing by hand.

Should you run this code utilizing Linux or Home windows, the dynamic library file will probably be known as libTextView.so beneath Linux and libTextView.dll on Home windows.

So simply place this file beneath your own home listing we’ll want the complete path to entry it utilizing the TextApp’s fundamental file. We’ll use the dlopen name to load the dylib, this may exchange our construct technique, then we shut it utilizing dlclose (on the supported platforms, extra on this later…).

import Basis
import TextUI


print(TextUI.construct())


let dylibPath = "/Customers/tib/libTextView.dylib"
guard let dylibReference = dlopen(dylibPath, RTLD_LAZY) else {
    if let err = dlerror() {
        fatalError(String(format: "dlopen error - %s", err))
    }
    else {
        fatalError("unknown dlopen error")
    }
}
defer {
    dlclose(dylibReference)
}


print(TextUI.construct())

The beauty of this method is that you do not have to fiddle with further dlsym calls and unsafe C pointers. There may be additionally a pleasant and detailed article about Swift and native technique swizzling, this focuses a bit extra on the emitted replacements code, however I discovered it a really nice learn.

Sadly there’s yet one more factor that we have now to speak about…

Drawbacks & conclusion

Dynamic technique substitute works good, this method is behind SwiftUI reside previews (or dlsym with some pointer magic, however who is aware of this for certain..). Anyway, every thing appears nice, till you begin involving Swift courses beneath macOS. What’s improper with courses?

Seems that the Goal-C runtime will get concerned beneath macOS in the event you compile a local Swift class. Simply compile the next instance supply and try it utilizing the nm instrument.

class A {}

Beneath macOS the output of nm will include traces of the Goal-C runtime and that’s greater than sufficient to trigger some troubles in the course of the dylib shut course of. Seems in case your library incorporates the ObjC runtime you will not have the ability to really shut the dylib, it doesn’t matter what. ⚠️

Previous to Mac OS X 10.5, solely bundles may very well be unloaded. Beginning in Mac OS X 10.5, dynamic libraries might also be unloaded. There are a
couple of circumstances wherein a dynamic library won’t ever be unloaded: 1) the principle executable hyperlinks in opposition to it, 2) an API that doesn’t help
unloading (e.g. NSAddImage()) was used to load it or another dynamic library that is dependent upon it, 3) the dynamic library is in dyld’s
shared cache.

Should you check out man 3 dlclose you will get a number of extra hints concerning the causes, plus you can even verify the supply code of the Goal-C runtime, if you wish to see extra particulars.

Anyway I assumed this must be talked about, as a result of it could trigger some hassle (solely on macOS), however every thing works simply nice beneath Linux, so if you’re planning to make use of this method on the server aspect, then I would say it will work simply effective. It is not secure, but it surely ought to work. 😈

Oh, I nearly overlook the hot-reload performance. Properly, you may add a listing or file watcher that may monitor your supply codes and if one thing modifications you may re-build the TextView dynamic library then load the dylib once more and name the construct technique if wanted. It is comparatively straightforward after you’ve got tackled the dylib half, as soon as you determine the smaller particulars, it really works like magic. 🥳