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 need to positively check out that put up 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 bundle = Package deal(
    title: "TextUI",
    merchandise: [
        .library(name: "TextUI", type: .dynamic, targets: ["TextUI"]),
    ],
    dependencies: [
        
    ],
    targets: [
        .target(name: "TextUI", swiftSettings: [
            .unsafeFlags(["-emit-module", "-emit-library"])
        ]),
    ]
)

The bundle manifest is sort of easy, though there are just a few 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. It will be certain that the precise .dylib (or .so / .dll) binary will likely be created if you construct the goal. 🎯

The second factor is that we might prefer to emit our Swift module data alongside the library, we will inform this to the compiler by means of some unsafe flags. Do not be afraid, these are literally not so harmful to make use of, these flags will likely be instantly 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 {
        "Howdy, 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 methodology) you inform the compiler that it ought to use dynamic dispatch to “resolve” the implementation when calling it.

We’ll make the most of the dynamic dispatch afterward, however earlier than we might transfer onto that half, we’ve to construct our dynamic library and make it accessible for others to make use of. 🔨

In the event you run swift construct (or run the challenge through Xcode) it’s going to construct all of the required recordsdata and place them beneath the right construct folder. You too can 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). In the event you listing the contents of the output listing, you need to discover the next recordsdata there:

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

So, what can we do with this construct folder and the output recordsdata? We’ll want them beneath a location the place the construct instruments can entry the associated recordsdata, for the sake of simplicity we’ll put every little 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 bundle clear

Now if you happen to run make or make set up all of the required recordsdata will likely be positioned beneath the precise location. Our dynamic library bundle is now prepared to make use of. The one query is how will we eat 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 bundle will use our beforehand created and put in shared dynamic library.

import PackageDescription

let bundle = Package deal(
    title: "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 will add some flags to the Swift compiler and the linker, so that they’ll know that we have ready some particular library and header (modulemap) recordsdata beneath the /usr/native/lib/ folder. We would additionally prefer to hyperlink the TextUI framework with our software, as a way to do that we’ve to move the title of the module as a flag. I’ve already defined these flags (-L, -I, -l) in my earlier posts so I suppose you are acquainted with them, if not please learn the linked articles. 🤓

import TextUI

print(TextUI.construct())

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

Are you prepared to switch the construct operate utilizing native methodology swizzling in Swift?

Dynamic methodology alternative

After publishing my authentic plugin system associated article, I’ve acquired an e mail from certainly one of my readers. To begin with thanks for letting me know concerning the @_dynamicReplacement attribute Corey. 🙏

The factor is that Swift helps dynamic methodology swizzling out of the field, though it’s by means of a non-public attribute (begins with an underscore), which suggests it isn’t prepared for public use but (yeah… similar to @_exported, @_functionBuilder and the others), however finally it is going to be finalized.

You’ll be able to learn the unique dynamic methodology alternative pitch on the Swift boards, there’s additionally this nice little snippet that comprises a minimal showcase concerning the @_dynamicReplacement attribute.

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

import TextUI

extension TextUI {

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

print(TextUI.construct()) 

In the event you alter the principal.swift file and run the challenge you need to see that even we’re calling the construct methodology, it will be dispatched dynamically and our _customBuild() methodology will likely 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 another dynamic library and cargo that at runtime, then change the unique construct implementation with the dynamically loaded lib code? The reply is sure, let me present you the way to do that. 🤩

import PackageDescription

let bundle = Package deal(
    title: "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 will place our TextUI extension into this library as an alternative of the TextApp goal.

To date 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 bundle and construct it as a dynamic library.

import TextUI

extension TextUI {

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

We will use the same makefile (to the earlier one) or just run the swift construct -c launch command and replica the libTextView.dylib file from the construct listing by hand.

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

So simply place this file beneath your property listing we’ll want the total path to entry it utilizing the TextApp’s principal file. We’ll use the dlopen name to load the dylib, this can change our construct methodology, 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 extra dlsym calls and unsafe C pointers. There’s additionally a pleasant and detailed article about Swift and native methodology swizzling, this focuses a bit extra on the emitted replacements code, however I discovered it a really nice learn.

Sadly there may be another factor that we’ve to speak about…

Drawbacks & conclusion

Dynamic methodology alternative works good, this method is behind SwiftUI dwell previews (or dlsym with some pointer magic, however who is aware of this for certain..). Anyway, every little thing seems to be nice, till you begin involving Swift courses beneath macOS. What’s mistaken with courses?

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

class A {}

Beneath macOS the output of nm will comprise traces of the Goal-C runtime and that’s greater than sufficient to trigger some troubles through the dylib shut course of. Seems in case your library comprises the ObjC runtime you will not be capable of truly shut the dylib, it doesn’t matter what. ⚠️

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

In the event you check out man 3 dlclose you will get just a few extra hints concerning the causes, plus you may as well examine the supply code of the Goal-C runtime, if you wish to see extra particulars.

Anyway I believed this ought to be talked about, as a result of it will probably trigger some hassle (solely on macOS), however every little thing works simply nice beneath Linux, so if you’re planning to make use of this method on the server facet, then I might say it’s going to work simply high quality. It isn’t secure, nevertheless it ought to work. 😈

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