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’re merely a bit extra about how the Swift compiler works, you need to undoubtedly check out that publish 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(
    identify: "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 a couple of particular issues that we had so as to add. The very very first thing is that we outlined the product sort as a dynamic library. It will be sure that the proper .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 information alongside the library, we are able to inform this to the compiler via some unsafe flags. Do not be afraid, these are literally not so harmful to make use of, these flags will likely 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 {
        "Whats up, 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 reap the benefits of the dynamic dispatch afterward, however earlier than we may transfer onto that half, we’ve got to construct our dynamic library and make it obtainable for others to make use of. 🔨

Should you run swift construct (or run the venture by way of Xcode) it will construct all of the required information and place them below the correct 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). Should you listing the contents of the output listing, you need to 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 below 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 bundle clear

Now for those who run make or make set up all of the required information will likely be positioned below the proper 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 utility referred to 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(
    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, so that they’ll know that we have ready some particular library and header (modulemap) information below the /usr/native/lib/ folder. We would additionally prefer to hyperlink the TextUI framework with our utility, so as to do that we’ve got to go 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 aware of them, if not please learn the linked articles. 🤓

import TextUI

print(TextUI.construct())

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

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

Dynamic methodology substitute

After publishing my authentic plugin system associated article, I’ve bought an e mail from certainly one of my readers. Initially thanks for letting me know in regards to the @_dynamicReplacement attribute Corey. 🙏

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

You possibly can learn the unique dynamic methodology substitute pitch on the Swift boards, there’s additionally this nice little snippet that accommodates a minimal showcase in regards to the @_dynamicReplacement attribute.

Lengthy story brief, you should use this attribute to override a customized dynamic methodology with your individual implementation (even when it comes from a dynamically loaded library). In our case we have already ready a dynamic construct methodology, 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 primary.swift file and run the venture you need to see that even we’re calling the construct methodology, it will be dispatched dynamically and our _customBuild() methodology will likely be referred to as below 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 the way to do that. 🤩

import PackageDescription

let bundle = 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.

Thus far 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 prolong 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 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 likely be referred to as libTextView.so below Linux and libTextView.dll on Home windows.

So simply place this file below your house listing we’ll want the total path to entry it utilizing the TextApp’s primary file. We’ll use the dlopen name to load the dylib, it will exchange 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 strategy is that you do not have to fiddle with extra dlsym calls and unsafe C pointers. There may be 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’s yet one more factor that we’ve got to speak about…

Drawbacks & conclusion

Dynamic methodology substitute works good, this strategy is behind SwiftUI reside previews (or dlsym with some pointer magic, however who is aware of this for certain..). Anyway, every thing seems to be nice, till you begin involving Swift lessons below macOS. What’s mistaken with lessons?

Seems that the Goal-C runtime will get concerned below macOS for those who compile a local Swift class. Simply compile the next instance supply and check out it utilizing the nm instrument.

// a.swift
class A {}

// swiftc a.swift -emit-library
// nm liba.dylib|grep -i objc

Below macOS the output of nm will comprise traces of the Goal-C runtime and that’s greater than sufficient to trigger some troubles throughout the dylib shut course of. Seems in case your library accommodates the ObjC runtime you will not be capable to 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 can also be unloaded. There are a few circumstances wherein a dynamic library won’t ever be unloaded: 1) the primary 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 will depend on it, 3) the dynamic library is in dyld’s shared cache.

Should you check out man 3 dlclose you may get a couple of extra hints in regards to the causes, plus you may also test the supply code of the Goal-C runtime, if you wish to see extra particulars.

Anyway I believed this must be talked about, as a result of it could actually trigger some hassle (solely on macOS), however every thing works simply nice below Linux, so if you’re planning to make use of this strategy on the server facet, then I might say it will work simply effective. It isn’t protected, however it ought to work. 😈

Oh, I virtually overlook the hot-reload performance. Properly, 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 straightforward after you have tackled the dylib half, as soon as you determine the smaller particulars, it really works like magic. 🥳