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 put up first.

This time we will 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 will 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 couple 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. It will make sure that the precise .dylib (or .so / .dll) binary can be created if you construct the goal. 🎯

The second factor is that we might wish to emit our Swift module information alongside the library, we are able to 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 can be immediately 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 {
        "Hi there, World!"
    }
}

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

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

When you run swift construct (or run the venture by way of Xcode) it’s going to construct all of the required information and place them below the correct construct folder. You may as well print the construct folder by working the swift construct -c launch --show-bin-path (-c launch is for launch builds, we will construct the library utilizing the discharge configuration for apparent causes… we’re releasing them). When you record 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 will 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 package deal clear

Now if you happen to run make or make set up all of the required information can be positioned below the precise location. Our dynamic library package deal 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 referred to 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, 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 wish to hyperlink the TextUI framework with our software, to be able to do that we’ve 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 acquainted with them, if not please learn the linked articles. 🤓

import TextUI

print(TextUI.construct())

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

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

Dynamic technique alternative

After publishing my unique plugin system associated article, I’ve obtained an e-mail from considered one of my readers. Initially 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 means of 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 will probably be finalized.

You may learn the unique dynamic technique alternative 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 should use this attribute to override a customized dynamic technique with your individual 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()) 

When you alter the important.swift file and run the venture you need to see that even we’re calling the construct technique, it’ll be dispatched dynamically and our _customBuild() technique can 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 doable to construct another 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.

Up to now 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 will prolong 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.

When you run this code utilizing Linux or Home windows, the dynamic library file can be referred to as libTextView.so below Linux and libTextView.dll on Home windows.

So simply place this file below your private home listing we will want the complete path to entry it utilizing the TextApp’s important 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 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 technique swizzling, this focuses a bit extra on the emitted replacements code, however I discovered it a really nice learn.

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

Drawbacks & conclusion

Dynamic technique alternative works good, this strategy is behind SwiftUI stay previews (or dlsym with some pointer magic, however who is aware of this for certain..). Anyway, every little thing seems nice, till you begin involving Swift courses below macOS. What’s fallacious with courses?

Seems that the Goal-C runtime will get concerned below macOS if you happen to 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

Underneath macOS the output of nm will include 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 incorporates the ObjC runtime you will not have the ability to truly shut the dylib, it doesn’t matter what. ⚠️

Previous to Mac OS X 10.5, solely bundles could possibly be unloaded. Beginning in Mac OS X 10.5, dynamic libraries may additionally be unloaded. There are a few instances wherein a dynamic library won’t ever be unloaded: 1) the principle 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 will depend on it, 3) the dynamic library is in dyld’s shared cache.

When you check out man 3 dlclose you may get a couple of extra hints concerning the causes, plus you too can 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 may possibly trigger some bother (solely on macOS), however every little thing works simply nice below Linux, so in case you are planning to make use of this strategy on the server facet, then I might say it’s going to work simply high-quality. It isn’t secure, but it surely 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 technique 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. 🥳