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simject is a command-line tool and iOS dynamic library that allows iOS tweak developers to easily test their tweaks on the iOS Simulator.

simject is BSD-licensed. See LICENSE for more information.

Setting up the simject environment

  1. Ensure that you have Xcode installed and ran at least once.
  2. Ensure that you have added your developer account in Xcode > Preferences > Accounts tab. This is required for code-signing some binaries used in simject.
  3. Ensure that there is "iOS Development" certificate listed once you clicked on "Manage Certificates", create one if there is not.
  4. Run these commands in a Terminal instance:
sudo xcode-select -s /Applications/
git clone
cd simject/
make all

Note: During the process, you may be asked by sudo to enter your login password. Please note that it is normal for nothing to be displayed as you type your password.

Now, we need to create a version of CydiaSubstrate.framework that has support for the x86_64 or arm64 (64-bit) and/or i386 (32-bit) architectures.

Getting Cydia Substrate to function properly with simject

Unless you want to do it manually, you can use script to symlink CydiaSubstrate.framework to the appropriate directory to every iOS runtime. Otherwise, continue reading.

If you use Xcode 10 (and above) and target iOS 12 (and above), you need to rely on substitute rather than cycript's included CydiaSubstrate.framework.

This is due to the incompatible libstdc++ in iOS 12 simulator of Xcode 10. The Cydia Substrate from cycript that requires libstdc++ will not work, but will complain /usr/lib/libstdc++.6.dylib: mach-o, but not built for iOS simulator. You can run the following commands to compile a working substitute framework:

# Xcode 10+ and iOS 12+
cd simject/
git clone
cd substitute/
./configure --xcode-sdk=iphonesimulator --xcode-archs=$(uname -m) && make
mv -v out/libsubstitute.dylib out/CydiaSubstrate
codesign -f -s - out/CydiaSubstrate
mkdir -p ../CydiaSubstrate.framework
mv -v out/CydiaSubstrate ../CydiaSubstrate.framework/CydiaSubstrate
cd .. && rm -rf substitute

Otherwise, run these commands to compile a working Cydia Substrate:

# Xcode 9 and below
cd simject/
curl -Lo /tmp/
unzip /tmp/ -d /tmp/simject_cycript
mkdir -p CydiaSubstrate.framework
mv -v /tmp/simject_cycript/Cycript.lib/libsubstrate.dylib CydiaSubstrate.framework/CydiaSubstrate
rm -rfv /tmp/simject_cycript /tmp/
  1. Copy the resulting CydiaSubstrate.framework bundle to /Library/Developer/CoreSimulator/Profiles/Runtimes/iOS${SDK_VERSION}.simruntime/Contents/Resources/RuntimeRoot/Library/Frameworks/ where ${SDK_VERSION} is your desired SDK version. If /Library/Frameworks does not exist, you can create it manually.

    • Let ${XCODE} be Xcode or Xcode-beta.
    • For runtimes bundled in Xcode 11+, copy the framework to /Applications/${XCODE}.app/Contents/Developer/Platforms/iPhoneOS.platform/Library/Developer/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot/Library/Frameworks/.
    • For runtimes bundled in Xcode 9 - 10, copy the framework to /Applications/${XCODE}.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot/Library/Frameworks/.
  2. Note that, from the step 1, you can symlink instead of copying CydiaSubstrate.framework to multiple places. This is what does to ease your life.

simject usage

  1. Place your dynamic libraries and accompanying property lists inside /opt/simject to load them in the iOS Simulator. Do not delete simject.plist and simject.dylib.

  2. Inside the bin subdirectory, you will find the resim command-line tool. Execute it to cause booted iOS Simulator(s) to respring and to be able to load tweaks.

  3. IMPORTANT: Please note that you will need to run resim every time the iOS Simulator reboots or if SpringBoard crashes by itself.

  4. resim can respring multiple simulators (check its usage notes), provided that the selected Xcode version is 9.0 or above.

  5. Happy developing! (And don't make SpringBoard cry too hard... it has feelings, too! Probably.)

Targeting the iOS Simulator

  1. Open your project's Makefile.

  2. Set the correct TARGET for your machine.

    • For Intel Mac: Set TARGET = simulator:clang::7.0 (a must for Xcode 10, with 64-bit compatibility) or TARGET = simulator:clang::5.0 otherwise (with 32-bit compatibility). Note that this is just an example, you can change the SDK version used and iOS version to target if you wish.
    • For Apple Silicon Mac: Set TARGET = simulator:clang::12.0.
  3. Set the correct ARCHS for your machine.

    • For Intel Mac: If you want to support both 64-bit and 32-bit iOS Simulators, set ARCHS = x86_64 i386 to your Makefile. If not (or if you're targeting iOS 11), set ARCHS = x86_64.
    • For Apple Silicon Mac: Set ARCHS = arm64. There is no 32-bit support.
  4. make your project and copy .theos/obj/iphone_simulator/${YOUR_TWEAK}.dylib to /opt/simject/${YOUR_TWEAK}.dylib.

  5. If there is already /opt/simject/${YOUR_TWEAK}.dylib, you have to delete it first before copying.

  6. Also make sure to copy ${YOUR_TWEAK}.plist to /opt/simject/${YOUR_TWEAK}.plist. simject will not load your tweak if you miss this step!

  7. An example tweak project is available in the simjectExampleTweak/ subfolder. Use it as reference if you want. Its Makefile is also written so that you can just run make setup to copy over the dylib and its plist automatically.

  8. If you encountered a problem while signing dylibs: error: The specified item could not be found in the keychain., you can try to fix it by following this or this.

  9. If you encountered a problem while signing dylibs: iPhone Developer: no identity found, check out this comment by iAlex11 (a neat way to get your identity is to run security find-identity -p codesigning -v | head -n 1 | xargs | cut -d " " -f 2).

Final notes

Please understand that that testing your tweaks on the iOS Simulator is not necessarily a 100% accurate representation of how it will behave on an actual iOS device.

Certain features and frameworks are usually missing from or incomplete in the iOS Simulator (such as anything related to the Weather frameworks).

Yes, in most cases, it will work identically across both the iOS Simulator and a real iOS device, but there will always be some edge cases where this does not apply.

For those who want to test their preference bundle inside the simulator, you need to compile a simulator version of preferenceloader and do some extra steps that you can consult from here. It may not be as convenient as using only simject, but it works, at least.

Also, special thanks to PoomSmart for his countless contributions to the simject project! Without him, I would have never even had the idea of creating such a tool.