simject is a command-line tool and iOS dynamic library that allows developers to easily test their tweaks on the iOS Simulator.
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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 (requires the latest version of Theos)

First, make sure that you have added your developer account in Xcode > Preferences > Accounts tab. This is required for code-signing some binaries used in simject.

Run these commands in a Terminal instance.

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

git clone
cd simject/
make setup

Now, we will need to create a version of CydiaSubstrate.framework that has support for the x86_64 and i386 (if you want 32-bit support) architectures.

Getting Cydia Substrate to function properly with simject

Unless you want to do it manually, you can use either or script to copy over this framework to the target runtime.

If you use Xcode 10 (and above) and target iOS 12 (and above), you need to use substitute instead of Cydia Substrate.

Due to the incompatible libstdc++ in iOS 12 simulator of Xcode 10, the Cydia Substrate binary from cycript that requires this library will not work (it will complain /usr/lib/libstdc++.6.dylib: mach-o, but not built for iOS simulator). While Cydia Substrate is not open-sourced to allow us to recompile the binary, we still have substitute as a replacement. You can run the following commands:

# Xcode 10+, iOS 12+
cd simject/
git clone
cd substitute/
./configure --xcode-sdk=iphonesimulator --xcode-archs=x86_64 && 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 in a Terminal instance.

# 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 $SDK_VERSION is your desired SDK version. If /Library/Frameworks does not exist, you can just create it manually.
    • Note: For runtimes bundled in Xcode 9.0+, instead copy the framework to /Applications/$ where $XCODE is usually either Xcode or Xcode-beta.
  2. Finally, select your Xcode directory with sudo xcode-select -s /Applications/ (or wherever Xcode is located on your system).

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 or simject.dylib.

  2. Inside the bin subdirectory, you will find the respring_simulator 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 respring_simulator every time the iOS Simulator reboots or if SpringBoard crashes by itself.

  4. respring_simulator 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. Change your TARGET variable to 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.

  3. If you want to support both 64-bit and 32-bit iOS Simulators, add ARCHS = x86_64 i386 to your Makefile. If not (or if you're targeting iOS 11), add ARCHS = x86_64.

  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.

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.