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Bundle assets into iOS static libraries

Project description

Baler is a Python-based tool that makes it easy to bundle and use resources (images, strings files, etc.) in an iOS static library.

The and PayPal iOS SDK projects use baler to include images and other non-code assets in their SDK static libraries. Developers only need to add the .h headers and .a archive, which includes all resources.

Baler is not a good way to handle assets in a regular consumer-facing app!


  • Preserves all subdirectory structure (useful for localization via .lproj subbundles)

  • Optional transparent libz compression (good for text, not helpful for images only).

  • Resolution- and device-aware image loading very similar to UIImage's +imageNamed:.

To see the full set of options, run bale -h.

To use the generated code, look at the generated header file. It should be documented and self-explanatory. If the help flag to the python script or the header file is not sufficient documentation, please file an issue!

The included Xcode project is not needed to use baler. It is useful for developing baler itself, and for an example integration.


  • Python 2 >= 2.6 (pull requests to also support Python 3 welcomed)

  • ARC, deployment target of iOS 6.0+, and Xcode 5+

  • pip


Install with pip:

pip install baler

Or download and install from source:

python install


Baler generates code. A python script (bale) accepts a directory of assets and generates an Objective-C class that bundles those assets.

Options are documented in the built-in help:

bale -h

Example invocation:

bale resources/bundle_assets/,resources/strings Classes/ --overwrite-delay 0 -z -c BalerBundle

This would use the files in resources/bundle_assets and resources/strings to generate a bundle class written to Classes/BalerBundle.h and Classes/BalerBundle.m, with compression and no overwrite delay. Be sure to use a class name more appropriate to your SDK than “BalerBundle”, to avoid possible collisions with other SDKs using baler.

Once the class files are generated, go ahead and add them to your project, then in your code import the generated header:

#import "MyBalerBundle.h"

And then get and use your bundle normally:

NSBundle *aBundle = [[PPZebraBundle sharedInstance] NSBundle]

Or take advantage of the imageNamed method:

UIImage *img = [[MyBalerBundle sharedInstance] imageNamed:@"baler-logo.jpg"];


You can integrate baler into your build process in two ways:

  1. Manually, by running it whenever you alter your assets.

  2. Automatically, by integrating it into your Xcode build.

Manual Use

A quick way to get running is to manually invoke baler whenever you change, add, delete, or move assets.

  1. Install baler as described above.

  2. Run bale <dir_containing_assets> <dir_for_output_code>.

  3. Add the output code to your project. Refer to the header file (e.g. BalerBundle.h) for usage.

  4. If using compression, add libz.dylib in the Link Binary With Libraries build phase.

Automated Use

Instead of running bale manually each time you want to update the generated code, you can add a bale step into your Xcode project.

  1. Install baler as described above.

  2. Place the assets that you want to include in a subdirectory (e.g. baled_assets), usually alongside your .xcodeproj bundle. Optional: You can add this directory to your Xcode project if you want, but be sure not add them to any targets.

  3. Create a Run Script build phase. In your project/target’s Build Phases tab – before the Compile Sources phase – add an appropriate invocation of bale, e.g.

    /path/to/python/env/bin/bale baled_assets Classes --overwrite-delay 0 -c BalerBundle

    This will regenerate the bundle from the assets each time you build. See tips below for more further advice.

  4. Add the generated files (e.g. Classes/BalerBundle.[h|m]) to your Xcode project as usual. Refer to the header file (e.g. BalerBundle.h) for usage.

  5. If using compression, add libz.dylib in the Link Binary With Libraries build phase.

  6. Optional: Add the generated classes’ self-tests to your unit tests.

    Set BALER_DEBUG=1 in your test target settings, then use the bundle instance’s passesSelfTest method:

    NSError *bundleSelfTestError = nil;
    BOOL pass = [[BalerBundle sharedInstance] passesSelfTest:&bundleSelfTestError];
    STAssertTrue(pass, @"BalerBundle failed self-test with error %@", bundleSelfTestError);

Run Script tips

Skip if baler isn’t set up

You can ensure the build progresses even if the baler invocation fails by having the shell command swallow the non-zero return code from baler, e.g.

/path/to/python/env/bin/bale baled_assets Classes --overwrite-delay 0 || echo "Failed to generate bundle"

This keeps the coupling with baler a little looser, so other contributors won’t need baler to build the project.

BYOB (Bring Your Own Baler)

You may want to let your teammates specify which baler to use in a .gitignore’d shell script. For example, a .env:

export PATH="/path/to/python/env/bin:$PATH"

Then your Run Script would look like this:

[ -f .env ] && source .env 2>/dev/null
bale ...


Pull requests and new issues are welcome. See for details.


Logo image

The baler logo is modified and shared with permission of Wikimedia Commons using the same Creative Commons Attribution-Share Alike 3.0 Unported license. See also the original image and license.


Brought to you by PayPal.

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