Xcode: Running a script before every build that modifies source code directly

What I did:

I have a script that

1. Read some configuration files to generate source code snippets
2. Find relevant Objective-C source files and
3. Replace some portions of the source code with the generated code in step 1.

and a Makefile that has a special timestamp file as a make target and the configuration files as target sources:

SRC = $(shell find ../config -iname "*.txt")
STAMP = $(PROJECT_TEMP_DIR)/$(CONFIGURATION)$(EFFECTIVE_PLATFORM_NAME).stamp
$(STAMP): $(SRC)
	python inject.py
	touch $(STAMP)

I added this Makefile as a "Run Script Build Phase" on top of the stack of build phases for the project target.

What happened:

The script build phase was run before compiling the source.

However, since the script modifies source code during its execution, I needed to build twice to get the most recent version of the build product. Here is what I imagine to be happening:

1. 1st run: Xcode collects dependency information ---> no changes
2. 1st run: Xcode runs "Run Script Build Phase" ---> source is changed behind Xcode's back
3. 1st run: Xcode finishes build, thinking nothing needs to be updated
4. 2nd run: Xcode collects dependency information ---> source has changed, needs rebuild!
5. 2nd run: Xcode runs Run Script Build Phase" ---> everything is up-to-date
6. 2nd run: Xcode proceeds to compilation

After reading Xcode documentation on Build Phases, I tried adding a source file which is known to be updated every time the script is run as the output of "Run Script Build Phases", but nothing changed. Since the number of configuration files may vary in my project, I don't want to specify every input and output file.

Question:

How do I make Xcode aware of source file changes made during "Run Script Build Phase"?

Edit:

• Added that I placed the script build phase before the other build phases

Every technique mentioned so far is an overkill. Reproducing steve kim's comment for visibility:

In the build phases tab, simply drag the "Run Script" step to a higher location (e.g. before "Compile Sources").

Tested on Xcode 6

This solution is probably outdated. See the higher voted answer instead; I no longer actively use Xcode and am not qualified to vet a solution.

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Use "External Target":

1. Select "Project" > "New Target..." from the menu
2. Select "Mac OS X" > "Other" > "External Target" and add it to your project
3. Open its settings and fill in your script setup
4. Open the "General" tab of the main target's settings and add the new target as it's direct dependency

Now the new "External Target" runs before the main target even starts gathering dependency information, so that any changes made during the script execution should be included in the build.

There is another, slightly simpler option that doesn't require a separate target, but it's only viable if your script tends to modify the same source files every time.

First, here's a brief explanation for anyone who's confused about why Xcode sometimes requires you to build twice (or do a clean build) to see certain changes reflected in your target app. Xcode compiles a source file if the object file it produces is missing, or if the object file's last-modified date is earlier than the source file's last-modified date was at the beginning of the first build phase. If your project runs a script that modifies a source file in a pre-compilation build phase, Xcode won't notice that the source file's last-modified date has changed, so it won't bother to recompile it. It's only when you build the project a second time that Xcode will notice the date change and recompile the file.

Here's a simple solution if your script happens to modify the same source files every time. Just add a Run Script build phase at the end of your build process like this:

touch Classes/FirstModifiedFile.m Classes/SecondModifiedFile.m
exit $?

Running touch on these source files at the end of your build process guarantees that they will always have a later last-modified date than their object files, so Xcode will recompile them every time.

I as well struggled with this for a long time. The answer is to use ento's "External Target" solution. He is WHY this problem occurs and how we use it in practice...

Xcode 4 build steps do not execute until AFTER the plist has been compiled. This is silly, of course, because it means that any pre-build steps that modify the plist won't take effect. But if you think about it, they actually DO take effect...on the NEXT build. That's why some people have talked about "caching" of plist values or "I have to do 2 builds to make it work." What happens is the plist is built, then your script runs. Next time you build, the plist builds using your modified files, hence the second build.

ento's solution is the one way I've found to actually do a true pre-build step. Unfortunately I also found that it does not cause the plist to update without a clean build and I fixed that. Here is how we have data-driven user values in the plist:

1. Add an External Build System project that points to a python script and passes some arguments
2. Add user-defined build settings to the build. These are the arguments that you pass to python (more on why we do this later)
3. The python script reads some input JSON files and builds a plist preprocessor header file AND touches the main app plist
4. The main project has "preprocess plist files" turned on and points to this preprocessor file

Using touch on the main app plist file causes the main target to generate the plist every time. The reason we pass in build settings as parameters is so our command-line build can override settings:

1. Add a user-defined variable "foo" to the prebuild project.
2. In your prebuild you can use $(foo) to pass the value into the python script.
3. On the command-line you can add foo=test to pass in a new value.

The python script uses base settings files and allows for user-defined settings files to override the defaults. You make a change and immediately it ends up in the plist. We only use this for settings that MUST be in the plist. For anything else it's a waste of effort....generate a json file or something similar instead and load it at run-time :)

I hope this helps...it's been a couple rough days figuring this out.

As of Xcode 4, it looks like if you add the generated files to the output section of the build phase, it will respect that setting, and not generate the ... has been modified since the precompiled header was built error messages.

This is a good option if your script is only generating a handful of files each time.

The External Target solution from @ento no longer works as of Xcode 11.5. The solution is to add all files that will be changed under Output Files in the Run Script.

Another option is to create a subproject framework with your scripts and just add it as a dependency to all targets. The phase scripts of this subproject should now be executed before all targets.