So this is a misleading error.
The fastest way to get to the root cause is to run:
bash gradlew assembleDebug --debug
then scroll up and look for the real error happening.
However, if it still doesn't seem like you have the answer you are looking for, then read on.
I'm going to explain the 30,000 foot view of what is happening. This is not EXACT order or EXACT flow, it is just pretty damn close ;) so if you know more then I do of the exact order and care to make corrections with links, feel free I won't stop ya :).
The Process
The R file is generated code.
There is an order to the generation.
Gradle will do it's magic, pull it's dependencies and kick off it's
warning and error tree first,
then Android converts all Kotlin to Java behind the scenes. Yup that's
right, our beloved Kotlin still has to be Java to compile for our
beloved ART virtual machine.
Then it runs through and does the adapters that you have created for
JVM Statics and a few other tasks.
Next up it compiles all the xml databinding files first to create the
generated databinding files.
If everything succeeds it moves on to processing the assets and
resources. Which creates pointers or IDs for each resource that you
reference in code. Next it will run through and begin compiling the
code and packaging process after that.
Pretty straight forward process, but here in lies the problem.
The misleading Error
If any step fails before the R generation is complete, then the R does not get generated. Sometimes a simple rebuild is all you need to do, sometimes a simple File->Invalidate Cache and Restart is all you need. However, more often than not you have a code issue in your gradle, your xml, your databinding or your adapters that are preventing the compiler from even reaching the R generation stage.
So the next question is
"Well shoot, how do we troubleshoot it if the errors are worthless or
non-existent".
Well first let's talk about the many ways these errors present themselves.
- Duplicate Databinding class found
- xml Binding Error at line #
- Couldn't find matching signature of bind:customAdapterMethod
- Can't find R file of the correct project, only shows import options for sub modules or incorrect namespace R files.
- Couldn't find DataBindingUtility or DataBinding for activity/fragment
- And many other various ways as well, too many to list them all
Next, let's talk about potential candidates causing the problem. As there are sooo many lol.
- Gradle Syncing issues
- Bad versions of Gradle or Tools, you may have gone too far forward in your last gradle file modification. Try stepping back one version and "invalidate cache and restart" if that fixed it, great, if not, read on.
- Caching Issues (File->Restart and Invalidate Cache)
- xml elements with wrong namespace
- xml elements with bad IDs or references IDs out of order (i.e. you say align to right of an element that is lower in the xml document then the sibling element that is trying to reference it)
- xml data binding issues referencing namespace or member that doesn't exist or is not typed correctly
- xml data binding issues in non-auto-filled spots like custom attributes using adapters as those are harder to spot. i.e. bind:myCustomMethod=@"myObject.mistypedProperty()"
- JVM Static adapters with issues or duplicated signatures
- Duplicated or bad character in the Strings or Dimens file or any other xml file for that matter
- Private variable marked for @Binding without properties to access it
- Member variable marked for @Binding that matches a parent class method causing duplications that manifests itself in almost impossible errors
- Mismatch of types like using an adapter that takes (Int) but you are passing (Int?) via data binding and it isn't recognized with JVM Statics until compile time
- You selected IMPORT on a popup to import R file of a sub module instead of the application file
- Having bindable members in a child or parent class, but not giving fully qualified namespace to class cast in the XML usage of the parent or child class. As the databinding compiler is not smart enough to realize the variable provided for class Foo is also parentFoo baseclass, so you have to qualify it as android:text="@((com.path.parentFoo)foo).parentMethod"
- Having a method name in a class, that matches a "generated property from @Binding member variable" i.e. firstName as a variable, but then having a method called getFirstName in a parent or child class, because you are now matching a method name that will get auto generated, thus causing dataBindingUtility duplication class errors.
- There are more causes, but this should give you a series of places to look, but the list can go on and on seriously.
Unfortunately this happens a lot in bleeding edge technologies where the UI tools are not up to speed with the terminal based tools yet. So I suggest you run from the project root in a terminal with
bash gradlew assembleDebug --debug
When it fails, and it will. Start scrolling up through the logs until you find the red where you see what is actually failing and preventing the next stage from occurring.
You will find this especially useful when you start dealing with databinding.
TIP:
When you start dealing with databinding, make sure you compile and run often because the goal is to recognize right away before doing other files to make sure you didn't break generation and make your life MUCH easier to know code you just added caused the issue before getting too far.
Times to compile and run to confirm no issues before moving on.
- If you add a few JVM statics compile and run
- If you add variables to your XML to use
- If you bind to properties of your model in 1 file
- If you add a binding to a JVMStatic
- If you add bindable members or properties to an model
- If you refactor moving observable member variables or properties into children or base classes
- Any other xml or binding related elements that can effect the generated code.
Like I mentioned above, the reason is to avoid getting so many changes, that it becomes a troubleshooting nightmare to find a generic vague, horrible error related to generated databinding code. I'm sure the tools will improve, but for now, do yourself a favor and compile and run often when changing Databinding related items.
Happy Coding