Custom lint rules

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Was trying to create a silly custom lint rule the other day.

Of course, I played around with https://github.com/googlesamples/android-custom-lint-rules (monkey-see-monkey-do).

I was under the impression that for editor highlighting to start working, I only had to build the app. (oh boy)

When I did manage to make it show up, I had issues editing, deleting or even adding more rules.

So, without further ado..

It’s not enough just writing a custom lint rule#

Rule here.

As evidenced, no highlighting:

class Test {
    // We have a custom lint check bundled with :library
    // that this module depends on. The lint check looks
    // for mentions of "lint", which should trigger an
    // error
    val s = "lint"

    fun lint() { }
}

Run the lint check in Android Studio:

Voila!

Editing rules#

Alter this rule to check for hello .

return object : UElementHandler() {
    override fun visitLiteralExpression(node: ULiteralExpression) {
        val string = node.evaluateString() ?: return
        if (string.contains("hello") && string.matches(Regex(".*\\bhello\\b.*"))) {
            context.report(
                ISSUE, node, context.getLocation(node),
                "This code mentions `hello`: **Congratulations**"
            )
        }
    }
}

SampleCodeDetector

The string literal that contains the word lint is still highlighted even after building because we need to…

Run lint again#

This pops up 🫠:

C:\code\android-custom-lint-rules\library\build\intermediates\lint_publish_jar\global\lint.jar: The process cannot access the file because it is being used by another process.

Let’s have a look what is this even and why would it complain:

..okay?#

I am not a smart man but I am going to assume something like this happens:

• lint command does stuff on first run.
• At the end, this lint.jar file is placed into the build folder.
• Once in there, this file is actively “watched” by Android Studio/some java process. That’s how we get highlighting in editor.
• Trying to replace the lint.jar by re-running lint will meet resistance. It’s busy being actively used.

Exit Android Studio#

Open the build folder in explorer and delete all files in it.

Now re-open AS. The highlighting should be gone.

Run lint again.

hello is highlighted:

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Considerations#

Few things that are nagging me after messing with this for about an hour:

• Should this lint.jar file be included in a project’s repository? (I couldn’t find any way to make this work outside the build folder)
• Do we have to do the delete/replace dance everytime there is a change? (how many times is a lint rule going to change anyway 😅)
• Hook lint in the normal build task via gradle? (not a good idea)

But also positives:

• Everyone can see the rules in the project. Easily accessible.
• Anything CI related.

Bundling lint rules in a separate library#

Timber is a classic example of shipping lint rules with a library.

• The rules are built and packaged in the library as part of the publishing process. (you can explore inside the timber aar file and have a look)
• The consumers don’t have to take any action just to see highlighting in the editor.

But!

• Flexibility is lost due to using an external library that is not in the main project.

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Anyways#

Hope you found this somewhat useful.

All credit goes to https://github.com/googlesamples/android-custom-lint-rules.