# Lint Check Unit Testing Lint has a dedicated testing library for lint checks. To use it, add this dependency to your lint check Gradle project: ``` testImplementation "com.android.tools.lint:lint-tests:$lintVersion" ``` This lends itself nicely to test-driven development. When we get bug reports of a false positive, we typically start by adding the text for the repro case, ensure that the test is failing, and then work on the bug fix (often setting breakpoints and debugging through the unit test) until it passes. ## Creating a Unit Test Here's a sample lint unit test for a simple, sample lint check which just issues warnings whenever it sees the word “lint” mentioned in a string: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers package com.example.lint.checks import com.android.tools.lint.checks.infrastructure.TestFiles.java import com.android.tools.lint.checks.infrastructure.TestLintTask.lint import org.junit.Test class SampleCodeDetectorTest { @Test fun testBasic() { lint().files( java( """ package test.pkg; public class TestClass1 { // In a comment, mentioning "lint" has no effect private static String s1 = "Ignore non-word usages: linting"; private static String s2 = "Let's say it: lint"; } """ ).indented() ) .issues(SampleCodeDetector.ISSUE) .run() .expect( """ src/test/pkg/TestClass1.java:5: Warning: This code mentions lint: Congratulations [ShortUniqueId] private static String s2 = "Let's say it: lint"; ∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼∼ 0 errors, 1 warnings """ ) } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Lint's testing API is a “fluent API”; you chain method calls together, and the return objects determine what is allowed next. Notice how we construct a test object here on line 10 with the `lint()` call. This is a “lint test task”, which has a number of setup methods on it (such as the set of source files we want to analyze), the issues it should consider, etc. Then, on line 23, the `run()` method. This runs the lint unit test, and then it returns a result object. On the result object we have a number of methods to verify that the test succeeded. For a test making sure we don't have false positives, you can just call `expectClean()`. But the most common operation is to call `expect(output)`. !!! Tip Notice how we're including the whole text output here; including not just the error message and line number, but lint's output of the relevant line and the error range (using ~~~~ characters). This is the recommended practice for lint checks. It may be tempting to avoid “duplication” of repeating error messages in the tests (“DRY”), so some developers have written tests where they just assert that a given test has say “2 warnings”. But this isn't testing that the error range is exactly what you expect (which matters a lot when users are seeing the lint check from the IDE, since that's the underlined region), and it could also continue to pass even if the errors flagged are no longer what you intended. Finally, even if the location is correct today, it may not be correct tomorrow. Several times in the past, some unit tests in lint's built-in checks have started failing after an update to the Kotlin compiler because of some changes to the AST which required tweaks here and there. ## Computing the Expected Output You may wonder how we knew what to paste into our `expect` call to begin with. We didn't. When you write a test, simply start with `expect("")`, and run the test. It will fail. You can now copy the actual output into the `expect` call as the expected output, provided of course that it's correct! ## Test Files On line 11, we construct a Java test file. We call `java(...)` and pass in the source file contents. This constructs a `TestFile`, and there are a number of different types of test source files, such as for Kotlin files, manifest files, icons, property files, and so on. Using test file descriptors like this has a number of advantages over the traditional approach of checking in test files as sources: * Everything is kept together, so it's easier to look at a test and see what it analyzes and what the expected results are. This is particularly important for complex lint checks which test a lot of scenarios. As of this writing, `ApiDetectorTest` has 157 individual unit tests. * Lint can provide a DSL to construct test files easily. For example, `projectProperties().compileSdk(17)` and `manifest().minSdk(5).targetSdk(17)` construct a `project.properties` and an `AndroidManifest.xml` file with the correct contents to specify for example the right element setting up the `minSdkVersion` and `targetSdkVersion`. For icons, we can construct bitmaps like this: ``` image("res/mipmap-hdpi/my_launcher2_round.png", 50, 50) .fillOval(0, 0, 50, 50, 0xFFFFFFFF) .text(5, 5, "x", 0xFFFFFFFF)) ``` * Similarly, when we construct `java()` or `kotlin()` test sources, we don't have to name the files, because lint will analyze the source code and figure out what the class file should be named and where to place it. * We can easily “parameterize” our test files. For example, if you want to run your unit test against a 100K json file, you can construct it programmatically; you don't have to check one in. * Since test sources often (deliberately!) have errors in them, this sometimes causes problems with the tooling; for example, some code review tools will flag “disallowed” constructs or things like tabs or trailing spaces, which may be deliberate in a lint unit test. * Lint originally checked in test sources as individual files. Unfortunately over time, source files ended up getting reused by multiple tests. And that made it harder to make changes, or figure out whether test sources are still in use, and so on. * Last but not least, because all the test construction methods specify the correct mime type for their string parameters, IntelliJ will actually syntax highlight the test source declarations! Here's how this looks:  ## Trimming indents? Notice how in the above Kotlin unit tests we used raw strings, **and** we indented the sources to be flush with the opening “”“ string delimiter. You might be tempted to call `.trimIndent()` on the raw string. However, doing that would break the above nested syntax highlighting method (or at least it used to). Therefore, instead, call `.indented()` on the test file itself, not the string, as shown on line 20. Note that we don't need to do anything with the `expect` call; lint will automatically call `trimIndent()` on the string passed in to it. ## Dollars in Raw Strings Kotlin requires that raw strings have to escape the dollar ($) character. That's normally not a problem, but for some source files, it makes the source code look **really** messy and unreadable. For that reason, lint will actually convert $ into ＄ (a unicode wide dollar sign). Lint lets you use this character in test sources, and it always converts the test output to use it (though it will convert in the opposite direction when creating the test sources on disk). ## Quickfixes If your lint check registers quickfixes with the reported incidents, it's trivial to test these as well. For example, for a lint check result which flags two incidents, with a single quickfix, the unit test looks like this: ``` lint().files(...) .run() .expect(expected) .expectFixDiffs( "" + "Fix for res/layout/textsize.xml line 10: Replace with sp:\n" + "@@ -11 +11\n" + "- android:textSize=\"14dp\" />\n" + "+ android:textSize=\"14sp\" />\n" + "Fix for res/layout/textsize.xml line 15: Replace with sp:\n" + "@@ -16 +16\n" + "- android:textSize=\"14dip\" />\n" + "+ android:textSize=\"14sp\" />\n"); ``` The `expectFixDiffs` method will iterate over all the incidents it found, and in succession, apply the fix, diff the two sources, and append this diff along with the fix message into the log. When there are multiple fixes offered for a single incident, it will iterate through all of these too: ``` lint().files(...) .run() .expect(expected) .expectFixDiffs( + "Fix for res/layout/autofill.xml line 7: Set autofillHints:\n" + "@@ -12 +12\n" + " android:layout_width=\"match_parent\"\n" + " android:layout_height=\"wrap_content\"\n" + "+ android:autofillHints=\"|\"\n" + " android:hint=\"hint\"\n" + " android:inputType=\"password\" >\n" + "Fix for res/layout/autofill.xml line 7: Set importantForAutofill=\"no\":\n" + "@@ -13 +13\n" + " android:layout_height=\"wrap_content\"\n" + " android:hint=\"hint\"\n" + "+ android:importantForAutofill=\"no\"\n" + " android:inputType=\"password\" >\n" + " \n"); ``` ## Library Dependencies and Stubs Let's say you're writing a lint check for something like the AndroidX library's `RecyclerView` widget. In this case, it's highly likely that your unit test will reference `RecyclerView`. But how does lint know what `RecyclerView` is? If it doesn't, type resolve won't work, and as a result the detector won't. You could make your test ”depend“ on the recyclerview. This is possible, using the `LibraryReferenceTestFile`, but is not recommended. Instead, the recommended approach is to just use ”stubs“; create skeleton classes which represent only the **signatures** of the library, and in particular, only the subset that your lint check cares about. For example, for lint's own recycler view test, the unit test declares a field holding the recycler view stub: ``` private val recyclerViewStub = java( """ package android.support.v7.widget; import android.content.Context; import android.util.AttributeSet; import android.view.View; import java.util.List; // Just a stub for lint unit tests public class RecyclerView extends View { public RecyclerView(Context context, AttributeSet attrs) { super(context, attrs); } public abstract static class ViewHolder { public ViewHolder(View itemView) { } } public abstract static class Adapter { public abstract void onBindViewHolder(VH holder, int position); public void onBindViewHolder(VH holder, int position, List