# Data Flow Analyzer The dataflow analyzer is a helper in lint which makes writing certain kinds of lint checks a lot easier. Let's say you have an API which creates an object, and then you want to make sure that at some point a particular method is called on the same instance. There are a lot of scenarios like this; * Calling `show` on a message in a Toast or Snackbar * Calling `commit` or `apply` on a transaction * Calling `recycle` on a TypedArray * Calling `enqueue` on a newly created work request and so on. I didn't include calling close on a file object since you typically use try-with-resources for those. Here are some examples: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers getFragmentManager().beginTransaction().commit() // OK val t1 = getFragmentManager().beginTransaction() // NEVER COMMITTED val t2 = getFragmentManager().beginTransaction() // OK t2.commit() ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here we are creating 3 transactions. The first one is committed immediately. The second one is never committed. And the third one is. This example shows us creating multiple transactions, and that demonstrates that solving this problem isn't as simple as just visiting the method and seeing if the code invokes `Transaction#commit` anywhere; we have to make sure that it's invoked on all the instances we care about. ## Usage To use the dataflow analyzer, you basically extend the `DataFlowAnalyzer` class, and override one or more of its callbacks, and then tell it to analyze a method scope. !!! Tip In recent versions of lint, there is a new special subclass of the `DataFlowAnalyzer`, `TargetMethodDataFlowAnalyzer`, which makes it easier to write flow analyzers where you are looking for a specific “cleanup” or close function invoked on an instance. See the separate section on `TargetMethodDataFlowAnalyzer` below for more information. For the above transaction scenario, it might look like this: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers override fun getApplicableMethodNames(): List = listOf("beginTransaction") override fun visitMethodCall(context: JavaContext, node: UCallExpression, method: PsiMethod) { val containingClass = method.containingClass val evaluator = context.evaluator if (evaluator.extendsClass(containingClass, "android.app.FragmentManager", false)) { // node is a call to FragmentManager.beginTransaction(), // so this expression will evaluate to an instance of // a Transaction. We want to track this instance to see // if we eventually call commit on it. var foundCommit = false val visitor = object : DataFlowAnalyzer(setOf(node)) { override fun receiver(call: UCallExpression) { if (call.methodName == "commit") { foundCommit = true } } } val method = node.getParentOfType(UMethod::class.java) method?.accept(visitor) if (!foundCommit) { context.report(Incident(...)) } } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Aas you can see, the `DataFlowAnalyzer` is a visitor, so when we find a call we're interested in, we construct a `DataFlowAnalyzer` and initialize it with the instance we want to track, and then we visit the surrounding method with this visitor. The visitor will invoke the `receiver` method whenever the instance is invoked as the receiver of a method call; this is the case with `t2.commit()` in the above example; here “t2” is the receiver, and `commit` is the method call name. With the above setup, basic value tracking is working; e.g. it will correctly handle the following case: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers val t = getFragmentManager().beginTransaction().commit() val t2 = t val t3 = t2 t3.commit() ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ However, there's a lot that can go wrong, which we'll need to deal with. This is explained in the following sections ## Self-referencing Calls The Transaction API has a number of utility methods; here's a partial list: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers public abstract class FragmentTransaction { public abstract int commit(); public abstract int commitAllowingStateLoss(); public abstract FragmentTransaction show(Fragment fragment); public abstract FragmentTransaction hide(Fragment fragment); public abstract FragmentTransaction attach(Fragment fragment); public abstract FragmentTransaction detach(Fragment fragment); public abstract FragmentTransaction add(int containerViewId, Fragment fragment); public abstract FragmentTransaction add(Fragment fragment, String tag); public abstract FragmentTransaction addToBackStack(String name); ... } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The reason all these methods return a `FragmentTransaction` is to make it easy to chain calls; e.g. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers final int id = getFragmentManager().beginTransaction() .add(new Fragment(), null) .addToBackStack(null) .commit(); ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In order to correctly analyze this, we'd need to know what the implementation of `add` and `addToBackStack` return. If we know that they simply return “this”, then it's easy; we can transfer the instance through the call. And this is what the `DataFlowAnalyzer` will try to do by default. When it encounters a call on our tracked receivers, it will try to guess whether that method is returning itself. It has several heuristics for this: * The return type is the same as its surrounding class, or a subtype of it * It's an extension method returning the same type * It's not named something which indicates a new instance (such as clone, copy, or to*X*), unless `ignoreCopies()` is overridden to return false In our example, the above heuristics work, so out of the box, the lint check would correctly handle this scenario. But there may be cases where you either don't want these heuristics, or you want to add your own. In these cases, you would override the `returnsSelf` method on the flow analyzer and apply your own logic: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers val visitor = object : DataFlowAnalyzer(setOf(node)) { override fun returnsSelf(call: UCallExpression): Boolean { return super.returnsSelf(call) || call.methodName == "copy" } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ## Kotlin Scoping Functions With this in place, lint will track the flow through the method. This includes handling Kotlin's scoping functions as well. For example, it will automatically handle scenarios like the following: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers transaction1.let { it.commit() } transaction2.apply { commit() } with (transaction3) { commit() } transaction4.also { it.commit() } getFragmentManager.let { it.beginTransaction() }.commit() // complex (contrived and unrealistic) example: transaction5.let { it.also { it.apply { with(this) { commit() } } } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ## Limitations It doesn't try to “execute”, constant evaluation (maybe) if/else ## Escaping Values What if your check gets invoked on a code snippet like this: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers fun createTransaction(): FragmentTransaction = getFragmentManager().beginTransaction().add(new Fragment(), null) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here, we're not calling `commit`, so our lint check would issue a warning. However, it's quite possible and likely that elsewhere, there's code using it, like this: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers val transaction = createTransaction() ... transaction.commit() ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Ideally, we'd perform global analysis to handle this, but that's not currently possible. However, we *can* analyze some additional non-local scenarios, and more importantly, we need to ensure that we don't offer false positive warnings in the above scenario. ### Returns In the above case, our tracked transaction “escapes” the method that we're analyzing through either an implicit return as in the above Kotlin code or via an explicit return. The analyzer has a callback method to let us know when this is happening. We can override that callback to remember that the value escapes, and if so, ignore the missing commit: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers var foundCommit = false var escapes = false val visitor = object : DataFlowAnalyzer(setOf(node)) { override fun returns(expression: UReturnExpression) { escapes = true } override fun argument(call: UCallExpression, reference: UElement) { super.argument(call, reference) } override fun field(field: UElement) { super.field(field) } } node.getParentOfType(UMethod::class.java)?.accept(visitor) if (!escapes && !foundCommit) { context.report(Incident(...)) } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ### Parameters Another way our transaction can “escape” out of the method such that we no longer know for certain whether it gets committed is via a method call. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers fun test) { val transaction = getFragmentManager().beginTransaction() process(transaction) } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here, it's possible that the `process` method will proceed to actually commit the transaction. If we have source, we could resolve the call and take a look at the method implementation (see the “Non Local Analysis” section below), but in the general case, if a value escapes, we'll want to do something similar to a returned value. The analyzer has a callback for this, `argument`, which is invoked whenever our tracked value is passed into a method as an argument. The callback gives us both the argument and the call in case we want to handle conditional logic based on the specific method call. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers var escapes = false val visitor = object : DataFlowAnalyzer(setOf(node)) { ... override fun argument(call: UCallExpression, reference: UElement) { escapes = true } ... } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (By default, the analyzer will ignore calls that look like logging calls since those are probably safe and not true escapes; you can customize this by overriding `ignoreArgument()`.) ### Fields Finally, a value may escape a local method context if it gets stored into a field: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers fun initialize() { this.transaction = createTransaction() } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ As with returns and method calls, the analyzer has a callback to make it easy to handle when this is the case: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers var escapes = false val visitor = object : DataFlowAnalyzer(setOf(node)) { ... override fun field(field: UElement) { escapes = true } ... } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ As you can see, it's passing in the field that is being stored to, in case you want to perform additional analysis to track field values; see the next section. !!! Tip There is a special subclass of the `DataFlowAnalyzer`, called `EscapeCheckingDataFlowAnalyzer`, which you can extend instead. This handles recording all the scenarios where the instance escapes from the method, and at the end you can just check its `escaped` property. ## Non Local Analysis In the above examples, if we found that the value escaped via a return or method call or storage in a field, we simply gave up. In some cases we can do better than that. * If the field we stored it into is a private field, we can visit the surrounding class, and check each reference to the field. If we can see that the field never escapes the class, we can perform the same analysis (using the data flow analyzer!) on each method where it's referenced. * Similarly, if the method which returns the value is private, we can visit the surrounding class and see how the method is invoked, and track the value returned from it in each usage. * Finally, if the value escapes as an argument to a call, we can resolve that call, and if it's to a method we have source for (which doesn't have to be in the same class, as long as it's in the same module), we can perform the analysis in that method as well, even reusing the same flow analyzer! Complications: - storing in a field, returning, intermediate variables, self-referencing methods, scoping functions, ## Examples Here are some existing usages of the data flow analyzer in lint's built-in rules. ### Simple Example For WorkManager, ensure that newly created work tasks eventually get enqueued: [Source](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/WorkManagerDetector.kt) [Test](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/WorkManagerDetectorTest.kt) ### Complex Example For the Slices API, apply a number of checks on chained calls constructing slices, checking that you only specify a single timestamp, that you don't mix icons and actions, etc etc. [Source](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/SliceDetector.kt) [Test](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/SliceDetectorTest.kt) ## TargetMethodDataFlowAnalyzer The `TargetMethodDataFlowAnalyzer` is a special subclass of the `DataFlowAnalyzer` which makes it simple to see if you eventually wind up calling a target method on a particular instance. For example, calling `close` on a file that was opened, or calling `start` on an animation you created. In addition, there is an extension function on `UMethod` which visits this analyzer, and then checks for various conditions, e.g. whether the instance “escaped” (for example by being stored in a field or passed to another method), in which case you probably don't want to conclude (and report) that the close method is never called. It also handles failures to resolve, where it remembers whether there was a resolve failure, and if so it looks to see if it finds a likely match (with the same name as the target function), and if so also makes sure you don't report a false positive. A simple way to do this is as follows: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers val targets = mapOf("show" to listOf("android.widget.Toast", "com.google.android.material.snackbar.Snackbar") val analyzer = TargetMethodDataFlowAnalyzer.create(node, targets) if (method.isMissingTarget(analyzer)) { context.report(...) } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You can subclass `TargetMethodDataFlowAnalyzer` directly and override the `getTargetMethod` methods and any other UAST visitor methods if you want to customize the behavior further. One advantage of using the `TargetMethodDataFlowAnalyzer` is that it also correctly handles method references.