Recorded Uses

Tree-shaking can cause APIs and resources to be removed from programs. We developed a mechanism in dart2js to help developers understand what resources are still in use after an application is optimized.

Status

(experimental, in progress)

Currently we have a mechanism that only supports tracking static member functions. It is not rich enough to track the use of resource classes or constants, like IconData in flutter applications. That would be an ideal expansion in the future.

How it works

Developers can annotate top-level functions or methods with RecordUse from package:record_use. This annotation signals to dart2js that the annotated member is a resource that needs to be tracked.

Example:

import 'package:record_use/record_use.dart';

@RecordUse()
void myTopLevelMethod() {
  // ... resource usage ...
}

When providing dart2js with the experimental --write-resources flag, the compiler will emit a .resources.json file. This file lists whether any top-level methods annotated with the special pragma was invoked in the program. It will also include some additional static information, like the source location of the call, or even which parameters where provided (if the parameters are constant).

Example output

Example outputs can be found in pkg/vm/testcases/transformations/record_use/, and can be read with package:record_use.