Writing an Aggregate Transformer

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An aggregate transformer processes multiple assets in a single pass–for example, collaging multiple images into a single image.

Most of the steps for writing an aggregate transformer are the same as for writing a normal transformer. This document only describes the differences specific to aggregate transformers. If you aren’t familiar with how to write a normal Pub tranformer, see Writing a Pub Transformer.

This page uses the aggregate_transformer example which you can find through Examples of Transformer Code.

Implementing an aggregate transformer

An aggregate transformer extends the Dart class, AggregateTransform, from the barback package.

Extend AggregateTransformer

In the Dart file with your transformer subclass, extend the AggregateTransformer class from the barback package:

class MyTransformer extends AggregateTransformer { ... }

Claim input assets

An aggregate transformer claims its input assets by implementing the classifyPrimary method. For an asset that you want to process, return a value, or key. For assets that you do not care about, return null. Pub calls the classifyPrimary method on every potential input asset. Assign the same key to all assets that you want to be processed together; these assets will be available in the apply method through the AggregateTransform parameter.

The following example, from aggregate_transformer, only accepts assets whose filename ends with the string .recipe.html For assets that it wants to process, it returns the path of the source directory, which is where it wants the output asset to be placed.

import 'package:path/path.dart' as path;


classifyPrimary(AssetId id) {
    // Only process assets where the filename ends with ".recipe.html".
    if (!id.path.endsWith('.recipe.html')) return null;

    // Return the path string, minus the recipe itself.
    // This is where the output asset will be written.
    return path.url.dirname(id.path);

Process input assets

To process assets, implement the apply() method. In this method, you access all of the relevant assets through the passed-in AggregateTransform, manipulate them, and write out the new asset.

To request a handle to all input assets, you can use the primaryInputs property in AggregateTransformer. This provides a stream of type Asset. Note that the assets returned from primaryInputs have no guaranteed order and might change each time the transformer is run, so you might need to re-order the assets before processing.

Because the inputs are provided asynchronously, you must write your code carefully. Functions that return a Future must be chained together and, before creating the output asset, use Future.wait to ensure that all assets have been processed.

The following apply method, from AggregateTransformer, reads the recipes, sorts them alphabetically according to the assets’ ID, and creates an output asset with the recipes compiled into a complete HTML file.

Future apply(AggregateTransform transform) async {
  var buffer = new StringBuffer()..write('<html><body>');

  var assets = await transform.primaryInputs.toList();
  assets.sort((x, y) => x.id.compareTo(y.id));
  for (var asset in assets) {
    var content = await asset.readAsString();
  // Write the output back to the same directory,
  // in a file named recipes.html.
  var id = new AssetId(
      transform.package, p.url.join(transform.key, "recipes.html"));
  transform.addOutput(new Asset.fromString(id, buffer.toString()));

If you wish to request a specific secondary input, you can use the getInput or readInput methods.

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