added hasInput funtion to Reader/BuildStep

Author: jakemac53

lib/src/asset/reader.dart

@@ -8,4 +8,6 @@ import 'id.dart';
 
 abstract class AssetReader {
   Future<String> readAsString(AssetId id, {Encoding encoding: UTF8});
+
+  Future<bool> hasInput(AssetId id);
 }

lib/src/builder/build_step.dart

@@ -13,6 +13,12 @@ abstract class BuildStep {
   /// The primary input for this build step.
   Asset get input;
 
+  /// Checks if an [Asset] by [id] exists as an input for this [BuildStep].
+  ///
+  /// If [trackAsDependency] is true, then [id] will be marked as a dependency
+  /// of all [expectedOutputs].
+  Future<bool> hasInput(AssetId id, {bool trackAsDependency: true});
+
   /// Reads an [Asset] by [id] as a [String] using [encoding].
   ///
   /// If [trackAsDependency] is true, then [id] will be marked as a dependency

lib/src/builder/build_step_impl.dart

@@ -49,6 +49,15 @@ class BuildStepImpl implements BuildStep {
     _dependencies.add(input.id);
   }
 
+  /// Checks if an [Asset] by [id] exists as an input for this [BuildStep].
+  ///
+  /// If [trackAsDependency] is true, then [id] will be marked as a dependency
+  /// of all [expectedOutputs].
+  Future<bool> hasInput(AssetId id, {bool trackAsDependency: true}) {
+    if (trackAsDependency) _dependencies.add(id);
+    return _reader.hasInput(id);
+  }
+
   /// Reads an [Asset] by [id] as a [String] using [encoding].
   ///
   /// If [trackAsDependency] is true, then [id] will be marked as a dependency

lib/src/generate/build.dart

@@ -122,6 +122,12 @@ Stream<Asset> _runBuilder(Builder builder, List<AssetId> inputs) async* {
 class _SimpleAssetReader implements AssetReader {
   const _SimpleAssetReader();
 
+  @override
+  Future<bool> hasInput(AssetId id) async {
+    assert(id.package == _localPackageName);
+    return new File(id.path).exists();
+  }
+
   @override
   Future<String> readAsString(AssetId id, {Encoding encoding: UTF8}) async {
     assert(id.package == _localPackageName);

lib/src/transformer/transformer.dart

@@ -89,6 +89,10 @@ class _TransformAssetReader implements AssetReader {
 
   _TransformAssetReader(this.transform);
 
+  @override
+  Future<bool> hasInput(build.AssetId id) =>
+      transform.hasInput(_toBarbackAssetId(id));
+
   @override
   Future<String> readAsString(build.AssetId id, {Encoding encoding: UTF8}) =>
       transform.readInputAsString(_toBarbackAssetId(id), encoding: encoding);

test/builder/build_step_impl_test.dart

@@ -35,6 +35,26 @@ main() {
         var a2 = makeAssetId();
         await buildStep.readAsString(a2);
         expect(buildStep.dependencies, [primary.id, a1, a2]);
+
+        var a3 = makeAssetId();
+        await buildStep.readAsString(a3, trackAsDependency: false);
+        expect(buildStep.dependencies.contains(a3), isFalse);
+      });
+
+      test('tracks dependencies on hasInput', () async {
+        expect(buildStep.dependencies, [primary.id]);
+
+        var a1 = makeAssetId();
+        await buildStep.hasInput(a1);
+        expect(buildStep.dependencies, [primary.id, a1]);
+
+        var a2 = makeAssetId();
+        await buildStep.hasInput(a2);
+        expect(buildStep.dependencies, [primary.id, a1, a2]);
+
+        var a3 = makeAssetId();
+        await buildStep.hasInput(a3, trackAsDependency: false);
+        expect(buildStep.dependencies.contains(a3), isFalse);
       });
 
       test('addDependency adds dependencies', () {

test/common/in_memory_reader.dart

@@ -11,8 +11,12 @@ class InMemoryAssetReader implements AssetReader {
 
   InMemoryAssetReader(this.assets);
 
+  Future<bool> hasInput(AssetId id) {
+    return new Future.value(assets.containsKey(id));
+  }
+
   Future<String> readAsString(AssetId id, {Encoding encoding: UTF8}) async {
-    if (!assets.containsKey(id)) throw new AssetNotFoundException(id);
+    if (!await hasInput(id)) throw new AssetNotFoundException(id);
     return assets[id];
   }
 }

test/common/stub_reader.dart

@@ -7,6 +7,8 @@ import 'dart:convert';
 import 'package:build/build.dart';
 
 class StubAssetReader implements AssetReader {
+  Future<bool> hasInput(AssetId id) => new Future.value(null);
+
   Future<String> readAsString(AssetId id, {Encoding encoding: UTF8}) =>
       new Future.value(null);
 }