Improve Dart plugin registration handling (#122046)

Improve Dart plugin registration handling

packages/flutter_tools/lib/src/flutter_plugins.dart

@@ -1191,13 +1191,13 @@ bool hasPlugins(FlutterProject project) {
 
 /// Resolves the platform implementation for Dart-only plugins.
 ///
-///   * If there are multiple direct pub dependencies on packages that implement the
-///     frontend plugin for the current platform, fail.
+///   * If there is only one dependency on a package that implements the
+///     frontend plugin for the current platform, use that.
 ///   * If there is a single direct dependency on a package that implements the
-///     frontend plugin for the target platform, this package is the selected implementation.
-///   * If there is no direct dependency on a package that implements the frontend
-///     plugin for the target platform, and the frontend plugin has a default implementation
-///     for the target platform the default implementation is selected.
+///     frontend plugin for the current platform, use that.
+///   * If there is no direct dependency on a package that implements the
+///     frontend plugin, but there is a default for the current platform,
+///     use that.
 ///   * Else fail.
 ///
 ///  For more details, https://flutter.dev/go/federated-plugins.
@@ -1214,11 +1214,15 @@ List<PluginInterfaceResolution> resolvePlatformImplementation(
     MacOSPlugin.kConfigKey,
     WindowsPlugin.kConfigKey,
   ];
-  final Map<String, PluginInterfaceResolution> directDependencyResolutions
-      = <String, PluginInterfaceResolution>{};
+  final Map<String, List<PluginInterfaceResolution>> possibleResolutions
+      = <String, List<PluginInterfaceResolution>>{};
   final Map<String, String> defaultImplementations = <String, String>{};
-  bool didFindError = false;
+  // Generates a key for the maps above.
+  String getResolutionKey({required String platform, required String packageName}) {
+    return '$packageName:$platform';
+  }
 
+  bool hasPubspecError = false;
   for (final Plugin plugin in plugins) {
     for (final String platform in platforms) {
       if (plugin.platforms[platform] == null &&
@@ -1257,11 +1261,12 @@ List<PluginInterfaceResolution> resolvePlatformImplementation(
               '\n'
             );
           }
-          didFindError = true;
+          hasPubspecError = true;
           continue;
         }
+        final String defaultImplementationKey = getResolutionKey(platform: platform, packageName: plugin.name);
         if (defaultImplementation != null) {
-          defaultImplementations['$platform/${plugin.name}'] = defaultImplementation;
+          defaultImplementations[defaultImplementationKey] = defaultImplementation;
           continue;
         } else {
           // An app-facing package (i.e., one with no 'implements') with an
@@ -1281,52 +1286,87 @@ List<PluginInterfaceResolution> resolvePlatformImplementation(
             minFlutterVersion.compareTo(semver.Version(2, 11, 0)) >= 0;
           if (!isDesktop || hasMinVersionForImplementsRequirement) {
             implementsPackage = plugin.name;
-            defaultImplementations['$platform/${plugin.name}'] = plugin.name;
+            defaultImplementations[defaultImplementationKey] = plugin.name;
+          } else {
+            // If it doesn't meet any of the conditions, it isn't eligible for
+            // auto-registration.
+            continue;
           }
         }
       }
+      // If there's no Dart implementation, there's nothing to register.
       if (plugin.pluginDartClassPlatforms[platform] == null ||
           plugin.pluginDartClassPlatforms[platform] == 'none') {
         continue;
       }
-      final String resolutionKey = '$platform/$implementsPackage';
-      if (directDependencyResolutions.containsKey(resolutionKey)) {
-        final PluginInterfaceResolution? currResolution = directDependencyResolutions[resolutionKey];
-        if (currResolution != null && currResolution.plugin.isDirectDependency) {
-          if (plugin.isDirectDependency) {
-            if (throwOnPluginPubspecError) {
-              globals.printError(
-                'Plugin `${plugin.name}` implements an interface for `$platform`, which was already '
-                'implemented by plugin `${currResolution.plugin.name}`.\n'
-                'To fix this issue, remove either dependency from pubspec.yaml.'
-                '\n\n'
-              );
-            }
-            didFindError = true;
-          }
-          // Use the plugin implementation added by the user as a direct dependency.
-          continue;
-        }
+
+      // If it hasn't been skipped, it's a candidate for auto-registration, so
+      // add it as a possible resolution.
+      final String resolutionKey = getResolutionKey(platform: platform, packageName: implementsPackage);
+      if (!possibleResolutions.containsKey(resolutionKey)) {
+        possibleResolutions[resolutionKey] = <PluginInterfaceResolution>[];
       }
-      directDependencyResolutions[resolutionKey] = PluginInterfaceResolution(
+      possibleResolutions[resolutionKey]!.add(PluginInterfaceResolution(
         plugin: plugin,
         platform: platform,
-      );
+      ));
     }
   }
-  if (didFindError && throwOnPluginPubspecError) {
+  if (hasPubspecError && throwOnPluginPubspecError) {
     throwToolExit('Please resolve the errors');
   }
+
+  // Now resolve all the possible resolutions to a single option for each
+  // plugin, or throw if that's not possible.
+  bool hasResolutionError = false;
   final List<PluginInterfaceResolution> finalResolution = <PluginInterfaceResolution>[];
-  for (final MapEntry<String, PluginInterfaceResolution> resolution in directDependencyResolutions.entries) {
-    if (resolution.value.plugin.isDirectDependency) {
-      finalResolution.add(resolution.value);
-    } else if (defaultImplementations.containsKey(resolution.key)) {
-      // Pick the default implementation.
-      if (defaultImplementations[resolution.key] == resolution.value.plugin.name) {
-        finalResolution.add(resolution.value);
+  for (final MapEntry<String, List<PluginInterfaceResolution>> entry in possibleResolutions.entries) {
+    final List<PluginInterfaceResolution> candidates = entry.value;
+    // If there's only one candidate, use it.
+    if (candidates.length == 1) {
+      finalResolution.add(candidates.first);
+      continue;
+    }
+    // Next, try direct dependencies of the resolving application.
+    final Iterable<PluginInterfaceResolution> directDependencies = candidates.where((PluginInterfaceResolution r) {
+      return r.plugin.isDirectDependency;
+    });
+    if (directDependencies.isNotEmpty) {
+      if (directDependencies.length > 1) {
+        globals.printError(
+          'Plugin ${entry.key} has conflicting direct dependency implementations:\n'
+          '${directDependencies.map((PluginInterfaceResolution r) => '  ${r.plugin.name}\n').join()}'
+          'To fix this issue, remove all but one of these dependencies from pubspec.yaml.\n'
+        );
+        hasResolutionError = true;
+      } else {
+        finalResolution.add(directDependencies.first);
+      }
+      continue;
+    }
+    // Next, defer to the default implementation if there is one.
+    final String? defaultPackageName = defaultImplementations[entry.key];
+    if (defaultPackageName != null) {
+      final int defaultIndex = candidates
+          .indexWhere((PluginInterfaceResolution r) => r.plugin.name == defaultPackageName);
+      if (defaultIndex != -1) {
+        finalResolution.add(candidates[defaultIndex]);
+        continue;
       }
     }
+    // Otherwise, require an explicit choice.
+    if (candidates.length > 1) {
+      globals.printError(
+        'Plugin ${entry.key} has multiple possible implementations:\n'
+        '${candidates.map((PluginInterfaceResolution r) => '  ${r.plugin.name}\n').join()}'
+        'To fix this issue, add one of these dependencies to pubspec.yaml.\n'
+      );
+      hasResolutionError = true;
+      continue;
+    }
+  }
+  if (hasResolutionError) {
+    throwToolExit('Please resolve the errors');
   }
   return finalResolution;
 }

packages/flutter_tools/lib/src/plugins.dart

@@ -418,4 +418,9 @@ class PluginInterfaceResolution {
       'dartClass': plugin.pluginDartClassPlatforms[platform] ?? '',
     };
   }
+
+  @override
+  String toString() {
+    return '<PluginInterfaceResolution ${plugin.name} for $platform>';
+  }
 }