Add configuration provider and simple provider functions

.gitignore

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+# Files and directories created by pub
+.dart_tool/
+.packages
+
+# Conventional directory for build outputs
+build/
+
+# Directory created by dartdoc
+doc/api/

CHANGELOG.md

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+## 1.0.0
+
+- Initial version, created by Stagehand
+- Add configurable interface, created by @SomboChea

README.md

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+# Dart Configurable Environment
+- [ ] Allow to get property from env file
+- [ ] Cache property for runtime 

analysis_options.yaml

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+# Defines a default set of lint rules enforced for
+# projects at Google. For details and rationale,
+# see https://github.com/dart-lang/pedantic#enabled-lints.
+include: package:pedantic/analysis_options.yaml
+
+# For lint rules and documentation, see http://dart-lang.github.io/linter/lints.
+# Uncomment to specify additional rules.
+# linter:
+#   rules:
+#     - camel_case_types
+
+analyzer:
+#   exclude:
+#     - path/to/excluded/files/**

bin/configurable.dart

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+void main(List<String> arguments) {
+  print('Just, say Hello to You!');
+}

lib/configuration_provider.dart

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+abstract class ConfigurationProvider {
+  bool containsKey(String key);
+  String? getOrNull(String key, {String? defaultValue});
+}

lib/simple_configuration_provider.dart

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+import 'package:configurable/configuration_provider.dart';
+
+class SimpleConfigurationProvider implements ConfigurationProvider {
+  Map<String, String?> configs = {};
+
+  @override
+  String? getOrNull(String key, {String? defaultValue}) {
+    var value = configs[key];
+
+    if (value == null) {
+      configs[key] = defaultValue;
+      return defaultValue;
+    }
+
+    return value;
+  }
+
+  @override
+  bool containsKey(String key) {
+    return configs.containsKey(key);
+  }
+}

lib/system_config.dart

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+import 'package:configurable/configuration_provider.dart';
+import 'package:configurable/simple_configuration_provider.dart';
+
+class SystemConfig {
+  static ConfigurationProvider? provider;
+
+  static void setProvider(ConfigurationProvider provider) {
+    SystemConfig.provider = provider;
+  }
+
+  static ConfigurationProvider getProvider() {
+    if (provider == null) {
+      setProvider(SimpleConfigurationProvider());
+    }
+
+    return provider!;
+  }
+
+  static String get(String key, {String? defaultValue}) {
+    return getOrNull(key, defaultValue: defaultValue)!;
+  }
+
+  static String? getOrNull(String key, {String? defaultValue}) {
+    return getProvider().getOrNull(key);
+  }
+}

pubspec.lock

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+# Generated by pub
+# See https://dart.dev/tools/pub/glossary#lockfile
+packages:
+  pedantic:
+    dependency: "direct dev"
+    description:
+      name: pedantic
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.11.0"
+sdks:
+  dart: ">=2.12.0 <3.0.0"

pubspec.yaml

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+name: configurable
+description: A simple command-line application.
+# version: 1.0.0
+# homepage: https://www.example.com
+
+environment:
+  sdk: '>=2.12.0 <3.0.0'
+
+#dependencies:
+#  path: ^1.7.0
+
+dev_dependencies:
+  pedantic: ^1.9.0