[jni] Add the ability to check if a JObject is a subclass of a Java class. (#1943)

Author: brianquinlan

pkgs/jni/CHANGELOG.md

@@ -9,6 +9,8 @@
   instead use the default value of `build/jni_libs`.
 - Added `JArray.of`, which allows a `JArray` to be constructed from an
   `Iterable`.
+- Added `JObject.isA`, which checks whether a `JObject` is a instance of a
+  java class.
 
 ## 0.13.0
 

pkgs/jni/lib/src/jobject.dart

@@ -9,6 +9,18 @@ import '../jni.dart';
 import 'jreference.dart';
 import 'types.dart';
 
+// Error thrown when casting between incompatible `JObject` subclasses.
+final class CastError extends Error {
+  final String _message;
+
+  CastError(this._message);
+
+  @override
+  String toString() {
+    return _message;
+  }
+}
+
 final class JObjectNullableType extends JObjType<JObject?> {
   @internal
   const JObjectNullableType();
@@ -125,22 +137,36 @@ class JObject {
     reference.release();
   }
 
+  /// Whether this object is of the given [type].
+  ///
+  /// For example:
+  ///
+  /// ```dart
+  /// if (object.isA(JLong.type)) {
+  ///   final i = object.as(JLong.type).longValue;
+  ///   ...
+  /// }
+  /// ```
+  bool isA<T extends JObject?>(JObjType<T> type) {
+    final targetJClass = type.jClass.reference.toPointer();
+    final canBeCasted = Jni.env.IsInstanceOf(reference.pointer, targetJClass);
+    Jni.env.DeleteGlobalRef(targetJClass);
+    return canBeCasted;
+  }
+
   /// Casts this object to another [type].
   ///
   /// If [releaseOriginal] is `true`, the casted object will be released.
+  ///
+  /// Throws [CastError] if this object is not an instance of [type].
   T as<T extends JObject?>(
     JObjType<T> type, {
     bool releaseOriginal = false,
   }) {
-    assert(
-      () {
-        final jClass = type.jClass.reference.toPointer();
-        final canBeCasted = Jni.env.IsInstanceOf(reference.pointer, jClass);
-        Jni.env.DeleteGlobalRef(jClass);
-        return canBeCasted;
-      }(),
-      'The object must be of type "${type.signature}".',
-    );
+    if (!isA(type)) {
+      throw CastError('not a subtype of "${type.signature}"');
+    }
+
     if (releaseOriginal) {
       final ret = type.fromReference(JGlobalReference(reference.pointer));
       reference.setAsReleased();

pkgs/jni/test/jobject_test.dart

@@ -277,6 +277,24 @@ void run({required TestRunnerCallback testRunner}) {
     expect(maxLong, maxLongInJava);
   });
 
+  testRunner('isA returns true', () {
+    final long = JLong(1);
+    expect(long.isA(JLong.type), isTrue);
+    expect(long.isA(JLong.nullableType), isTrue);
+    expect(long.isA(JNumber.type), isTrue);
+    expect(long.isA(JNumber.nullableType), isTrue);
+    expect(long.isA(JObject.type), isTrue);
+    expect(long.isA(JObject.nullableType), isTrue);
+  });
+
+  testRunner('isA returns false', () {
+    final long = JLong(1);
+    expect(long.isA(JInteger.type), isFalse);
+    expect(long.isA(JInteger.nullableType), isFalse);
+    expect(long.isA(JString.type), isFalse);
+    expect(long.isA(JString.nullableType), isFalse);
+  });
+
   testRunner('Casting correctly succeeds', () {
     final long = JLong(1);
     final long2 = long.as(JLong.type, releaseOriginal: true);
@@ -287,7 +305,8 @@ void run({required TestRunnerCallback testRunner}) {
     final long = JLong(1);
     expect(
       () => long.as(JInteger.type, releaseOriginal: true),
-      throwsA(isA<AssertionError>()),
+      throwsA(isA<CastError>().having(
+          (e) => e.toString(), 'toString()', contains('java/lang/Integer'))),
     );
   });