[Swift] Memory usage fix (#8643)

Allows a complete reset for the underlying memory of the
_InternalByteBuffers within FlatBuffers and FlexBuffers.

tests/swift/Tests/Flatbuffers/FlatBuffersMonsterWriterTests.swift

@@ -43,6 +43,34 @@ class FlatBuffersMonsterWriterTests: XCTestCase {
     readVerifiedMonster(fb: _data)
   }
 
+  func testCreateMonsterData() {
+    let bytes = createMonster(withPrefix: false)
+    var buffer = ByteBuffer(data: bytes.data)
+    let monster: MyGame_Example_Monster = getRoot(byteBuffer: &buffer)
+    readMonster(monster: monster)
+    mutateMonster(fb: bytes.buffer)
+    readMonster(monster: monster)
+  }
+
+  func testCreateMonsterResetTests() {
+    var builder = createMonster(withPrefix: false)
+    var buffer = ByteBuffer(data: builder.data)
+    let monster: MyGame_Example_Monster = getRoot(byteBuffer: &buffer)
+    readMonster(monster: monster)
+    builder.clear()
+    XCTAssertEqual(builder.capacity, 1)
+    XCTAssertEqual(builder.size, 0)
+
+    write(fbb: &builder, prefix: false)
+    var _buffer = ByteBuffer(data: builder.data)
+    XCTAssertEqual(_buffer.capacity, 304)
+    let _monster: MyGame_Example_Monster = getRoot(byteBuffer: &_buffer)
+    readMonster(monster: _monster)
+    builder.clear(keepingCapacity: true)
+    XCTAssertEqual(builder.capacity, 512)
+    XCTAssertEqual(builder.size, 0)
+  }
+
   func testCreateMonster() {
     let bytes = createMonster(withPrefix: false)
     // swiftformat:disable all
@@ -257,6 +285,11 @@ class FlatBuffersMonsterWriterTests: XCTestCase {
 
   func createMonster(withPrefix prefix: Bool) -> FlatBufferBuilder {
     var fbb = FlatBufferBuilder(initialSize: 1)
+    write(fbb: &fbb, prefix: prefix)
+    return fbb
+  }
+
+  func write(fbb: inout FlatBufferBuilder, prefix: Bool = false) {
     let names = [
       fbb.create(string: "Frodo"),
       fbb.create(string: "Barney"),
@@ -313,7 +346,6 @@ class FlatBuffersMonsterWriterTests: XCTestCase {
     Monster.addVectorOf(testarrayoftables: sortedArray, &fbb)
     let end = Monster.endMonster(&fbb, start: mStart)
     Monster.finish(&fbb, end: end, prefix: prefix)
-    return fbb
   }
 
   func mutateMonster(fb: ByteBuffer) {

tests/swift/Tests/Flexbuffers/FlexBuffersReaderTests.swift

@@ -15,9 +15,10 @@
  */
 
 import Common
-import FlexBuffers
 import XCTest
 
+@testable import FlexBuffers
+
 final class FlexBuffersReaderTests: XCTestCase {
 
   func testReadingProperBuffer() throws {
@@ -30,6 +31,29 @@ final class FlexBuffersReaderTests: XCTestCase {
     try validate(buffer: buf)
   }
 
+  func testReset() throws {
+    var fbx = FlexBuffersWriter(
+      initialSize: 8,
+      flags: .shareKeysAndStrings)
+    write(fbx: &fbx)
+
+    try validate(buffer: ByteBuffer(data: fbx.data))
+    XCTAssertEqual(fbx.capacity, 512)
+    fbx.reset()
+    XCTAssertEqual(fbx.writerIndex, 0)
+    XCTAssertEqual(fbx.capacity, 8)
+
+    write(fbx: &fbx)
+    try validate(buffer: ByteBuffer(data: fbx.data))
+    fbx.reset(keepingCapacity: true)
+    XCTAssertEqual(fbx.writerIndex, 0)
+    XCTAssertEqual(fbx.capacity, 512)
+
+    write(fbx: &fbx)
+    try validate(buffer: ByteBuffer(data: fbx.data))
+    XCTAssertEqual(fbx.capacity, 512)
+  }
+
   private func validate(buffer buf: ByteBuffer) throws {
     let reference = try getRoot(buffer: buf)!
     XCTAssertEqual(reference.type, .map)

tests/swift/Tests/Flexbuffers/Mocks.swift

@@ -32,7 +32,11 @@ func createProperBuffer() -> FlexBuffersWriter {
   var fbx = FlexBuffersWriter(
     initialSize: 8,
     flags: .shareKeysAndStrings)
+  write(fbx: &fbx)
+  return fbx
+}
 
+func write(fbx: inout FlexBuffersWriter) {
   fbx.map { map in
     map.vector(key: "vec") { v in
       v.add(int64: -100)
@@ -57,5 +61,4 @@ func createProperBuffer() -> FlexBuffersWriter {
   }
 
   fbx.finish()
-  return fbx
 }