RUMM-1870 Improve documentation

Sources/Datadog/Core/Persistence/DataEncryption.swift

@@ -18,10 +18,10 @@ public protocol DataEncryption {
 
     /// Decrypts given `Data` with user-chosen encryption.
     ///
-    /// Beware that data to decrypt could be encrypted in a previous
-    /// app launch, so implementation should be aware of the case when decryption could
-    /// fail (for example, key used for encryption is different from key used for decryption, if
-    /// they are unique for every app launch).
+    /// Beware that data to decrypt could be encrypted in a previous app launch, so
+    /// implementation should be aware of the case when decryption could fail (for example,
+    /// key used for encryption is different from key used for decryption, if they are unique
+    /// for every app launch).
     ///
     /// - Parameter data: Data to decrypt.
     /// - Returns: The decrypted data.

Sources/Datadog/Core/Persistence/Reading/FileReader.swift

@@ -38,7 +38,7 @@ internal final class FileReader: Reader {
         }
 
         do {
-            let fileData = try decode(data: file.read())
+            let fileData = try decrypt(data: file.read())
             let batchData = dataFormat.prefixData + fileData + dataFormat.suffixData
             return Batch(data: batchData, file: file)
         } catch {
@@ -47,7 +47,16 @@ internal final class FileReader: Reader {
         }
     }
 
-    func decode(data: Data) -> Data {
+    /// Decrypts data if encryption is available.
+    ///
+    /// When encryption is provided, the data is splitted using data-format separator, each slices
+    /// is then decoded from base64 and decrypted. Data is finally re-joined with data-format separator.
+    ///
+    /// If no encryption, the data is returned.
+    ///
+    /// - Parameter data: The data to decrypt.
+    /// - Returns: Decrypted data.
+    private func decrypt(data: Data) -> Data {
         guard let encryption = encryption else {
             return data
         }

Sources/Datadog/Core/Persistence/Writing/FileWriter.swift

@@ -49,7 +49,13 @@ internal final class FileWriter: Writer {
         }
     }
 
-    func encode<T: Encodable>(value: T) throws -> Data {
+    /// Encodes the given encodable valud and encrypt it if encryption is available.
+    ///
+    /// If encryption is available, encryption result is base64 encoded,
+    ///
+    /// - Parameter value: The value to encode.
+    /// - Returns: Data representation of the value.
+    private func encode<T: Encodable>(value: T) throws -> Data {
         let data = try jsonEncoder.encode(value)
 
         guard let encryption = encryption else {