Redesign the request protocol hierarchy:

DatabaseRequest -> SelectStatementRequest -> FetchRequest

This commit has two goals:

- introduce a protocol without any associated type which can feed RxGRDB multiple-requests observation methods without user pain (RxSwiftCommunity/RxGRDB#27 (comment))
- introduce a protocol that can feed RxGRDB observers and is able to execute multiple SQL queries (the eventual future "fetch all authors with all their books" request).

GRDB/Core/FetchRequest.swift

@@ -1,38 +1,38 @@
-/// The protocol for all types that define a way to fetch database content.
-///
-///     struct Player: FetchableRecord { ... }
-///     let request: ... // Some FetchRequest that fetches Player
-///     try request.fetchCursor(db) // Cursor of Player
-///     try request.fetchAll(db)    // [Player]
-///     try request.fetchOne(db)    // Player?
-public protocol FetchRequest {
-    /// The type that can convert raw database rows to fetched values
-    associatedtype RowDecoder
-
+// MARK: - DatabaseRequest
+
+/// The protocol for all types that request database values.
+public protocol DatabaseRequest {
+    /// The database region that the request looks into.
+    ///
+    /// - parameter db: A database connection.
+    func fetchedRegion(_ db: Database) throws -> DatabaseRegion
+}
+
+// MARK: - SelectStatementRequest
+
+/// The protocol for all requests that run from a single select statement.
+public protocol SelectStatementRequest: DatabaseRequest {
     /// A tuple that contains a prepared statement that is ready to be
     /// executed, and an eventual row adapter.
+    ///
+    /// - parameter db: A database connection.
+    /// - returns: A prepared statement and an eventual row adapter.
     func prepare(_ db: Database) throws -> (SelectStatement, RowAdapter?)
 
     /// The number of rows fetched by the request.
     ///
     /// Default implementation builds a naive SQL query based on the statement
     /// returned by the `prepare` method: `SELECT COUNT(*) FROM (...)`.
     ///
-    /// Adopting types can refine this countRequest method and return more
+    /// Adopting types can refine this method in order to use more
     /// efficient SQL.
     ///
     /// - parameter db: A database connection.
     func fetchCount(_ db: Database) throws -> Int
-
-    /// The database region that the request looks into.
-    ///
-    /// This method has a default implementation.
-    ///
-    /// - parameter db: A database connection.
-    func fetchedRegion(_ db: Database) throws -> DatabaseRegion
 }
 
-extension FetchRequest {
+extension SelectStatementRequest {
+    // Default implementation
     /// The number of rows fetched by the request.
     ///
     /// This default implementation builds a naive SQL query based on the
@@ -45,6 +45,7 @@ extension FetchRequest {
         return try Int.fetchOne(db, sql, arguments: statement.arguments)!
     }
 
+    // Default implementation
     /// The database region that the request looks into.
     ///
     /// - parameter db: A database connection.
@@ -54,6 +55,21 @@ extension FetchRequest {
     }
 }
 
+// MARK: - FetchRequest
+
+/// The protocol for all requests that run from a single select statement, and
+/// tell how fetched rows should be interpreted.
+///
+///     struct Player: FetchableRecord { ... }
+///     let request: ... // Some FetchRequest that fetches Player
+///     try request.fetchCursor(db) // Cursor of Player
+///     try request.fetchAll(db)    // [Player]
+///     try request.fetchOne(db)    // Player?
+public protocol FetchRequest: SelectStatementRequest {
+    /// The type that tells how fetched database rows should be interpreted.
+    associatedtype RowDecoder
+}
+
 extension FetchRequest {
     /// Returns a request bound to type T.
     ///
@@ -78,6 +94,8 @@ extension FetchRequest {
     }
 }
 
+// MARK: - AdaptedFetchRequest
+
 /// An adapted request.
 public struct AdaptedFetchRequest<Base: FetchRequest> : FetchRequest {
     public typealias RowDecoder = Base.RowDecoder
@@ -113,6 +131,8 @@ public struct AdaptedFetchRequest<Base: FetchRequest> : FetchRequest {
     }
 }
 
+// MARK: - AnyFetchRequest
+
 /// A type-erased FetchRequest.
 ///
 /// An AnyFetchRequest forwards its operations to an underlying request,
@@ -166,6 +186,8 @@ public struct AnyFetchRequest<T> : FetchRequest {
     }
 }
 
+// MARK: - SQLRequest
+
 /// A Request built from raw SQL.
 public struct SQLRequest<T> : FetchRequest {
     public typealias RowDecoder = T

GRDB/QueryInterface/RequestProtocols.swift

@@ -1,3 +1,5 @@
+// MARK: - SelectionRequest
+
 /// The protocol for all requests that can refine their selection.
 public protocol SelectionRequest {
     /// Creates a request with a new net of selected columns.
@@ -59,6 +61,8 @@ extension SelectionRequest {
     }
 }
 
+// MARK: - FilteredRequest
+
 /// The protocol for all requests that can be filtered.
 public protocol FilteredRequest {
     /// Creates a request with the provided *predicate* added to the
@@ -89,6 +93,8 @@ extension FilteredRequest {
     }
 }
 
+// MARK: - AggregatingRequest
+
 /// The protocol for all requests that can aggregate.
 public protocol AggregatingRequest {
     /// Creates a request grouped according to *expressions*.
@@ -127,6 +133,8 @@ extension AggregatingRequest {
     }
 }
 
+// MARK: - OrderedRequest
+
 /// The protocol for all requests that be ordered.
 public protocol OrderedRequest {
     /// Creates a request with the provided *orderings*.
@@ -201,3 +209,4 @@ extension OrderedRequest {
         return order([expression])
     }
 }
+