Merge pull request #165 from loki-project/database-3

Add Storage Utility

SignalServiceKit/src/Loki/Database/Storage.swift

@@ -0,0 +1,32 @@
+
+/// Some important notes about YapDatabase:
+///
+/// • Connections are thread-safe.
+/// • Executing a write transaction from within a write transaction is NOT allowed.
+@objc(LKStorage)
+public final class Storage : NSObject {
+
+    private static var owsStorage: OWSPrimaryStorage { OWSPrimaryStorage.shared() }
+
+    /// Some important points regarding reading from the database:
+    ///
+    /// • Background threads should use `OWSPrimaryStorage`'s `dbReadPool`, whereas the main thread should use `OWSPrimaryStorage`'s `uiDatabaseConnection` (see the `YapDatabaseConnectionPool` documentation for more information).
+    /// • Multiple read transactions can safely be executed at the same time.
+    @objc(readWithBlock:)
+    public static func read(with block: @escaping (YapDatabaseReadTransaction) -> Void) {
+        let isMainThread = Thread.current.isMainThread
+        let connection = isMainThread ? owsStorage.uiDatabaseConnection : owsStorage.dbReadConnection
+        connection.read(block)
+    }
+
+    /// Some important points regarding writing to the database:
+    ///
+    /// • There can only be a single write transaction per database at any one time, so all write transactions must use `OWSPrimaryStorage`'s `dbReadWriteConnection`.
+    /// • Executing a write transaction from within a write transaction causes a deadlock and must be avoided.
+    @objc(writeWithBlock:)
+    public static func write(with block: @escaping (YapDatabaseReadWriteTransaction) -> Void) {
+        // TODO: Right now this is kind of pointless, but the idea is to eventually
+        // somehow manage nested write transactions in this class.
+        owsStorage.dbReadWriteConnection.readWrite(block)
+    }
+}