GRDB.swift: DatabaseValueConvertible use throws methods instead of properties

@kdubb Your questions did ring a bell in me. Although I still stand by non-failable DatabaseValueConvertible and RowConvertible conversion protocols, I have to admit that the state of handling errors that happen when reading from the database lacks a lot:

// A chore
let request = Message.all()
let (statement, adapter) = try request.prepare(db)
let rows = Row.fetch(statement, adapter: adapter)
let iterator = rows.makeIterator()
while let row = try iterator.step() {
    let message = try Message(row: row)
}

That’s why I’ve pushed to the DatabaseCursor branch support for a new type: DatabaseCursor.

Unlike DatabaseSequence which can not throw (because of Swift), DatabaseCursor throws whenever it can.

The code above can now read:

let request = Message.all()
let cursor = try Row.fetchCursor(db, request)
while let row = try cursor.step() {
    let message = try Message(row: row)
}

Of course, with records that can always initialize from rows, it’s even shorter:

let cursor = try Message.fetchCursor(db)
while let message = try cursor.step() {
    ...
}

The fetchCursor() methods completes the list of fetching methods:

try Type.fetchCursor(...) // DatabaseCursor<Type>
Type.fetch(...)           // DatabaseSequence<Type>
Type.fetchAll(...)        // [Type]
Type.fetchOne(...)        // Type?

I think this branch may help you.