Introduced Batch

[sgr-ksmt]

Introduced Batch class. It can avoid try keyword on create/update compared to Batch+Extension.

• Before

let batch = Firestore.firestore().batch()
try! batch.create(user)
try! batch.create(task)

• After

let batch = Batch()
    .create(user)
    .create(task) // It can method chain style.

batch.commit{ error in
    // Emit error if extracting snapshot failed or `commit` failed.
}