CollorAPI.BuilderDecorationBlock.swift

public func startDecorationBlock(type: DecorationBlockType) -> DecorationBlock {
    let startindex = cells.indices.last.flatMap { $0 + 1 } ?? 0 // start the decorationView after the last cell
    let decorationBlock = DecorationBlock(type: type, startItemIndex: startindex, sectionDescriptor: sectionDescriptor)
    return decorationBlock
}

public func endDecorationBlock(_ block: DecorationBlock?) {
    guard let block = block else {
        return
    }
    // error if empty section
    guard let lastIndex = cells.indices.last else {
        fatalError("End block in a no empty section")
    }
    // check if startItemIndex does exist
    guard cells.indices.contains(block.startItemIndex) else {
        fatalError("Add a cell after startDecorationBlock(:) and before calling endDecorationBlock()")
    }

    block.endItemIndex = lastIndex
    decorationBlocks.append(block)
}