Update Plist data without erasing old data - ios

Question

I'm trying to make a file downloader application using swift and cocoa. I am using plist for the download history. Reading the data works however, writing the data will erase the previous data and replace it for the new data.
Here is the code
let newdownloaditem = downloadList(root: [downloadListt(downloadURL: response.url!.absoluteString, fileName: response.suggestedFilename!)])
// This is a codeable method
let encoder = PropertyListEncoder()
encoder.outputFormat = .xml
let pListFilURL = uniqueDataDir()?.appendingPathComponent("downloads.plist")
do {
let data = try encoder.encode(newdownloaditem)
try data.write(to: pListFilURL!)
// Here is the problem
} catch {
print(error)
}
// Here is the codeable
public struct downloadList: Codable {
let root: [downloadListt]
}
public struct downloadListt: Codable {
let downloadURL: String
let fileName: String
}
Here is an image of what happens
The contents got erased
Thanks!

Answer 1

You are indeed replacing the previous data with your new one.
You need to retrieve the previous data.
Append your new data to it.
Save that combination
let newItem = downloadListt(downloadURL: response.url!.absoluteString,
fileName: response.suggestedFilename!)
var allItems: [downloadListt] = []
allItems.append(contentsOf: previousList.root)
allitems.append(newItem)
let newList = downloadList(root: allItems)
...
let data = try encoder.encode(newList)
try data.write(to: pListFilURL!)
Unrelated but recommended (it's convention):
You should starting naming your struct/classes with an uppercase: downloadList => DownloadList
I would avoid naming downloadListt, it's unredable it's hard at first look to make a difference between downloadListt and downloadList. Instead, name it maybe DownloadItem. More readable.