集合视图单元格背景颜色在 reloadData() Swift/Firebase 上反转

Question

NOTE - This question was edited significantly on 22/08/2020. Any answers or comments before this edit may not (or may still) make sense. As the question (and possibly the existing answers) may be valid, I don't believe deleting the question is of benefit to others.

=======================================

SWIFT 5

---

The Problem

I have a collection view, which is filled with data from Firebase. As part of this data, a "colour" is passed for each item. This colour is an int, which the cell looks up in a table and draws.

A fragment of the code used to assign the colour:

let purpleGrad = [#colorLiteral(red: 0.4392156863, green: 0.4193316882, blue: 1, alpha: 1),#colorLiteral(red: 0.4392156863, green: 0.4193316882, blue: 1, alpha: 1)]
let blueGrad = [#colorLiteral(red: 0.2392156869, green: 0.6745098233, blue: 0.9686274529, alpha: 1),#colorLiteral(red: 0.2392156869, green: 0.6745098233, blue: 0.9686274529, alpha: 1)]
let pinkGrad = [#colorLiteral(red: 0.9098039269, green: 0.4784313738, blue: 0.6431372762, alpha: 1),#colorLiteral(red: 0.9098039269, green: 0.4784313738, blue: 0.6431372762, alpha: 1)]
let yellowGrad = [#colorLiteral(red: 0.7254902124, green: 0.4784313738, blue: 0.09803921729, alpha: 1),#colorLiteral(red: 0.7254902124, green: 0.4784313738, blue: 0.09803921729, alpha: 1)]
        
let colorsTable2: [Int: [UIColor]] = [0: purpleGrad, 1: blueGrad, 2: pinkGrad, 3: yellowGrad, 4: greenBlue]

When the collection view first draws, the colours show in the correct order. When the prices update from Apple (when reloadData() is called), the collection view then draws backwards.

In the following picture, the left side is how the collection view loads before the prices (which is correct, and always loads with the correct colours). The right side is after the prices have loaded (which is a reversal of the left side, and always loads this exact way).

---

Possible Solutions

Previous comments suggested dictionaries may be the cause of this error - whilst this may still be the case, I do not believe this is the issue. The ordering is always correct, and the colours ALWAYS reverse exactly, rather than just redrawing with random colours.

I am unsure as to what else to try - to my knowledge, reloadData() is required to update the collection view when the price is pulled.

Answer 1

Sometimes when tableview cells get messy by refreshing or scrolling, it is because there is no default value for data which is changing in each cell. So you might want to consider a primary value for color Int (like 0) and set cell's background color to nil, before fetching and setting actual values.

Answer 2

This is a wild guess because there's too much code but you're using a lot of NSDictionaries (please use Swift Dictionary).

Dictionaries are unordered pairs so you cannot rely on the order of the data when iterating over it. So for example here:

let course = course as? NSDictionary
for (courseID, courseData) in course! {
   let courseData = courseData as? NSDictionary ?? NSDictionary()
   let subjects = courseData["subjects"] ?? NSDictionary()

That's going to iterate over the data in course potentially different each time it's run (the child data is unordered).

So you probably have some disconnect between the ordering of the data and getting the associated background color.

EDIT

The data returned from Firebase is being cast to a Dictionary by your code, that will then loose the ordering.

let downloadedCourses = snapshot.value as? NSDictionary

If you want to maintain the ordering and use the child data as snapshots (recommended) do this

let orderedCourses = snapshot.children.allObjects as! [DataSnapshot]