iOS 14, Swift 5.x
I watched this excellent WWDC from 2018
https://developer.apple.com/videos/play/wwdc2018/223/
And I wrote a shapes editor... and have been trying to use partition as Dave in the video says you should. I got the first three to work, but the last one I had to use a loop- cannot for the life of me figure out how to get it to work with partition.
Can someone see how I might do this?
The first method moves the selected object to the end of the list, works perfectly.
func bringToFrontEA() {
let subset = objects.partition(by: { $0.selected })
let selected = objects[subset...]
let unselected = objects[..<subset]
let reordered = unselected selected
objects = Array(reordered)
}
The second method moves the selected object to the front of the list. Works prefectly.
func sendToBackEA() {
let subset = objects.partition(by: { !$0.selected })
let selected = objects[subset...]
let unselected = objects[..<subset]
let reordered = unselected selected
objects = Array(reordered)
}
The third method moves the element just one element back in the list. Works perfectly.
func sendBackEA() {
if let i = objects.firstIndex(where: { $0.selected }) {
if i == 0 { return }
let predecessor = i - 1
let shapes = objects[predecessor...].partition(by: { !$0.selected })
let slice = objects[predecessor...]
let row = objects[..<predecessor]
let selected = Array(slice[..<shapes])
let unselected = Array(slice[shapes...])
objects = row selected unselected
}
}
The last method moves the element forward in the list, works perfectly... but unlike the other methods it will not scale as described in the WWDC video.
func bringForwardEA() {
let indexes = objects.enumerated().filter { $0.element.selected == true }.map{$0.offset}
for i in indexes {
if objects[i 1].unused {
return
}
objects.swapAt(i 1, i)
}
}
Objects is an array of shapes with a property indicating if it is selected or not. I want to exchange the loop in the last method by using a partition as I did in the first three. It needs to work for one or more selected shapes.
CodePudding user response:
Looking at the WWDC video, it appears that what you are calling sendBackEA
is what WWDC calls bringForward
, and what you are calling bringForwardEA
is what WWDC calls sendBack
.
Just like how you move the first selected element forward one index (index decreases) in sendBackEA
, then move all the other selected elements to immediately after that first selected element. bringForwardEA
should do the reverse: move the last selected element backward one index (index increases), then move all the other selected elements to immediately before the last selected element. (See circa 19:10 in the video)
You seem to have confused yourself by trying to increase the indices of all the selected index by 1. This obviously cannot be done with a partition in general.
Also note that partition(by:)
already modifies the collection, you don't need to get each partition, then recombine.
Your 4 methods can be written like this:
func bringToFrontEA() {
objects.partition(by: { $0.selected })
}
func sendToBackEA() {
objects.partition(by: { !$0.selected })
}
func sendBackEA() {
if let i = objects.indices.first(where: { objects[$0].selected }) {
if i == 0 { return }
let predecessor = i - 1
objects[predecessor...].partition(by: { !$0.selected })
}
}
func bringForwardEA() {
if let i = objects.indices.last(where: { objects[$0].selected }) {
if i == objects.indices.last { return }
let successor = i 1
objects[...successor].partition(by: { !$0.selected })
}
}
Notice the symmetry between sendBackEA
and bringForwardEA
.