i have an question about the Mirror reflection .
i convert my struct to mirror to iterate through all the properties to get values and after i iterate through it and change the values in properties i need to convert mirror again to the original struct with values which i edited but i can't , is swift language have way to do this conversion ?
the code below
//MARK:- loop get tags
func getTags(filter: Any){
let getTags = Mirror(reflecting: filter)
for (tag) in getTags.children {
if let getTag = tag.value as? String {
if let _ = Int(getTag) {
}else {
if getTag != "" && getTag != "All" {
arrayOfTags.append(getTag)
}
}
}// if let
}// end for loop
}
thanks
You can't construct a struct without hardcore memory manipulation. You could create objets with functions that are still available from Objective C. You could set the property with the setValue forKey function. Your objects needs to be derived from NSObject.
Doing this and taking into account all scenario's is quite a challenge. There is a CocoaPod library that could help with this. Have a look at EVReflection You could create a dictionary from your object and an object from your dictionary.
it's been asked a while a go.. but just in case I think you may be looking for https://cocoapods.org/pods/EVReflection
Related
I'm learning how to store custom types in AppStorage, and came across an issue. In this simplified example, I'm trying to save an empty Int array to AppStorage once the view is created.
The following code gives me the error, No exact matches in call to initializer . I know that this error usually means there are mismatching types somewhere, but I'm not sure what the types should be, or how to fix it.
struct test: View {
init() {
let emptyList = [Int]()
guard let encodedList = try? JSONEncoder().encode(emptyList) else { return }
self.storedList = encodedList
}
#AppStorage("stored_list") var storedList: Data //NO EXACT MATCHES TO CALL IN INITIALIZER
//"body" implementation not shown
}
Why is this error occurring, and how can I fix it?
It should be either with default value or optional, so correct variants are
#AppStorage("stored_list") var storedList: Data = Data()
or
#AppStorage("stored_list") var storedList: Data?
I have a global var notes: Set<Note> that contains notes initialized with downloaded data.
In the code below, does Swift know to skip the initialization of my Note object if notes already contains it?
for dictionary in downloadedNoteDictionaries {
let note = Note(dictionary: dictionary)
notes.insert(note)
}
I'm wondering because my app downloads dozens of notes per request and initializing a Note object seems rather computationally expensive.
If the answer to my question is no, then how could I improve my code's performance?
My Note class—which I just realized should probably be a struct instead—has the property let id: Int64 as its sole essential component, but apparently, you can't access an element of a set by its hash value? I don't want to use Set's instance method first(where:) because it has a complexity of O(n), and notes could contain millions of Note objects.
You cannot rely on Swift to eliminate the construction of a new Note in your code. Your Set needs to ask the Note for its hashValue, and may need to call == with your Note as an argument. Those computations require the Note object. Possibly if Swift can inline everything, it can notice that your hashValue and == depend only on the id property, but it is certainly not guaranteed to notice or to act on that information.
It sounds like you should be using an [Int64: Note] instead of a Set<Note>.
No, Swift will not avoid creating the new Note object. The problem here is trying to determine if an object already exists in a set. In order to check if an object already exists in the set, you must have some way to identify this object consistently and have that identification persist across future reads and writes to that set. Assuming we want to adopt Swift's hashing enhancements which deprecates the old methods for having to manually provide a hashValue for any object conforming to the Hashable, we should not use a Set as a solution to this problem. The reason is because Swift's new recommended hashing methods use a random seed to generate hashes for added security. Depending on hash values to identify an element in a set alone would therefore not be possible.
It seems that your method of identifying these objects are by an id. I would do as Rob suggests and use a dictionary where the keys are the id. This would help you answer existential questions in order avoid instantiating a Note object.
If you need the resulting dictionary as a Set, you can still create a new Set from this resulting dictionary sequence.
It is possible to pull out a particular element from a set as long as you know how to identify it, and the operations would be O(1). You would use these two methods:
https://developer.apple.com/documentation/swift/set/2996833-firstindex
https://developer.apple.com/documentation/swift/set/2830801-subscript
Here is an example that I was able to run in a playground. However, this assumes that you have a way to identify in your data / model the id of the Note object beforehand in order to avoid creating the Note object. In this case, I am assuming that a Note shares an id with a Dictionary it holds:
import UIKit
struct DictionaryThatNoteUses {
var id: String
init(id: String = UUID().uuidString) {
self.id = id
}
}
struct Note: Hashable {
let dictionary: DictionaryThatNoteUses
var id: String {
return dictionary.id
}
func hash(into hasher: inout Hasher) {
hasher.combine(id)
}
static func == (lhs: Note, rhs: Note) -> Bool {
return lhs.id == rhs.id
}
}
var downloadedNoteDictionaries: [DictionaryThatNoteUses] = [
DictionaryThatNoteUses(),
DictionaryThatNoteUses(),
DictionaryThatNoteUses()
]
var notesDictionary: [String: Note] = [:]
var notesSet: Set<Note> = []
// Add a dictionary with the same id as the first dictionary
downloadedNoteDictionaries.append(downloadedNoteDictionaries.first!) // so now there are four Dictionary objects in this array
func createDictionaryOfNotes() {
func avoidCreatingDuplicateNotesObject(with id: String) {
print("avoided creating duplicate notes object with id \(id)") // prints once because of the duplicated note at the end matching the first, so a new Note object is not instantiated
}
downloadedNoteDictionaries.forEach {
guard notesDictionary[$0.id] == nil else { return avoidCreatingDuplicateNotesObject(with: $0.id) }
let note = Note(dictionary: $0)
notesDictionary[note.id] = note
}
}
createDictionaryOfNotes()
// Obtain a set for set operations on those unique Note objects
notesSet = Set<Note>(notesDictionary.values)
print("number of items in dictionary = \(notesDictionary.count), number of items in set = \(notesSet.count)") // prints 3 and 3 because the 4th object was a duplicate
// Grabbing a specific element from the set
// Let's test with the second Note object from the notesDictionary
let secondNotesObjectFromDictionary = notesDictionary.values[notesDictionary.values.index(notesDictionary.values.startIndex, offsetBy: 1)]
let thirdNotesObjectFromDictionary = notesDictionary.values[notesDictionary.values.index(notesDictionary.values.startIndex, offsetBy: 2)]
if let secondNotesObjectIndexInSet = notesSet.firstIndex(of: secondNotesObjectFromDictionary) {
print("do the two objects match: \(notesSet[secondNotesObjectIndexInSet] == secondNotesObjectFromDictionary)") // prints true
print("does the third object from dictionary match the second object from the set: \(thirdNotesObjectFromDictionary == notesSet[secondNotesObjectIndexInSet])") // prints false
}
In my app I translate objects from custom classes into dictionaries so that they can be saved locally in a plist as well as on a server. I use the following to turn the properties of a class into a dictionary:
func dictionary() -> [String : Any] {
var count: UInt32 = 0;
let myClass: AnyClass = self.classForCoder;
let properties = class_copyPropertyList(myClass, &count);
var dictionaryRepresentation: [String:Any] = [:]
for i in 0..<count {
let property = properties![Int(i)]
let cStringKey = property_getName(property);
let key = String(cString: cStringKey!)
dictionaryRepresentation[key] = self.value(forKey: key) as Any
}
return dictionaryRepresentation
}
I have a problem, however, with computed properties. It seems that those are computed and the returned value gets put into the dictionary as well, which I would like to avoid. So here is my question:
Is it possible to check whether is a property computed programatically using only its name?
I am assuming this could be possible by trying to assign a value to it which would give me an error or some similar approach.
Here is what seems to be a working solution, based on suggestion by dasblinkenlight.
Rather than using the Objective-C method outlined above, create a Mirror of the class which has a children made up of all settable properties, therefore excluding computables.
Used like this:
let mirror = Mirror(reflecting: MyObject)
for case let (label?, value) in mirror.children {
print (label, value)
}
Here label is the name of the variable and value is obviously the value.
EDIT: In case anyone wants to convert objects into dictionary, I am posting the full code here as well. Do however remember that if values are custom objects as well, those will need to be converted too.
func dictionary() -> [String:Any] {
let mirror = Mirror(reflecting: self)
var dictionaryRepresentation = [String:Any]()
for case let (label, value) in mirror.children {
guard let key = label else { continue }
dictionaryRepresentation[key] = value
}
return dictionaryRepresentation
}
You can try property_copyAttributeList(_:_:) function, it may contain a read-only marker for swift's computed properties. Although I guess let properties also will have that marker, so you must find a way to differ them.
I have an array of data that doesn't need to be a managed List, meaning I don't need Realm to create a new model for the items with links and the ability to query on the items, etc. I just want a simple array, typically of primitives that don't inherit from Object anyway, that will be persisted with my main object.
The only solution I can think of is to use NSData and NSKeyedArchiver/NSKeyedUnarchiver. Is that the best/only way to do this? Should I just use List even if I don't think I'll need it — what's the best practice for this situation?
Realm doesn't support arrays of primitives (although that functionality is coming soon), so the most straightforward solution is to use a List filled with model objects that just wrap your primitives. There's nothing wrong with archiving to and from NSData and storing the data in your Realm model, though, if you feel that better suits your particular use case.
Here's how I decided to deal with this:
var instructions: [String] {
get {
if _instructions == nil {
_instructions = NSKeyedUnarchiver.unarchiveObject(with: instructionsData) as? [String]
}
return _instructions!
}
set {
instructionsData = NSKeyedArchiver.archivedData(withRootObject: newValue)
_instructions = newValue
}
}
fileprivate var _instructions: [String]? = nil
dynamic var instructionsData = Data()
override static func ignoredProperties() -> [String] {
return ["instructions", "_instructions"]
}
That way I can use the array as I normally would and still have it persisted in a simple way (without having to create an actual List and having to manage a bunch of new models/objects).
var objects:RLMCollection!
if selectedObject != nil {
objects = selectedObject.childobjects as RLMArray
} else {
objects = RealmObject.objectsInRealm(realm, "isFavourite == 1")
}
println(couplets.objectAtIndex(indexPath.row) as! RealmObject)
I had a senario, where I won't know whether I need to work with RLMArray or RLMResults, so I used RLMCollection.
How can I achieve the same in RealmSwift 0.93.1
In general, in Swift it's good enough to work with a CollectionType, which defines subscripting, enumeration, and many of the other 'array-like' methods that Results and List expose. (Though we are interested in adding a RealmCollectionType protocol, just haven't gotten around to it yet!)