How to access deeply nested dictionaries in Swift

问题:

I have a pretty complex data structure in my app, which I need to manipulate. I am trying to keep track of how many types of bugs a player has in thier garden. There are ten types of bugs, each with ten patterns, each pattern having ten colors. So there are 1000 unique bugs possible, and I want to track how many of each of these types the player has. The nested dictionary looks like:

var colorsDict: [String : Int]
var patternsDict: [String : Any] // [String : colorsDict]
var bugsDict: [String : Any] // [String : patternsDict]

I do not get any errors or complaints with this syntax.

When I want to increment the player's bug collection though, doing this:

bugs["ladybug"]["spotted"]["red"]++

I get this error: String is not convertible to 'DictionaryIndex< String, Any >' with the error's carrot under the first string.

Another similar post suggested using "as Any?" in the code, but the OP of that post only had a dictionary one deep so could do that easily with: dict["string"] as Any? ...

I am not sure how to do this with a multilevel dictionary. Any help would be appreciated.

回答1:

When working with dictionaries you have to remember that a key might not exist in the dictionary. For this reason, dictionaries always return optionals. So each time you access the dictionary by key you have to unwrap at each level as follows:

bugsDict["ladybug"]!["spotted"]!["red"]!++

I presume you know about optionals, but just to be clear, use the exclamation mark if you are 100% sure the key exists in the dictionary, otherwise it's better to use the question mark:

bugsDict["ladybug"]?["spotted"]?["red"]?++

Addendum: This is the code I used for testing in playground:

var colorsDict = [String : Int]()
var patternsDict =  [String : [String : Int]] ()
var bugsDict = [String : [String : [String : Int]]] ()

colorsDict["red"] = 1
patternsDict["spotted"] = colorsDict
bugsDict["ladybug"] = patternsDict


bugsDict["ladybug"]!["spotted"]!["red"]!++ // Prints 1
bugsDict["ladybug"]!["spotted"]!["red"]!++ // Prints 2
bugsDict["ladybug"]!["spotted"]!["red"]!++ // Prints 3
bugsDict["ladybug"]!["spotted"]!["red"]! // Prints 4

回答2:

Another option: You could try calling dict.value( forKeyPath: "ladybug.spotted.red" )!

回答3:

My primary use case was reading ad-hoc values from a deep dictionary. None of the answers given worked for me in my Swift 3.1 project, so I went looking and found Ole Begemann's excellent extension for Swift dictionaries, with a detailed explanation on how it works.

I've made a Github gist with the Swift file I made for using it, and I welcome feedback.

To use it, you can add the Keypath.swift into your project, and then you can simply use a keyPath subscript syntax on any [String:Any] dictionary as follows.

Considering you have a JSON object like so:

{
    "name":"John",
    "age":30,
    "cars": {
        "car1":"Ford",
        "car2":"BMW",
        "car3":"Fiat"
    }
}

stored in a dictionary var dict:[String:Any]. You could use the following syntax to get to the various depths of the object.

if let name = data[keyPath:"name"] as? String{
    // name has "John"
}
if let age = data[keyPath:"age"] as? Int{
    // age has 30
}
if let car1 = data[keyPath:"cars.car1"] as? String{
    // car1 has "Ford"
}

Note that the extension supports writing into nested dictionaries as well, but I haven't yet used this.

I still haven't found a way to access arrays within dictionary objects using this, but it's a start! I'm looking for a JSON Pointer implementation for Swift but haven't found one, yet.

回答4:

If it's only about retrieval (not manipulation) then here's a Dictionary extension for Swift 3 (code ready for pasting into XCode playground) :

//extension
extension Dictionary where Key: Hashable, Value: Any {
    func getValue(forKeyPath components : Array<Any>) -> Any? {
        var comps = components;
        let key = comps.remove(at: 0)
        if let k = key as? Key {
            if(comps.count == 0) {
                return self[k]
            }
            if let v = self[k] as? Dictionary<AnyHashable,Any> {
                return v.getValue(forKeyPath : comps)
            }
        }
        return nil
    }
}

//read json
let json = "{\"a\":{\"b\":\"bla\"},\"val\":10}" //
if let parsed = try JSONSerialization.jsonObject(with: json.data(using: .utf8)!, options: JSONSerialization.ReadingOptions.mutableContainers) as? Dictionary<AnyHashable,Any>
{
    parsed.getValue(forKeyPath: ["a","b"]) //-> "bla"
    parsed.getValue(forKeyPath: ["val"]) //-> 10
}

//dictionary with different key types
let test : Dictionary<AnyHashable,Any> = ["a" : ["b" : ["c" : "bla"]], 0 : [ 1 : [ 2 : "bla"]], "four" : [ 5 : "bla"]]
test.getValue(forKeyPath: ["a","b","c"]) //-> "bla"
test.getValue(forKeyPath: ["a","b"]) //-> ["c": "bla"]
test.getValue(forKeyPath: [0,1,2]) //-> "bla"
test.getValue(forKeyPath: ["four",5]) //-> "bla"
test.getValue(forKeyPath: ["a","b","d"]) //-> nil

//dictionary with strings as keys
let test2 = ["one" : [ "two" : "three"]]
test2.getValue(forKeyPath: ["one","two"]) //-> "three"

回答5:

I had the same issue, where I wanted to get boolValue nested in dictionary.

{
  "Level1": {
    "leve2": {
      "code": 0,
      "boolValue": 1
    }
  }
}

I tried a lot of solution but those didn't worked for me as i was missing type casting. So I used following code to get the boolValue from json, where json is a nested dictionary of type [String:Any].

let boolValue = ((json["level1"]
    as? [String: Any])?["level2"]
    as? [String: Any])?["boolValue"] as? Bool

回答6:

You can use the following syntax on Swift 3/4:

    if let name = data["name"] as? String {
        // name has "John"
    }

    if let age = data["age"] as? Int {
        // age has 30
    }

    if let car = data["cars"] as? [String:AnyObject],
        let car1 = car["car1"] as? String {
        // car1 has "Ford"
    }