Settings bundle values returning nil

问题:

I added a Settings bundle to my app and in Xcode it appears in the root of my project tree view.

The Root.plist file looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>StringsTable</key>
    <string>Root</string>
    <key>PreferenceSpecifiers</key>
    <array>
        <dict>
            <key>Type</key>
            <string>PSGroupSpecifier</string>
            <key>Title</key>
            <string>Service</string>
        </dict>
        <dict>
            <key>Type</key>
            <string>PSTextFieldSpecifier</string>
            <key>Title</key>
            <string>Hostname</string>
            <key>Key</key>
            <string>service_hostname</string>
   <!-- and so on -->

When I open the Settings app on iOS the entry appears at the bottom and I can display and edit my settings perfectly fine.

However I cannot retrieve these values from code. Here's my code:

static func loadSettings() {

    let ud = NSUserDefaults.standardUserDefaults()
    ud.synchronize()

    Settings.hostName = ud.stringForKey("service_hostname")
    // etc
}

I also tried ud.objectForKey and ud.valueForKey - both return nil as well.

After setting Settings.hostName the Xcode debugger reports it has a value of nil despite me setting an explicit value in the Settings app.

I saw this thread ( iPhone App : How to get default value from root.plist? ) where someone posted a chunk of Objective-C code that manually loads the Root.plist file directly into an NSMutableDictionary and calls NSUserDefaults.standardUserDefaults().registerDefaults but that seems like a hack (and I can't get it to work in Swift because the compiler says that stringByAppendingPathComponent doesn't exist anymore)

Why isn't NSUserDefaults picking up the settings from the Settings app?

回答1:

Apparently the cause is that if my settings in the plist have defaults defined and the user has not explicitly set a value, then the value displayed in the Settings app will be the defaults from the plist file, however the NSUserDefaults API will still return nil.

Unfortunately this means that if the default value is meaningful (such as a default web-service address URI: "http://www.example.com") it must exist twice in my project: as a default in the plist and in my program code:

Root.plist:

  <dict>
      <key>Key</key>          <string>mySettingKey</string>
      <key>Title</key>        <string>Some address</string>
      <key>Type</key>         <string>PSTextFieldSpecifier</string>
      <key>DefaultValue</key> <string>http://www.example.com</string>
      <key>IsSecure</key>     <false />
      <key>KeyboardType</key> <string>Alphabet</string>
  </dict>

Program.swift:

let ud = NSUserDefaults.standardUserDefaults()
ud.synchronize()

var mySettingValue = ud.stringForKey("mySettingKey")
if mySettingValue == nil {
    mySettingValue = "http://www.example.com"
}

That's surprising.

回答2:

Default values can be taken from Settings.bundle and added to UserDefaults. Following function can be called in AppDelegate.swift from didFinishLaunchingWithOptions.

func setDefaultsFromSettingsBundle() {
    //Read PreferenceSpecifiers from Root.plist in Settings.Bundle
    if let settingsURL = Bundle.main.url(forResource: "Root", withExtension: "plist", subdirectory: "Settings.bundle"),
        let settingsPlist = NSDictionary(contentsOf: settingsURL),
        let preferences = settingsPlist["PreferenceSpecifiers"] as? [NSDictionary] {

        for prefSpecification in preferences {

            if let key = prefSpecification["Key"] as? String, let value = prefSpecification["DefaultValue"] {

                //If key doesn't exists in userDefaults then register it, else keep original value
                if UserDefaults.standard.value(forKey: key) == nil {

                    UserDefaults.standard.set(value, forKey: key)
                    NSLog("registerDefaultsFromSettingsBundle: Set following to UserDefaults - (key: \(key), value: \(value), type: \(type(of: value)))")
                }
            }
        }
    } else {
        NSLog("registerDefaultsFromSettingsBundle: Could not find Settings.bundle")
    }
}

回答3:

You should register your defaults so that it will sync up. source from here

// Swift 3
var appDefaults = Dictionary<String, AnyObject>()
appDefaults["mySettingKey"] = "http://www.example.com" // Default Value

UserDefaults.standard.register(appDefaults)
UserDefaults.standard.synchronize()

let mySettingValue = UserDefaults.standard.string(forKey: "mySettingKey")

Keep in mind that you should also use registerDefaults: when your app uses a Settings Bundle. Since you already specified default values inside the settings bundle’s plist, you may expect that your app picks these up automatically. However, that is not the case. The information contained in the settings bundle is only read by the iOS Settings.app and never by your app. In order to have your app use the same defaults as shown inside the Settings.app, you have to manually copy the user defaults keys and their default values into a separate plist file and register it with the defaults database as shown above.