Working with User Defaults
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last updated: 2016-06
This article covers working with NSUserDefault to save default settings in a Xamarin iOS App or Extension.
NSUserDefaults class provides a way for iOS Apps and Extensions to programmatically interact with the system-wide Default System. By using the Defaults System, the user can configure an app's behavior or styling to meet their preferences (based on the design of the app). For example, to present data in Metric vs Imperial measurements or select a given UI Theme.
When used App Groups,
NSUserDefaults also provides a way to communicate between apps (or Extensions) within a given group.
About User Defaults
As stated above, User Defaults (
NSUserDefaults) can be added to an App (or Extension) and used to provide configurable options that the end user can modify to adjust the look or operation of the app at runtime.
When your app first executes,
NSUserDefaults reads the keys and values from the app's User Defaults Database and caches them to memory to avoid opening and reading the database each time a value is required.
Note: Apple no longer suggest that the developer call the
Synchronizemethod to sync the in-memory cache with the database directly. Instead, it will be automatically called at periodic intervals to keep the in-memory cache in sync with a user’s defaults database.
NSUserDefaults class contains several convenience methods for reading and writing preference values for common data types such as: string, integer, float, boolean and URLs. Other types of data can be archived using
NSData, then read from or written to the User Defaults Database. For more information, please see Apple's Preferences and Settings Programming Guide.
Accessing the Shared NSUserDefaults Instance
The Shared User Defaults Instance provides access to the User Defaults for the current user of the device. If the Shared Defaults object doesn't exist, it is created the first time it is accessed and initialized with the following information:
NSArgumentDomainconsisting of the defaults parsed from the current app.
- The app's Bundle Identifier domain.
NSGlobalDomainconsisting of the defaults shared by all apps.
- A separate domain for each of the user's preferred languages.
NSRegistationDomainwith a set of temporary defaults that can be modified by the app to ensure searches are always successful.
To access the Shared User Defaults Instance, use the following code:
// Get Shared User Defaults var plist = NSUserDefaults.StandardUserDefaults;
Accessing an App Group NSUserDefaults Instance
As stated above, by using App Groups,
NSUserDefaults can be used to communicate between Apps (or Extensions) within a given group. First, you will need to ensure that the App Group and the required App IDs have been properly configured in the Certificates, Identifiers & Profiles section on iOS Dev Center and have been installed on in the development environment.
Next, your App and/or Extension projects will need to be one of the valid App IDs created above, that the
Entitlements.plist file has the App Groups enabled and specified and that it get's included in the App Bundle.
With this all in place, the shared App Group user Defaults can be accessed using the following code:
// Get App Group User Defaults var plist = new NSUserDefaults ("group.com.xamarin.todaysharing", NSUserDefaultsType.SuiteName);
group.com.xamarin.todaysharing is the App Group created in Certificates, Identifiers & Profiles that you want to access. For more information, please see the App Group Capabilities documentation.
Reading Default Values
After you have accessed the desired User Default Database, you can read values from the defaults using Key/Value Pairs and several convenience methods based on the type of data being read:
ArrayForKey- Returns an array of
NSObjectsfor the given key value.
BoolForKey- Returns a boolean value for the given key.
DataForKey- Returns an
NSDataobject for the given key.
DictionaryForKey- Returns an
NSDictionaryfor the given key.
DoubleForKey- Returns a double value for the given key.
FloatForKey- Returns a float value for the given key.
IntForKey- Returns an integer value for the given key.
StringArrayForKey- Returns an array of
Stringobjects from the given key value.
StringForKey- Returns a string value for the given key.
URLForKey- Returns an
NSUrlvalue for the given key.
For example, the following code would read a boolean value from the User Defaults:
// Get Shared User Defaults var plist = NSUserDefaults.StandardUserDefaults; ... // Get value var useHeader = plist.BoolForKey("UseHeader");
Writing Default Values
Just like reading values above, after you have accessed the desired User Default Database, you can write values to the defaults using Key/Value Pairs and several convenience methods based on the type of data being written:
SetBool- Writes the given boolean value to the given key.
SetDouble- Writes the given double value to the given key.
SetFloat- Writes the given float value to the given key.
SetString- Writes the given string value to the given key.
SetURL- Writes the given URL (
NSUrl) value to the given key.
For example, the following code would write a boolean value to the User Defaults:
// Get Shared User Defaults var plist = NSUserDefaults.StandardUserDefaults; ... // Save value plist.SetBool(useHeader, "UseHeader"); ...
NOTE: When your App first executes,
NSUserDefaultsreads the keys and values from the app's User Defaults Database and caches them to memory to avoid opening and reading the database each time a value is required.
This article has covered the
NSUserDefaults class and how it can be used to provide a set of options that the end user can use to configure your Xamarin.iOS App. Additionally, it covered using App Groups to communicate between an extension and its Parent App or between apps in a group.