Ios – Sharing data between an iOS 8 share extension and main app

iosios-app-extensionswiftuserdefaults

Recently, I've been making a simple iOS 8 share extension to understand how the system works. As Apple states in its App Extension Programming Guide:

By default, your containing app and its extensions have no direct access to each other’s containers.

Which means the extension and the containing app do not share data. But in the same page Apple brings a solution:

If you want your containing app and its extensions to be able to share data, use Xcode or the Developer portal to enable app groups for the app and its extensions. Next, register the app group in the portal and specify the app group to use in the containing app.

Then it becomes possible to use NSUserDefaults to share data between the containing app and the extension. This is exactly what I would like to do. But for some reason, it does not work.

In the same page, Apple suggests the standard defaults:

var defaults = NSUserDefaults.standardUserDefaults()

In a WWDC presentation (217), they suggest a common package:

var defaults = NSUserDefaults(suiteName: kDefaultsPackage)

Also, I enabled App Groups for both the containing app target and the extension target, with the same App Group name:
XCode Target capabilities, App Groups

But all this setup is for nothing. I cannot retrieve the data I stored in the containing app, from the extension. It is like two targets are using completely different NSUserDefaults storages.

So,

Is there a solution for this method?
How can I share simple data between the containing app and the share extension? The data is just user credentials for an API.

Best Answer

You should use NSUserDefaults like this:

Save data:

objc

NSUserDefaults *shared = [[NSUserDefaults alloc] initWithSuiteName:@"group.yougroup"];
[shared setObject:object forKey:@"yourkey"];
[shared synchronize];

swift

let defaults = UserDefaults(suiteName: "group.yourgroup")
defaults?.set(5.9, forKey: "yourKey")

Read data:

objc

NSUserDefaults *shared = [[NSUserDefaults alloc] initWithSuiteName:@"group.yougroup"];
id value = [shared valueForKey:@"yourkey"];
NSLog(@"%@",value);

swift

let defaults = UserDefaults(suiteName: "group.yourgroup")
let x = defaults?.double(forKey: "yourKey")
print(x)

This will work fine!

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The last two are identical; "atomic" is the default behavior (~~note that it is not actually a keyword; it is specified only by the absence of nonatomic~~ -- atomic was added as a keyword in recent versions of llvm/clang).

Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).

With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.

In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".

What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.

Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.

Adding to this:

atomicity of a single property also cannot guarantee thread safety when multiple dependent properties are in play.

Consider:

 @property(atomic, copy) NSString *firstName;
 @property(atomic, copy) NSString *lastName;
 @property(readonly, atomic, copy) NSString *fullName;

In this case, thread A could be renaming the object by calling setFirstName: and then calling setLastName:. In the meantime, thread B may call fullName in between thread A's two calls and will receive the new first name coupled with the old last name.

To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName while the dependent properties are being updated.

Best Answer

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