FetchAndActivate is asynchronously executed?

[shuniiiig]

[REQUIRED] Step 1: Describe your environment

• Xcode version: 11.5
• Firebase SDK version: 6.27.0
• Firebase Component:
  FirebaseAnalyticsBinary 6.27.0
  FirebaseDatabaseBinary 6.27.0
  FirebaseDynamicLinksBinary 6.27.0
  FirebaseMessagingBinary 6.27.0
  FirebaseProtobufBinary 6.27.0
  FirebaseRemoteConfigBinary 6.27.0
• Installation method: Carthage

[REQUIRED] Step 2: Describe the problem

I called "fetchAndActivateWithCompletionHandler" in AppDelegate, after callback, I tried to present ViewController from root in AppDelegate, I got this error below.

[Animation] +[UIView setAnimationsEnabled:] being called from a background thread. Performing any operation from a background thread on UIView or a subclass is not supported and may result in unexpected and insidious behavior. trace=(
0 UIKitCore 0x00000001a54670d4 BD57BD6E-12B4-3F92-85CA-754932DA499D + 15552724
1 libdispatch.dylib 0x000000010b4fb730 dispatch_client_callout + 16
2 libdispatch.dylib 0x000000010b4fd044 dispatch_once_callout + 84
3 UIKitCore 0x00000001a5467038 BD57BD6E-12B4-3F92-85CA-754932DA499D + 15552568
4 UIKitCore 0x00000001a54671c0 BD57BD6E-12B4-3F92-85CA-754932DA499D + 15552960
5 UIKitCore 0x00000001a498f9c4 BD57BD6E-12B4-3F92-85CA-754932DA499D + 4184516
6 UIKitCore 0x00000001a498fe64 BD57BD6E-12B4-3F92-85CA-754932DA499D + 4185700
7 UIKitCore 0x00000001a498f8ac BD57BD6E-12B4-3F92-85CA-754932DA499D + 4184236
8 UIKitCore 0x00000001a498fb18 BD57BD6E-12B4-3F92-85CA-754932DA499D + 4184856
9 cast beta 0x0000000101232bbc $s4cast11AppDelegateC23setupRootViewController33_8F4EC175BF072ACA9581F0048C6E0225LLyyF + 1668
10 cast beta 0x0000000101231d54 $s4cast11AppDelegateC22latestVersionInstalled33_8F4EC175BF072ACA9581F0048C6E0225LLyyF + 60
11 cast beta 0x0000000101232130 $s4cast11AppDelegateC08checkNewB33VersionAndSetUpRootViewController33_8F4EC175BF072ACA9581F0048C6E0225LLyyFyAA20FirebaseRemoteConfigC13ConfigurationVSgcfU + 972
12 cast beta 0x00000001026879e8 $s4cast20FirebaseRemoteConfigC5fetch6resultyyAC13ConfigurationVSgc_tFZySo09FIRRemoteD22FetchAndActivateStatusV_s5Error_pSgtcfU + 1724
13 cast beta 0x0000000102687b48 $sSo37FIRRemoteConfigFetchAndActivateStatusVs5Error_pSgIegyg_ABSo7NSErrorCSgIeyByy_TR + 136
14 libdispatch.dylib 0x000000010b4fa338 _dispatch_call_block_and_release + 24
15 libdispatch.dylib 0x000000010b4fb730 _dispatch_client_callout + 16
16 libdispatch.dylib 0x000000010b4fde88 _dispatch_queue_override_invoke + 872
17 libdispatch.dylib 0x000000010b50cd74 _dispatch_root_queue_drain + 376
18 libdispatch.dylib 0x000000010b50d698 _dispatch_worker_thread2 + 152
19 libsystem_pthread.dylib 0x00000001a0c30b38 _pthread_wqthread + 212
20 libsystem_pthread.dylib 0x00000001a0c33740 start_wqthread + 8
)

DispatchQueue.main.async {
rootViewController.present(appTitleVC, animated: true)
}

To avoid this error, I wrote the code above.

I would like to ask remoteConfig.fetchAndActivate is changed to asynchronous?

Thank you.

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[ryanwilson]

Hi @shuniiiig - thanks for the report.

You're right, looks like the completion handler is called on the Remote Config queue instead of the main queue:

1. Wrapping the completion handler in a fetch completion block
2. Calling fetchWithCompletionHandler: with the block declared in 1
3. Dispatching to a private queue, assigned here.

We'll have to discuss the appropriate approach moving forward and it may not be able to change until the next major version since some developers may be relying on this behaviour explicitly, but we'll chat about it on the team.

Thanks for the report!

[paulb777]

Adding to Firebase 7 milestone to consider for next major release.

[ryanwilson]

Follow up for this: for Firebase 7 we should make sure that all completion handlers are called on the main thread for RC.

[karenyz]

Looks like the completion handler is eventually called on the main thread here? @paulb777

[ryanwilson]

@karenyz that looks like it's a different code path.

I was able to verify that the completion handler isn't called on the main thread by putting a breakpoint here:

firebase-ios-sdk/FirebaseRemoteConfig/Tests/Unit/RCNRemoteConfigTest.m

Line 320 in bde8e07

XCTAssertEqual(self->_configInstances[i].lastFetchStatus, FIRRemoteConfigFetchStatusSuccess);

and running that unit test (see attached screenshot).

By wrapping this call

firebase-ios-sdk/FirebaseRemoteConfig/Sources/FIRRemoteConfig.m

Line 249 in 62a98dc

completionHandler(status, nil);

and the one on line 257 I was able to get it to run on the main thread.