Realm writes are blocking main thread - swift

Question

I have a json file on a remote server that I'm grabbing data from (36k records) using dataTaskWithURL. I'm the resulting JSON (SwiftyJSON) is returned. Below is my completion handler.
The problem is that as soon as I start rolling through the realm create's, all my other callbacks stop executing until the realm commit fires. My desired outcome is that this task simply runs and inserts the data in the background allowing the user to continue on their merry way while this rolls.
The blocking seems to happen once the realm.beginWrite() fires.
RemoteAPI().getMetafile({JSONData, error -> Void in
if (JSONData != nil) {
do {
print("***** Loading realm")
let realm = try Realm()
realm.beginWrite()
for (_, subJSON) in JSONData {
realm.create(Meta.self, value: ["xxxxxx": subJSON["xxxxx"].int!, "xxxxx": subJSON["xxxxx"].stringValue, "xxxxx": subJSON["xxxxx"].stringValue, "xxxxxx": subJSON["xxxxxx"].int!, "xxxxxx": subJSON["xxxxx"].stringValue, "xxxxx": subJSON["xxxxx"].stringValue, "xxxxxx": subJSON["xxxxx"].stringValue], update: true)
}
try realm.commitWrite()
print("***** Finished loading realm")
} catch _ {}
} else {
print("api data fetch failed")
print(error)
}
})
While the above call is happening, I have another call:
RemoteAPI().getLatestActivityData({JSONData, error -> Void in
if (JSONData != nil) {
// do stuff
dispatch_async(dispatch_get_main_queue(), {
NSNotificationCenter.defaultCenter().postNotificationName("refreshTableView", object: nil)
})
}
})
Both of these calls fire from the app delegate. The problem however, is that the observer in this second call does not fire until after the first call above completes.

Answer 1

You should perform the transaction in the background if you don't want to block the main thread.
...
do {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
let realm = try! Realm()
realm.beginWrite()
for (_, subJSON) in JSONData {
realm.create(Meta.self, value: [...], update: true)
}
try realm.commitWrite()
}
}
...
See Realm docs about threading.

Answer 2

The issue had nothing to do with thread blocking. Even though I was running the requests asynchronously something I figured out is if I have Request A and Request B running asynchronously and they both start trying to do Realm writes. They basically get treated as serial requests and the writes will be queued up until the initial writes are completed.
As noted in the documentation (not in the threads section) -- Realm writes will block each other. They note that you can use threading to resolve this, but even using the dispatch_async methods, the writes continued to block each other.
However what I realized was that the initial request is simply loading data to be used for an in-app search and not for anything else. The rest of the system uses Realm for storing user specific data.
My Solution
Request A represents my search meta data request populating, for that I created a new realm file:
let config = Realm.Configuration(
path: utility.getDocumentsDirectory().stringByAppendingPathComponent("Meta.realm"),
readOnly: false)
let realm = try! Realm(configuration: config)
Request B Which was writing all my user data, and then triggering a tableview refresh I had read/write from the default realm.
As soon as I implemented this change in usage, no more blocking and everything worked correctly and as expected.
Honestly I don't know if this is best practice for Swift/iOS or not (I'm not a Swift developer) but it works, and the performance is quite acceptable.

Answer 3

You can try this:
for (_, subJSON) in JSONData {
realm.beginWrite()
realm.create(Meta.self, value: ["xxxxxx": subJSON["xxxxx"].int!, "xxxxx": subJSON["xxxxx"].stringValue, "xxxxx": subJSON["xxxxx"].stringValue, "xxxxxx": subJSON["xxxxxx"].int!, "xxxxxx": subJSON["xxxxx"].stringValue, "xxxxx": subJSON["xxxxx"].stringValue, "xxxxxx": subJSON["xxxxx"].stringValue], update: true)
try realm.commitWrite()
}
I don't know why, but it works for me.