Mike from Crashlytics here.
To use custom logging in Swift, just use CLSLogv or CLSNSLogv. You need to make an array and then call getVaList function on that array.
Here’s a snippet:
CLSLogv("Log something %d %d %@", getVaList([1, 2, "three"]))
For CLSNSLogv:
CLSNSLogv("hello %@", getVaList(["goodbye"]))