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Counters - Java SDK
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Realm offers MutableRealmInteger, a wrapper around numeric values, to help better synchronize numeric changes across multiple clients.
Typically, incrementing or decrementing a
byte, short, int, or long field of a Realm
object looks something like this:
Read the current value of the field.
Update that value in memory to a new value based on the increment or decrement.
Write a new value back to the field.
When multiple distributed clients attempt this at the same time,
updates reaching clients in different orders can
result in different values on different clients. MutableRealmInteger
improves on this by translating numeric updates into sync operations
that can be executed in any order to converge to the same value.
MutableRealmInteger fields are backed by traditional numeric types,
so no migration is required when changing a field from byte, short,
int or long to MutableRealmInteger.
The following example demonstrates a MutableRealmInteger field that
counts the number of ghosts found in a haunted house:
Important
Counter Fields Must be Final
MutableRealmInteger is a live object like RealmObject,
RealmResults and RealmList. This means the value contained
inside the MutableRealmInteger can change when a realm is
written to. For this reason MutableRealmInteger fields must be
marked final in Java and val in Kotlin.
Usage
The counter.increment() and counter.decrement() operators ensure that increments and decrements from multiple distributed clients are aggregated correctly.
To change a MutableRealmInteger value, call increment() or
decrement() within a write transaction:
You can assign a MutableRealmInteger a new value with a call to
counter.set()
within a write transaction.
Warning
Counter Resets
Use the set() operator with extreme care. set() ignores
the effects of any prior calls to increment() or decrement().
Although the value of a MutableRealmInteger always converges
across devices, the specific value on which it converges depends on
the actual order in which operations took place.
Mixing set() with increment() and decrement() is
not advised unless fuzzy counting is acceptable.
Since MutableRealmInteger instances retain a reference to their
parent object, neither object can be garbage collected while you still
retain a reference to the MutableRealmInteger.