// initial utc_ms_delta: 0 uvarint(utc_ms_delta) + uvarint(payload.length) + payload
payload = JSON({"pid": 123, "app": "com.example.Foo"}) (only log when there's a change)
payload = JSON({"url": "file://localhost/Users/satoshi/Desktop/foo.txt"}) (only log when there's a change)
payload = 0x01 for BECAME_ACTIVE 0x02 for BECAME_IDLE (only log when there's a change)
// initial x, y: 0 payload = uvarint(x_delta) + uvarint(y_delta) (only log when there's a change)