// 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)