Reserved Events

Reserved events (prefixed with $) are automatically tracked by SDKs. They provide consistent lifecycle tracking across platforms.

Lifecycle Events

Event	When Tracked	Properties
$app_installed	First launch after install	$version
$app_updated	First launch after version change	$version, $previous_version
$app_opened	App became active (foreground)	-
$app_backgrounded	App went to background	-

$app_installed

Tracked on the very first launch after installation.

{
  "name": "$app_installed",
  "properties": {
    "$version": "1.0.0"
  }
}

Use this for:

• Install attribution
• New user funnels
• Install-to-activation metrics

$app_updated

Tracked on the first launch after the app version changes.

{
  "name": "$app_updated",
  "properties": {
    "$version": "2.0.0",
    "$previous_version": "1.9.0"
  }
}

Use this for:

• Update adoption rates
• Version migration tracking
• Feature rollout analysis

$app_opened

Tracked when the app comes to the foreground.

Use this for:

• Daily/monthly active users
• Session frequency
• Return user analysis

$app_backgrounded

Tracked when the app goes to the background.

Use this for:

• Session duration (with $app_opened)
• Drop-off analysis

Reserved Properties

Property	Description	Example
$version	Current app version	"1.0.0"
$previous_version	Previous app version	"0.9.0"
$device_type	Device category	"phone", "tablet", "desktop"
$device_model	Device model	"iPhone15,2", "Pixel 8"
$sdk	SDK identifier	"swift", "android", "javascript"
$storage_type	Storage mechanism	"persistent", "memory"

Enabling/Disabling

Lifecycle events are enabled by default. To disable:

MostlyGoodMetrics.configure('api_key', {
  trackAppLifecycleEvents: false,
});

Don't Use $ Prefix

The $ prefix is reserved for system events and properties. Don't use it for your own events:

// Wrong - will conflict with system events
track('$my_event');

// Correct
track('my_event');