Title: Understanding getCurrentPositionAsync vs getLastKnownPositionAsync in React Native Expo: When to Use Each for Location Tracking

Introduction

When developing a location-based app in React Native using Expo, you’ll often need to decide how to get the device’s location. The Expo Location API provides two methods: getCurrentPositionAsync and getLastKnownPositionAsync. Although they may seem similar, they serve different purposes and can significantly impact your app's behavior. In this article, we'll dive into the differences, explore their use cases, and discuss when to use each method.

What are getCurrentPositionAsync and getLastKnownPositionAsync?

getCurrentPositionAsync

This function retrieves the current GPS location of the device by actively querying the hardware, such as the GPS, Wi-Fi, or cellular network. It can take a few seconds to complete, depending on factors like signal strength and the device’s location. The function will always provide the most accurate and up-to-date location, making it suitable for apps requiring real-time tracking.

getLastKnownPositionAsync

This function retrieves the last cached GPS location from the device’s system. The operating system constantly updates the cached location as various apps or system processes access location services. getLastKnownPositionAsync is much faster than getCurrentPositionAsync because it doesn't need to actively query the hardware. However, the accuracy may vary depending on when the location was last updated.

How Does the Device Manage Location Services?

Mobile devices have built-in location services managed by the operating system (Android or iOS). These services use various sources, such as GPS, Wi-Fi, cellular networks, and Bluetooth, to determine the device’s position. The system maintains a cached record of the latest location, which can be accessed via getLastKnownPositionAsync.

When an app or system process requests the device’s location, the cached data is updated in the background. This cached location is what you get with getLastKnownPositionAsync. However, if you need a fresh reading, you should use getCurrentPositionAsync, which triggers an active query.

Key Differences

• Accuracy: getCurrentPositionAsync provides the most recent and accurate location, as it fetches the current GPS data. In contrast, getLastKnownPositionAsync returns the last known location, which might be outdated.
• Speed: getLastKnownPositionAsync is faster since it fetches the location from the cache without querying the GPS hardware.
• Battery Consumption: Actively querying the GPS (using getCurrentPositionAsync) consumes more battery, while getLastKnownPositionAsync is less demanding.
• Use Cases:

a. Real-Time Tracking: Use getCurrentPositionAsync when accuracy is crucial, such as in navigation or fitness tracking apps.

b. Quick Location Checks: Use getLastKnownPositionAsync for non-critical tasks, like showing the user's location on a map or setting an initial position.

Practical Use Cases and Examples

1. Real-Time Navigation

In navigation apps, users expect real-time location updates. Using getCurrentPositionAsync ensures that you provide the most accurate position. However, you may want to combine it with getLastKnownPositionAsync as a fallback in case the GPS takes too long to respond.

import * as Location from 'expo-location';  

async function getRealTimeLocation() {   
  let location = await Location.getLastKnownPositionAsync({});   
  if (!location) {     
    location = await Location.getCurrentPositionAsync({       
      accuracy: Location.Accuracy.High,     
    });   
  }   
  return location; 
}

2. Quick Location Access

If you need the user’s location just to display it on a map or to set an initial position, getLastKnownPositionAsync is ideal. It's fast and doesn't consume much battery.

import * as Location from 'expo-location';  

async function getQuickLocation() {   
  const location = await Location.getLastKnownPositionAsync({});   
  return location; 
}

Why Does the Location Differ Between These Methods?

The difference in location accuracy and timing is primarily due to the source of data:

• getLastKnownPositionAsync accesses the cached location maintained by the system. It is updated whenever an app or system process requests the location.
• getCurrentPositionAsync triggers a fresh query to GPS hardware, potentially resulting in a more accurate but slower response.

If you see different locations using these methods, it’s often due to the time elapsed between updates to the cached location.

When to Use Each Method

• Use getCurrentPositionAsync when:

a. You need the most accurate location for tasks like navigation, fitness tracking, or real-time updates.

b. You want to get a fresh reading, ensuring the data reflects the current environment.

• Use getLastKnownPositionAsync when:

a. You want a quick response and don’t need real-time accuracy, such as showing the user’s approximate location.

b. The location is used for background tasks that don’t require precision.

c. You’re displaying a map where an approximate initial location is sufficient.

Common Pitfalls and Tips

1. Handling Errors and Timeouts

Sometimes, getCurrentPositionAsync can take longer to respond or fail if the device is in a poor signal area. Always handle errors gracefully and consider providing a fallback using getLastKnownPositionAsync.

2. Battery Management

Continuously polling getCurrentPositionAsync can drain the battery. Use it selectively and consider using getLastKnownPositionAsync when the app is in the background.

3. Managing Permissions

Both methods require location permissions. Make sure your app requests the appropriate permissions (foreground or background) based on your use case.

Conclusion

Choosing between getCurrentPositionAsync and getLastKnownPositionAsync depends on your app's requirements. While getCurrentPositionAsync provides real-time accuracy, getLastKnownPositionAsync offers speed and efficiency for non-critical tasks. Understanding these differences and using them together smartly can significantly enhance your app's location-based features.

Happy coding! 🚀