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Taking a Look at UserDefaults' synchronize()

iOS, User Defaults, Synchronization2 min read

User Defaults is a widely used mechanism in iOS app development for storing simple user preferences and app settings. It provides a convenient way to save and retrieve data, such as user preferences, app settings, and small amounts of application-specific data. While User Defaults automatically handles the persistence of data, there are scenarios where you may need to ensure that the data is immediately written to disk. This is where the synchronize() method comes into play.

Understanding User Defaults

Before diving into synchronize(), let's have a brief understanding of User Defaults. User Defaults is a key-value store provided by Apple as part of the Foundation framework. It allows developers to save and retrieve data using a simple and straightforward API. The data is saved in a property list format and is accessible throughout the app.

To use User Defaults, you typically create an instance of UserDefaults, like this:

1let defaults = UserDefaults.standard

You can then save and retrieve data using various methods provided by User Defaults, such as set(_:forKey:), object(forKey:), and so on. However, when you call these methods to update the defaults, the changes are not immediately written to disk. Instead, they are synchronized periodically by the operating system for performance reasons.

The synchronize() Method

The synchronize() method in User Defaults allows you to manually trigger the synchronization process and ensure that any changes made to User Defaults are immediately written to disk. This can be useful in certain scenarios where you need to ensure data consistency or persistence at a specific point in your app.

To use synchronize(), simply call the method on your User Defaults instance:


By calling synchronize(), you ensure that any changes made to User Defaults are persisted immediately. However, it's important to note that calling synchronize() too frequently can have a negative impact on performance, as it forces an immediate disk write operation.

Example Usage

Let's take a look at a couple of examples where using synchronize() can be beneficial.

Example 1: Saving User Preferences

Consider a scenario where your app allows users to customize certain preferences, such as the app's theme color. When the user changes the theme color, you can save the new value using User Defaults:

1defaults.set(themeColor, forKey: "AppThemeColor")

Since the theme color change is an important preference, you want to ensure that it is immediately persisted to User Defaults. In such a case, you can call synchronize() to make sure the change is written to disk:


Example 2: Data Consistency

Imagine you have an app that manages a to-do list. When the user adds or updates a task, you store it in User Defaults. However, if the app crashes or terminates unexpectedly, there is a chance that the latest changes are lost if they haven't been synchronized yet. To mitigate this risk, you can call synchronize() after adding or updating a task:

1// Add or update a task
4// Synchronize the changes immediately

By doing this, you ensure that the latest task additions or updates are immediately persisted to User Defaults, reducing the risk of data loss.

In Closing

While calling synchronize() is not always necessary, it can be a handy tool in scenarios where data consistency and persistence are critical. However, it's important to use it judiciously, considering the impact on performance. With a proper understanding of User Defaults and the synchronize() method, you can ensure that your app's data is reliably persisted.