Introduction

Apple does not let one iPhone app read another app's private sandbox directly. If you need data sharing, you must use one of the supported mechanisms such as App Groups, Keychain sharing, URL-based handoff, or a backend service.

The best option for related apps: App Groups

If the apps are owned by the same developer team, App Groups are usually the right answer. They provide a shared container that multiple apps and extensions can read and write.

After enabling the same App Group entitlement in each target, you can access the shared directory like this:

1if let containerURL = FileManager.default.containerURL(
2    forSecurityApplicationGroupIdentifier: "group.com.example.shared"
3) {
4    let fileURL = containerURL.appendingPathComponent("settings.json")
5    let data = Data("{\"theme\":\"dark\"}".utf8)
6    try data.write(to: fileURL)
7}

Another app in the same group can read the same file:

1if let containerURL = FileManager.default.containerURL(
2    forSecurityApplicationGroupIdentifier: "group.com.example.shared"
3) {
4    let fileURL = containerURL.appendingPathComponent("settings.json")
5    let data = try Data(contentsOf: fileURL)
6    let text = String(data: data, encoding: .utf8)
7    print(text ?? "")
8}

App Groups also work well with shared UserDefaults:

let sharedDefaults = UserDefaults(suiteName: "group.com.example.shared")
sharedDefaults?.set("dark", forKey: "theme")

This is ideal for preferences, cached state, and lightweight coordination between related apps.

Sharing credentials with Keychain access groups

If the shared data is sensitive, such as login tokens, use Keychain sharing instead of plain files or defaults. Keychain access groups let multiple apps from the same team access the same secure item.

This is the right choice for:

• authentication tokens
• passwords
• small secrets that must survive reinstall scenarios better than defaults

Unlike App Group files, the Keychain is designed for secure storage, not general document exchange.

Opening one app from another

Sometimes you do not need shared storage at all. You only need one app to send a piece of information to another. In that case, custom URL schemes or Universal Links can be enough.

if let url = URL(string: "myapp://open?article=123") {
    UIApplication.shared.open(url)
}

The receiving app parses the URL and decides what to do. This is good for deep linking and simple handoff, but it is not a general-purpose storage mechanism.

Pasteboard and temporary exchange

UIPasteboard can be used for quick, user-visible copy and paste workflows. It is fine for transient text or a shared token in a controlled interaction, but it is rarely the best long-term architecture for app-to-app state.

Pasteboard is better thought of as temporary transfer rather than shared persistence.

When a backend is the real solution

If the apps are not under the same developer account, or they need to share data across multiple devices, the cleanest design is often a backend service. Each app syncs through the server instead of trying to exchange local data directly.

That approach avoids many of the limitations of local inter-app communication and usually scales better once users expect their data to move between phone, tablet, and web.

Choosing the right mechanism

Use App Groups when the apps are related and need shared local files or preferences. Use Keychain sharing for secrets. Use URL schemes or Universal Links for navigation and small payload handoff. Use a backend when the data needs to be durable, cross-device, or available to apps that are not tightly coupled on the same device.

There is no single "share data between apps" API because the right tool depends on whether the requirement is storage, security, invocation, or synchronization.

Common Pitfalls

• Expecting one iOS app to read another app's sandbox directly.
• Using UserDefaults.standard instead of group-scoped defaults for shared preferences.
• Storing secrets in a shared file when Keychain sharing is more appropriate.
• Using custom URL schemes as if they were a durable shared database.
• Designing for local device sharing when the real product requirement is server-backed sync.

Summary

• iPhone apps cannot arbitrarily read each other's private storage.
• App Groups are the standard solution for shared local files and preferences between related apps.
• Keychain sharing is better for credentials and other secrets.
• URL schemes and Universal Links are useful for launching and passing small pieces of context.
• For cross-device or loosely coupled sharing, a backend service is usually the right architecture.