On-Demand Resources: What It Is, Request Mechanism, and Resource Storage

What is On-Demand Resources in iOS

On-Demand Resources (ODR) is a content management system built into iOS, tvOS, and macOS. It allows developers to split app resources into mandatory (included in the installation package) and on-demand (stored in App Store and downloaded when needed).

Key ODR Concepts

The ODR system operates with three key entities: tags, resource packages, and requests. A tag is a string identifier that groups a set of files. A resource package is a data set associated with a tag. A request is a programmatic call to NSBundleResourceRequest that initiates package loading.

Resource Request Lifecycle

Resource loading goes through several stages: initializing a request with specified tags, checking availability in local storage, downloading from Apple servers if absent, notifying the app when ready, and releasing the resource after use. The system automatically manages caching and removal of unused resources.

How ODR Works in iOS

ODR mechanism is based on a background service that manages resource requests from the App Store cloud storage. When the app requests a tag, the system checks if the resource exists in the local cache. If not — it starts downloading via a background URL session with resume support for interrupted connections.

Managing Requests via NSBundleResourceRequest

import Foundation

class ResourceManager {
    let odrRequest = NSBundleResourceRequest(tags: ["level-3"])

    func loadLevel3() {
        odrRequest.conditionallyBeginAccessingResources { available in
            if available {
                self.startLevel3()
            } else {
                self.odrRequest.beginAccessingResources { error in
                    guard error == nil else { return }
                    self.startLevel3()
                }
            }
        }
    }

    func releaseResources() {
        odrRequest.endAccessingResources()
    }
}

The conditionallyBeginAccessingResources method checks for resource availability in cache without immediate loading. If the availability flag is positive — the app continues without delay. If the resource is absent, beginAccessingResources starts downloading with progress indication via the progress property.

Background Loading and Priorities

iOS manages ODR resource loading priorities based on current app activity. When the app is in the foreground, loading is performed with high priority. In the background, iOS may delay loading or pause it during low battery.

• High priority — immediate loading for resources needed right now
• Low priority — deferred loading for prefetching and preliminary caching
• Background — loading only with active Wi-Fi and sufficient battery

Resource Types and ODR Tags

ODR resources can include images, videos, audio files, sprites, level data, and any other files not critical for app launch. Each resource is tied to one or more tags that determine its functional block membership.

Content Tagging in Xcode

In Xcode, tags are configured through the Resource Tags inspector in the Target Membership tab. The developer specifies for each project file whether it is included in the main bundle or loaded by tag. One file can belong to multiple tags, allowing resource reuse across different app sections.

<!-- Example of Resource Tags configuration in Info.plist -->
<key>NSBundleResourceRequestTags</key>
<dict>
    <key>level-3</key>
    <array>
        <string>levels/level3.scnassets</string>
        <string>textures/level3_tex.png</string>
    </array>
</dict>

Tag Categories by Lifetime

Apple distinguishes three ODR tag categories based on how long a resource should remain on the device. Initial Install Tags are loaded with the app and are never deleted by the system. Prefetch Tag Order determines the background preloading order immediately after installation. Main tags are requested on demand and may be deleted by iOS when storage is low.

In addition to categories, each tag can have a download priority. Tags with high priority are loaded before low-priority ones, even if they were requested later. This allows developers to control content appearance order: first critical resources for the main screen, then additional materials for deep app sections.

• Initial install tags — loaded during first installation, not removed by system
• Prefetched tags — downloaded in background after installation to improve UX
• On-demand tags — requested programmatically, may be deleted by iOS when storage is tight

Configuring On-Demand Resources in Xcode

ODR setup in Xcode includes three stages: categorizing resources by tags, configuring download parameters in Info.plist, and programmatic request implementation via NSBundleResourceRequest. Most of the work is done on the build side.

Categorization in Target Membership

// Checking ODR status and progress monitoring
func monitorODRProgress(tag: String) {
    let request = NSBundleResourceRequest(tags: [tag])
    let observer = request.progress.observe(\.fractionCompleted) { progress, _ in
        DispatchQueue.main.async {
            let percent = Int(progress.fractionCompleted * 100)
            print("ODR: \(percent)% loaded")
        }
    }
}

Download progress is tracked via the progress property of the NSProgress class with KVO observation support. The app can display a loading indicator to the user during resource downloading. After download completion, resources are available in the main app bundle through standard FileManager and NSDataAsset mechanisms.

Quota and Limitation Parameters

ODR quotas depend on iOS version. Starting with iOS 13, a device can store up to 20 GB of on-demand resources per account. This space is shared among all apps installed on the device — if one app takes up a lot of space, less remains for others.

Used space monitoring is done via NSBundleResourceRequest. The app can check available ODR storage and decide which resources to load first accordingly. It is recommended to implement a fallback mechanism: if ODR storage is full, use reduced quality resources from the main bundle.

Developers must consider that iOS may decide to delete On-Demand resources at any time — the app must correctly handle situations where a previously loaded resource suddenly becomes unavailable. It is recommended to intercept resource access errors and re-request them via beginAccessingResources. For critical resources without which the app cannot function, use Initial Install Tags that are not removed by the system.

Advantages and Limitations of ODR

On-Demand Resources give developers a flexible tool for managing app size, but impose a number of limitations related to network availability and iOS storage policies.

Advantages of Using ODR

The main advantage of ODR is a radical reduction in initial installation size. Games with multiple levels, apps with video content, or large image sets can load content in portions. The user gains access to basic functionality immediately, while additional resources are downloaded in the background.

Technology Limitations

ODR requires a constant internet connection for loading resources on first access. Users in areas with poor coverage may experience delays and download errors. Additionally, Apple does not guarantee that downloaded resources will remain on the device — when storage is low, iOS may delete On-Demand tags without warning.

• Network requirement — first resource request requires internet connection
• Automatic deletion — iOS may clear cache when device storage is low
• Debugging complexity — reproducing loading issues requires simulating various network conditions
• Tag size — Apple recommends not exceeding 512 MB per tag for optimal performance
• Error handling — the app must correctly handle loading failures and retry requests