咨询：应用终止后仍执行后台进程（时间检查）的实现步骤

Hey there! I get that figuring out how to run time-check operations when your app is terminated can be confusing, especially after digging into Background Modes and not quite wrapping your head around it. Let me walk you through the exact steps using Apple's official, approved mechanisms—since iOS is pretty strict about background/terminated app behavior.

There are two main reliable approaches depending on your needs: one for periodic checks, and another for more targeted, scheduled tasks. Let's break them down:

方案1：Background App Refresh（适合定期自动检查） #

This mechanism lets the system wake your app periodically (even if it's terminated) to run short tasks like time checks or data syncs.

Step 1: Enable the required capability #

• Open your Xcode project, select your app target
• Go to the Signing & Capabilities tab
• Click + Capability, search for and add Background Modes
• In the Background Modes list, check Background fetch
• Add NSBackgroundRefreshUsageDescription to your Info.plist with a user-friendly explanation (e.g., "Needs to check time periodically to send you reminders")—this is mandatory for App Store approval.

Step 2: Code setup and check logic #

First, request permission and set the refresh interval in your AppDelegate (or SceneDelegate if using Scenes):

// AppDelegate.swift
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    // Request background refresh access and set minimum interval
    UIApplication.shared.setMinimumBackgroundFetchInterval(UIApplication.backgroundFetchIntervalMinimum)
    return true
}

Then implement the fetch callback where you'll run your time check:

// AppDelegate.swift
func application(_ application: UIApplication, performFetchWithCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
    // Run your time check logic here
    let currentTime = Date()
    let targetTime = // Load your target time from local storage/UserDefaults
    if currentTime >= targetTime {
        // Do what you need—like trigger a local notification
        sendTimeReminderNotification()
    }
    
    // Always call this to let the system know you're done
    completionHandler(.newData)
}

方案2：Background Tasks Framework（iOS 13+，适合精准调度） #

If you need to run the check at a specific time (not just periodically), this framework is more flexible and supports waking terminated apps.

Step 1: Enable the capability #

Same as above, add Background Modes and check Background processing.

Step 2: Register your background task #

In AppDelegate's launch method, register your task identifier (make sure it matches what you'll add to Info.plist):

import BackgroundTasks

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    // Register your task with a unique identifier
    BGTaskScheduler.shared.register(forTaskWithIdentifier: "com.yourApp.timeCheckTask", using: nil) { task in
        self.handleTimeCheckTask(task: task as! BGProcessingTask)
    }
    return true
}

Add BGTaskSchedulerPermittedIdentifiers to your Info.plist as an array, and include your task identifier (com.yourApp.timeCheckTask) in it.

Step 3: Schedule the task #

When you need to set up the time check (e.g., when the user sets a target time), submit the task request:

func scheduleTimeCheckTask() {
    let request = BGProcessingTaskRequest(identifier: "com.yourApp.timeCheckTask")
    // Set the earliest time the task should run (e.g., 1 hour from now)
    request.earliestBeginDate = Date().addingTimeInterval(3600)
    // Optional: Require network if your check needs it
    request.requiresNetworkConnectivity = false
    
    do {
        try BGTaskScheduler.shared.submit(request)
    } catch {
        print("Failed to schedule task: \(error.localizedDescription)")
    }
}

Step 4: Handle the task execution #

Write the logic for your time check and mark the task as complete when done:

func handleTimeCheckTask(task: BGProcessingTask) {
    // Set an expiration handler in case the system cuts off the task early
    task.expirationHandler = {
        task.setTaskCompleted(success: false)
    }
    
    // Run your time check
    let currentTime = Date()
    let targetTime = // Load your saved target time
    if currentTime >= targetTime {
        sendTimeReminderNotification()
    }
    
    // Mark the task as successful
    task.setTaskCompleted(success: true)
    
    // If you need to repeat the check, re-schedule it here
    scheduleTimeCheckTask()
}

Critical Notes to Remember #

• iOS prioritizes battery life and performance, so it doesn't guarantee your task will run exactly when you want—system conditions (battery, network, device activity) affect scheduling.
• Local notifications are the best way to alert users when your check triggers, since the app is in the background/terminated and can't interact directly.
• Always call the completion handlers (completionHandler or setTaskCompleted)—failing to do so will make the system restrict your app's background access over time.
• For testing, don't just kill the app manually. Use Xcode's Debug > Simulate Background Fetch or test on a real device—simulators don't fully replicate real background behavior.

内容的提问来源于stack exchange，提问作者jay123456