iOS系统在后台执行程序时，有严格的限制，为了更好地管理资源和电池寿命，iOS会限制应用程序在后台的运行时间。然而，iOS提供了一些特定的策略和技术，使得应用程序可以在特定场景下保持后台运行（即“后台保活”）。以下是iOS中几种常见的后台保活方案，并附上示例代码:

一、后台任务

利用beginBackgroundTask和endBackgroundTask来执行后台任务。后台任务将在应用程序进入后台时仍能保持有限的时间执行任务.

#import <UIKit/UIKit.h>

@interface AppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;
@property (assign, nonatomic) UIBackgroundTaskIdentifier bgTask;

@end

@implementation AppDelegate

- (void)applicationDidEnterBackground:(UIApplication *)application {
    self.bgTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
        [[UIApplication sharedApplication] endBackgroundTask:self.bgTask];
        self.bgTask = UIBackgroundTaskInvalid;
    }];
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 在这里执行你的后台任务
        for (int i = 0; i < 100; i++) {
            NSLog(@"Background task running %d", i);
            [NSThread sleepForTimeInterval:1];
        }
        
        [[UIApplication sharedApplication] endBackgroundTask:self.bgTask];
        self.bgTask = UIBackgroundTaskInvalid;
    });
}

@end

二、使用Background Fetch

利用Background Fetch，系统会间歇性地唤醒应用程序，以便它可以执行任务或获取数据。需要在Xcode的“Capabilities”中开启Background Modes，并勾选“Background fetch”.

#import <UIKit/UIKit.h>

@interface AppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    [application setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];
    return YES;
}

- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler {
    // 在这里执行你的后台数据获取任务
    NSLog(@"Background fetch started");

    // 模拟数据获取
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"Background fetch completed");
        completionHandler(UIBackgroundFetchResultNewData);
    });
}

@end

三、使用远程通知（Silent Push Notification）

利用远程通知，在接收到通知时，系统会唤醒应用程序执行指定的任务。需要开启Remote notifications，在Application Capabilities中勾选“Remote notifications”.

#import <UIKit/UIKit.h>
#import <UserNotifications/UserNotifications.h>

@interface AppDelegate : UIResponder <UIApplicationDelegate, UNUserNotificationCenterDelegate>

@property (strong, nonatomic) UIWindow *window;

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    [UNUserNotificationCenter currentNotificationCenter].delegate = self;
    [[UNUserNotificationCenter currentNotificationCenter] requestAuthorizationWithOptions:(UNAuthorizationOptionAlert | UNAuthorizationOptionSound | UNAuthorizationOptionBadge) completionHandler:^(BOOL granted, NSError * _Nullable error) {
        if (granted) {
            [[UIApplication sharedApplication] registerForRemoteNotifications];
        }
    }];
    return YES;
}

- (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo fetchCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler {
    // 在这里处理收到的远程通知
    NSLog(@"Received remote notification");

    // 模拟处理任务
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"Handled remote notification");
        completionHandler(UIBackgroundFetchResultNewData);
    });
}

@end

四、使用特定的后台模式（Background Modes）

iOS提供了一些特定的后台模式，允许程序在后台持续运行。常见的后台模式包括:

• Audio: 允许应用程序在后台播放音频。
• Location: 允许应用程序在后台持续获取位置更新。
• VoIP: 允许应用程序在后台侦听VoIP事件。
• Bluetooth: 允许应用程序与蓝牙设备通信。

1. Audio后台模式

需要在Xcode的“Capabilities”中开启Background Modes，并勾选“Audio, AirPlay, and Picture in Picture”.

#import <AVFoundation/AVFoundation.h>

@interface AppDelegate ()

@property (nonatomic, strong) AVAudioPlayer *audioPlayer;

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    NSError *error = nil;
    NSURL *audioURL = [[NSBundle mainBundle] URLForResource:@"audioFileName" withExtension:@"mp3"];
    self.audioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:audioURL error:&error];
    [self.audioPlayer prepareToPlay];
    
    AVAudioSession *audioSession = [AVAudioSession sharedInstance];
    [audioSession setCategory:AVAudioSessionCategoryPlayback error:&error];
    [audioSession setActive:YES error:&error];
    
    return YES;
}

- (void)applicationDidEnterBackground:(UIApplication *)application {
    [self.audioPlayer play];
}

@end

2. Location后台模式

需要在Xcode的“Capabilities”中开启Background Modes，并勾选“Location updates”.

#import <CoreLocation/CoreLocation.h>

@interface AppDelegate () <CLLocationManagerDelegate>

@property (nonatomic, strong) CLLocationManager *locationManager;

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    self.locationManager = [[CLLocationManager alloc] init];
    self.locationManager.delegate = self;
    [self.locationManager requestAlwaysAuthorization];
    return YES;
}

- (void)applicationDidEnterBackground:(UIApplication *)application {
    self.locationManager.desiredAccuracy = kCLLocationAccuracyBest;
    [self.locationManager startUpdatingLocation];
}

- (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray<CLLocation *> *)locations {
    CLLocation *location = [locations lastObject];
    NSLog(@"Background location: %@", location);
}

@end

五、使用后台URLSession

使用NSURLSession来执行后台下载和上传任务。需要在后台配置中开启Background Modes，并勾选“Background fetch”和“Remote notifications”.

#import <UIKit/UIKit.h>

@interface AppDelegate : UIResponder <UIApplicationDelegate, NSURLSessionDelegate, NSURLSessionDownloadDelegate>

@property (strong, nonatomic) UIWindow *window;
@property (nonatomic, strong) NSURLSession *backgroundSession;

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    NSURLSessionConfiguration *config = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:@"com.example.background"];
    self.backgroundSession = [NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:nil];
    return YES;
}

- (void)applicationDidEnterBackground:(UIApplication *)application {
    NSURL *url = [NSURL URLWithString:@"http://example.com/largefile.zip"];
    NSURLSessionDownloadTask *downloadTask = [self.backgroundSession downloadTaskWithURL:url];
    [downloadTask resume];
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
    NSLog(@"Download completed: %@", location);
    // 处理下载结果，比如保存文件
}

@end

通过上述几种方案，我们可以在iOS应用程序中实现各种场景下的后台保活。每种方案都有其适用的场景和限制，开发者需要根据应用的实际需求和系统提供的特性，选择合适的后台保活方案.

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