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：
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
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
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
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：
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
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
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。