iOS后台保活

iOS后台保活按时间可分为短时保活和长时间保活

短时保活的方式
通过beginBackgroundTaskWithName来实现。在iOS7-iOS13可以申请到大约3分钟的保活时间，在iOS 13以后只能申请30秒左右的时间。
1. 先通过监听UIApplicationWillEnterForegroundNotification（应用进入前台通知）和UIApplicationDidEnterBackgroundNotification（应用进入后台通知）。

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appWillEnterForeground) name:UIApplicationWillEnterForegroundNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appDidEnterBackground) name:UIApplicationDidEnterBackgroundNotification object:nil];
- (void)appWillEnterForeground {}

- (void)appDidEnterBackground {}

使用Background Task在应用进入后台时开启保活，进入前台时关闭保活。

@property (assign, nonatomic) UIBackgroundTaskIdentifier backgroundId;
- (void)appWillEnterForeground {
   [self stopKeepAlive];
}

- (void)appDidEnterBackground {
    _backgroundId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
        //申请的时间即将到时回调该方法
        NSLog(@"BackgroundTask time gone");
        [self stopKeepAlive];
    }];
}

- (void)stopKeepAlive{
  if (_backgroundId) {
        [[UIApplication sharedApplication] endBackgroundTask:_backgroundId];
        _backgroundId = UIBackgroundTaskInvalid;
    }
}

如果想申请多一点时间，可以使用NSTimer循环申请保活时间，但是建议不要无限申请保活时间，因为系统如果发现该app一直在后台运行，可能会直接杀掉app。

//开启定时器 不断向系统请求后台任务执行的时间
NSTimer *_timer = [NSTimer scheduledTimerWithTimeInterval:5 target:self selector:@selector(applyForMoreTime) userInfo:nil repeats:YES];
[_timer fire];

//在这里我判断了申请次数，加上第一次申请保活时间的次数一共6次。
@property(nonatomic,assign) int applyTimes;
-(void)applyForMoreTime {
    if ([UIApplication sharedApplication].backgroundTimeRemaining < 10) {
        _applyTimes += 1;
        NSLog(@"Try to apply for more time:%d",_applyTimes);
        [[UIApplication sharedApplication] endBackgroundTask:_backIden];
        _backIden = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
            [self stopKeepAlive];
        }];
        if(_applyTimes == 5){
            [_timer invalidate];
            _applyTimes = 0;
            [self stopKeepAlive];
        }
    }
}

长时间保活
App长时间保活的方式有：播放无声音乐、后台持续定位、后台下载资源、BGTaskScheduler等，这些需要在苹果后台开通后台权限，并且在xcode中也开启相关权限。

播放无声音乐，适用于音乐类app。像腾讯视频、爱奇艺等用了播放无声音乐保活的方式。
在app进入后台时开启无声音乐，进入前台后停止无声音乐。（更好的处理方式是先获取短时保活，短时快过时再播放无声音乐）示例如下：
监听进入前后台：

@property (nonatomic, strong) BackgroundPlayer* player;

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appWillEnterForeground) name:UIApplicationWillEnterForegroundNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appDidEnterBackground) name:UIApplicationDidEnterBackgroundNotification object:nil];
- (void)appWillEnterForeground {
    if (self.player) {
        [self.player stopPlayBackgroundAlive];
    }
}

- (void)appDidEnterBackground {
    if (_player == nil) {
        _player = [[BackgroundPlayer alloc] init];
    }
    [self.player startPlayer];  
}

编写音乐播放类：

#import <Foundation/Foundation.h>
#import <AVFoundation/AVFoundation.h>

@interface BackgroundPlayer : NSObject <AVAudioPlayerDelegate>
{
    AVAudioPlayer* _player;
}
- (void)startPlayer;

- (void)stopPlayer;
@end

#import "BackgroundPlayer.h"

@implementation BackgroundPlayer

- (void)startPlayer
{
    if (_player && [_player isPlaying]) {
        return;
    }
    AVAudioSession *session = [AVAudioSession sharedInstance];
    [[AVAudioSession sharedInstance] setMode:AVAudioSessionModeDefault error:nil];

    NSString* route = [[[[[AVAudioSession sharedInstance] currentRoute] outputs] objectAtIndex:0] portType];
    
    if ([route isEqualToString:AVAudioSessionPortHeadphones] || [route isEqualToString:AVAudioSessionPortBluetoothA2DP] || [route isEqualToString:AVAudioSessionPortBluetoothLE] || [route isEqualToString:AVAudioSessionPortBluetoothHFP]) {
        if (@available(iOS 10.0, *)) {
            [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord
                                             withOptions:(AVAudioSessionCategoryOptionMixWithOthers | AVAudioSessionCategoryOptionAllowBluetooth | AVAudioSessionCategoryOptionAllowBluetoothA2DP)
                                                   error:nil];
        } else {
            // Fallback on earlier versions
        }
    }else{
        [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord
                                         withOptions:(AVAudioSessionCategoryOptionMixWithOthers | AVAudioSessionCategoryOptionDefaultToSpeaker)
                                               error:nil];
    }
    
    [session setActive:YES error:nil];
    
    NSURL *url = [[NSBundle bundleWithPath:WECAST_CLOUD_BUNDLE_PATH]URLForResource:@"你的音乐资源" withExtension:nil];
    _player = [[AVAudioPlayer alloc] initWithContentsOfURL:url error:nil];
    [_player prepareToPlay];
    [_player setDelegate:self];
    _player.numberOfLoops = -1;
    BOOL ret = [_player play];
    if (!ret) {
        NSLog(@"play failed,please turn on audio background mode");
    }
}

- (void)stopPlayer
{
    if (_player) {
        [_player stop];
        _player = nil;
        AVAudioSession *session = [AVAudioSession sharedInstance];
        [session setActive:NO error:nil];
        NSLog(@"stop in play background success");
    }
}

@end

后台持续定位
后台下载资源
创建指定标识的后台NSURLSessionConfiguration，配置好。

    NSURL *url = [NSURL URLWithString:@"https://images.pexels.com/photos/3225517/pexels-photo-3225517.jpeg"];
    NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:@"com.qishare.ios.wyw.backgroundDownloadTask"];
// 低于iOS13.0设备资源下载完后 可以得到通知 AppDelegate.m 文件中的 - (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)(void))completionHandler
// iOS13.0+的设备资源下载完后 直接在下载结束的代理方法中会有回调
    sessionConfig.sessionSendsLaunchEvents = YES;
// 当传输大数据量数据的时候，建议将此属性设置为YES，这样系统可以安排对设备而言最佳的传输时间。例如，系统可能会延迟传输大文件，直到设备连接充电器并通过Wi-Fi连接到网络为止。 此属性的默认值为NO。
    sessionConfig.discretionary = YES;
    NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfig delegate:self delegateQueue:nil];
    NSURLSessionDownloadTask *downloadTask = [session downloadTaskWithURL:url];
    [downloadTask resume];