使用

1，申请后台权限

勾选后，会自动在app的info.plist文件中添加Required background modes一项，包含了所勾选的后台运行模式。如下所示：

    <key>UIBackgroundModes</key>
    <array>
        <string>fetch</string>
    </array>

2，设置获取时间间隔

在- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions方法中设置时间间隔

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

如果不对最小后台获取间隔进行设定的话，系统将使用默值UIApplicationBackgroundFetchIntervalNever，也就是永远不进行后台获取。而最小的时间间隔则有系统根据电量、网络状态、用户使用习惯等综合考量后来设定一定的差值，执行fetch任务

3，fetch任务

在- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler中执行任务

-(void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler{
    // 刷新数据
    completionHandler(UIBackgroundFetchResultNewData);
}

completionHandler回调返回状态

typedef NS_ENUM(NSUInteger, UIBackgroundFetchResult) {
    UIBackgroundFetchResultNewData,
    UIBackgroundFetchResultNoData,
    UIBackgroundFetchResultFailed
} NS_ENUM_AVAILABLE_IOS(7_0);

查看是否开启 Background Fetch 功能

[UIApplication sharedApplication].backgroundRefreshStatus

typedef NS_ENUM(NSInteger, UIBackgroundRefreshStatus) {
    UIBackgroundRefreshStatusRestricted, //< unavailable on this system due to device configuration; the user cannot enable the feature
    UIBackgroundRefreshStatusDenied,     //< explicitly disabled by the user for this application
    UIBackgroundRefreshStatusAvailable   //< enabled for this application
} API_AVAILABLE(ios(7.0), tvos(11.0));

如何测试 Background Fetch 功能

在实际的IOS7环境中，Fetch事件是由系统管理的，app开发者无法预先知道Fetch事件达到的时机。

但XCode也提供了Fetch事件的调试办法，在XCode上运行程序后，在Debug->Simulate Background Fetch.

还有一种情况是app没有运行(不在前台也不在后台)，被Fetch事件唤醒执行.这种情况的测试方法如下:
Product->Scheme->Edit scheme 在Debug模式选中Options,点选Launch due to a background fetch event，运行即可