iOS后台进行数据更新和下载

三种方式使得iOS程序即使在关闭或崩溃的情况下也能够在后台持续进行一些任务，比如更新程序界面快照，下载文件等。这三个方法分别是

Background Fetch，Remote Notification和NSURLSession的backgroundSessionConfiguration

Background Fetch

开启

首先在info plist文件中开启UIBackgroundModes的Background fetch。或者手动编辑这个值

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

iOS默认不进行background fetch，需要设置一个时间的间隔

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
//UIApplicationBackgroundFetchIntervalMinimum表示尽可能频繁去获取，如果需要指定至少多少时间更新一次就需要给定一个时间值
[application setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];
return YES;
}

最后在App Delegate里实现下面的方法，这个方法只能在30秒内完成。

- (void) application:(UIApplication *)application
performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfiguration];

NSURL *url = [[NSURL alloc] initWithString:@"http://yourserver.com/data.json"];
NSURLSessionDataTask *task = [session dataTaskWithURL:url
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {

if (error) {
completionHandler(UIBackgroundFetchResultFailed);
return;
}

// 解析响应/数据以决定新内容是否可用
BOOL hasNewData = ...
if (hasNewData) {
completionHandler(UIBackgroundFetchResultNewData);
} else {
completionHandler(UIBackgroundFetchResultNoData);
}
}];

// 开始任务
[task resume];
}

测试

4000

通过查看UIApplication的applicationState

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
NSLog(@"Launched in background %d", UIApplicationStateBackground == application.applicationState);

return YES;
}

Remote Notification

在普通的远程通知里带上content-available标志就可以在通知用户同时在后台进行更新。通知结构如下

{
"aps" : {
"content-available" : 1
},
"content-id" : 42
}

接收一条带有content-available的通知会调用下面的方法

- (void)application:(UIApplication *)application
didReceiveRemoteNotification:(NSDictionary *)userInfo
fetchCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
NSLog(@"Remote Notification userInfo is %@", userInfo);

NSNumber *contentID = userInfo[@"content-id"];
// 根据 content ID 进行操作
completionHandler(UIBackgroundFetchResultNewData);
}

利用NSURLSession进行background transfer task

使用[NSURLSessionConfiguration backgroundSessionConfiguration]创建一个后台任务，当应用退出后，崩溃或进程被关掉都还是会运行。

范例，先处理一条远程通知，并将NSURLSessionDownloadTask添加到后台传输服务队列。

- (NSURLSession *)backgroundURLSession
{
static NSURLSession *session = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
NSString *identifier = @"io.objc.backgroundTransferExample";
NSURLSessionConfiguration* sessionConfig = [NSURLSessionConfiguration backgroundSessionConfiguration:identifier];
session = [NSURLSession sessionWithConfiguration:sessionConfig
delegate:self
delegateQueue:[NSOperationQueue mainQueue]];
});

return session;
}

- (void) application:(UIApplication *)application
didReceiveRemoteNotification:(NSDictionary *)userInfo
fetchCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
NSLog(@"Received remote notification with userInfo %@", userInfo);

NSNumber *contentID = userInfo[@"content-id"];
NSString *downloadURLString = [NSString stringWithFormat:@"http://yourserver.com/downloads/%d.mp3", [contentID intValue]];
NSURL* downloadURL = [NSURL URLWithString:downloadURLString];

NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL];
NSURLSessionDownloadTask *task = [[self backgroundURLSession] downloadTaskWithRequest:request];
task.taskDescription = [NSString stringWithFormat:@"Podcast Episode %d", [contentID intValue]];
//执行resume保证开始了任务
[task resume];

completionHandler(UIBackgroundFetchResultNewData);
}

下载完成后调用NSURLSessionDownloadDelegate的委托方法，这些委托方法全部是必须实现的。了解所有类型session task的生命周期可以参考官方文档：https://developer.apple.com/library/ios/documentation/cocoa/Conceptual/URLLoadingSystem/NSURLSessionConcepts/NSURLSessionConcepts.html#//apple_ref/doc/uid/10000165i-CH2-SW42

#Pragma Mark - NSURLSessionDownloadDelegate

- (void) URLSession:(NSURLSession *)session
downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location
{
NSLog(@"downloadTask:%@ didFinishDownloadingToURL:%@", downloadTask.taskDescription, location);

// 必须用 NSFileManager 将文件复制到应用的存储中，因为临时文件在方法返回后会被删除
// ...

// 通知 UI 刷新
}

- (void) URLSession:(NSURLSession *)session
downloadTask:(NSURLSessionDownloadTask *)downloadTask
didResumeAtOffset:(int64_t)fileOffset
expectedTotalBytes:(int64_t)expectedTotalBytes
{
}

- (void) URLSession:(NSURLSession *)session
downloadTask:(NSURLSessionDownloadTask *)downloadTask
didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
}
后台的任务完成后如果应用没有在前台运行，需要实现UIApplication的两个delegate让系统唤醒应用

- (void) application:(UIApplication *)application
handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler
{
// 你必须重新建立一个后台 seesiong 的参照
// 否则 NSURLSessionDownloadDelegate 和 NSURLSessionDelegate 方法会因为
// 没有 对 session 的 delegate 设定而不会被调用。参见上面的 backgroundURLSession
NSURLSession *backgroundSession = [self backgroundURLSession];

NSLog(@"Rejoining session with identifier %@ %@", identifier, backgroundSession);

// 保存 completion handler 以在处理 session 事件后更新 UI
[self addCompletionHandler:completionHandler forSession:identifier];
}

- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session
{
NSLog(@"Background URL session %@ finished events.
", session);

if (session.configuration.identifier) {
// 调用在 -application:handleEventsForBackgroundURLSession: 中保存的 handler
[self callCompletionHandlerForSession:session.configuration.identifier];
}
}

- (void)addCompletionHandler:(CompletionHandlerType)handler forSession:(NSString *)identifier
{
if ([self.completionHandlerDictionary objectForKey:identifier]) {
NSLog(@"Error: Got multiple handlers for a single session identifier. This should not happen.
");
}

[self.completionHandlerDictionary setObject:handler forKey:identifier];
}

- (void)callCompletionHandlerForSession: (NSString *)identifier
{
CompletionHandlerType handler = [self.completionHandlerDictionary objectForKey: identifier];

if (handler) {
[self.completionHandlerDictionary removeObjectForKey: identifier];
NSLog(@"Calling completion handler for session %@", identifier);

handler();
}
}