NSURLSessionConfiguration
创建 NSURLSessionConfiguration 对象的三种方式：
// 默认会话配置
NSURLSessionConfiguration *defaultConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
// 短暂会话配置，无缓存（caches），cookies 和证书（credentials）
NSURLSessionConfiguration *ephemeralConfig = [NSURLSessionConfiguration ephemeralSessionConfiguration];
// 后台会话配置
NSURLSessionConfiguration *backgroundConfig = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:@"com.Apple.BackgroundDownload.BackgroundSession"];
NSURLSession
创建 NSURLSession 对象的三种方式：
// 1. 单例类会话,便捷创建方法
NSURLSession *sharedSession = [NSURLSession sharedSession];
// 2. sessionWithConfiguration:
// 一般用 Block 处理服务器回调时使用
// 默认会话
NSURLSession *defaultSession = [NSURLSession sessionWithConfiguration:defaultConfig];
// 短暂会话
NSURLSession *ephemeralSesson = [NSURLSession sessionWithConfiguration:ephemeralConfig];
// 后台会话
NSURLSession *backgroundSession = [NSURLSession sessionWithConfiguration:backgroundConfig];
// 3. sessionWithConfiguration:delegate:delegateQueue:
// 需要实现委托协议或者指定线程的创建方法
NSOperationQueue *operationQueue = [NSOperationQueue mainQueue];
// 默认会话
NSURLSession *defalutSession1 = [NSURLSession sessionWithConfiguration:defaultConfig delegate:self delegateQueue:operationQueue];
// 短暂会话
NSURLSession *ephemeralSesson1 = [NSURLSession sessionWithConfiguration:ephemeralConfig delegate:self delegateQueue:operationQueue];
// 后台会话
NSURLSession *backgroundSession1 = [NSURLSession sessionWithConfiguration:backgroundConfig delegate:self delegateQueue:operationQueue];
NSURLSessionTask
创建 NSURLSessionTask 对象有四种种方式：
- 数据任务(NSURLSessionDataTask)
- 上传任务(NSURLSessionUploadTask)
- 下载任务(NSURLSessionDownloadTask)
- 流任务（NSURLSessionStreamTask）：WWDC2015新增，提供 TCP/IP 连接接口。
NSURLSessionDelegate 委托协议
示例代码
注：该项目的 源码
// *************************************************
// SCViewController.h
#import <UIKit/UIKit.h>
@interface SCViewController : UIViewController
@property (strong, nonatomic) NSURLSessionDownloadTask *resumableTask; // 可恢复任务
@property (strong, nonatomic) NSURLSessionDownloadTask *backgroundTask; // 后台任务
@property (strong, nonatomic, readonly) NSURLSession *backgroundSession; // 后台会话
@property (weak, nonatomic) IBOutlet UIImageView *imageView;
@property (weak, nonatomic) IBOutlet UIProgressView *progressIndicator;
@property (strong, nonatomic) IBOutletCollection(UIBarButtonItem) NSArray *startButtons;
- (IBAction)startCancellable:(id)sender; // 开始任务
- (IBAction)cancelCancellable:(id)sender; // 取消任务
- (IBAction)startResumable:(id)sender; // 恢复任务
- (IBAction)startBackground:(id)sender; // 后台任务
@end
// *************************************************
// SCViewController.m
#import "SCViewController.h"
#import "SCAppDelegate.h"
@interface SCViewController () <NSURLSessionDownloadDelegate> {
NSURLSessionDownloadTask *cancellableTask; // 可取消任务
NSURLSession *inProcessSession; // 私有会话
NSData *partialDownload; // 可恢复任务下载的临时数据
}
@end
@implementation SCViewController
#pragma mark - Lifecycle
- (void)viewDidLoad
{
[super viewDidLoad];
// 从 nib 文件加载视图后，进行额外的初始化工作
self.progressIndicator.hidden = YES;
self.progressIndicator.progress = 0;
// Make sure that we've attached to the background session
self.backgroundSession.sessionDescription = @"Background session";
}
#pragma mark - Custom Accessors
// 后台会话，单例类
- (NSURLSession *)backgroundSession
{
static NSURLSession *backgroundSession = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
NSURLSessionConfiguration *config = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:@"com.shinobicontrols.BackgroundDownload.BackgroundSession"];
backgroundSession = [NSURLSession sessionWithConfiguration:config
delegate:self
delegateQueue:nil];
});
return backgroundSession;
}
#pragma mark - IBActions
// 开始任务
- (IBAction)startCancellable:(id)sender {
if (!cancellableTask) {
if(!inProcessSession) {
// 创建默认会话
NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
inProcessSession = [NSURLSession sessionWithConfiguration:sessionConfig
delegate:self
delegateQueue:nil];
inProcessSession.sessionDescription = @"in-process NSURLSession";
}
// Image CreativeCommons courtesy of flickr.com/charliematters
NSString *url = @"http://farm6.staticflickr.com/5505/9824098016_0e28a047c2_b_d.jpg";
NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:url]];
// 创建可取消任务
cancellableTask = [inProcessSession downloadTaskWithRequest:request];
[self setDownloadButtonsAsEnabled:NO];
self.imageView.hidden = YES;
// 开启下载任务
[cancellableTask resume];
}
}
// 取消任务
- (IBAction)cancelCancellable:(id)sender {
if(cancellableTask) {
// 1. 可取消任务
[cancellableTask cancel];
cancellableTask = nil;
} else if(self.resumableTask) {
// 2. 可恢复任务
[self.resumableTask cancelByProducingResumeData:^(NSData *resumeData) {
// ❇️ 缓存已下载数据
partialDownload = resumeData;
self.resumableTask = nil;
}];
} else if(self.backgroundTask) {
// 3. 后台任务
[self.backgroundTask cancel];
self.backgroundTask = nil;
}
}
// 恢复任务
- (IBAction)startResumable:(id)sender {
if(!self.resumableTask) {
if(!inProcessSession) {
// 创建默认会话
NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
inProcessSession = [NSURLSession sessionWithConfiguration:sessionConfig delegate:self delegateQueue:nil];
inProcessSession.sessionDescription = @"in-process NSURLSession";
}
if(partialDownload) {
// 1. 如果存在缓存数据，继续下载此任务
self.resumableTask = [inProcessSession downloadTaskWithResumeData:partialDownload];
} else {
// 2. 如果不存在缓存，新建可恢复任务
// Image CreativeCommons courtesy of flickr.com/charliematters
NSString *url = @"http://farm3.staticflickr.com/2846/9823925914_78cd653ac9_b_d.jpg";
NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:url]];
// 创建可恢复任务
self.resumableTask = [inProcessSession downloadTaskWithRequest:request];
}
[self setDownloadButtonsAsEnabled:NO];
self.imageView.hidden = YES;
// 开启下载任务
[self.resumableTask resume];
}
}
// 开始后台任务
- (IBAction)startBackground:(id)sender {
// Image CreativeCommons courtesy of flickr.com/charliematters
NSString *url = @"http://farm3.staticflickr.com/2831/9823890176_82b4165653_b_d.jpg";
NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:url]];
// self.backgroundSession 是单例对象
self.backgroundTask = [self.backgroundSession downloadTaskWithRequest:request];
[self setDownloadButtonsAsEnabled:NO];
self.imageView.hidden = YES;
// 开启下载任务
[self.backgroundTask resume];
}
#pragma mark - Private
- (void)setDownloadButtonsAsEnabled:(BOOL)enabled
{
// 更新按钮 enable 状态
for(UIBarButtonItem *btn in self.startButtons) {
btn.enabled = enabled;
}
}
#pragma mark - NSURLSessionDownloadDelegate
// 追踪下载任务进度，更新进度条
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
double currentProgress = totalBytesWritten / (double)totalBytesExpectedToWrite;
dispatch_async(dispatch_get_main_queue(), ^{
self.progressIndicator.hidden = NO;
self.progressIndicator.progress = currentProgress;
});
}
// 恢复下载后调用
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didResumeAtOffset:(int64_t)fileOffset expectedTotalBytes:(int64_t)expectedTotalBytes
{
// Leave this for now
}
// 下载任务成功后调用
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
{
// We've successfully finished the download. Let's save the file
// 保存到文件
NSFileManager *fileManager = [NSFileManager defaultManager];
NSArray *URLs = [fileManager URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask];
NSURL *documentsDirectory = URLs[0];
NSURL *destinationPath = [documentsDirectory URLByAppendingPathComponent:[location lastPathComponent]];
NSError *error;
// 先删除文件中已经存在的任何文件
[fileManager removeItemAtURL:destinationPath error:NULL];
BOOL success = [fileManager copyItemAtURL:location toURL:destinationPath error:&error];
if (success) {
// 保存到文件后，更新主线程 UI
dispatch_async(dispatch_get_main_queue(), ^{
UIImage *image = [UIImage imageWithContentsOfFile:[destinationPath path]];
self.imageView.image = image;
self.imageView.contentMode = UIViewContentModeScaleAspectFill;
self.imageView.hidden = NO;
});
}else {
NSLog(@"Couldn't copy the downloaded file");
}
// ❇️ 会话置空，解除循环引用
if(downloadTask == cancellableTask) {
cancellableTask = nil;
} else if (downloadTask == self.resumableTask) {
self.resumableTask = nil;
partialDownload = nil;
} else if (session == self.backgroundSession) {
// 后台会话
self.backgroundTask = nil;
// Get hold of the app delegate
SCAppDelegate *appDelegate = (SCAppDelegate *)[[UIApplication sharedApplication] delegate];
if(appDelegate.backgroundURLSessionCompletionHandler) {
// Need to copy the completion handler
void (^handler)() = appDelegate.backgroundURLSessionCompletionHandler;
appDelegate.backgroundURLSessionCompletionHandler = nil;
handler();
}
}
}
#pragma mark - NSURLSessionTaskDelegate
// 每一个任务结束的时候执行-NSURLSessionDelegate,不管任务是不是正常结束:
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
// 主线程更新 UI
dispatch_async(dispatch_get_main_queue(), ^{
self.progressIndicator.hidden = YES;
[self setDownloadButtonsAsEnabled:YES];
});
}
@end
开启后台下载任务时，即使关闭了应用程序，这个下载任务依然在后台运行。
当这个下载任务完成后，iOS将会重新启动app来让任务知道，对这个我们不必放在心上。为了达到这个目的，我们可以在app的代理中调用下面的代码:
- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler
{
self.backgroundURLSessionCompletionHandler = completionHandler;
}
效果
