URL Download delegate

--显示进度条,逐渐显示已下载的图片
#import "ViewController.h"

@interface ViewController ()<NSURLSessionDelegate,NSURLSessionTaskDelegate,NSURLSessionDataDelegate>
@property (weak, nonatomic) IBOutlet UIProgressView *progressView;
@property (weak, nonatomic) IBOutlet UIImageView *imageView;
@property (nonatomic,strong) NSURLSession *session;
@property (nonatomic,strong) NSURLSessionDataTask *dataTask;
@property (nonatomic,strong) NSMutableData *imageData;//保存图片数据
@property (nonatomic,assign) long long totalLength;//数据总长度
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
}
-(NSURLSession *)session{
    if (_session ==nil) {
        NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration ephemeralSessionConfiguration];
        self.session = [NSURLSession sessionWithConfiguration:sessionConfig delegate:self delegateQueue:[NSOperationQueue mainQueue]];
    }
    return _session;
}

-(NSURLSessionDataTask *)dataTask{
    if (_dataTask == nil) {
        NSURL *url = [NSURL URLWithString:@"http://120.25.226.186:32812/resources/videos/minion_01.mp4"];
        self.dataTask = [self.session dataTaskWithURL:url];
    }
    return _dataTask;
}


- (IBAction)CancelButtonClicked:(id)sender {
    
    if (_dataTask) {
        if (self.dataTask.state == NSURLSessionTaskStateRunning ||self.dataTask.state == NSURLSessionTaskStateSuspended) {
            [self.dataTask cancel];
            self.dataTask = nil;
            self.session = nil;
        }
    }
}
- (IBAction)SuspendedButtonClicked:(id)sender {
    if (self.dataTask.state == NSURLSessionTaskStateRunning ) {
        [self.dataTask suspend];
    }
    
}
- (IBAction)ResumeButtonClicked:(id)sender {
    //开启下载任务
    [self.dataTask resume];
}
#pragma mark -----------------delegate-------------------
//获取服务器返回的相应信息
-(void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition))completionHandler{
    //将urlresponse ->nshttpurlresponse
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
    //获取资源的大小
    self.totalLength = httpResponse.expectedContentLength;
    
    //创建保存这个数据的容器
    self.imageData = [NSMutableData dataWithCapacity:_totalLength];
    //确定下面如何操作
    if (_totalLength != -1) {
        //继续下载或者上传任务
        completionHandler(NSURLSessionResponseAllow);
    }else{
        //取消数据传输，请求的数据没有
        completionHandler(NSURLSessionResponseCancel);
        
        [self.dataTask cancel];
        self.dataTask = nil;
    }
}

//一个数据包过来了
-(void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data{
    //将当前的这个数据添加到imageData
    [self.imageData appendData:data];
    //计算进度比例，数据包的百分比
    float rate = data.length/(_totalLength*1.0);
    
    //进度条改变
    [self.progressView setProgress:(_progressView.progress + rate) animated:YES];
}
//下载完毕

-(void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error{
    NSLog(@"complete");
    if (error) {
        NSLog(@"error 下载失败");
    }else{
        UIImage *image = [UIImage imageWithData:_imageData];
        self.imageView.image = image;
    }
    self.session = nil;
    self.dataTask = nil;
}
@end