1.用downloadTask下载图片

优点：简单

缺点：不能监听下载的进度

代码示例：

    NSURL *url = [NSURL URLWithString:@"http://pic1.win4000.com/pic/b/03/21691230681.jpg"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    
    NSURLSession *session = [NSURLSession sharedSession];
    NSURLSessionDownloadTask *downloadTask = [session downloadTaskWithRequest:request completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
       //默认把数据写到磁盘中：tmp/...随时可能被删除
        NSLog(@"location= %@", location);
        
        //转移文件
        NSString *cache = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES)  lastObject];
        NSString *filePath = [cache stringByAppendingPathComponent:response.suggestedFilename];
        NSLog(@"filePath = %@",filePath);
        NSURL *toURL = [NSURL fileURLWithPath:filePath];
        [[NSFileManager defaultManager] moveItemAtURL:location toURL:toURL error:nil];
    }];
    [downloadTask resume];

 

2.downloadTask下载比较大的文件

优点：已经解决内存飚升的问题

缺点：无法实现断点下载的功能

代码如下：

#import "ViewController.h"

@interface ViewController ()<NSURLSessionDownloadDelegate>

@property (nonatomic,strong) NSURLSession *session;
@property (nonatomic,strong) NSURLSessionDownloadTask *downloadTask;
@property (nonatomic,strong) NSData *resumeData ;

@end

@implementation ViewController

- (IBAction)startClick:(id)sender {
    
    [self.downloadTask resume];
}

- (IBAction)suspendClick:(id)sender {
    [self.downloadTask suspend];
}

- (IBAction)cancleClick:(id)sender {
    //用cancel的方法，操作不可以恢复
//    [self.downloadTask cancel];
    
    //取消，可以恢复的取消操作
    //resumeDta可以用来恢复下载的数据
    [self.downloadTask cancelByProducingResumeData:^(NSData * _Nullable resumeData) {
        //data不是沙盒中已经下载好的数据
        self.resumeData = resumeData;
    }];
    
    self.downloadTask = nil;
}

- (IBAction)resumeClick:(id)sender {
    //在恢复下载的时候，判断是否可以用来进行恢复下载的数据，如果有那么就根据该数据创建一个新的网络请求
    if (self.resumeData) {
        //取消 再恢复，在恢复的时候，需要重新创建
        self.downloadTask = [self.session downloadTaskWithResumeData:self.resumeData];
    }
    [self.downloadTask resume];
}

- (NSURLSession *)session {
    if (!_session) {
        _session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:[NSOperationQueue mainQueue]];
    }
    return _session;
}

- (NSURLSessionDownloadTask *)downloadTask {
    if (!_downloadTask) {
        NSURL *url = [NSURL URLWithString:@"http://meiye-mbs.oss-cn-shenzhen.aliyuncs.com/mbsFiles/0e3d0e4a0d5d4da5963e9e7617e8de101565841097849.mp4"];
        NSURLRequest *request = [NSURLRequest requestWithURL:url];
        _downloadTask = [self.session downloadTaskWithRequest:request];
    }
    return _downloadTask;
}

#pragma mark - 代理
//01 写数据的时候调用
/**
 bytesWritten:本次写入的数据大小
 totalBytesWritten：写入数据的总大小
 totalBytesExpectedToWrite:文件的总大小
 */
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
    NSLog(@"111111=%f", 1.0 * totalBytesWritten / totalBytesExpectedToWrite);
}

//02 下载完成的时候调用
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
    //转移文件
    NSString *cache = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES)  lastObject];
    NSString *filePath = [cache stringByAppendingPathComponent:downloadTask.response.suggestedFilename];
    NSLog(@"filePath = %@",filePath);
    NSURL *toURL = [NSURL fileURLWithPath:filePath];
    [[NSFileManager defaultManager] moveItemAtURL:location toURL:toURL error:nil];
}

//03  整个请求结束或者失败的时候调用
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    NSLog(@"结束");
}
@end