废话不说，上效果图：
Untitled1.gif
部分一：NSURLSession
@interface sessionViewController ()<NSURLSessionDataDelegate>
// 进度条
@property (nonatomic,strong)UIProgressView *progressView;
// 下载百分比显示
@property (nonatomic,strong)UILabel *textLabel;
// 下载按钮
@property (nonatomic,strong)UIButton *btn;
// 取消按钮
@property (nonatomic,strong)UIButton *closeBtn;
@property (nonatomic,strong)NSURLSessionDataTask *downloadTask;
@property (nonatomic,strong)NSURLSession *session;
// 文件句柄
@property (nonatomic,strong)NSFileHandle *filehandle;
// 大文件的总大小
@property (nonatomic,assign)double totalLenght;
// 已下载的文件大小
@property (nonatomic,assign)double currentLenght;
界面的布局
- (void)viewDidLoad {
[super viewDidLoad];
self.view.backgroundColor = [UIColor whiteColor];
// 界面
UIProgressView *progressView = [[UIProgressView alloc] initWithFrame:CGRectMake(20, 100, 300, 5)];
progressView.tintColor = [UIColor redColor];
progressView.trackTintColor = [UIColor lightGrayColor];
self.progressView = progressView;
[self.view addSubview:progressView];
UIButton *closeBtn = [[UIButton alloc] initWithFrame:CGRectMake(330, 90, 20, 20)];
[closeBtn setImage:[UIImage imageNamed:@"close"] forState:UIControlStateNormal];
[closeBtn addTarget:self action:@selector(closeClick:) forControlEvents:UIControlEventTouchUpInside];
self.closeBtn = closeBtn;
self.closeBtn.enabled = NO;
[self.view addSubview:closeBtn];
UILabel *textLabel = [[UILabel alloc] initWithFrame:CGRectMake(20, 130, 100, 20)];
textLabel.font = [UIFont fontWithName:@"American Typewriter" size:24];
self.textLabel = textLabel;
[self.view addSubview:textLabel];
UIButton *btn = [[UIButton alloc] initWithFrame:CGRectMake(150, 200, 50, 50)];
[btn setImage:[UIImage imageNamed:@"start"] forState:UIControlStateNormal];
[btn setImage:[UIImage imageNamed:@"stop"] forState:UIControlStateSelected];
[btn addTarget:self action:@selector(btnClick:) forControlEvents:UIControlEventTouchUpInside];
self.btn = btn;
[self.view addSubview:btn];
}
效果：
Snip20170524_7.png
下载任务的创建，因为需要断点续传所以设置请求头，并且调用停止任务的方法[self.downloadTask suspend],点击下载时，需要每次调用createDownTask方法判断需要从哪里开始下载，已经下载好的无需再次下载。
// 创建任务
- (void)createDownTask{
NSURLSessionConfiguration *cfg = [NSURLSessionConfiguration defaultSessionConfiguration];// 默认配置
NSURLSession *session = [NSURLSession sessionWithConfiguration:cfg delegate:self delegateQueue:[NSOperationQueue mainQueue]];
self.session = session;
NSURL *downURL = [NSURL URLWithString:@"http://dlsw.baidu.com/sw-search-sp/soft/9d/25765/sogou_mac_32c_V3.2.0.1437101586.dmg"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:downURL];
//设置请求头
NSString *cache = CACHE;
NSString *fileName = [cache stringByAppendingPathComponent:DOWNNAME];
NSString *range = [NSString stringWithFormat:@"bytes=%zd-",[[[NSFileManager defaultManager] attributesOfItemAtPath:fileName error:nil][NSFileSize] intValue]];
[request setValue:range forHTTPHeaderField:@"Range"];
NSURLSessionDataTask *downloadTask = [self.session dataTaskWithRequest:request];
if(self.downloadTask){
self.downloadTask = nil;
}
self.downloadTask = downloadTask;
if ([[[NSFileManager defaultManager] attributesOfItemAtPath:fileName error:nil][NSFileSize] intValue] != 0) {
self.closeBtn.enabled = YES;
}
}
// 开始/暂停任务
- (void)btnClick:(UIButton *)btn{
btn.selected = !btn.isSelected;
if (btn.isSelected) {
self.closeBtn.enabled = YES;
[self createDownTask];
[self.downloadTask resume];
}else {
[self.downloadTask suspend];
}
}
// 默认下session会一直下载，需要手动关闭
- (void)viewDidDisappear:(BOOL)animated{
[super viewDidDisappear:animated];
[self.downloadTask suspend];
}
关闭按钮，点击取消下载任务。思路是讲内存中保存的文件删去。至于关闭按钮什么时候点击，什么时候不能点击，可以根据自己的实际情况进行选择。我的demo是只要没有下载完或者还没有开始下载就可以进行点击进行取消。[self.session invalidateAndCancel];方法进行取消，注意在实际中invalidateAndCancel需要对session再进行创建才能再下载。
// 手动关闭下载任务，清除内存
- (void)closeClick:(UIButton *)sender{
[self.session invalidateAndCancel];
UIAlertController *alertC = [UIAlertController alertControllerWithTitle:@"温馨提示" message:@"确定取消任务？" preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction *alertA = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
self.progressView.progress = 0.00;
self.textLabel.text = @"0.00%";
NSString *cache = CACHE;
NSString *fileName = [cache stringByAppendingPathComponent:DOWNNAME];
NSFileManager *mgr = [NSFileManager defaultManager];
[mgr removeItemAtPath:fileName error:nil];
self.closeBtn.enabled = NO;
self.btn.selected = NO;
}];
UIAlertAction *alertB = [UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleCancel handler:^(UIAlertAction * _Nonnull action) {
[alertC dismissViewControllerAnimated:YES completion:nil];
_btn.selected = NO;
[self btnClick:self.btn];
}];
[alertC addAction:alertA];
[alertC addAction:alertB];
[self presentViewController:alertC animated:YES completion:nil];
}
NSURLSession代理方法，用文件的大小是否一样判断是否需要下载，用句柄进行文件保存
/**
* 1.接收到服务器的响应就会调用
*
* @param response 响应
*/
#pragma mark
#pragma make - 判断是否已经下载过
- (void)URLSession:(NSURLSession *)session dataTask:(nonnull NSURLSessionDataTask *)dataTask didReceiveResponse:(nonnull NSURLResponse *)response completionHandler:(nonnull void (^)(NSURLSessionResponseDisposition))completionHandler{
NSFileManager *mgr = [NSFileManager defaultManager];
NSString *cache = CACHE;
NSString *fileName = [cache stringByAppendingPathComponent:DOWNNAME];
self.currentLenght = [[mgr attributesOfItemAtPath:fileName error:nil][NSFileSize] doubleValue];
// 下载文件的总长度
self.totalLenght = (double)response.expectedContentLength + self.currentLenght;
if (![mgr fileExistsAtPath:fileName]) {
[mgr createDirectoryAtPath:cache withIntermediateDirectories:YES attributes:nil error:nil];
[mgr createFileAtPath:fileName contents:nil attributes:nil];
}else if([[mgr attributesOfItemAtPath:fileName error:nil][NSFileSize] doubleValue] == response.expectedContentLength + self.currentLenght){
self.progressView.progress = 1.0;
self.textLabel.text = @"1.00%";
completionHandler(NSURLSessionResponseCancel);
[self createAlterView];
return;
}
// 文件的句柄
self.filehandle = [NSFileHandle fileHandleForUpdatingAtPath:fileName];
self.progressView.progress = self.currentLenght / self.totalLenght;
self.textLabel.text = [NSString stringWithFormat:@"%.2f%%",self.progressView.progress];
completionHandler(NSURLSessionResponseAllow);
}
/**
* 2.当接收到服务器返回的实体数据时调用（具体内容，这个方法可能会被调用多次）
*
* @param data 这次返回的数据
*/
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data{
// 句柄移动到文件的末尾
[self.filehandle seekToEndOfFile];
// 写入文件
[self.filehandle writeData:data];
// 累计长度
self.currentLenght += data.length;
// 显示进度
self.progressView.progress = self.currentLenght / self.totalLenght;
self.textLabel.text = [NSString stringWithFormat:@"%.2f%%",self.currentLenght / self.totalLenght];
}
/**
* 3.加载完毕后调用（服务器的数据已经完全返回后）
*/
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error{
[self.filehandle closeFile];
self.filehandle = nil;
[self createAlterView];
}
// 提示出下载文件的大小和存储的位置
- (void)createAlterView{
self.btn.selected = NO;
self.closeBtn.enabled = NO;
NSString *cache = CACHE;
NSString *fileName = [cache stringByAppendingPathComponent:DOWNNAME];
NSFileManager *mgr = [NSFileManager defaultManager];
NSDictionary<NSFileAttributeKey, id> *dict = [mgr attributesOfItemAtPath:fileName error:nil];
NSInteger fileSize = [dict[NSFileSize] doubleValue] / 1024.0 / 1024.0;
NSString *fileSizeString = [NSString stringWithFormat:@"%.2ld",(long)fileSize];
if ([dict[NSFileSize] doubleValue] == self.totalLenght) {
NSString *message = [NSString stringWithFormat:@"文件大小:%@M 文件地址：%@",fileSizeString,fileName];
UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"下载完成" message:message preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction *sureBtn = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
[self dismissViewControllerAnimated:YES completion:nil];
}];
// 下载文件的属性
[alertController addAction:sureBtn];
[self presentViewController:alertController animated:YES completion:nil];
}
}
至于NSURLConnection，我就不就行阐述了，思路一样。感兴趣的朋友可以看github中的WMDownTask，其中NSURLConnection中的文件保存由句柄改成数据流方法。
总结
可以在下载页面开始时展示之前下载的进度，思路是在viewDidAppear或者viewWillAppear方法里面将文件大小从内存中读出，并且总大小保存在内存中。此外对于大文件的多线程的下载可以分成多个线程放到不同队列中下载。
