前段时间AFNetworking 更新到3.0 ，彻底废弃NSURLConnection的API转由NSURLSession来实现，代码并没有改动很大，AF封装的很好了，读完源码感觉收获很大。
下载不可避免的会遇到多线程异步下载问题，iOS中多线程解决方案一般是GCD和NSOperation。为了下载器便于管理，这里是创建一个类继承于NSOperation重写其start方法即可（GCD版本基本一致，最后会提到其实现代码和思路）
相关代码和下载库地址都在这里：https://github.com/LeeBlaze/LNDownload.git
NSOperation 版本
typedef NS_ENUM(NSInteger, DownloadState){
DownloadState_Ready = ,
DownloadState_Suspend,
DownloadState_Doing,
DownloadState_Success,
DownloadState_Cancel,
DownloadState_Fail,
};
首先定义了下载状态的枚举类型，不多解释，下面看看相关的属性
@protocol LNDownloaderDelegate ; @interface LNDownloader : NSOperation @property (nonatomic, weak)id<LNDownloaderDelegate>delegate;
@property (nonatomic, assign,readonly) DownloadState state; @property (nonatomic, assign,readonly) float downloadRate; @property (nonatomic, assign,readonly) float progress; @property (nonatomic, copy, readonly)NSURL *downloadURL; @property (nonatomic, copy, readonly)NSString *downloadPath; @property (nonatomic, copy, readonly)NSString *filePath;
@property (nonatomic, copy, readwrite)NSString *fileName; @property (nonatomic, strong, readonly) NSMutableURLRequest *fileRequest; @property (readonly,getter=isSuspend) BOOL isSuspend;
一开始声明了一个代理，用来提供下载进度，下载状态和下载地址等信息，相关代码待会再说。
可以看到几乎所有属性都是只读状态，这里避免用户对下载状态，进度，下载地址等进行更改，由下载库统一管理，只暴露出相关只读属性返回当前任务状态进度等相关信息（只读属性的赋值参见KVO）
在下载回调时，一般有两种常用方式，代理和block，AF中采用了block得方式，这里提供两种方式：
代理和block
/* 利用代理回调的初始化方式； */ - (instancetype) initWithDownloadURL:(NSURL *)url
downloafPath:(NSString *)path;
/*
block回调的初始化方式；
*/
- (instancetype) initWithDownloadURL:(NSURL *)url
downloafPath:(NSString *)path
progress:(void (^)(int64_t writtenByte,int64_t totalByte,float progress))progress
error:(void (^)(NSError *error))error
complete:(void (^)(BOOL downloadFinished, NSURLSessionDownloadTask *task))completBlock;
当然还要有下载的开启，取消，删除等方法：
- (void)cancelDownloaderAndRemoveFile:(BOOL)remove; - (void)pause; - (void)resume;
下载库写的比较简略，一些对于错误的处理没有涉及，实际项目中可以加上。
.m文件中：
// Copyright © 2015年 Lee. All rights reserved.
// #define FILE_MANAGER [NSFileManager defaultManager]
#import "LNDownloader.h" @interface LNDownloader()<NSURLSessionDownloadDelegate> @property (nonatomic, strong) NSMutableURLRequest *fileRequest;
@property (nonatomic, copy) NSURL *downloadURL;
@property (nonatomic, copy) NSString *downloadPath;
@property (nonatomic, assign) DownloadState state;
@property (nonatomic, assign) float progress;
@property (nonatomic, strong) NSMutableData *receiveData;
@property (nonatomic, strong) NSData *resumeData; @property (nonatomic, assign)int64_t writtenByte;
@property (nonatomic, assign)int64_t expectTotalByte;
@property (nonatomic, strong)NSURLSession *session;
@property (nonatomic, strong)NSURLSessionDownloadTask *downloadTask; @property (nonatomic,copy)void(^progressBlock)(int64_t writtenByte,int64_t totalByte,float progress);
@property (nonatomic,copy)void(^errorBlock)(NSError *error);
@property (nonatomic,copy)void(^completeBlock)(BOOL downloadFinished, NSURLSessionDownloadTask *task);
这里定义了三个block，第一个用来回调下载进度，第二个是下载失败的回调，第三个是下载完成的回调。因为我们在.h文件中声明了一个isSuspend属性，算是对NSOperation属性的一个扩充吧，在.m中我们要实现其getter方法，因此我们需要@dynamic isSuspend
@dynamic isSuspend;
#pragma mark init
- (instancetype)initWithDownloadURL:(NSURL *)url
downloafPath:(NSString *)path
{
self = [super init];
if (self)
{
self.downloadURL = url;
self.state = DownloadState_Ready;
self.downloadPath = path;
self.fileRequest = [[NSMutableURLRequest alloc] initWithURL:url
cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:];
}
return self;
} - (instancetype)initWithDownloadURL:(NSURL *)url
downloafPath:(NSString *)path
progress:(void (^)(int64_t, int64_t,float))progress
error:(void (^)(NSError *))error
complete:(void (^)(BOOL, NSURLSessionDownloadTask *))completBlock
{
self = [self initWithDownloadURL:url downloafPath:path];
if (self)
{
self.progressBlock = progress;
self.errorBlock = error;
self.completeBlock = completBlock;
}
return self;
}
在初始化方法中，分别对downloadUrl state downloadPath和fileRequest等进行了赋值和request的初始化,对于一些progress state属性等是readonly声明，所以在内部赋值的时候需要使用KVC
- (void)start
{
[self willChangeValueForKey:@"isExecuting"];
self.state = DownloadState_Doing;
[self didChangeValueForKey:@"isExecuting"]; self.session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]
delegate:self
delegateQueue:nil]; self.downloadTask = [self.session downloadTaskWithRequest:self.fileRequest];
[self.downloadTask resume];
}
在重写resmue的时候因为可能是暂停下载任务继续下载的情况，所以通过判断resumeData是否为空来获得或新建对应的task.下载的进度和状态完成情况都是通过代理回调给我们需要注意的是
- (void)URLSession:(NSURLSession *)session
task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error这个方法在我们暂停或者下载失败的时候都会回调，因此我们需要通过当前task的状态进行判断：
if (error)
{
if (self.state == DownloadState_Suspend)
{ }
else
{
[self downloadWithError:error task:self.downloadTask];
} }
在暂停时，这里没有做任何处理。
下载的progress和状态等通过通知和代理传值，监听相关的通知即可获取需要的所有信息：
- (void)URLSession:(NSURLSession *)session
downloadTask:(NSURLSessionDownloadTask *)downloadTask
didWriteData:(int64_t)bytesWritten
totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
self.writtenByte = totalBytesWritten;
self.expectTotalByte = totalBytesExpectedToWrite; dispatch_async(dispatch_get_main_queue(), ^{
if (self.progressBlock) {
self.progressBlock(totalBytesWritten,totalBytesExpectedToWrite,self.progress);
}
if (self.writtenByte *1.0 / self.expectTotalByte > 0.01 || self.writtenByte *1.0 /self.expectTotalByte >= 0.99)
{
self.progress = (float)self.writtenByte / self.expectTotalByte;
if ([self.delegate respondsToSelector:@selector(download:progress:)])
{
[self.delegate download:self progress:self.progress];
}
} });
} - (void)downloadWithError:(NSError *)error task:(NSURLSessionDownloadTask *)task
{
if (error)
{
dispatch_async(dispatch_get_main_queue(), ^{
if (self.errorBlock)
{
self.errorBlock(error);
}
if ([self.delegate respondsToSelector:@selector(download:didStopWithError:)]) {
[self.delegate download:self didStopWithError:error];
}
});
}
BOOL success = error == nil; dispatch_async(dispatch_get_main_queue(), ^{ if (self.completeBlock)
{
self.completeBlock(success,task);
}
if (self.state == DownloadState_Suspend) {
return ;
}
if ([self.delegate respondsToSelector:@selector(download:didFinishWithSuccess:atPath:)]) {
[self.delegate download:self didFinishWithSuccess:success atPath:[self.downloadPath stringByAppendingPathComponent:self.fileName]];
} });
if (self.state == DownloadState_Suspend) {
return ;
}
DownloadState state = success ? DownloadState_Success :DownloadState_Fail;
[self cancelAndSetDownloadStateWhenDownloadFinish:state];
} - (void)cancelAndSetDownloadStateWhenDownloadFinish:(DownloadState)state
{
[self.downloadTask cancel];
[self willChangeValueForKey:@"isFinished"];
[self willChangeValueForKey:@"isExecuting"];
self.state = state;
[self didChangeValueForKey:@"isExecuting"];
[self didChangeValueForKey:@"isFinished"];
}
- (void)cancelDownloaderAndRemoveFile:(BOOL)remove
{
[self.downloadTask cancel];
self.delegate = nil;
[self removeFile];
[self cancel];
}