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The basic principle of segmented download of original ecology

File downloading often takes a long time, and the realization of segmented download is the premise of breakpoint download.

The following two problems need to be solved to realize segmented download

Question 1: before downloading, you need to know the length of the file.

Question 2: a section of each request to download a file

Get the file length for the problem 1:HEAD request

The HEAD request only gets the response message, not the actual data of the resource.

It is usually used to obtain the file length and to detect whether the server-side resources have changed.

The property exceptedContentLength of the response object NSURLResponse indicates the length of the file

For problem 2: request header parameter Range for segmented download

The HTTP protocol stipulates that the Range parameter in the request header is used to request a piece of data.

Such as:

The first 500bytes of bytes=0-499 from 0 to 499

Second 500byte of bytes=500-999 from 500to 999

Bytes=500- all bytes after 500byte

The last 500bytes of bytes=-500

Bytes=500-599800-899 specifies several ranges simultaneously

That is to say, download in segments means sending multiple requests and setting the Range field for each request.

The following code completes the segmented download of a file:

The following code completes a download of the file:

When using asynchronous downloads, agents, notifications, and block should be used for post-download processing actions.

Download task for NSURLSession

Download task creation:

-(NSURLSessionDownloadTask *) downloadTaskWithURL: (NSURL*) url- (NSURLSessionDownloadTask *) downloadTaskWithURL: (NSURL*) url completionHandler: (void (^) (NSURL*location, NSURLResponse * response, NSError*error)) completionHandler- (NSURLSessionDownloadTask *) downloadTaskWithRequest: (NSURLRequest *) request- (NSURLSessionDownloadTask *) downloadTaskWithRequest: (NSURLRequest *) request completionHandler: (void (^) (NSURL*location, NSURLResponse * response, NSError*error)) completionHandler

Proxy methods related to NSURLSession performing download tasks:

/ / periodic call. Parameters describe the download progress-(void) URLSession: (NSURLSession *) session downloadTask: (NSURLSessionDownloadTask*) downloadTask didWriteData: (int64_t) bytesWritten totalBytesWritten: (int64_t) totalBytesWritten totalBytesExpectedToWrite: (int64_t) totalBytesExpectedToWrite//. Call-(void) URLSession: (NSURLSession *) session downloadTask: (NSURLSessionDownloadTask*) downloadTask didFinishDownloadingToURL: (NSURL *) location when the download is complete.

The example shows:

1) create a NSURLSession object and specify a proxy

2) create a download task and start it

3) Agent method for download completion: the downloaded files should be moved to the specified directory and renamed

4) proxy method to monitor download progress:

Download the implementation of pause / resume

Pause method for NSURLSessionDownloadTask:

-(void) cancelByProducingResumeData: (void (^) (NSData * resumeData)) completionHandler

Parameter resumeData: describes breakpoint information

NSURLSession creates a download task with breakpoint information:

-(NSURLSessionDownloadTask *) downloadTaskWithResumeData: (NSData *) resumeData- (NSURLSessionDownloadTask *) downloadTaskWithResumeData: (NSData *) resumeDatacompletionHandler: (void (^) (NSURL * location, NSURLResponse * response, NSError*error)) completionHandler

Related agent methods:

-(void) URLSession: (NSURLSession *) session downloadTask: (NSURLSessionDownloadTask*) downloadTask didResumeAtOffset: (int64_t) fileOffset expectedTotalBytes: (int64_t) expectedTotalBytes

An example shows:

1) download pause

2) download continues

3) proxy method for breakpoint to continue downloading

Download using AFNetworking

Downloading using AFNetworking essentially creates a NSURLSessionDownloadTask object

Simply encapsulate the proxy method in an AFURLSession object and specify the behavior with block

AFURLSessionManager create download task

/ / create a download task based on the request object-(NSURLSessionDownloadTask *) downloadTaskWithRequest: (NSURLRequest *) request progress: (NSProgress * _ nullable _ _ autoreleasing * _ nullable) progress destination: (nullable NSURL * (^) (NSURL * targetPath, NSURLResponse * response) destination completionHandler: (nullable void (^) (NSURLResponse * response, NSURL * filePath) NSError * error)) completionHandler// creates a love download task based on breakpoint information-(NSURLSessionDownloadTask *) downloadTaskWithResumeData: (NSData *) resumeData progress: (NSProgress * _ nullable _ autoreleasing * _ nullable) progress destination: (nullable NSURL * (^) (NSURL * targetPath, NSURLResponse * response) destination completionHandler: (nullable void (^) (NSURLResponse * response, NSURL * filePath, NSError * error)) completionHandler

Parameter destination: this block is used to specify the path after the file is downloaded.

Parameter completionHandler: this block is executed when the download task is completed

Including when the task is paused (cancelByProducingResumeData)

Output parameter progress: if it is not NULL, a NSProgress object is returned to describe the download progress

The example shows:

1) create a NSURLSession object and specify a proxy

2) block wrapper 1: the final storage URL of the downloaded file should be returned.

3) Task completion block encapsulation: including processing in case of error, prompt in case of successful request, etc.

4) start / continue the creation of the download task

5) suspension of download task

Use NSProgress to monitor download progress

In the above code, there is a parameter of type NSProgress, which is passed as NULL, which is used by the AFN framework to listen for download progress.

NSProgress is introduced in iOS7.0, using observer mode

Notify all observers when the value of its fractionCompleted attribute changes

The example shows:

1) start / continue the creation of the download task

2) the response method of KVO

Where self.progressView is a custom view object that displays the progress of the download

Download tasks often encounter situations

Condition 1: how to save the download status when the program exits

Download the task management class and register as a responder to the following UIApplication object notifications

UIApplicationDidEnterBackgroundNotification

UIApplicationWillTerminateNotification

Response method implementation: pause all download tasks and save breakpoint information to the local operation

Read breakpoint information locally in response to the following notification

UIApplicationWillEnterForegroundNotification

Or the operation of reading breakpoint information locally when the download task management object is created

Condition 2: what should I do if the file on the server changes or is deleted when I continue to download from the breakpoint?

You can send a HEAD request to determine whether the file exists and whether the length of the file has changed before continuing the download from the breakpoint.

Thank you for reading! This is the end of this article on "how to download iOS files". I hope the above content can be helpful to you, so that you can learn more knowledge. if you think the article is good, you can share it out for more people to see!

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