Implementing HTTP Resume for Large File Downloads on iOS
When working with large audio and video files, it’s common for users to experience network outages or program closures during the download process. To mitigate this issue, we need to implement HTTP resume functionality, allowing users to pause and restart downloads at any time, picking up where they left off.
Understanding HTTP Resume
To achieve HTTP resume, the client must record the current download progress and notify the server when a resume is required. The HTTP protocol defines the associated HTTP Range header field and Content-Range HTTP/1.1 Protocol (RFC2616).
A simple realization of HTTP resume can be achieved as follows:
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The client downloads a file of 1,024 KB, but network outages occur, and the client requests a resume.
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The client sends an HTTP request with the following header:
Range: bytes=512000-
* The server receives the request and responds with the following header:
```http
Content-Range: bytes 512000- / 1024000
- The server returns an HTTP status code of 206 instead of 200, indicating a partial content response.
Challenges in Implementing HTTP Resume
There are two primary challenges in implementing HTTP resume:
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Obtaining the Number of Bytes Already Downloaded
To implement HTTP resume, the client needs to record the number of bytes already downloaded. This can be achieved by directly accessing the file size using the following code:
let fileManager = NSFileManager.defaultManager()
let fileAttributes = fileManager.attributesOfItemAtPath(FileStorePath, error: nil)
let fileSize = fileAttributes?[NSFileSize] as? Int
2. **Obtaining the Total Number of Bytes in the File**
To calculate the download progress, the client needs to obtain the total number of bytes in the file. This can be achieved by using the `Content-Length` field in the HTTP header.
The `Content-Length` field describes the transfer length of the message-body, which is different from the physical length of the message. For example, under gzip compression, the message length is the physical length before compression, while the transmission message length is the length of the entity after gzip compression.
To calculate the total number of bytes in the file, the client must add the number of bytes already downloaded to the `Content-Length` value. The `Content-Length` value can be obtained from the HTTP header.
To store the total number of bytes in the file, the client can use `NSUserDefaults` to record the value with a unique key. To prevent duplication, the client can set the file name to the hash value of the URL.
**Conclusion**
Implementing HTTP resume on iOS requires understanding the principles of HTTP resume and addressing the challenges involved. By recording the number of bytes already downloaded and obtaining the total number of bytes in the file, the client can calculate the download progress and implement HTTP resume functionality.