如何确定使用 NSURLConnection 下载的文件的大小（以字节为单位）？

Question

我需要知道使用 NSURLConnection (GET) 下载到我的应用程序中的文件的大小（以字节为单位）。如果有帮助的话，下面是我收到的字节代码。我需要知道的是如何获取文件大小（以字节为单位），以便我可以使用它来显示 UIProgressView。

- (void)connection:(NSURLConnection *)theConnection didReceiveData:(NSData *)data
// A delegate method called by the NSURLConnection as data arrives.  We just 
// write the data to the file.
{
#pragma unused(theConnection)
    NSInteger       dataLength;
    const uint8_t * dataBytes;
    NSInteger       bytesWritten;
    NSInteger       bytesWrittenSoFar;

    assert(theConnection == self.connection);

    dataLength = [data length];
    dataBytes  = [data bytes];

    bytesWrittenSoFar = 0;
    do {
        bytesWritten = [self.fileStream write:&dataBytes[bytesWrittenSoFar] maxLength:dataLength - bytesWrittenSoFar];
        assert(bytesWritten != 0);
        if (bytesWritten == -1) {
            [self _stopReceiveWithStatus:@"File write error"];
            break;
        } else {
            bytesWrittenSoFar += bytesWritten;

    } while (bytesWrittenSoFar != dataLength);
}

Answer 1

[数据长度]确实返回数据的大小（以字节为单位），这应该为您提供您正在寻找的数字。

但是 NSData 提供了 -writeToFile:atomically: 和 -writeToFile:options:error 方法将数据写入磁盘，所以我不知道你为什么要编写自己的文件 I/O。

Answer 2

我相信该代码来自 Apple 的 SimpleURLConnections 示例代码。这里直接引用苹果的 NSURLConnection 方法“connection:didReceiveData:”

“委托应该连接
传递的每个数据对象的内容
建立完整的数据
网址加载。”

不提供增量写入文件的方法，它会一次性写入所有内容，因此我认为这种增量写入 NSOutputStream（此处名为 fileStream）的方法是正确的。在此代码中使用 dataLength 只会返回在一次调用“connection:didReceiveData:”中从 NSURLConnection 发送到委托的数据长度，而不是整个连接的数据总长度。为了实现此目的，我添加了额外的属性：

@property (nonatomic, assign)   long long dataWrittenForWholeConnection;
@property (nonatomic, assign)   long long dataLengthOfWholeConnection;

在连接中 。 ：didReceiveResponse：我使用 NSURLResponse 方法预期内容长度来获取整个连接的接收数据文件大小的正确估计

self.dataWrittenForWholeConnection = 0;
self.dataLengthOfWholeConnection = [httpResponse expectedContentLength];

在连接中：didReceiveData：我更改了以下内容：

// bytes written for current "connection:didReceiveData:" call
bytesWrittenSoFar += bytesWritten;
// cumulative data written for connection so far
self.dataWrittenForWholeConnection += bytesWritten;

// also update the progress bar
if (self.dataLengthOfWholeConnection != NSURLResponseUnknownLength) {
    self.progressView.progress =
        ((float)self.dataWrittenForWholeConnection / (float)self.dataLengthOfWholeConnection);
}

在连接结束时，这应该是正确的：

assert(dataWrittenForWholeConnection == dataLengthOfWholeConnection);

这是：来自相同 SimpleURLConnections 代码的打印输出，经过我的修改以显示真正发生的情况：

connection:didReceiveResponse:
Full size of data is 8789 bytes

connection:didReceiveData:
Data has 1408 bytes
Progress bar is at: 0.160200
1408 bytes of data were written, 1408 bytes so far

connection:didReceiveData:
Data has 1408 bytes
Progress bar is at: 0.320401
1408 bytes of data were written, 2816 bytes so far

connection:didReceiveData:
Data has 1408 bytes
Progress bar is at: 0.480601
1408 bytes of data were written, 4224 bytes so far

connection:didReceiveData:
Data has 1408 bytes
Progress bar is at: 0.640801
1408 bytes of data were written, 5632 bytes so far

connection:didReceiveData:
Data has 1408 bytes
Progress bar is at: 0.801001
1408 bytes of data were written, 7040 bytes so far

connection:didReceiveData:
Data has 1749 bytes
Progress bar is at: 1.000000
1749 bytes of data were written, 8789 bytes so far

connectionDidFinishLoading:
Connection Finished. 8789/8789 bytes of data were written.

此外，NSOutputStream 方法“write:maxLength:”返回“实际写入的字节数”，因此出于某种原因，它可能选择不写入所有数据在一次“connection:didReceiveData:”调用中立即执行，因此使用 do/while 循环来检查这一点。但在本例中，它在一次 do/while 迭代中写入所有数据。

另外，您可以使用 NSFileHandle 代替，这将始终写入每个连接发送的整个数据：didReceiveData：调用，而且它是增量的。但请注意 writeData: 是同步的。

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    // write data to file handler
    unsigned long long dataLength = [data length];

    NSLog(@"Writing %qu bytes at %qu bytes", dataLength, [self.fileHandle offsetInFile]);
    [self.fileHandle writeData:data];
    NSLog(@"Wrote %qu bytes, now at %qu bytes", dataLength, [self.fileHandle offsetInFile]);
}