How to integrate NSURLConnection with UIProgressView?

Question

I'm trying to integrate a NSURLConnection object with UIProgressView, so I can update the user while a file download is happening in the background.

I created a separate object to download the file in the background, and I'm having problems figuring out how to update the progress property in the UIProgressView object with the correct value. It's probably something very simple, but I cannot figure it out with Googling around.

Here's the code that I have:

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    [self.resourceData setLength:0];
    self.filesize = [NSNumber numberWithLongLong:[response expectedContentLength]];
    NSLog(@"content-length: %d bytes", self.filesize);
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    [self.resourceData appendData:data];

    NSNumber *resourceLength = [NSNumber numberWithUnsignedInteger:[self.resourceData length]];
    NSLog(@"resourceData length: %d", [resourceLength intValue]);
    NSLog(@"filesize: %d", self.filesize);
    NSLog(@"float filesize: %f", [self.filesize floatValue]);
    progressView.progress = [resourceLength floatValue] / [self.filesize floatValue];
    NSLog(@"progress: %f", [resourceLength floatValue] / [self.filesize floatValue]);
}

As you ca see, the resourceData member variable holds the file data as it is being downloaded. The filesize member variable holds the full size of the file, as returned by my web service in its Content-Length header.

It all works sort of OK, I keep getting the downloaded data with multiple executions of didReceiveData as I should, but when I try to calculate the progress value, no proper value is returned. See below for a small sample of what I get in my console log:

content-length: 4687472 bytes
resourceData length: 2904616
filesize: 4687472
float filesize: -1.000000
progress: -2904616.000000

For reference, progressView.progress is a float. filesize is a NSNumber that holds a long long. Finally, resourceLength is a NSNumber that holds a NSUInteger.

What am I missing here?

Answer 1

Not sure what I'm missing here, but your filesize being -1 seems to be your problem. The API docs clearly state that expectedContentLength may not be available and that NSURLResponseUnknownLength is returned in these cases. NSURLResponseUnknownLength is defined as:

#define NSURLResponseUnknownLength ((long long)-1)

In these cases, you cannot get an accurate progress. You'll need to handle this and display an indeterminate progress meter of some sort.

Answer 2

I think you are printing out the file size wrong. If self.filesize is indeed an NSNumber, you print it using the %@ format, because it is an object, not a primitive:

NSLog(@"filesize: %@", self.filesize);

By using the %d, your are just printing the pointer value of self.filesize. To print out the actual long long value, use %lli (%d is only for 32-bit values):

NSLog(@"filesize: %lli", [self.filesize longLongValue]);

So your self.filesize is actually -1 and the division is correct.

Answer 3

In your code, filesize appears to be an NSNumber object (!). So

NSLog(@"filesize: %d", self.filesize);

and

NSLog(@"content-length: %d bytes", self.filesize);

will likely report something like the address (id) of that object (or something else). This is the

filesize: 4687472

you see. As pointed out by others, the file size returned by the response is indeed -1, i.e.,

NSURLResponseUnknownLength

i.e., the server did not return the file size.