Memory issue with [NSData dataWithContentsOfFile]

问题:

I am developing application which required image caching. For doing this, I am using JMImageCache library. It is work fine for caching. But It can not release memory occupied by following line.

[NSData dataWithContentsOfFile]

Here, is function which content code for cache image from disk.

- (UIImage *) imageFromDiskForURL:(NSString *)url {
    NSData *data = [NSData dataWithContentsOfFile:cachePathForURL(url) options:0 error:NULL];
    UIImage *i = [[[UIImage alloc] initWithData:data] autorelease];
    data = nil;
    [data release];
    return i;
}

I have check it with instruments and it alloc 2.34 MB each time.

回答1:

data = nil;
[data release];

Why do you expect this at all to work? Why should this release the original data? You're sending the release message to nil, which is a no-op.

Furthermore, if you don't create the object using alloc or copy, then it's autoreleased. That means if you release it once more, it will be overreleased and most likely your app is going to crash. What you need is:

One. Wrap the method call in an explicit autorelease pool:

- (UIImage *)imageFromDiskForURL:(NSString *)url
{
    UIImage *i;
    @autoreleasepool {
        NSData *data = [NSData dataWithContentsOfFile:cachePathForURL(url) options:0 error:NULL];
        i = [[UIImage alloc] initWithData:data];
    }
    return [i autorelease];
}

Two, alloc-init or manually release the data object:

- (UIImage *)imageFromDiskForURL:(NSString *)url
{
    NSData *data = [[NSData alloc] initWithContentsOfFile:cachePathForURL(url) options:0 error:NULL];
    UIImage *i = [[[UIImage alloc] initWithData:data] autorelease];
    [data release];
    return i;
}

回答2:

alter the sequence nil and release

[data release];
data = nil;

and for clearing the cache use following delegate methods

[[JMImageCache sharedCache] removeAllObjects];
[[JMImageCache sharedCache] removeImageForURL:@"http://dundermifflin.com/i/MichaelScott.png"];

read the read me file of library https://github.com/jakemarsh/JMImageCache/blob/master/README.markdown

回答3:

What you are trying to do cannot work due to the way how UIImage uses a data you pass it. The data object is retained by the image, or more precisely by a CGImageSource that the UIImage has internally. From this data it is able to decompress and create the image any time. There is an option on CGImageSource to also keep around the decompressed data, but UIImage does not use that because it optimized for small UI graphics.

One thing that you can do to alleviate memory pressure is to not load the entire NSData into memory, but to memory-map it instead. This makes creation or recreation of the image a tad slower, but the created NSData object is very small in comparison.