Re: Downloading image from web and memory usage questions

FROM : Shawn Erickson
DATE : Fri Jan 24 22:54:03 2003


On Friday, January 24, 2003, at 09:53  PM, John Tsombakos wrote:

> Second question deals with retain and release. I originally did not 
> have the [image release] statement in there and while watching 'top', 
> I could see that memory was being taken up and not released every time 
> I clicked the button (assuming the image actually loaded). I added the 
> release and the memory stopped being eaten up. I then figured I also 
> needed to release the imageData object (marked **2). When that's in 
> there, the app crashes. My question is why don't I need to release 
> that? I understand the concept of the release and retain, but just 
> can't figure out which routines return something that I need to 
> release. Any hints or help would be nice.


Try to keep this in mind when working with Cocoa frameworks (and 
hopefully follow it in your own code): If you alloc, copy or retain an 
object then it is your responsibility to release it.

I suggest you read one or all of the following...
http://www.stepwise.com/Articles/Technical/2001-03-11.01.html
http://cocoadev.com/index.pl?MemoryManagement
http://developer.apple.com/techpubs/macosx/Cocoa/TasksAndConcepts/
ProgrammingTopics/MemoryMgmt/index.html

> NSData *imageData = [url resourceDataUsingCache:NO];


Here you are not allocating the data, resourceDataUsingCache is and it 
is its responsibility to release the object. It also has to return the 
object to you so you get a chance to use it, so it autoreleases it 
before it returns.

> [[NSImage alloc] initWithData:imageData];


Here you are allocating a NSImage object so it is your responsibility 
to release it when you are done with it.

As a side NSImage retains (or possibly copies) imageData so that 
imageData stays around while NSImage needs it. When NSImage goes away 
it will release imageData (sends it a release message).

> NSURL* url = [NSURL URLWithString:string];


One last note... the above is just like resourceDataUsingCache in that 
URLWithString is creating a URL instance and so it must be responsible 
for making sure it gets released.

-Shawn