Re: Downloading image from web and memory usage questions

FROM : John Tsombakos
DATE : Fri Jan 24 23:46:08 2003


On Saturday, January 25, 2003, at 01:52  AM, Shawn Erickson wrote:

>
> 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.
>


That in concept I understand.

> 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
>


Yup. Read those...

>> 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.
>

This is the explanation that I needed to understand what is going on... 
thanks! I guess I didn't make the connection that 
resourceDataUsingCache allocated the object and I didn't need to do 
anything with it.


>> [[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.
>


I did figure out this one.


> 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).
>

I did NSLog the retainCount of imageData and did see that it went to 2 
after this line. Now it makes sense why.

This is very helpful too..

>> 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.
>


This I got to. The best piece of advice I read somewhere, and I wish I 
could remember where, is if the method has the class name in it, it's 
most likely a convenience constructor, and the returned object is 
autoreleased.

> -Shawn



Thanks! Now I just have to figure out how to work around the partial 
loading of the image and the "Application transferred too few 
scanlines" message.

john