iPhone Development - Lessons in memory management

Question

I need lessons in memory management. I have an application that uses multiple views (around 10), some of these are attached to tab controller. Problem is that I'm using images (many images that I load from a web service). I'm facing the following issues.

1. The memory keeps increasing when I scroll in the table view (why?) - I checked the CustomTableViewCell application from Apple's site, and it's showing the same signs when I run it with Instruments.

2. I'm using autorelease with many objects, but I see that these objects don't actually get released and the memory is wired. How can I get rid of these objects?

3. How can I tell the NSAutoreleasePool to periodically release the objects that are not being used? I think this can help me get rid of wired memory. But can I do that?

Is there any example by Apple or someone else (book or online articles) explaining how to use Instruments (in a little detail with example?) and to fine tune the application for memory and performance?

Thanks.

Answer 1

The autoreleased memory is released when control is returned back to the system, but only when it chooses to. If you wish to force memory to be released use "release" which works there and then.

It should be noted that because of memory fragmentation that allocating and deallocating a block of memory may not seem to get you back to where you started in terms of measurable "free" memory.

Tony

Answer 2

For performance reasons, Apple recommends that you retain/release objects yourself whenever possible. autoreleasing them can cause excess memory usage as the autoreleased objects aren't always released immediately.

In other words, if you know you're done with an object, explicitly release it.

Answer 3

According to Apple, you should not use autorelease and instead should retain and release objects explicitly as needed. autorelease will not release an object as soon as its function is over. If in the tableview you are using images downloaded from a webservice, try and cache these images and reuse them if possible. Another option is to only get those images which are being displayed.

Answer 4

Now that we have the "just say no" answers to autorelease out of the way, I thought I'd add a tip on how to use autorelease more effectively. For better or worse not everyone's going to completely avoid autorelease-- if for no other reason than because Apple provides so many convenience methods that hand you autoreleased objects.

You can't just tell the autorelease pool to free up any objects that you're not using. There's no garbage collection, and how else is it going to know?

What you CAN do is to create a local autorelease pool and then release that when you no longer need the local autoreleased objects. If you have a block where you're creating autoreleased objects, you'll ensure they get freed by creating a local autorelease pool at the start of the block (just alloc/init it, no magic required) and then releasing the pool at the end of the block. And voila, and objects in the pool are also released.

Autorelease pools nest, so keep that in mind if you do this. If you release an autorelease pool, make sure it's the most-recently-allocated pool and not some other one.

Answer 5

The UITableView has a way to reuse table cells that aren't being displayed anymore. That way if you only display 6 cells on the screen at once it doesn't keep on creating more as you scroll, but reuses the cells that have gone off screen. whenever you want to create a new cell, first query the tableview to see if it has any to reuse and if not then create a new one.

an example of this can be found on slide 55 of the standford iphone course note found here: http://www.scribd.com/doc/7671058/Standford-CS-193P-11Performance