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iOS Interview Prep 2 — Autorelease Pool

The purpose of this interview preparation series is to assist you in quickly refining your interview skills and thoroughly preparing for the typical questions asked during an iOS interview. If you find it useful, please leave a comment or tap the like button.

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Interview Questions

  • Can you explain what is an autorelease pool?
  • Can you explain how it works?
  • How does ARC (Automatic Reference Counting) work with autoreleasepool in iOS development?
  • What are common use cases of autorelease pool?

How it works?

Autorelease pool is an object that manages the memory of multiple objects allocated in a block of code. It provides a mechanism whereby you can relinquish ownership of an object, but avoid the possibility of it being deallocated immediately. It works by keeping track of all the objects that are created within a specific block of code.

All the objects created inside the block will receives a release message for each time it was sent an autorelease message. An autorelease pool is actually a collection of objecst that will be released at some point in the future. When pool is drained all the objects in the pool are sent the release message.

Reduce Peak Memory Footprint

A common use case of autorelease pool is reducing the memory footprint. When creating a large number of objects in a loop, your memory footprint are increase significantly with all the temporary objects that are created inside the loop.

To avoid the memory hikes, we can wrap the loop in an autorelease pool block. This way, all the objects are released at the end of each iteration, preventing the memory from being consumed. All the temporary objects will be disposed before the next iteration.

// Use Local Autorelease Pool Blocks to Reduce Peak Memory Footprint
for (NSURL *url in urls) {
    @autoreleasepool {
        NSError *error;
        NSString *fileContents = [NSString stringWithContentsOfURL:url
                                         encoding:NSUTF8StringEncoding error:&error];
        /* Process the string, creating and autoreleasing more objects. */
    }
}

Another common use case is when parsing large amounts of JSON data. We can use autorelease pool to release the objects that are no longer used.

NSData *jsonData = [NSData dataWithContentsOfFile:@"mydata.json"];
NSDictionary *jsonObject = [NSJSONSerialization JSONObjectWithData:jsonData options:kNilOptions error:&error];

NSArray *dataArray = jsonObject[@"data"];
    for (NSDictionary *dataDict in dataArray) {
        @autoreleasepool {
            // Parse data from the dictionary
            NSString *name = dataDict[@"name"];
            NSString *age = dataDict[@"age"];
            NSString *address = dataDict[@"address"];
            // Do something with the parsed data
            // ...
        }
    }

Autorelease and ARC

Autorelease and ARC were introduced to make memory management easier and less error-prone. Autorelease is a mechanism in Objective-C that allows objects to be released later. You don’t have to worry about releasing the object manually.

With ARC, the compiler inserts retain, release, and autorelease calls on your behalf. ARC uses autorelease as well as release. The main thread of every Cocoa app already has an autorelease pool in it. ARC helps to simplify memory management and prevent crashes caused by accessing released objects.

iOS Interview Prep Series

iOS Interview Prep 1 — Memory management iOS Interview Prep 2 — Autorelease Pool iOS Interview Prep 3 — Blocks and Closures iOS Interview Prep 4 — Event Handling & Responder Chain iOS Interview Prep 5 — Singletons iOS Interview Prep 6 — Dependency Injection iOS Interview Prep 7 — Concurrency Part 1 iOS Interview Prep 7 — Concurrency Part 2 iOS Interview Prep 8 — View and Layout

iOS
iOS App Development
Swift
Interview Preparation
Objective C
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