Setting a large NSDictionary property on a non-main thread - ios

Question

I have a class called Dictionary, where the init method looks like this:
- (id) init{
self = [super init];
if (self){
[self makeEmojiDictionaries];
}
return self;
}
- (void)makeEmojiDictionaries{
//next line triggers bad_exc_access error
self.englishEmojiAllDictionary = #{#"hi" : #"👋"}; //this is a strong, atomic property of NSDictionary
};
My issue is that the actual emoji dictionary is quite large, and I want to do all the heavy lifting in a non-main thread using GCD. However, whenever I get to the line where I set self.englishEmojiAllDictionary, I always get a bad_access error.
I am using GCD in the most normal way possible:
dispatch_queue_t myQueue = dispatch_queue_create("My Queue",NULL);
dispatch_async(myQueue, ^{
//Do long process activity
Dictionary *dictionary = [[Dictionary alloc] init];
});
Are there particular nuances to GCD or non-main thread work that I am missing? Any help is much appreciated - thank you!
Edit 1:
In case you'd like to try it yourself. I have uploaded a sample project that replicates this exception. My theory is that the NSDictionary I am initialization is simply too large.

Answer 1

I have moved your data from code to a plist file in the form:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>arancia meccanica</key><string>⏰🍊</string>
<key>uno freddo</key><string>🍺</string>
<key>un drink</key><string>🍸</string>
...
<key>bacio</key><string>💋</string>
<key>baci</key><string>💋👐</string>
</dict>
</plist>
(I took your data and used find-replace three times: ", => </string>, then ":#" => </key><string> and #" => <key>).
Then I have loaded the data using:
NSString *filePath = [[NSBundle mainBundle] pathForResource:#"dictionary"
ofType:#"plist"]
dictionary = [NSDictionary dictionaryWithContentsOfFile:filePath];
That has fixed the problem. Note that you should never hardcode your data into source code.
The exact reason for the bug was pretty hard to pinpoint. The NSDictionary literal uses method +[NSDictionary dictionaryWithObjects:forKeys:count:].
My assembler knowledge is very poor but I think that before calling this initializer, all the keys & values are put on the stack.
However, there is a difference between the stack size of the main thread and the stack size of the background thread (see Creating Threads in Thread Programming Guide).
That's why the issue can be seen when executing the code on the background thread. If you had more data, the issue would probably appear on the main thread too.
The difference between stack size on main thread and background thread can be also demonstrated by the following simple code:
- (void)makeEmojiDictionaries {
// allocate a lot of data on the stack
// (approximately the number of pointers we need for our dictionary keys & values)
id pointersOnStack[32500 * 2];
NSLog(#"%i", sizeof(pointersOnStack));
}

Answer 2

First of all, I suggest you use a file (plist, txt, xml, ...) to store large data, then read it at runtime, or download it from a remote server.
For your issue, it is because of the limitation of stack size. On iOS, the default stack size for the main thread is 1 MB, and 512 KB for the secondary threads. You can check it out via [NSThread currentThread].stackSize.
Your hardcoded dictionary costs almost 1 MB of stack, that is why your app will be crash on a secondary thread, but be OK on the main thread.
If you want to do this work on a background thread, you must increase the stack size for that thread.
For example:
// NSThread way:
NSThread *thread = [[NSThread alloc] initWithTarget:self selector:#selector(populateDictionaries) object:nil];
thread.stackSize = 1024*1024;
[thread start];
Or
// POSIX way:
#include <pthread.h>
static void *posixThreadFunc(void *arg) {
Dictionary *emojiDictionary = [[Dictionary alloc] init];
return NULL;
}
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
pthread_t posixThread;
pthread_attr_t stackSizeAttribute;
size_t stackSize = 0;
pthread_attr_init (&stackSizeAttribute);
pthread_attr_getstacksize(&stackSizeAttribute, &stackSize);
if (stackSize < 1024*1024) {
pthread_attr_setstacksize (&stackSizeAttribute, REQUIRED_STACK_SIZE);
}
pthread_create(&posixThread, &stackSizeAttribute, &posixThreadFunc, NULL);
}
#end
Or
// Create mutable dictionary to prevent stack from overflowing
- (void)makeEmojiDictionaries {
NSMutableDictionary *dict = [NSMutableDictionary dictionary];
dict[#"arancia meccanica"] = #"⏰🍊";
dict[#"uno freddo"] = #"🍺";
dict[#"un drink"] = #"🍸";
.....
self.englishEmojiAllDictionary = [dict copy];
}
FYI:
Thread Costs
Customizing Process Stack Size

Answer 3

The correct pattern when you need to do something slow is to do the work privately on a background queue, and then dispatch back to the main queue to make the completed work available to the rest of the app. In this case, you don't need to create your own queue. You can use one of the global background queues.
#import "ViewController.h"
#import "Dictionary.h"
#interface ViewController ()
#property (nonatomic, strong, readonly) Dictionary *dictionary;
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
[self updateViews];
dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0), ^{
Dictionary *dictionary = [[Dictionary alloc] init];
dispatch_async(dispatch_get_main_queue(), ^{
_dictionary = dictionary;
[self updateViews];
});
});
}
- (void)updateViews {
if (self.dictionary == nil) {
// show an activity indicator or something
} else {
// show UI using self.dictionary
}
}
#end
Loading the dictionary from a file is a good idea, and you can do that in the background queue and then dispatch back to the main thread with the loaded dictionary.