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Learning Cocoa with Objc reading notes

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Learning Cocoa with Objective-C 这本书我差不多读了一半，感觉这本书比较像一本参考书，要做某个功能可以参考下了解一下基本的东西。跳过了一些章节，把自己之前不知道的一些东西记一下。

NSData

NSData contains bytes with no assumptions of the type of data.

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//load file into NSData
NSData *loadedData = [NSData dataWithContentsOfFile:<filePath>];

NSData itself is useless

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NSString *loadedString = [[NSString alloc] initWithData:loadedData
                                        encoding:NSUTF8Encoding];
//encoding:NSUTF8Encoding tells NSString how to interrprate the bytes in NSData

NSCoding

Serialization and Deserialization in Cocoa

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// confirming to NSCoding
- (void)encodeWithCoder:(NSKeyedArchiver *)aCoder{
    [aCoder encodeObject:<obj> forKey:<key>];
}
- (void)initWithCoder:(NSKeyedArchiver *)aCoder{
    ...
    _obj = [aDecoder decodeObjectForKey:<key>]
    return self;
}

Working with NSData

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// store
NSData *storedData = [NSKeyedArchiver archivedDataWithRootObject:myObject];
[storedData writeToFile:<filePath> atomically:YES];
// retrive
SomeObj *obj = [NSKeyedUnarchiver unarchiveObjectWithData:storedData];

Using NSBundle to find resources

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NSString *path = [[NSBundle mainBundle] pathForResource:@"SomeImage" ofType:@"png"];

MultiTasking on iOS

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- (void)applicationDidEnterBacktround:(UIApplication *)application

Any task like saving to disk needs to be performed at this moment. Background app can be killed at anytime and no code can be executed at while being suspended.

Request to run in background

App can request to run in background for no more 10 mins

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- (void)applicationDidEnterBackground:(UIApplication *)application
{
    // beginBackgroundTaskWithExpirationHandler returns a unqie ident for the task
    bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }
    dispatch_async(dispatch_get_global_queue(DISPTACH_QUEU_PRIORITY_DEFAULT, 0), ^{
        ....bgTasks... 
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    })
}

Tasks that can run for a longer time

  1. playing audio
  2. tracking location
  3. VoIP

Nib

Nib – “freeze-drying” objects and storing them in a serialized form inside the file.
When displaying, loads the nib file, “rehydrates” the stored bojects, and presents them to the user.

How Nib Files are Loaded

  1. re-create every objects in the nib
  2. connect evey outlet
  3. every single object loaded receives awakeFromNib: message

Blocks

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<return type>(^ <variable name>) (<params>)

Blocks exist in stack when created, thus then assign to like instance variable, the block needs to be copied to the heap.

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_var = [bomeBlock copy];

Blocks keep strong ref to variables captured.
To modify captured variables, add __block modifier.

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__block int i = 0;

KVC

  1. accessing key that doesn’t exist causes exception
  2. is able to access even private vriables
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    // determine the varibale to set/get at run time
    for (NSString * key in dict) {
        NSObject *value = [dict objectForKey:key];
        [aProduct setValue:value forKey:key];
    }

KVO

  1. synthesized property auto notify changes when setter is called
  2. overried setter -> needs to fire notifiction manually
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    - (void)setName:(NSString *)name {
        [self willChangeValueForKey:@"name"];
        _name = name;
        [self didChangeValueForKey:@"name"];
    }

NSNotification

broadcast notifications, typically one singleton notification center.