Xcode 4.4发行说明中提到的"Objective-C Literals"的细节是什么？

Question

我正在浏览Xcode 4.4的发行说明并注意到这一点:

LLVM 4.0编译器

Xcode现在包含Apple LLVM Compiler 4.0版,包括以下newObjective-C语言特性:[...]
- Objective-C文字:为NSArray,NSDictionary和NSNumber创建文字,与NSString的文字相同

我对这个功能很感兴趣.这并不完全清楚,我是多么的文字NSString工作,以及如何一个可以在使用它们NSArray,NSDictionary和NSNumber.

有什么细节？

Answer 1

从http://cocoaheads.tumblr.com/post/17757846453/objective-c-literals-for-nsdictionary-nsarray-并逐字复制:

Objective-C文字:现在可以为NSArray,NSDictionary和NSNumber创建文字(就像可以为NSString创建文字一样)

NSArray文字

先前:

array = [NSArray arrayWithObjects:a, b, c, nil];

现在:

array = @[ a, b, c ];

NSDictionary文字

先前:

dict = [NSDictionary dictionaryWithObjects:@[o1, o2, o3]
                                   forKeys:@[k1, k2, k3]];

现在:

dict = @{ k1 : o1, k2 : o2, k3 : o3 };

NSNumber文字

先前:

NSNumber *number;
number = [NSNumber numberWithChar:'X'];
number = [NSNumber numberWithInt:12345];
number = [NSNumber numberWithUnsignedLong:12345ul];
number = [NSNumber numberWithLongLong:12345ll];
number = [NSNumber numberWithFloat:123.45f];
number = [NSNumber numberWithDouble:123.45];
number = [NSNumber numberWithBool:YES];

现在:

NSNumber *number;
number = @'X';
number = @12345;
number = @12345ul;
number = @12345ll;
number = @123.45f;
number = @123.45;
number = @YES;

[编辑]

zxoq在http://news.ycombinator.com/item?id=3672744添加了更多有趣的新订阅.(添加文字):

arr[1]      === [arr objectAtIndex:1]
dict[@"key"] === [dict objectForKey:@"key"]

[编辑2]

在多个WWDC 2012会议中讨论了新的ObjC文字.我故意没有删除每个幻灯片的文件名和时间,所以如果你愿意,你可以自己找到它们.它们与本文中所述的内容基本相同,但在图像上方还会提到一些新的内容.

请注意,图像都很大.只需将它们拖到另一个选项卡中即可以原始大小查看它们

[NSNumber numberWithint:42]
[NSNumber numberWithDouble:10.8]
[NSNumber numberWithBool:YES]
[NSNumber numberWithint:6 + x * 2012]

@42
@10.8
@YES
@(6 + x * 2012)

[NSArray arrayWithObjects: a, b, c, nil]
[array objectAtIndex:i]
[NSDictionary dictionaryWithObjectsAndKeys: v1, k1, v2, k2, nil];
[dictionary valueForKey:k]

@[a, b, c]
array[i]
@{k1:v1, k2:v2}
dictionary[k]

这部分是新的.表达文字

当你有一个表达式(M_PI / 16例如)时,你应该把它放在括号内.

此语法适用于数字表达式,布尔值,在(C-)字符串中查找索引,布尔值,枚举常量甚至字符串!

NSNumber *piOverSixteen = [NSNumber numberWithDouble: (M_PI / 16)];

NSNumber *hexDigit = [NSNumber numberWithChar:"0123456789ABCDEF"[i % 16]];

NSNumber *usesScreenFonts = [NSNumber numberWithBool:[NSLayoutManager usesScreenFonts]];

NSNumber *writingDirection = [NSNumber numberWithInt:NSWritingDirectionLeftToRight];

NSNumber *path = [NSString stringWithUTF8String: getenv("PATH")];

NSNumber *piOverSixteen = @( M_PI / 16 );

NSNumber *hexDigit = @( "0123456789ABCDEF"[i % 16] );

NSNumber *usesScreenFonts = @( [NSLayoutManager usesScreenFonts] );

NSNumber *writingDirection = @( NSWritingDirectionLeftToRight );

NSNumber *path = @( getenv("PATH") );

有关字符串以及如何/何时可以使用此文字语法的更多信息:

NSString *path = [NSString stringWithUTF8String: getenv("PATH")];
for (NSString *dir in [path componentsSeparatedByString: @":"]) {
    // search for a file in dir...
}

NSString *path = @( getenv("PATH") );
for (NSString *dir in [path componentsSeparatedByString: @":"]) {
    // search for a file in dir...
}

数组文字如何工作

// when you write this:
array = @[a, b, c ];

// compiler generates:
id objects[] = { a, b, c };
NSUInteger count = sizeof(objects) / sizeof(id);
array = [NSArray arrayWithObjects:objects count:count];

字典文字如何工作

// when you write this:
dict = @{k1 : o1, k2 : o2, k3 : o3 };

// compiler generates:
id objects[] = { o1, o2, o3 };
id keys[] = { k1, k2, k3 };
NSUInteger count = sizeof(objects) / sizeof(id);
dict = [NSDictionary dictionaryWithObjects:objects
                                   forKeys:keys
                                     count:count];

有关数组下载的更多信息

@implementation SongList {
    NSMutableArray *_songs;
}

- (Song *)replaceSong:(Song *)newSong atindex:(NSUinteger)idx {
    Song *oldSong = [_songs objectAtIndex:idx];
    [_songs replaceObjectAtindex:idx withObject:newSong];
    return oldSong;
}

@implementation SongList {
    NSMutableArray *_songs;
}

- (Song *)replaceSong:(Song *)newSong atindex:(NSUinteger)idx {
    Song *oldSong = _songs[idx];
    _songs[idx] = newSong;
    return oldSong;
}    

更多关于字典下载

@implementation Database {
    NSMutableDictionary *_storage;
}

- (id)replaceObject:(id)newObject forKey:(id <NSCopying>)key {
    id oldObject = [_storage objectForKey:key];
    [_storage setObject:object forKey:key];
    return oldObject;
}

@implementation Database {
    NSMutableDictionary *_storage;
}

- (id)replaceObject:(id)newObject forKey:(id <NSCopying>)key {
    id oldObject = _storage[key];
    _storage[key] = newObject;
    return oldObject;
}

[编辑3]

Mike Ash对这些新文字有很好的写作.如果您想了解更多有关此内容的信息,请务必查看.

Answer 2

Objective-C编译器具有NSConstantString类的实例(即类)的内存布局的硬编码知识__CFConstantString.查看clang源代码中的RewriteObjCStringLiteral函数lib/Rewrite/RewriteModernObjC.cpp.编译器只发出与NSConstantString类实例布局匹配的数据.

文字NSArray和NSDictionary实例有几种可能性.他们可以做类似于文字字符串的操作 - 在编译器中硬编码实例布局(用于特殊子类)并在该布局中发出数据.或者他们可以让编译器发出只在运行时创建实例的代码.