ios static NSDictionary* const letterValues = @{ ..... } 在方法中不编译
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static NSDictionary* const letterValues = @{ ..... } in a method does not compile
提问by Alexander Farber
I'm trying to use the following code in my custom view Tile.m:
我正在尝试在我的自定义视图Tile.m 中使用以下代码：
- (void)awakeFromNib
{
[super awakeFromNib];
static NSDictionary* const letterValues = @{
@"A": @1,
@"B": @4,
@"C": @4,
// ...
@"X": @8,
@"Y": @3,
@"Z": @10,
};
NSString* randomLetter = [kLetters substringWithRange:[kLetters rangeOfComposedCharacterSequenceAtIndex:arc4random_uniform(kLetters.length)]];
int letterValue = [letterValues[randomLetter] integerValue];
_smallLetter.text = _bigLetter.text = randomLetter;
_smallValue.text = _bigValue.text = [NSString stringWithFormat:@"%d", letterValue];
}
Unfortunately this gives me compile error Initializer element is not a compile-time constantand I have to remove the statickeyword to get my app compile in Xcode (here fullscreen):
不幸的是，这给了我编译错误Initializer element is not a compile-time constant，我必须删除static关键字才能在 Xcode 中编译我的应用程序（这里是全屏）：
I think I initialize the NSDictionarycorrectly - by using the new Objective-C Literalssyntax.
我想我NSDictionary正确地初始化了- 通过使用新的Objective-C 文字语法。
But why can't I use statichere?
但是为什么我static这里不能用呢？
I thought it would be appropriate here to make sure that my letterValuesconstant is set only once?
我认为在这里确保我的letterValues常量只设置一次是合适的？
回答by Dave Wood
You can only set a static variable during initialization with a constant. @{} creates an object, thus not a constant.
您只能在初始化期间使用常量设置静态变量。@{} 创建一个对象，因此不是一个常量。
Do this instead:
改为这样做：
- (void)awakeFromNib
{
[super awakeFromNib];
static NSDictionary* letterValues = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
letterValues = @{
@"A": @1,
@"B": @4,
@"C": @4,
// ...
@"X": @8,
@"Y": @3,
@"Z": @10,
};
});
...
}
Some other answers here suggest a check for nil instead of dispatch once, but that can cause issues when creating multiple tiles at the same time (via threads). dispatch_once implements the required locking.
这里的其他一些答案建议检查 nil 而不是调度一次，但是在同时创建多个图块时（通过线程）可能会导致问题。dispatch_once 实现所需的锁定。
回答by Austin
You can use static, but the assignment can't be made on the same line. Try this:
您可以使用静态，但不能在同一行上进行分配。尝试这个：
- (void)awakeFromNib {
[super awakeFromNib];
static NSDictionary* letterValues = nil;
if (!letterValues) {
letterValues = @{@"A": @1,
@"B": @4,
@"C": @4,
// ...
@"X": @8,
@"Y": @3,
@"Z": @10};
}
...
}
The reason is that the @{<key> : <value>}syntax is translated by the compiler into an Objective-C method ([[NSPlaceholderDictionary alloc] initWithObjects:forKeys:count:]), which cannot be resolved at compile-time.
原因是@{<key> : <value>}编译器将语法翻译成 Objective-C 方法 ( [[NSPlaceholderDictionary alloc] initWithObjects:forKeys:count:])，无法在编译时解析。
回答by Adam
NSDictionaryobjects can't be created at compile time. However, if you need a static object, you can create one. You can, for example, use the initializemethod, like this:
NSDictionary不能在编译时创建对象。但是，如果您需要一个静态对象，则可以创建一个。例如，您可以使用该initialize方法，如下所示：
static NSDictionary* letterValues;
+ (void)initialize
{
if (self == [MyClass class]) {
letterValues = @{
@"A": @1,
@"B": @4,
@"C": @4,
@"X": @8,
@"Y": @3,
@"Z": @10,
};
}
}
The if statement is there to prevent multiple calls of your code in MyClasssubclasses.
if 语句用于防止在MyClass子类中多次调用您的代码。
回答by mgyky
Another alternative is using the Macro Approach:
另一种选择是使用宏方法：
Macro (The Easiest) Approach (2020)
宏观（最简单）方法（2020）
#define kScreenNames @{ \
@"HOME": @"Home Page", \
@"SEARCH": @"Search Page", \
@"MYLISTS": @"My List", \
@"MYACCOUNT": @"My Aaccount" \
}
it's short and elegant.
它简短而优雅。
Then, you can use it like any other NSDictionary.
然后，您可以像使用任何其他 NSDictionary 一样使用它。
I hope this will help someone on the planet.
我希望这会帮助地球上的某个人。
Best Regards
此致
