我在寻找Unit(symbol: String)
类型的初始化程序,它会识别,未定义符号。从字符串初始化单元
下面是一个代码片段,通过向其提供有限解决方案来说明我的问题。
虽然下面的工作方案,明确列出在Foundation框架预定义了所有单位是乏味的,没有隐含的向前兼容性应该在苹果的iOS即将更新推出新单位。
func matchedUnit<S: Sequence>(symbol: String, valid sequence: S) -> S.Iterator.Element? where S.Iterator.Element: Unit {
let symbols = sequence.map { $0.symbol };
let candidate = zip(sequence, symbols).first {
return $0.1 == symbol;
}?.0
return candidate;
}
let knownUnitsOfLength = [UnitLength.meters, UnitLength.centimeters, UnitLength.kilometers, UnitLength.decimeters];
if let aUnit = matchedUnit(symbol: "dm", valid: knownUnitsOfLength) {
let measurement = Measurement(value: 1, unit: aUnit);
}
也许我错过了SDK中一个明显的难题 - 识别符号而不是定义一个符号的初始化工具?
UPDATE:
已经证实,有没有苹果提供的API来做到这一点。我最终使用了Objective-C运行时来查找从NSDimension
继承的所有类,并遍历其名称中以NSUnit
开头的所有类属性。我映射了所有符号,类和属性名称,并且可以在运行时基于用户提供的符号创建NSUnit
实例。
当然,在语言支持方面,它与NSMeasurementFormatter
不在同一水平。 但是我会自动获得Apple推出的任何新单位和符号的未来版本。
下面是一个示例客户端代码:
NSUnit *kg = [NSUnit unitRecognizingSymbol:@"kg"];
NSMeasurement *mkg = [[NSMeasurement alloc] initWithDoubleValue:3.0 unit:kg];
NSUnit *g = [NSUnit unitRecognizingSymbol:@"g"];
NSMeasurement *mg = [[NSMeasurement alloc] initWithDoubleValue:200 unit:g];
NSMeasurement *total = [mkg measurementByAddingMeasurement:mg];
NSMeasurementFormatter *formatter = [[NSMeasurementFormatter alloc] init];
[formatter setUnitOptions:NSMeasurementFormatterUnitOptionsProvidedUnit];
NSLog(@"%@", [formatter stringFromMeasurement:total]);
// 3.2 kg
而且这里是实施方的核心部分是什么样子。
// Count receives its value from objc_copyClassList().
unsigned int classCount;
Class *classes = objc_copyClassList(&classCount);
// Inspect all available classes.
for (int classIterator = 0; classIterator < classCount; classIterator++) {
char const *className = class_getName(classes[classIterator]);
// Classes we are looking for are derived from NSDimension or NSUnit.
Class superClass = class_getSuperclass(classes[classIterator]);
char const *superClassName = class_getName(superClass);
// Single out NSDimension exception because NSDimension itself inherits from NSUnit.
if ((strcmp(superClassName, "NSDimension") == 0 || strcmp(superClassName, "NSUnit") == 0) && strcmp(className, "NSDimension") != 0) {
// To access available class properties, gain access to meta class.
Class meta = objc_getMetaClass(class_getName(classes[classIterator]));
// Count receives its value from class_copyPropertyList()
unsigned int propertyCount = 0;
objc_property_t *properties = class_copyPropertyList(meta, &propertyCount);
// Inspect all available class properties.
for (int propertyIterator = 0; propertyIterator < propertyCount; propertyIterator++) {
char const *propertyName = property_getName(properties[propertyIterator]);
// ...
}
// ...
}
// ...