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为什么NumberFormatter函数func string(from number: NSNumber) -> String?返回String?而不是String？NumberFormatter这个函数返回的位置是否有特定的输入或状态为零？有时我听说这些不太理想的返回类型来自Objective-C保持文件,这是其中一种情况吗？

这是我的代码

NSNumberFormatter *currencyStyle = [[NSNumberFormatter alloc] init];

    [currencyStyle setFormatterBehavior:NSNumberFormatterBehavior10_4];
    [currencyStyle setNumberStyle:NSNumberFormatterCurrencyStyle];

    NSNumber *amount = [[NSNumber alloc] init];

    NSLog(@"the price string is %@", price);

    amount = [currencyStyle numberFromString:price];

    NSLog(@"The converted number is %@",[currencyStyle numberFromString:price]);
    NSLog(@"The NSNumber is %@", amount);

    NSLog(@"The formatted version is %@", [currencyStyle stringFromNumber:amount]);

    NSLog(@"--------------------");
    self.priceLabel.text = [currencyStyle stringFromNumber:amount]; 

    [amount release];
    [currencyStyle release];

这是日志吐出的内容

价格字符串是5转换后的数字是(null)NSNumber是(null)格式化的版本是(null)

我错过了什么吗？

编辑:更新的代码

NSNumberFormatter *currencyStyle = [[NSNumberFormatter alloc] init];
    [currencyStyle setFormatterBehavior:NSNumberFormatterBehavior10_4];
    [currencyStyle setNumberStyle:NSNumberFormatterCurrencyStyle];

    NSNumber *amount = [currencyStyle numberFromString:price];

    NSLog(@"the price string is %@", price);
    NSLog(@"The …