参考文档

常用NSDateFormatter，但少见NSNumberFormatter ，但后者似乎功能更丰富，也更实用。

类方法

/*
     eg. 12345.345
     NSNumberFormatterNoStyle //四舍五入整数
     NSNumberFormatterDecimalStyle //小数形式
     NSNumberFormatterCurrencyStyle //本地货币样式，保留两位小数 $12,345.34
     NSNumberFormatterPercentStyle   //百分比样式  1,234,534%
     NSNumberFormatterScientificStyle  //科学计数样式  1.2345345E4
     NSNumberFormatterSpellOutStyle //朗读样式 twelve thousand three hundred forty-five point three four five
     
     iOS 9.0后
     NSNumberFormatterOrdinalStyle            //序数样式  12,345th
     NSNumberFormatterCurrencyISOCodeStyle   //货币代码样式  USD12,345.34
     NSNumberFormatterCurrencyPluralStyle    //本地货币样式  12,345.34 US dollars
     NSNumberFormatterCurrencyAccountingStyle //会计计数  $12,345.34
     */
    NSNumber *number = [NSNumber numberWithDouble:12345.345];
    NSString *numberNoStyleStr = [NSNumberFormatter localizedStringFromNumber:number numberStyle:NSNumberFormatterCurrencyAccountingStyle];

实例方法

    //实例方法
    NSNumberFormatter *formatter = [[NSNumberFormatter alloc]init];
    //舍入值
    formatter.roundingIncrement = @10;
    formatter.roundingMode = kCFNumberFormatterRoundUp;
    NSLog(@"%@",[formatter numberFromString:@""]);

    //样式
    formatter.numberStyle = NSNumberFormatterDecimalStyle ;
    //宽度，包括前后缀在内的宽度
    formatter.formatWidth = 12;
    //填充位置符（单个字符）
    formatter.paddingCharacter = @"X";
    formatter.paddingPosition = NSNumberFormatterDecimalStyle;
    /*
     NSNumberFormatterPadBeforePrefix
     NSNumberFormatterPadAfterPrefix
     NSNumberFormatterPadBeforeSuffix
     NSNumberFormatterPadAfterSuffix
     */

    //后缀
    formatter.positiveSuffix = @"元";
    //前缀
    formatter.positivePrefix = @"￥";
    //缩放因子
    formatter.multiplier = @10;
    //负数样式
    formatter.negativeFormat = @"-";
    //正数样式
    formatter.positiveFormat = @"+";
    //小数点的样式
    formatter.decimalSeparator = @".";
    //零的样式
    formatter.zeroSymbol = @"0";
    
    //设置了这个后，会自动用0补充而不用位置符
    //整数的最多/少位数
    formatter.maximumIntegerDigits = 100;
    formatter.minimumIntegerDigits = 10;
    //小数的最多/少位数
    formatter.maximumFractionDigits = 10;
    formatter.minimumFractionDigits = 7;

    NSLog(@"%@",[formatter stringFromNumber:number]);