Docker源码解读:1.flag解读

type float64Value float64

func newFloat64Value(val float64, p *float64) *float64Value {
	*p = val
	return (*float64Value)(p)
}

func (f *float64Value) Set(s string) error {
	v, err := strconv.ParseFloat(s, 64)
	*f = float64Value(v)
	return err
}

func (f *float64Value) Get() interface{} { return float64(*f) }

func (f *float64Value) String() string { return fmt.Sprintf("%v", *f) }

针对上面两个重要的结构，分别绑定了很多重要的方法，具体如下：

// Name returns the name of the FlagSet.
func (fs *FlagSet) Name() string {
	return fs.name
}

// Out returns the destination for usage and error messages.
func (fs *FlagSet) Out() io.Writer {
         //如果fs中的output为空，则设置为默认的os.Stderr
	if fs.output == nil {
		return os.Stderr
	}
	return fs.output
}

// SetOutput sets the destination for usage and error messages.
// If output is nil, os.Stderr is used.
func (fs *FlagSet) SetOutput(output io.Writer) {
	fs.output = output
}

// VisitAll visits the flags in lexicographical order, calling fn for each.
// It visits all flags, even those not set.
//这个方法会以字典顺序来访问其中的每一个flag，哪怕这个flag没有被设置，在每一个flag上调用指定的函数
func (fs *FlagSet) VisitAll(fn func(*Flag)) {
	for _, flag := range sortFlags(fs.formal) {
		fn(flag)
	}
}
// Visit visits the flags in lexicographical order, calling fn for each.
// It visits only those flags that have been set.
//这个方法只能以字典顺序访问已经被设置的flag，并在访问到的flag上执行指定的函数
func (fs *FlagSet) Visit(fn func(*Flag)) {
	for _, flag := range sortFlags(fs.actual) {
		fn(flag)
	}
}

// Lookup returns the Flag structure of the named flag, returning nil if none exists.
func (fs *FlagSet) Lookup(name string) *Flag {
	return fs.formal[name]
}

// IsSet indicates whether the specified flag is set in the given FlagSet
//判断某个flag是否已经被设置了
func (fs *FlagSet) IsSet(name string) bool {
	return fs.actual[name] != nil
}

// Require adds a requirement about the number of arguments for the FlagSet.
// The first parameter can be Exact, Max, or Min to respectively specify the exact,
// the maximum, or the minimal number of arguments required.
// The actual check is done in FlagSet.CheckArgs().
func (fs *FlagSet) Require(nArgRequirementType nArgRequirementType, nArg int) {
	fs.nArgRequirements = append(fs.nArgRequirements, nArgRequirement{nArgRequirementType, nArg})
}

// CheckArgs uses the requirements set by FlagSet.Require() to validate
// the number of arguments. If the requirements are not met,
// an error message string is returned.
func (fs *FlagSet) CheckArgs() (message string) {
	for _, req := range fs.nArgRequirements {
		var arguments string
		if req.N == 1 {
			arguments = "1 argument"
		} else {
			arguments = fmt.Sprintf("%d arguments", req.N)
		}

		str := func(kind string) string {
			return fmt.Sprintf("%q requires %s%s", fs.name, kind, arguments)
		}

		switch req.Type {
		case Exact:
			if fs.NArg() != req.N {
				return str("")
			}
		case Max:
			if fs.NArg() > req.N {
				return str("a maximum of ")
			}
		case Min:
			if fs.NArg() < req.N {
				return str("a minimum of ")
			}
		}
	}
	return ""
}


//设置flag
// Set sets the value of the named flag.
func (fs *FlagSet) Set(name, value string) error {
	flag, ok := fs.formal[name]
	if !ok {
		return fmt.Errorf("no such flag -%v", name)
	}
	if err := flag.Value.Set(value); err != nil {
		return err
	}
	if fs.actual == nil {
		fs.actual = make(map[string]*Flag)
	}
	fs.actual[name] = flag
	return nil
}
// FlagCount returns the number of flags that have been defined.
func (fs *FlagSet) FlagCount() int { return len(sortFlags(fs.formal)) }

// FlagCountUndeprecated returns the number of undeprecated flags that have been defined.
//返回未过时的flag数量
func (fs *FlagSet) FlagCountUndeprecated() int {
	count := 0
	for _, flag := range sortFlags(fs.formal) {
		for _, name := range flag.Names {
			if name[0] != ‘#‘ {
				count++
				break
			}
		}
	}
	return count
}

// NFlag returns the number of flags that have been set.
//返回已经被设置的flag的数量
func (fs *FlagSet) NFlag() int { return len(fs.actual) }

// Arg returns the i‘th argument.  Arg(0) is the first remaining argument
// after flags have been processed.
//获得某参数
func (fs *FlagSet) Arg(i int) string {
	if i < 0 || i >= len(fs.args) {
		return ""
	}
	return fs.args[i]
}

// NArg is the number of arguments remaining after flags have been processed.
//返回剩下的参数数量
func (fs *FlagSet) NArg() int { return len(fs.args) }
// Args returns the non-flag arguments.
//返回非flag参数
func (fs *FlagSet) Args() []string { return fs.args }

除了上述绑定的函数外，在这里面还定义基本类型的从Var到类似StringVar，再到String的的函数，我们看其中的一个：

func (fs *FlagSet) Var(value Value, names []string, usage string) {
	// Remember the default value as a string; it won‘t change.
	flag := &Flag{names, usage, value, value.String()}
	for _, name := range names {
		name = strings.TrimPrefix(name, "#")
		_, alreadythere := fs.formal[name]
		if alreadythere {
			var msg string
			if fs.name == "" {
				msg = fmt.Sprintf("flag redefined: %s", name)
			} else {
				msg = fmt.Sprintf("%s flag redefined: %s", fs.name, name)
			}
			fmt.Fprintln(fs.Out(), msg)
			panic(msg) // Happens only if flags are declared with identical names
		}
		if fs.formal == nil {
			fs.formal = make(map[string]*Flag)
		}
		fs.formal[name] = flag
	}
}

这个方法的基本含义就是就是将names对应的value是指到fs中的formal中，如果已经存在就报错。

//设置fs中那么的值类型为string的name
func (fs *FlagSet) StringVar(p *string, names []string, value string, usage string) {
	fs.Var(newStringValue(value, p), names, usage)
}

//比StringVar更进一步的封装
func (fs *FlagSet) String(names []string, value string, usage string) *string {
	p := new(string)
	fs.StringVar(p, names, value, usage)
	return p
}

好了，重头戏来了。

// parseOne parses one flag. It reports whether a flag was seen.
func (fs *FlagSet) parseOne() (bool, string, error) {
	if len(fs.args) == 0 {
		return false, "", nil
	}
	s := fs.args[0]
	//第一字符必须是‘-‘
	if len(s) == 0 || s[0] != ‘-‘ || len(s) == 1 {
		return false, "", nil
	}
	//形如‘--不接任何字符串‘  这种格式也是错误的,其实这里不算是错误，只是把把单纯的‘--‘格式给过滤掉了
	if s[1] == ‘-‘ && len(s) == 2 { // "--" terminates the flags
		fs.args = fs.args[1:]
		return false, "", nil
	}
	name := s[1:] //得到flag的下一个字符
	//形如‘--=’这种格式是错误的
	if len(name) == 0 || name[0] == ‘=‘ {
		return false, "", fs.failf("bad flag syntax: %s", s)
	}
	//到这里，说明这个flag是合法的了
	// it‘s a flag. does it have an argument?
	fs.args = fs.args[1:] //将指针移动到下一个flag
	hasValue := false
	value := ""
	//如果是形如‘name=’的格式,那么，value=[name[i+1:]
	//此时，真正的name=name[:i]
	if i := strings.Index(name, "="); i != -1 {
		value = trimQuotes(name[i+1:])
		hasValue = true
		name = name[:i]
	}

	m := fs.formal
	//查看已经规整的formal中是否存在这个name
	flag, alreadythere := m[name] // BUG
	//如果规整过的参数中还没有这个名字
	if !alreadythere {
		if name == "-help" || name == "help" || name == "h" { // special case for nice help message.
			fs.usage()
			return false, "", ErrHelp
		}
		//连续三个‘-‘也不行
		if len(name) > 0 && name[0] == ‘-‘ {
			return false, "", fs.failf("flag provided but not defined: -%s", name)
		}
		return false, name, ErrRetry
	}
	if fv, ok := flag.Value.(boolFlag); ok && fv.IsBoolFlag() { // special case: doesn‘t need an arg
		if hasValue {
			//这个flag有值，却不能设置，那么就要报错
			if err := fv.Set(value); err != nil {
				return false, "", fs.failf("invalid boolean value %q for  -%s: %v", value, name, err)
			}
		} else {
			//如果判断出来没有值，说明这个flag是一个bool类型的
			fv.Set("true") //咩有值的情况就设置为true
		}
	} else {
		// It must have a value, which might be the next argument.
		if !hasValue && len(fs.args) > 0 {
			// value is the next arg
			//需要有值，而且这个值是参数表里面的下一个值
			hasValue = true
			value, fs.args = fs.args[0], fs.args[1:]
		}
		if !hasValue {
			return false, "", fs.failf("flag needs an argument: -%s", name)
		}
		if err := flag.Value.Set(value); err != nil {
			return false, "", fs.failf("invalid value %q for flag -%s: %v", value, name, err)
		}
	}
	if fs.actual == nil {
		fs.actual = make(map[string]*Flag)
	}
	//规整后的值
	fs.actual[name] = flag
	for i, n := range flag.Names {
		if n == fmt.Sprintf("#%s", name) {
			replacement := ""
			for j := i; j < len(flag.Names); j++ {
				if flag.Names[j][0] != ‘#‘ {
					replacement = flag.Names[j]
					break
				}
			}
			if replacement != "" {
				fmt.Fprintf(fs.Out(), "Warning: ‘-%s‘ is deprecated, it will be replaced by ‘-%s‘ soon. See usage.\n", name, replacement)
			} else {
				fmt.Fprintf(fs.Out(), "Warning: ‘-%s‘ is deprecated, it will be removed soon. See usage.\n", name)
			}
		}
	}
	return true, "", nil
}

func (fs *FlagSet) Parse(arguments []string) error {
	fs.parsed = true
	fs.args = arguments
	for {
		seen, name, err := fs.parseOne()
		if seen {
			//seen代表已经规整过了，不用再重新规整
			continue
		}
		if err == nil {
			//这也是严重错误
			break
		}
		//连续三个‘-‘的情况
		if err == ErrRetry {
			if len(name) > 1 {
				err = nil
				for _, letter := range strings.Split(name, "") {
					//将这个-去掉，然后刚在参数的最前面，重新来解析
					fs.args = append([]string{"-" + letter}, fs.args...)
					seen2, _, err2 := fs.parseOne()
					if seen2 {
						continue
					}
					if err2 != nil {
						err = fs.failf("flag provided but not defined: -%s", name)
						break
					}
				}
				if err == nil {
					continue
				}
			} else {
				err = fs.failf("flag provided but not defined: -%s", name)
			}
		}
		switch fs.errorHandling {
		case ContinueOnError:
			return err
		case ExitOnError:
			os.Exit(2)
		case PanicOnError:
			panic(err)
		}
	}
	return nil
}

//将多个flagsets合并到一个flagset上面
func Merge(dest *FlagSet, flagsets ...*FlagSet) error {
	for _, fset := range flagsets {
		for k, f := range fset.formal {
			if _, ok := dest.formal[k]; ok {
				var err error
				if fset.name == "" {
					err = fmt.Errorf("flag redefined: %s", k)
				} else {
					err = fmt.Errorf("%s flag redefined: %s", fset.name, k)
				}
				fmt.Fprintln(fset.Out(), err.Error())
				// Happens only if flags are declared with identical names
				switch dest.errorHandling {
				case ContinueOnError:
					return err
				case ExitOnError:
					os.Exit(2)
				case PanicOnError:
					panic(err)
				}
			}
			newF := *f //flag
			newF.Value = mergeVal{f.Value, k, fset}
			dest.formal[k] = &newF
		}
	}
	return nil
}

最后再来看，这个包中定义的最后一个变量：

var CommandLine = NewFlagSet(os.Args[0], ExitOnError)

CommandLine也是一个FlagSet哦，其中os.Args[0]就是程序的名字。

上面的结构比较完整了，留给我们的就是最后一个函数，他的访问级别死公开的：

func Parse() {
	// Ignore errors; CommandLine is set for ExitOnError.
	CommandLine.Parse(os.Args[1:])
}