1. 首先定义Optionsstruct

type Options struct {
    // NatsURL is an URL (or comma separated list of URLs) to a node or nodes
    // in the cluster.
    NatsURL string

    // NatsConn is a user provided low-level NATS connection that the streaming
    // connection will use to communicate with the cluster. When set, closing
    // the NATS streaming connection does NOT close this NATS connection.
    // It is the responsibility of the application to manage the lifetime of
    // the supplied NATS connection.
    NatsConn *nats.Conn

    // ConnectTimeout is the timeout for the initial Connect(). This value is also
    // used for some of the internal request/replies with the cluster.
    ConnectTimeout time.Duration

    // AckTimeout is how long to wait when a message is published for an ACK from
    // the cluster. If the library does not receive an ACK after this timeout,
    // the Publish() call (or the AckHandler) will return ErrTimeout.
    AckTimeout time.Duration

    // DiscoverPrefix is the prefix connect requests are sent to for this cluster.
    // The default is "_STAN.discover".
    DiscoverPrefix string

    // MaxPubAcksInflight specifies how many messages can be published without
    // getting ACKs back from the cluster before the Publish() or PublishAsync()
    // calls block.
    MaxPubAcksInflight int

    // DEPRECATED: Please use PingInterval instead
    PingIterval int

    // PingInterval is the interval at which client sends PINGs to the server
    // to detect the loss of a connection.
    PingInterval int

    // PingMaxOut specifies the maximum number of PINGs without a corresponding
    // PONG before declaring the connection permanently lost.
    PingMaxOut int

    // ConnectionLostCB specifies the handler to be invoked when the connection
    // is permanently lost.
    ConnectionLostCB ConnectionLostHandler
}

2. 定义Option类型的字段

type Option func(*Options) error

1. 定义Option类型的包装

// NatsURL is an Option to set the URL the client should connect to.
// The url can contain username/password semantics. e.g. nats://derek:pass@localhost:4222
// Comma separated arrays are also supported, e.g. urlA, urlB.
func NatsURL(u string) Option {
    return func(o *Options) error {
        o.NatsURL = u
        return nil
    }
}

// ConnectWait is an Option to set the timeout for establishing a connection.
func ConnectWait(t time.Duration) Option {
    return func(o *Options) error {
        o.ConnectTimeout = t
        return nil
    }
}

// PubAckWait is an Option to set the timeout for waiting for an ACK for a
// published message.
func PubAckWait(t time.Duration) Option {
    return func(o *Options) error {
        o.AckTimeout = t
        return nil
    }
}

// MaxPubAcksInflight is an Option to set the maximum number of published
// messages without outstanding ACKs from the server.
func MaxPubAcksInflight(max int) Option {
    return func(o *Options) error {
        o.MaxPubAcksInflight = max
        return nil
    }
}

// NatsConn is an Option to set the underlying NATS connection to be used
// by a streaming connection object. When such option is set, closing the
// streaming connection does not close the provided NATS connection.
func NatsConn(nc *nats.Conn) Option {
    return func(o *Options) error {
        o.NatsConn = nc
        return nil
    }
}

// Pings is an Option to set the ping interval and max out values.
// The interval needs to be at least 1 and represents the number
// of seconds.
// The maxOut needs to be at least 2, since the count of sent PINGs
// increase whenever a PING is sent and reset to 0 when a response
// is received. Setting to 1 would cause the library to close the
// connection right away.
func Pings(interval, maxOut int) Option {
    return func(o *Options) error {
        // For tests, we may pass negative value that will be interpreted
        // by the library as milliseconds. If this test boolean is set,
        // do not check values.
        if !testAllowMillisecInPings {
            if interval < 1 || maxOut <= 2 {
                return fmt.Errorf("invalid ping values: interval=%v (min>0) maxOut=%v (min=2)", interval, maxOut)
            }
        }
        o.PingInterval = interval
        o.PingMaxOut = maxOut
        return nil
    }
}

// SetConnectionLostHandler is an Option to set the connection lost handler.
// This callback will be invoked should the client permanently lose
// contact with the server (or another client replaces it while being
// disconnected). The callback will not be invoked on normal Conn.Close().
func SetConnectionLostHandler(handler ConnectionLostHandler) Option {
    return func(o *Options) error {
        o.ConnectionLostCB = handler
        return nil
    }
}

4. 方法接收

func Connect(stanClusterID, clientID string, options ...Option) (Conn, error) {
    ...
}

这样，通过options接收的值，遍历调用，验证值得正确性。