constantvar

Documentation

Overview

Package constantvar provides a runtimevar implementation with Variables that never change. Use New, NewBytes, or NewError to construct a *runtimevar.Variable.

URLs

For runtimevar.OpenVariable, constantvar registers for the scheme "constant". For more details on the URL format, see URLOpener. See https://github.com/eliben/gocdkx/concepts/urls/ for background information.

As

constantvar does not support any types for As.

Example

package main

import (
	"context"
	"fmt"
	"log"

	"github.com/eliben/gocdkx/runtimevar/constantvar"
)

// MyConfig is a sample configuration struct.
type MyConfig struct {
	Server string
	Port   int
}

func main() {
	// cfg is our sample config.
	cfg := MyConfig{Server: "foo.com", Port: 80}

	// Construct a *runtimevar.Variable that always returns cfg.
	v := constantvar.New(cfg)
	defer v.Close()

	// We can now read the current value of the variable from v.
	snapshot, err := v.Latest(context.Background())
	if err != nil {
		log.Fatal(err)
	}
	cfg = snapshot.Value.(MyConfig)
	fmt.Printf("%s running on port %d", cfg.Server, cfg.Port)

}

Output:

foo.com running on port 80

Example (Bytes)

package main

import (
	"context"
	"fmt"
	"log"

	"github.com/eliben/gocdkx/runtimevar"
	"github.com/eliben/gocdkx/runtimevar/constantvar"
)

// MyConfig is a sample configuration struct.
type MyConfig struct {
	Server string
	Port   int
}

func main() {

	// Create a decoder for decoding JSON strings into MyConfig.
	decoder := runtimevar.NewDecoder(MyConfig{}, runtimevar.JSONDecode)

	// Construct a *runtimevar.Variable based on a JSON string.
	v := constantvar.NewBytes([]byte(`{"Server": "foo.com", "Port": 80}`), decoder)
	defer v.Close()

	// We can now read the current value of the variable from v.
	snapshot, err := v.Latest(context.Background())
	if err != nil {
		log.Fatal(err)
	}
	cfg := snapshot.Value.(MyConfig)
	fmt.Printf("%s running on port %d", cfg.Server, cfg.Port)

}

Output:

foo.com running on port 80

Example (Error)

package main

import (
	"context"
	"errors"
	"fmt"
	"log"

	"github.com/eliben/gocdkx/runtimevar/constantvar"
)

func main() {
	// Construct a runtimevar.Variable that always returns errFake.
	var errFake = errors.New("my error")
	v := constantvar.NewError(errFake)
	defer v.Close()

	// We can now use Watch to read the current value of the variable
	// from v. Note that Latest would block here since it waits for
	// a "good" value, and v will never get one.
	_, err := v.Watch(context.Background())
	if err == nil {
		log.Fatal("Expected an error!")
	}
	fmt.Println(err)

}

Output:

runtimevar (code=Unknown): my error

Example (OpenVariable)

package main

import (
	"context"
	"fmt"
	"log"

	"github.com/eliben/gocdkx/runtimevar"
)

func main() {
	// OpenVariable creates a *runtimevar.Variable from a URL.
	ctx := context.Background()
	v, err := runtimevar.OpenVariable(ctx, "constant://?val=hello+world&decoder=string")
	if err != nil {
		log.Fatal(err)
	}
	defer v.Close()

	snapshot, err := v.Latest(ctx)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(snapshot.Value.(string))

	// Output
	// hello world
}

Index

• Constants
• func New(value interface{}) *runtimevar.Variable
• func NewBytes(b []byte, decoder *runtimevar.Decoder) *runtimevar.Variable
• func NewError(err error) *runtimevar.Variable
• type URLOpener
  • func (o *URLOpener) OpenVariableURL(ctx context.Context, u *url.URL) (*runtimevar.Variable, error)

Examples

• Package
• Package (Bytes)
• Package (Error)
• Package (OpenVariable)

Constants

const Scheme = "constant"

Scheme is the URL scheme constantvar registers its URLOpener under on blob.DefaultMux.

Functions

func New

func New(value interface{}) *runtimevar.Variable

New constructs a *runtimevar.Variable holding value.

func NewBytes

func NewBytes(b []byte, decoder *runtimevar.Decoder) *runtimevar.Variable

NewBytes uses decoder to decode b. If the decode succeeds, it constructs a *runtimevar.Variable holding the decoded value. If the decode fails, it constructs a runtimevar.Variable that always fails with the error.

func NewError

func NewError(err error) *runtimevar.Variable

NewError constructs a *runtimevar.Variable that always fails. Runtimevar wraps errors returned by provider implementations, so the error returned by runtimevar will not equal err.

Types

type URLOpener

type URLOpener struct {
	// Decoder specifies the decoder to use if one is not specified in the URL.
	// Defaults to runtimevar.BytesDecoder.
	Decoder *runtimevar.Decoder
}

URLOpener opens constantvar URLs like "constant://?val=foo&decoder=string".

The host and path are ignored.

The following URL parameters are supported:

• val: The value to use for the constant Variable. The bytes from val are passed to NewBytes.
• err: The error to use for the constant Variable. A new error is created using errors.New and passed to NewError.
• decoder: The decoder to use. Defaults to runtimevar.BytesDecoder. See runtimevar.DecoderByName for supported values.

If both "err" and "val" are provided, "val" is ignored.

func (*URLOpener) OpenVariableURL

func (o *URLOpener) OpenVariableURL(ctx context.Context, u *url.URL) (*runtimevar.Variable, error)

OpenVariableURL opens the variable at the URL's path. See the package doc for more details.