影象拉著進度條寫在 Go

下面是使用 Go 和 Docker Engine API 進行影象拉動的示例，以及與在 CLI 中執行 docker pull your_image_name 時顯示的進度條相同的進度條。出於進度條的目的，使用一些 ANSI 程式碼 。

package yourpackage

import (
    "context"
    "encoding/json"
    "fmt"
    "io"
    "strings"

    "github.com/docker/docker/api/types"
    "github.com/docker/docker/client"
)

// Struct representing events returned from image pulling
type pullEvent struct {
    ID             string `json:"id"`
    Status         string `json:"status"`
    Error          string `json:"error,omitempty"`
    Progress       string `json:"progress,omitempty"`
    ProgressDetail struct {
        Current int `json:"current"`
        Total   int `json:"total"`
    } `json:"progressDetail"`
}

// Actual image pulling function
func PullImage(dockerImageName string) bool {
    client, err := client.NewEnvClient()

    if err != nil {
        panic(err)
    }

    resp, err := client.ImagePull(context.Background(), dockerImageName, types.ImagePullOptions{})

    if err != nil {
        panic(err)
    }

    cursor := Cursor{}
    layers := make([]string, 0)
    oldIndex := len(layers)

    var event *pullEvent
    decoder := json.NewDecoder(resp)

    fmt.Printf("\n")
    cursor.hide()

    for {
        if err := decoder.Decode(&event); err != nil {
            if err == io.EOF {
                break
            }

            panic(err)
        }

        imageID := event.ID

        // Check if the line is one of the final two ones
        if strings.HasPrefix(event.Status, "Digest:") || strings.HasPrefix(event.Status, "Status:") {
            fmt.Printf("%s\n", event.Status)
            continue
        }

        // Check if ID has already passed once
        index := 0
        for i, v := range layers {
            if v == imageID {
                index = i + 1
                break
            }
        }

        // Move the cursor
        if index > 0 {
            diff := index - oldIndex

            if diff > 1 {
                down := diff - 1
                cursor.moveDown(down)
            } else if diff < 1 {
                up := diff*(-1) + 1
                cursor.moveUp(up)
            }

            oldIndex = index
        } else {
            layers = append(layers, event.ID)
            diff := len(layers) - oldIndex

            if diff > 1 {
                cursor.moveDown(diff) // Return to the last row
            }

            oldIndex = len(layers)
        }

        cursor.clearLine()

        if event.Status == "Pull complete" {
            fmt.Printf("%s: %s\n", event.ID, event.Status)
        } else {
            fmt.Printf("%s: %s %s\n", event.ID, event.Status, event.Progress)
        }

    }

    cursor.show()

    if strings.Contains(event.Status, fmt.Sprintf("Downloaded newer image for %s", dockerImageName)) {
        return true
    }

    return false
}

為了更好的可讀性，使用 ANSI 程式碼的遊標操作將移動到單獨的結構，如下所示：

package yourpackage

import "fmt"

// Cursor structure that implements some methods
// for manipulating command line's cursor
type Cursor struct{}

func (cursor *Cursor) hide() {
    fmt.Printf("\033[?25l")
}

func (cursor *Cursor) show() {
    fmt.Printf("\033[?25h")
}

func (cursor *Cursor) moveUp(rows int) {
    fmt.Printf("\033[%dF", rows)
}

func (cursor *Cursor) moveDown(rows int) {
    fmt.Printf("\033[%dE", rows)
}

func (cursor *Cursor) clearLine() {
    fmt.Printf("\033[2K")
}

在你的主程式包中，你可以呼叫 PullImage 函式傳遞你要拉的影象名稱。當然，在呼叫之前，你必須登入到 Docker 登錄檔，其中包含影象。