Add thing extract command (#25)

Extract a template from an existing thing using this command:
$ iot-cloud-cli thing extract --id <thingID> --outfile <templateFile.json>
outfile parameter is optional, it specifies the json destination file for the template

* Add thing extract command

* Update readme

Author: polldo

Diff for: README.md

@@ -70,4 +70,8 @@ Print only the thing associated to the passed device:
 
 Delete a thing with the following command:
 
-`$ iot-cloud-cli thing delete --device-id <deviceID>`
+`$ iot-cloud-cli thing delete --device-id <deviceID>`
+
+Extract a template from an existing thing:
+
+`$ iot-cloud-cli thing extract --id <thingID> --outfile <templateFile.json>`

Diff for: cli/thing/extract.go

@@ -0,0 +1,43 @@
+package thing
+
+import (
+	"fmt"
+
+	"github.com/arduino/iot-cloud-cli/command/thing"
+	"github.com/spf13/cobra"
+)
+
+var extractFlags struct {
+	id      string
+	outfile string
+}
+
+func initExtractCommand() *cobra.Command {
+	extractCommand := &cobra.Command{
+		Use:   "extract",
+		Short: "Extract and save a thing",
+		Long:  "Extract a thing from Arduino IoT Cloud and save it in a template file",
+		RunE:  runExtractCommand,
+	}
+	extractCommand.Flags().StringVarP(&extractFlags.id, "id", "i", "", "Thing ID")
+	extractCommand.Flags().StringVarP(&extractFlags.outfile, "outfile", "o", "", "Template file destination path")
+	extractCommand.MarkFlagRequired("id")
+	return extractCommand
+}
+
+func runExtractCommand(cmd *cobra.Command, args []string) error {
+	fmt.Printf("Extracting thing %s\n", extractFlags.id)
+
+	params := &thing.ExtractParams{ID: extractFlags.id}
+	if extractFlags.outfile != "" {
+		params.Outfile = &extractFlags.outfile
+	}
+
+	err := thing.Extract(params)
+	if err != nil {
+		return err
+	}
+
+	fmt.Println("Thing successfully extracted")
+	return nil
+}

Diff for: cli/thing/thing.go

@@ -14,6 +14,7 @@ func NewCommand() *cobra.Command {
 	thingCommand.AddCommand(initCreateCommand())
 	thingCommand.AddCommand(initListCommand())
 	thingCommand.AddCommand(initDeleteCommand())
+	thingCommand.AddCommand(initExtractCommand())
 
 	return thingCommand
 }

Diff for: command/thing/extract.go

@@ -0,0 +1,86 @@
+package thing
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"os"
+
+	iotclient "github.com/arduino/iot-client-go"
+	"github.com/arduino/iot-cloud-cli/internal/config"
+	"github.com/arduino/iot-cloud-cli/internal/iot"
+)
+
+// ExtractParams contains the parameters needed to
+// extract a thing from Arduino IoT Cloud and save it on local storage.
+// Output indicates the destination path of the extraction.
+type ExtractParams struct {
+	ID      string
+	Outfile *string
+}
+
+// Extract command is used to extract a thing template
+// from a thing on Arduino IoT Cloud.
+func Extract(params *ExtractParams) error {
+	conf, err := config.Retrieve()
+	if err != nil {
+		return err
+	}
+	iotClient, err := iot.NewClient(conf.Client, conf.Secret)
+	if err != nil {
+		return err
+	}
+
+	thing, err := iotClient.GetThing(params.ID)
+	if err != nil {
+		err = fmt.Errorf("%s: %w", "cannot extract thing: ", err)
+		return err
+	}
+
+	template, err := templateFromThing(thing)
+	if err != nil {
+		return err
+	}
+
+	if params.Outfile == nil {
+		outfile := thing.Name + "-template.json"
+		params.Outfile = &outfile
+	}
+	err = ioutil.WriteFile(*params.Outfile, template, os.FileMode(0644))
+	if err != nil {
+		err = fmt.Errorf("%s: %w", "cannot write outfile: ", err)
+		return err
+	}
+
+	return nil
+}
+
+func templateFromThing(thing *iotclient.ArduinoThing) ([]byte, error) {
+	template := make(map[string]interface{})
+	template["properties_count"] = thing.PropertiesCount
+
+	var props []map[string]interface{}
+	for _, p := range thing.Properties {
+		prop := make(map[string]interface{})
+		prop["max_value"] = p.MaxValue
+		prop["min_value"] = p.MinValue
+		prop["name"] = p.Name
+		prop["permission"] = p.Permission
+		prop["persist"] = p.Persist
+		prop["tag"] = p.Tag
+		prop["type"] = p.Type
+		prop["update_parameter"] = p.UpdateParameter
+		prop["update_strategy"] = p.UpdateStrategy
+		prop["variable_name"] = p.VariableName
+		props = append(props, prop)
+	}
+	template["properties"] = props
+
+	// Extract json template from thing structure
+	file, err := json.MarshalIndent(template, "", "    ")
+	if err != nil {
+		err = fmt.Errorf("%s: %w", "thing marshal failure: ", err)
+		return nil, err
+	}
+	return file, nil
+}