Twitter API (#207)

* add twitter service

* update readme

twitter/handler/twitter.go

@@ -0,0 +1,84 @@
+package handler
+
+import (
+	"context"
+
+	"github.com/dghubble/go-twitter/twitter"
+	"github.com/micro/micro/v3/service/config"
+	"github.com/micro/micro/v3/service/errors"
+	"github.com/micro/micro/v3/service/logger"
+	pb "github.com/micro/services/twitter/proto"
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/clientcredentials"
+)
+
+type Twitter struct {
+	Client *twitter.Client
+}
+
+func New() *Twitter {
+	v, err := config.Get("twitter.api_key")
+	if err != nil {
+		logger.Fatalf("twitter.api_key config not found: %v", err)
+	}
+	apiKey := v.String("")
+	if len(apiKey) == 0 {
+		logger.Fatal("twitter.api_key config not found")
+	}
+	v, err = config.Get("twitter.api_secret")
+	if err != nil {
+		logger.Fatalf("twitter.api_secret config not found: %v", err)
+	}
+	apiSecret := v.String("")
+	if len(apiSecret) == 0 {
+		logger.Fatal("twitter.api_secret config not found")
+	}
+
+	// oauth2 configures a client that uses app credentials to keep a fresh token
+	config := &clientcredentials.Config{
+		ClientID:     apiKey,
+		ClientSecret: apiSecret,
+		TokenURL:     "https://api.twitter.com/oauth2/token",
+	}
+	// http.Client will automatically authorize Requests
+	httpClient := config.Client(oauth2.NoContext)
+
+	// Twitter client
+	client := twitter.NewClient(httpClient)
+
+	return &Twitter{
+		Client: client,
+	}
+}
+
+func (t *Twitter) Timeline(ctx context.Context, req *pb.TimelineRequest, rsp *pb.TimelineResponse) error {
+	if len(req.Username) == 0 {
+		return errors.BadRequest("twitter.timeline", "missing username")
+	}
+
+	if req.Limit <= 0 {
+		req.Limit = 20
+	}
+
+	tweets, _, err := t.Client.Timelines.UserTimeline(&twitter.UserTimelineParams{
+		ScreenName: req.Username,
+		Count:      int(req.Limit),
+	})
+	if err != nil {
+		logger.Errorf("Failed to retrieve tweets for %v: %v", req.Username, err)
+		return errors.InternalServerError("twitter.timeline", "Failed to retrieve tweets for %v: %v", req.Username, err)
+	}
+
+	for _, tweet := range tweets {
+		rsp.Tweets = append(rsp.Tweets, &pb.Tweet{
+			Id:              tweet.ID,
+			Text:            tweet.Text,
+			CreatedAt:       tweet.CreatedAt,
+			FavouritedCount: int64(tweet.FavoriteCount),
+			RetweetedCount:  int64(tweet.RetweetCount),
+			Username:        tweet.User.ScreenName,
+		})
+	}
+
+	return nil
+}