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Streak
Google APIs Client and OAuth2
Andrew Gerrand
adg@golang.org
* Streak
Streak is a command-line productivity tool based around the "Seinfeld method".
[Seinfeld] revealed a unique calendar system he uses to pressure
himself to write. Here's how it works.
He told me to get a big wall calendar that has a whole year on one page
and hang it on a prominent wall. The next step was to get a big red
magic marker.
He said for each day that I do my task of writing, I get to put a big
red X over that day. "After a few days you'll have a chain. Just keep
at it and the chain will grow longer every day. You'll like seeing that
chain, especially when you get a few weeks under your belt. Your only
job next is to not break the chain."
"Don't break the chain," he said again for emphasis.
Streak maintains a Google Calendar named "Streaks", adding and extending multi-day events to represent a chain or "streak."
* My streaks
.image screenshot.png
* User interface
Add today to a streak (or create a streak if none exists):
$ streak
Remove today from a streak:
$ streak -remove
Add yesterday to a streak (or create if none exists):
$ streak -offset -1
Remove yesterday from a streak:
$ streak -offset -1 -remove
* How it works
At startup:
- Authenticate with OAuth2 and get access to the user's Calendar
- Find the calendar named "Streaks"
When adding a day to a streak:
- Iterate through existing events
- Either add a new event or update an existing one
When removing a day from a streak:
- Iterate through existing events
- Either delete or or update the relevant event
* Inside Streak
Beyond the Go standard library, Streak has a two dependencies:
- `google-api-go-client`, to access the Google Calendar API, and
- `goauth2`, for OAuth2 authentication with Google.
* Using the Calendar API
Import the relevant package from the `google-api-go-client` repository:
import "code.google.com/p/google-api-go-client/calendar/v3"
With an OAuth-authenticated HTTP client (more on this later), create a calendar service:
service, err := calendar.New(transport.Client())
if err != nil {
log.Fatal(err)
}
* Anatomy of an API call
To list the user's calendars, first build an `*calendar.CalendarListCall` value:
call := service.CalendarList.List()
Invoke the call's `Do` method, which returns a `*calendar.CalendarList` value:
list, err := call.Do()
if err != nil {
return "", err
}
Then finally we can use the result:
for _, entry := range list.Items {
fmt.Println(entry.Summary)
}
* Finding the "Streaks" calendar
const calSummary = "Streaks"
.code streak.go /func streakCalendarId/,/^}/
* The Calendar type
All operations require a calendar service and the relevant calendar ID, so we'll put them in a `Calendar` type:
.code streak.go /type Calendar/,/^}/
At startup we create a service and find the Calendar ID, and store them in a `Calendar`:
.code streak.go /service, err/
.code streak.go /calId, err/
.code streak.go /cal :=/,/}/
* Listing events (1/2)
Similar to listing calendars.
First build an `*calendar.EventsListCall` value, and use its "method chaining" API to request only non-recurring events in chronological order:
call := service.Events.List(calId).SingleEvents(true).OrderBy("startTime")
Invoke the call's `Do` method, which returns a `*calendar.Events` value:
events, err := call.Do()
if err != nil {
return err
}
Then we can use the result:
for _, e := range events.Items {
// Do something with the event, e.
}
* Listing events (2/2)
Only 100 Events may be returned per API call, so we may need to make multiple calls to retrieve the full list. The `*Events` struct has a `NextPageToken` field for pagination.
var pageToken string
for {
call := service.Events.List(calId).SingleEvents(true).OrderBy("startTime")
if pageToken != "" {
call.PageToken(pageToken)
}
events, err := call.Do()
if err != nil {
return err
}
for _, e := range events.Items {
// Do something with the event, e.
}
pageToken = events.NextPageToken
if pageToken == "" {
break // This is the last page.
}
}
* Iterating over events (interface)
We need to iterate through the events for both the add and remove operations, so it would be nice to abstract away this functionality somehow.
What if we could just write this instead?
cal.iterateEvents(func(e *calendar.Event) error {
// Do something with the event, e.
})
* Iterating over events (implementation)
func (c *Calendar) iterateEvents(fn func(e *calendar.Event) error) error {
var pageToken string
for {
call := c.Events.List(c.Id).SingleEvents(true).OrderBy("startTime")
if pageToken != "" {
call.PageToken(pageToken)
}
events, err := call.Do()
if err != nil {
return err
}
for _, e := range events.Items {
if err := fn(e); err != nil {
return err
}
}
pageToken = events.NextPageToken
if pageToken == "" {
return nil
}
}
panic("unreachable")
}
* Iterating over events (explicit continue)
We don't always want to iterate over the entire list, so we add a `Continue` `error` value that the iterator function must return to continue the iteration (otherwise the iterator returns with the given error).
var Continue = errors.New("continue")
func (c *Calendar) iterateEvents(fn func(e *calendar.Event) error) error {
// ...
for _, e := range events.Items {
if err := fn(e); err != Continue {
return err
}
}
// ...
}
cal.iterateEvents(func(e *calendar.Event) error {
if e.Summary == "Foo" {
return nil // stop iterating
}
return Continue
})
* Iterating over events (more specialization)
There's some other common behavior we can put in the iterator.
We only want all-day events named "Streak", and since we use the `time` package to perform computations on dates, we must convert the Calendar API's date strings to Go `time.Time` values.
type iteratorFunc func(e *calendar.Event, start, end time.Time) error
func (c *Calendar) iterateEvents(fn iteratorFunc) error {
// ...
for _, e := range events.Items {
if e.Start.Date == "" || e.End.Date == "" || e.Summary != evtSummary {
// Skip non-all-day event or non-streak events.
continue
}
start, end := parseDate(e.Start.Date), parseDate(e.End.Date)
if err := fn(e, start, end); err != Continue {
return err
}
}
// ...
}
* Iterating over events (final form in use)
This code to uses the iterator to find the duration of the longest event:
var longest time.Duration
cal.iterateEvents(func(e *calendar.Event, start, end time.Time) error {
if d := end.Sub(start); d > longest {
longest = d
}
return Continue
})
fmt.Println("Longest streak:", longest)
Output:
Longest streak: 360h0m0s
* Adding days to a streak (1/2)
The `addToStreak` function adds the given date to a streak in the `Calendar`.
func (c *Calendar) addToStreak(today time.Time) (err error) {
create := true
err = c.iterateEvents(func(e *calendar.Event, start, end time.Time) error {
if start.After(today) {
if start.Add(-day).Equal(today) {
// This event starts tomorrow, update it to start today.
create = false
e.Start.Date = today.Format(dateFormat)
_, err = c.Events.Update(c.Id, e.Id, e).Do()
return err
}
// This event is too far in the future.
return Continue
}
if end.After(today) {
// Today fits inside this event, nothing to do.
create = false
return nil
}
// ...
* Adding days to a streak (2/2)
// ...
if end.Equal(today) {
// This event ends today, update it to end tomorrow.
create = false
e.End.Date = today.Add(day).Format(dateFormat)
_, err = c.Events.Update(c.Id, e.Id, e).Do()
if err != nil {
return err
}
}
return Continue
})
if err == nil && create {
// No existing events cover or are adjacent to today, so create one.
err = c.createEvent(today, today.Add(day))
}
return
}
(This code is slightly abridged; the real program also combines adjacent events.)
* Removing days from a streak (1/2)
The `removeFromStreak` function is just like `addToStreak`.
func (c *Calendar) removeFromStreak(today time.Time) (err error) {
err = c.iterateEvents(func(e *calendar.Event, start, end time.Time) error {
if start.After(today) || end.Before(today) || end.Equal(today) {
// This event is too far in the future or past.
return Continue
}
if start.Equal(today) {
if end.Equal(today.Add(day)) {
// Single day event; remove it.
return c.Events.Delete(c.Id, e.Id).Do()
}
// Starts today; shorten to begin tomorrow.
e.Start.Date = start.Add(day).Format(dateFormat)
_, err := c.Events.Update(c.Id, e.Id, e).Do()
return err
}
if end.Equal(today.Add(day)) {
// Ends tomorrow; shorten to end today.
e.End.Date = today.Format(dateFormat)
_, err := c.Events.Update(c.Id, e.Id, e).Do()
return err
}
* Removing days from a streak (2/2)
// ...
// Split into two events.
// Shorten first event to end today.
e.End.Date = today.Format(dateFormat)
_, err = c.Events.Update(c.Id, e.Id, e).Do()
if err != nil {
return err
}
// Create second event that starts tomorrow.
return c.createEvent(today.Add(day), end)
})
return
}
* Authentication
Streak uses OAuth2 to authenticate with the Calendar API.
OAuth2 in a nutshell:
- Application directs Client to Service to authenticate
- Client tells Service "OK, Application can represent me"
- Service redirects Client to Application, with a `code` query parameter
- Application exchanges `code` with Service for an Access Token
- Application provides Access Token when using Service to access Client's data
Streak uses the `goauth2` package, which helps with the first, fourth, and final steps.
import "code.google.com/p/goauth2/oauth"
* OAuth2 configuration
The client ID and secret are obtained from the Google APIs Console. The `Scope` specifies the service to access (Calendar), while the `AuthURL` and `TokenURL` point to Google's OAuth2 service.
.code streak.go /config :=/,/}/
The `TokenCache` field and `oauth.CacheFile` helpers transparently store the access token on disk. A flag specifies the location of the cache file:
defaultCacheFile = filepath.Join(os.Getenv("HOME"), ".streak-request-token")
cachefile = flag.String("cachefile", defaultCacheFile, "Authentication token cache file")
* Setting up an authenticated service
At startup, try to read the cached token.
If that fails for any reason, call `authenticate` to go do the OAuth2 flow.
If it succeeds, store the token in the transport and build a Calendar service.
.code streak.go /transport :=/,/calendar.New/
* The authenticate function
.code oauth.go /func authenticate/,/^}/
* Demo