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146 lines
4.5 KiB
Markdown
146 lines
4.5 KiB
Markdown
# Getting Started with the Google APIs for Go
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## Getting Started
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This is a quick walk-through of how to get started with the Google APIs for Go.
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## Background
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The first thing to understand is that the Google API libraries are auto-generated for
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each language, including Go, so they may not feel like 100% natural for any language.
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The Go versions are pretty natural, but please forgive any small non-idiomatic things.
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(Suggestions welcome, though!)
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## Installing
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Pick an API and a version of that API to install.
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You can find the complete list by looking at the
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[directories here](https://github.com/google/google-api-go-client/tree/master/).
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For example, let's install the
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[urlshortener's version 1 API](https://godoc.org/google.golang.org/api/urlshortener/v1):
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```
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$ go get -u google.golang.org/api/urlshortener/v1
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```
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Now it's ready for use in your code.
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## Using
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Once you've installed a library, you import it like this:
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```go
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package main
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import (
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"context"
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"golang.org/x/oauth2"
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"golang.org/x/oauth2/google"
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"google.golang.org/api/urlshortener/v1"
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)
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```
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The package name, if you don't override it on your import line, is the name of the
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API without the version number. In the case above, just `urlshortener`.
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## Instantiating
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Each API has a `New` function taking an `*http.Client` and returning an API-specific `*Service`.
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You create the service like:
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```go
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svc, err := urlshortener.New(httpClient)
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```
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## OAuth HTTP Client
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The HTTP client you pass in to the service must be one that automatically adds
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Google-supported Authorization information to the requests.
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There are several ways to do authentication. They will all involve the package
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[golang.org/x/oauth2](https://godoc.org/golang.org/x/oauth2) in some way.
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### 3-legged OAuth
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For 3-legged OAuth (your application redirecting a user through a website to get a
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token giving your application access to that user's resources), you will need to
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create an oauth2.Config,
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```go
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var config = &oauth2.Config{
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ClientID: "", // from https://console.developers.google.com/project/<your-project-id>/apiui/credential
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ClientSecret: "", // from https://console.developers.google.com/project/<your-project-id>/apiui/credential
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Endpoint: google.Endpoint,
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Scopes: []string{urlshortener.UrlshortenerScope},
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}
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```
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... and then use the AuthCodeURL, Exchange, and Client methods on it.
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For an example, see: https://godoc.org/golang.org/x/oauth2#example-Config
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For the redirect URL, see
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https://developers.google.com/identity/protocols/OAuth2InstalledApp#choosingredirecturi
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### Service Accounts
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To use a Google service account, or the GCE metadata service, see
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the [golang.org/x/oauth2/google](https://godoc.org/golang.org/x/oauth2/google) package.
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In particular, see [google.DefaultClient](https://godoc.org/golang.org/x/oauth2/google#DefaultClient).
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### Using API Keys
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Some APIs require passing API keys from your application.
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To do this, you can use
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[transport.APIKey](https://godoc.org/google.golang.org/api/googleapi/transport#APIKey):
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```go
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ctx := context.WithValue(context.Background(), oauth2.HTTPClient, &http.Client{
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Transport: &transport.APIKey{Key: developerKey},
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})
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oauthConfig := &oauth2.Config{ .... }
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var token *oauth2.Token = .... // via cache, or oauthConfig.Exchange
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httpClient := oauthConfig.Client(ctx, token)
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svc, err := urlshortener.New(httpClient)
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...
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```
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## Using the Service
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Each service contains zero or more methods and zero or more sub-services.
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The sub-services related to a specific type of "Resource".
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Those sub-services then contain their own methods.
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For instance, the urlshortener API has just the "Url" sub-service:
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```go
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url, err := svc.Url.Get(shortURL).Do()
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if err != nil {
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...
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}
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fmt.Printf("The URL %s goes to %s\n", shortURL, url.LongUrl)
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```
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For a more complete example, see
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[urlshortener.go](https://github.com/google/google-api-go-client/tree/master/examples/urlshortener.go)
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in the [examples directory](https://github.com/google/google-api-go-client/tree/master/examples/).
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(the examples use some functions in `main.go` in the same directory)
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## Error Handling
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Most errors returned by the `Do` methods of these clients will be of type
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[`googleapi.Error`](https://godoc.org/google.golang.org/api/googleapi#Error).
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Use a type assertion to obtain the HTTP status code and other properties of the
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error:
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```go
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url, err := svc.Url.Get(shortURL).Do()
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if err != nil {
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if e, ok := err.(*googleapi.Error); ok && e.Code == http.StatusNotFound {
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...
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}
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}
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```
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