取得特定版本的資源

每項資源都有版本欄位,每次資源都會變更 變更 — etag 欄位。Etag 是 HTTP 的標準部分 Calendar API 支援以下兩種情況:

  • 修改資源,確保在此期間未對這項資源執行其他寫入作業 (條件修改)
  • 資源擷取時,只擷取資源變更時的資源資料 (條件式擷取)

條件式修改

如果您只想更新或刪除資源,但現在資源在 上次擷取時,您可以指定包含 If-Match 標頭 與前一次擷取的 Etag 值。這很實用 不會因資源修改而遺失修改內容客戶可以選擇重新擷取 並重新套用變更

如果項目 (及其 Etag) 自上次擷取以來無變更,則 修改成功,使用新的 etag 的新版本資源為 。否則,您會收到 412 (前條件失敗) 回應代碼。

下列程式碼片段示範如何執行條件式 修改 Java 用戶端程式庫

  private static void run() throws IOException {
    // Create a test event.
    Event event = Utils.createTestEvent(client, "Test Event");
    System.out.println(String.format("Event created: %s", event.getHtmlLink()));

    // Pause while the user modifies the event in the Calendar UI.
    System.out.println("Modify the event's description and hit enter to continue.");
    System.in.read();

    // Modify the local copy of the event.
    event.setSummary("Updated Test Event");

    // Update the event, making sure that we don't overwrite other changes.
    int numAttempts = 0;
    boolean isUpdated = false;
    do {
      Calendar.Events.Update request = client.events().update("primary", event.getId(), event);
      request.setRequestHeaders(new HttpHeaders().setIfMatch(event.getEtag()));
      try {
        event = request.execute();
        isUpdated = true;
      } catch (GoogleJsonResponseException e) {
        if (e.getStatusCode() == 412) {
          // A 412 status code, "Precondition failed", indicates that the etag values didn't
          // match, and the event was updated on the server since we last retrieved it. Use
          // {@link Calendar.Events.Get} to retrieve the latest version.
          Event latestEvent = client.events().get("primary", event.getId()).execute();

          // You may want to have more complex logic here to resolve conflicts. In this sample we're
          // simply overwriting the summary.
          latestEvent.setSummary(event.getSummary());
          event = latestEvent;
        } else {
          throw e;
        }
      }
      numAttempts++;
    } while (!isUpdated && numAttempts <= MAX_UPDATE_ATTEMPTS);

    if (isUpdated) {
      System.out.println("Event updated.");
    } else {
      System.out.println(String.format("Failed to update event after %d attempts.", numAttempts));
    }
  }

條件擷取

如果您只想擷取自上次至今有變更的資源 擷取的物件,您可以指定 If-None-Match 標頭,其中包含 與前一次擷取的 Etag 值。如果項目 (及其 etag) 自上次擷取以來已有所變更,也就是使用 就會傳回新的 etag。否則,您會收到 304 (未修改) 回應代碼。

下列程式碼片段示範如何執行條件式 擷取 YAML 檔案 Java 用戶端程式庫

  private static void run() throws IOException {
    // Create a test event.
    Event event = Utils.createTestEvent(client, "Test Event");
    System.out.println(String.format("Event created: %s", event.getHtmlLink()));

    // Pause while the user modifies the event in the Calendar UI.
    System.out.println("Modify the event's description and hit enter to continue.");
    System.in.read();

    // Fetch the event again if it's been modified.
    Calendar.Events.Get getRequest = client.events().get("primary", event.getId());
    getRequest.setRequestHeaders(new HttpHeaders().setIfNoneMatch(event.getEtag()));
    try {
      event = getRequest.execute();
      System.out.println("The event was modified, retrieved latest version.");
    } catch (GoogleJsonResponseException e) {
      if (e.getStatusCode() == 304) {
        // A 304 status code, "Not modified", indicates that the etags match, and the event has
        // not been modified since we last retrieved it.
        System.out.println("The event was not modified, using local version.");
      } else {
        throw e;
      }
    }
  }