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;
}
}
}