每个资源都有一个版本字段，该字段每次资源都会更改
etag 字段。Etag 是 HTTP 的标准组成部分，
日历 API 中支持的两种情况：
- 针对资源修改执行，以确保在此期间没有对此资源执行任何其他写入操作（有条件地修改）
- 检索资源，以便仅在资源发生更改时检索资源数据（条件检索）
有条件的修改
如果您只想更新或删除未更改过的资源，请执行以下操作：
您可以指定一个 If-Match 标头，其中包含
从上一次检索中检索到的 etag 的值。这对于防止
会丢失对资源的修改客户端可以选择重新检索
然后重新应用更改
如果自上次检索以来条目（及其 etag）没有更改， 修改成功，使用新 etag 的资源的新版本为 返回。否则，您会收到 412（未满足前提条件）响应代码。
以下示例代码段演示了如何根据条件执行 使用 Java 客户端库。
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));
}
}
条件检索
如果您只想检索自上次更新后发生了更改的资源
您可以指定一个 If-None-Match 标头，其中包含
从上一次检索中检索到的 etag 的值。如果条目（及其 etag）
自上次检索以来发生了更改，即具有
则会返回新的 etag。否则，您会收到 304（未修改）错误
响应代码。
以下示例代码段演示了如何根据条件执行 使用 Java 客户端库。
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;
}
}
}