按照本指南中的步骤，从 Unity C# 代码获取应用的资源包。如果您尚未使用资源包构建 app bundle，请参阅针对 Unity 构建后再继续。
概览
Play Asset Delivery Unity API 提供了用于请求资源包、管理下载内容和获取资产的功能。您在该 API 中使用的函数取决于您创建资源包的方式。
如果您使用插件界面创建资源包，请选择插件配置的资源包。
如果您使用 API（或插件界面）创建资源包，请选择 API 配置的资源包。
根据您希望获取的资源包的分发类型实现该 API。这些步骤如以下流程图所示。
图 1. 获取资源包流程图
检索 AssetBundle
导入 Play Asset Delivery 库，然后调用 RetrieveAssetBundleAsync() 方法检索 AssetBundle。
using Google.Play.AssetDelivery; // Loads the AssetBundle from disk, downloading the asset pack containing it if necessary. PlayAssetBundleRequest bundleRequest = PlayAssetDelivery.RetrieveAssetBundleAsync(asset-bundle-name);
安装时分发
配置为 install-time 的资源包可以在应用启动后立即使用。您可以使用以下代码加载 AssetBundle 中的场景：
AssetBundle assetBundle = bundleRequest.AssetBundle; // You may choose to load scenes from the AssetBundle. For example: string[] scenePaths = assetBundle.GetAllScenePaths(); SceneManager.LoadScene(scenePaths[path-index]);
快速跟进式分发和按需分发
以下几部分适用于 fast-follow 和 on-demand 资源包。
查看状态
每个资源包都存储于应用的内部存储空间内单独的文件夹中。使用 isDownloaded() 方法确定资源包是否已下载。
监控下载
查询 PlayAssetBundleRequest 对象以监控请求的状态：
// Download progress of request, between 0.0f and 1.0f. The value will always be
// 1.0 for assets delivered as install-time.
// NOTE: A value of 1.0 will only signify the download is complete. It will still need to be loaded.
float progress = bundleRequest.DownloadProgress;
// Returns true if:
// * it had either completed the download, installing, and loading of the AssetBundle,
// * OR if it has encountered an error.
bool done = bundleRequest.IsDone;
// Returns status of retrieval request.
AssetDeliveryStatus status = bundleRequest.Status;
switch(status) {
case AssetDeliveryStatus.Pending:
// Asset pack download is pending - N/A for install-time assets.
case AssetDeliveryStatus.Retrieving:
// Asset pack is being downloaded and transferred to app storage.
// N/A for install-time assets.
case AssetDeliveryStatus.Available:
// Asset pack is downloaded on disk but NOT loaded into memory.
// For PlayAssetPackRequest(), this indicates that the request is complete.
case AssetDeliveryStatus.Loading:
// Asset pack is being loaded.
case AssetDeliveryStatus.Loaded:
// Asset pack has finished loading, assets can now be loaded.
// For PlayAssetBundleRequest(), this indicates that the request is complete.
case AssetDeliveryStatus.Failed:
// Asset pack retrieval has failed.
case AssetDeliveryStatus.WaitingForWifi:
// Asset pack retrieval paused until either the device connects via Wi-Fi,
// or the user accepts the PlayAssetDelivery.ShowCellularDataConfirmation dialog.
default:
break;
}
下载内容较大
超过 150MB 的资源包可以自动下载，但前提是必须已连接到 WLAN。如果用户未连接到 WLAN，PlayAssetBundleRequest 状态会设置为 AssetDeliveryStatus.WaitingForWifi，下载也会暂停。在这种情况下，要么等到设备连接到 WLAN 再恢复下载，要么提示用户批准通过移动网络连接下载资源包。
if(bundleRequest.Status == AssetDeliveryStatus.WaitingForWifi) {
var userConfirmationOperation = PlayAssetDelivery.ShowCellularDataConfirmation();
yield return userConfirmationOperation;
switch(userConfirmationOperation.GetResult()) {
case ConfirmationDialogResult.Unknown:
// userConfirmationOperation finished with an error. Something went
// wrong when displaying the prompt to the user, and they weren't
// able to interact with the dialog. In this case, we recommend
// developers wait for Wi-Fi before attempting to download again.
// You can get more info by calling GetError() on the operation.
case ConfirmationDialogResult.Accepted:
// User accepted the confirmation dialog - download will start
// automatically (no action needed).
case ConfirmationDialogResult.Declined:
// User canceled or declined the dialog. Await Wi-Fi connection, or
// re-prompt the user.
default:
break;
}
}
取消请求（仅限按需）
如果您在 AssetBundle 加载到内存中之前需要取消请求，请对 PlayAssetBundleRequest 对象调用 AttemptCancel() 方法：
// Will only attempt if the status is Pending, Retrieving, or Available - otherwise
// it will be a no-op.
bundleRequest.AttemptCancel();
// Check to see if the request was successful by checking if the error code is Canceled.
if(bundleRequest.Error == AssetDeliveryErrorCode.Canceled) {
// Request was successfully canceled.
}
异步请求资源包
在大多数情况下，您应使用协程异步请求资源包并监控进度，如下所示：
private IEnumerator LoadAssetBundleCoroutine(string assetBundleName) {
PlayAssetBundleRequest bundleRequest =
PlayAssetDelivery.RetrieveAssetBundleAsync(assetBundleName);
while (!bundleRequest.IsDone) {
if(bundleRequest.Status == AssetDeliveryStatus.WaitingForWifi) {
var userConfirmationOperation = PlayAssetDelivery.ShowCellularDataConfirmation();
// Wait for confirmation dialog action.
yield return userConfirmationOperation;
if((userConfirmationOperation.Error != AssetDeliveryErrorCode.NoError) ||
(userConfirmationOperation.GetResult() != ConfirmationDialogResult.Accepted)) {
// The user did not accept the confirmation - handle as needed.
}
// Wait for Wi-Fi connection OR confirmation dialog acceptance before moving on.
yield return new WaitUntil(() => bundleRequest.Status != AssetDeliveryStatus.WaitingForWifi);
}
// Use bundleRequest.DownloadProgress to track download progress.
// Use bundleRequest.Status to track the status of request.
yield return null;
}
if (bundleRequest.Error != AssetDeliveryErrorCode.NoError) {
// There was an error retrieving the bundle. For error codes NetworkError
// and InsufficientStorage, you may prompt the user to check their
// connection settings or check their storage space, respectively, then
// try again.
yield return null;
}
// Request was successful. Retrieve AssetBundle from request.AssetBundle.
AssetBundle assetBundle = bundleRequest.AssetBundle;
如需详细了解如何处理错误，请参阅 AssetDeliveryErrorCodes 列表。
其他 Play Core API 方法
以下是您可能希望在应用中使用的一些其他 API 方法。
检查下载内容大小
向 Google Play 发出异步调用并针对操作何时完成设置回调方法，从而检查 AssetBundle 的大小：
public IEnumerator GetDownloadSize() {
PlayAsyncOperation<long> getSizeOperation =
PlayAssetDelivery.GetDownloadSize(assetPackName);
yield return getSizeOperation;
if(operation.Error != AssetDeliveryErrorCode.NoError) {
// Error while retrieving download size.
} else {
// Download size is given in bytes.
long downloadSize = operation.GetResult();
}
}
移除 AssetBundle
您可以移除当前未加载到内存中的快速跟进式分发和按需分发 AssetBundle。发出以下异步调用，并针对何时完成设置回调方法：
PlayAsyncOperation<string> removeOperation = PlayAssetDelivery.RemoveAssetPack(assetBundleName);
removeOperation.Completed += (operation) =>
{
if(operation.Error != AssetDeliveryErrorCode.NoError) {
// Error while attempting to remove AssetBundles.
} else {
// Files were deleted OR files did not exist to begin with.
}
};
下一步
在本地和通过 Google Play 测试 Asset Delivery 情况。