我正在使用GOOGLE Drive API(v3)与Google的 PHP api客户端列出并显示来自我的云端硬盘的视频,其中我还显示了所述视频的缩略图.虽然我在请求参数中启用了thumbnailLink,但我很难从API中获取缩略图网址：

'fields' => 'nextPageToken,files(id,name,thumbnailLink,webContentLink,webViewLink)'

thumbnailLink的值仍为空,其他值成功返回.我在api文档中查找缩略图设置,但没有成功. API也不会返回任何错误.会是什么呢？

这是我正在使用的代码：

$drive_service = new Google_Service_Drive($client);

$optParams = array(
'pageSize' => 4,'q' => "'0B_nSMgQioOWwNy1ha0tFWEE0QWs' in parents and mimeTyPE contains 'video/'",'fields' => 'nextPageToken,modifiedTime,mimeType,createdTime,webViewLink)'
);

return $files_list = $drive_service->files->listFiles($optParams)->getFiles();

和响应的var_dump

array(4) {
[0]=>
object(Google_Service_Drive_DriveFile)#590 (56) {
["collection_key":PRotected]=>
string(6) "spaces"
["appProperties"]=>
NULL
["capabilITiesType":protected]=>
string(42) "Google_Service_Drive_DriveFileCapabilities"
["capabilitiesDataType":protected]=>
string(0) ""
["contentHintsType":protected]=>
string(42) "Google_Service_Drive_DriveFileContentHints"
["contentHintsDataType":protected]=>
string(0) ""
["createdTime"]=>
string(24) "2016-09-12T19:54:22.000Z"
["description"]=>
NULL
["explicitlyTrashed"]=>
NULL
["fileExtension"]=>
NULL
["folderColorRgb"]=>
NULL
["fullFileExtension"]=>
NULL
["headrevisionId"]=>
NULL
["iconLink"]=>
NULL
["id"]=>
string(35) "1St7nQ3X3-ocrMUC2t-r6NOVc4xIDFhBigg"
["imageMediaMetadataType":protected]=>
string(48) "Google_Service_Drive_DriveFileimageMediaMetadata"
["imageMediaMetadataDataType":protected]=>
string(0) ""
["isAppAuthorized"]=>
NULL
["kind"]=>
NULL
["lastModifyingUserType":protected]=>
string(25) "Google_Service_Drive_User"
["lastModifyingUserDataType":protected]=>
string(0) ""
["md5Checksum"]=>
NULL
["mimeType"]=>
string(9) "video/mp4"
["modifiedByMeTime"]=>
NULL
["modifiedTime"]=>
string(24) "2016-12-27T11:55:43.958Z"
["name"]=>
string(12) "MOV_1950.mp4"
["originalFilename"]=>
NULL
["ownedByMe"]=>
NULL
["ownersType":protected]=>
string(25) "Google_Service_Drive_User"
["ownersDataType":protected]=>
string(5) "array"
["parents"]=>
NULL
["permissionsType":protected]=>
string(31) "Google_Service_Drive_Permission"
["permissionsDataType":protected]=>
string(5) "array"
["properties"]=>
NULL
["quotaByteSUSEd"]=>
NULL
["shared"]=>
NULL
["sharedWithMeTime"]=>
NULL
["sharingUserType":protected]=>
string(25) "Google_Service_Drive_User"
["sharingUserDataType":protected]=>
string(0) ""
["size"]=>
NULL
["spaces"]=>
NULL
["starred"]=>
NULL
["thumbnailLink"]=>
NULL
["trashed"]=>
NULL
["version"]=>
NULL
["videoMediaMetadataType":protected]=>
string(48) "Google_Service_Drive_DriveFileVideoMediaMetadata"
["videoMediaMetadataDataType":protected]=>
string(0) ""
["viewedByMe"]=>
NULL
["viewedByMeTime"]=>
NULL
["viewersCanCopyContent"]=>
NULL
["webContentLink"]=>
string(82) "https://drive.google.COM/uc?id=1St7nQ3X3-ocrMUC2t-r6NOVc4xIdfhbIgg&export=download"
["webViewLink"]=>
string(85) "https://drive.google.com/file/d/1St7nQ3X3-ocrMUC2t-r6NOVc4xIdfhbIgg/view?usp=drivesdk"
["writersCanShare"]=>
NULL
["internal_gapi_mappings":protected]=>
array(0) {
}
["modelData":protected]=>
array(0) {
}
["processed":protected]=>
array(0) {
}
}...