day3:
PHAsset:图片库中图像、视频或实时照片的表示。
1、Fetching Assets 获取asset
2、Assets contain only metadata,可不存储在本地，可下载预览图，见PHImageManager
3、Asset objects are immutable，通过 PHAssetChangeRequest修改，见 PHPhotoLibrary
var mediaType: PHAssetMediaType//The type of the asset, such as video or audio.(enum PHAssetMediaType//Constants identifying the general type of an asset, such as image or video.)
var mediaSubtypes: PHAssetMediaSubtype//The subtypes of the asset, identifying special kinds of assets such as panoramic photo or high-framerate video.全景照片或高帧率视频。(struct PHAssetMediaSubtype//Constants identifying specific variations of asset media, such as panorama or screenshot photos and time lapse or high frame rate video.)
var sourceType: PHAssetSourceType//The means by which the asset entered the user’s Photos library. (struct PHAssetSourceType//Means by which an asset enters the Photos library, used by the PHAssetSourceTypeproperty. UserLibrary、CloudShared、iTunesSynced )
var pixelWidth: Int //pixel 像素
var creationDate: Date?//The date and time at which the asset was originally created.
var modificationDate: Date?//The date and time at which the asset was last modified. 修改
var location: CLLocation?//The location information saved with the asset.
var duration: TimeInterval//The duration, in seconds, of the video asset.
var isSyncFailureHidden: Bool 同步失败隐藏
var playbackStyle: PHAsset.PlaybackStyle//An enumerated value that describes how to present an asset to the user. case unsupported、case image、case imageAnimated、case livePhoto、case video、case videoLooping
var burstIdentifier: String?//The unique identifier shared by photo assets from the same burst sequence. 连拍
var burstSelectionTypes: PHAssetBurstSelectionType//The selection type of the asset in a burst photo sequence.//.UserPick 表示用户手动标记的资源，.AutoPick 表示用户可能标记的潜在资源，.None 表示没有标记的资源。
var representsBurst: Bool//A Boolean value that indicates whether the asset is the representative photo from a burst photo sequence.
Fetching Assets：Retrieve asset metadata or request full asset content.
检索asset元数据或请求完整的asset内容。
class func fetchAssets(in: PHAssetCollection, options: PHFetchOptions?) -> PHFetchResult<PHAsset>
Retrieves assets from the specified asset collection.
class func fetchAssets(with: PHAssetMediaType, options: PHFetchOptions?) -> PHFetchResult<PHAsset>
Retrieves assets with the specified media type.
Retrieves assets with the specified local-device-specific unique identifiers.
Retrieves assets marked as key assets in the specified asset collection.
class func fetchAssets(with: PHFetchOptions?) -> PHFetchResult<PHAsset>
Retrieves all assets matching the specified options.
class func fetchAssets(withBurstIdentifier: String, options: PHFetchOptions?) -> PHFetchResult
Retrieves assets with the specified burst photo sequence identifier.
Editing an Asset:Editing Asset Content//Make a request to modify and save a photo or video asset.
class PHContentEditingInput//A container that provides information about and access to the image, video, or Live Photo content of an asset to be edited.
class PHContentEditingOutput//A container to which you provide the results of editing the photo, video, or Live Photo content of a Photos asset.
class PHAdjustmentData//A description of the edits made to an asset's photo, video, or Live Photo content, which allows your app to reconstruct or revert the effects of prior editing sessions.
class PHContentEditingInputRequestOptions//A set of options affecting the delivery of image or video data when you request to edit the content of a Photos asset.
class PHLivePhotoEditingContext//An editing session for modifying the photo, video, and audio content of a Live Photo.
protocol PHLivePhotoFrame//A container that provides image content for a single frame of a Live Photo in an editing context.
func canPerform(PHAssetEditOperation) -> Bool//Returns whether the asset supports the specified editing operation.
enum PHAssetEditOperation//Values identifying possible actions an asset can support, used by the canPerform(_:) method.
func requestContentEditingInput(with: PHContentEditingInputRequestOptions?, completionHandler: (PHContentEditingInput?, [AnyHashable : Any]) -> Void) -> PHContentEditingInputRequestID//Requests asset information for beginning a content editing session.
func cancelContentEditingInputRequest(PHContentEditingInputRequestID)
Cancels a request for editing the asset’s content.
typealias PHContentEditingInputRequestID//An identifier for an asset content editing session.
Keys indicating the status of an asset content editing request, used in the completion handler of the requestContentEditingInput(with:completionHandler:)method.
Asset Retrieval
Fetching Objects and Requesting Changes
Get assets, asset collections, and collection lists matching a specified query.
A representation of a Photos asset grouping, such as a moment, user-created album, or smart album.
The abstract superclass for Photos asset collections and collection lists.
A group containing Photos asset collections, such as Moments, Years, or folders of user-created albums.
The abstract superclass for Photos model objects (assets and collections).
An ordered list of assets or collections returned from a Photos fetch method.
A set of options that affect the filtering, sorting, and management of results that Photos returns when you fetch asset or collection objects.
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