DownloadTarget 编辑

The size of the target file, in bytes, or zero if the download has not yet been completed.

It is possible for this property to have a non-zero value even if the target file no longer exists, if the value is available in download metadata. If that metadata is no longer available and the file has been deleted, then this value is zero.

For single-file downloads, this property's value will always match the actual size of the file on disk, while the Download.totalBytes property, when available, may indicate the size of the data as encoded for transfer instead.

For downloads of multiple files, such as downloading a web page and its required media to disk, the meaning of this property is undefined. It currently indicates the size of the main file (such as the HTML document) rather than the sum of all of the files' sizes, but you must not rely upon this behavior, as it is subject to change.

This is a dynamic property, which is updated when the download finishes or whenever the Download.refresh() method is called. You can use this value from the front-end to reduce file I/O that would be required to check the file directly.