wp_generate_attachment_metadata

wp_generate_attachment_metadata: 這個函式用來為一個上傳的檔案，如圖片，生成後設資料。它接受檔案路徑作為引數，並返回一個後設資料陣列，可用於顯示或操作該檔案。

生成附件的後設資料，併為影象建立影象子尺寸。

function wp_generate_attachment_metadata( $attachment_id, $file ) {
	$attachment = get_post( $attachment_id );

	$metadata  = array();
	$support   = false;
	$mime_type = get_post_mime_type( $attachment );

	if ( preg_match( '!^image/!', $mime_type ) && file_is_displayable_image( $file ) ) {
		// Make thumbnails and other intermediate sizes.
		$metadata = wp_create_image_subsizes( $file, $attachment_id );
	} elseif ( wp_attachment_is( 'video', $attachment ) ) {
		$metadata = wp_read_video_metadata( $file );
		$support  = current_theme_supports( 'post-thumbnails', 'attachment:video' ) || post_type_supports( 'attachment:video', 'thumbnail' );
	} elseif ( wp_attachment_is( 'audio', $attachment ) ) {
		$metadata = wp_read_audio_metadata( $file );
		$support  = current_theme_supports( 'post-thumbnails', 'attachment:audio' ) || post_type_supports( 'attachment:audio', 'thumbnail' );
	}

	/*
	 * wp_read_video_metadata() and wp_read_audio_metadata() return `false`
	 * if the attachment does not exist in the local filesystem,
	 * so make sure to convert the value to an array.
	 */
	if ( ! is_array( $metadata ) ) {
		$metadata = array();
	}

	if ( $support && ! empty( $metadata['image']['data'] ) ) {
		// Check for existing cover.
		$hash   = md5( $metadata['image']['data'] );
		$posts  = get_posts(
			array(
				'fields'         => 'ids',
				'post_type'      => 'attachment',
				'post_mime_type' => $metadata['image']['mime'],
				'post_status'    => 'inherit',
				'posts_per_page' => 1,
				'meta_key'       => '_cover_hash',
				'meta_value'     => $hash,
			)
		);
		$exists = reset( $posts );

		if ( ! empty( $exists ) ) {
			update_post_meta( $attachment_id, '_thumbnail_id', $exists );
		} else {
			$ext = '.jpg';
			switch ( $metadata['image']['mime'] ) {
				case 'image/gif':
					$ext = '.gif';
					break;
				case 'image/png':
					$ext = '.png';
					break;
				case 'image/webp':
					$ext = '.webp';
					break;
			}
			$basename = str_replace( '.', '-', wp_basename( $file ) ) . '-image' . $ext;
			$uploaded = wp_upload_bits( $basename, '', $metadata['image']['data'] );
			if ( false === $uploaded['error'] ) {
				$image_attachment = array(
					'post_mime_type' => $metadata['image']['mime'],
					'post_type'      => 'attachment',
					'post_content'   => '',
				);
				/**
				 * Filters the parameters for the attachment thumbnail creation.
				 *
				 * @since 3.9.0
				 *
				 * @param array $image_attachment An array of parameters to create the thumbnail.
				 * @param array $metadata         Current attachment metadata.
				 * @param array $uploaded         {
				 *     Information about the newly-uploaded file.
				 *
				 *     @type string $file  Filename of the newly-uploaded file.
				 *     @type string $url   URL of the uploaded file.
				 *     @type string $type  File type.
				 * }
				 */
				$image_attachment = apply_filters( 'attachment_thumbnail_args', $image_attachment, $metadata, $uploaded );

				$sub_attachment_id = wp_insert_attachment( $image_attachment, $uploaded['file'] );
				add_post_meta( $sub_attachment_id, '_cover_hash', $hash );
				$attach_data = wp_generate_attachment_metadata( $sub_attachment_id, $uploaded['file'] );
				wp_update_attachment_metadata( $sub_attachment_id, $attach_data );
				update_post_meta( $attachment_id, '_thumbnail_id', $sub_attachment_id );
			}
		}
	} elseif ( 'application/pdf' === $mime_type ) {
		// Try to create image thumbnails for PDFs.

		$fallback_sizes = array(
			'thumbnail',
			'medium',
			'large',
		);

		/**
		 * Filters the image sizes generated for non-image mime types.
		 *
		 * @since 4.7.0
		 *
		 * @param string[] $fallback_sizes An array of image size names.
		 * @param array    $metadata       Current attachment metadata.
		 */
		$fallback_sizes = apply_filters( 'fallback_intermediate_image_sizes', $fallback_sizes, $metadata );

		$registered_sizes = wp_get_registered_image_subsizes();
		$merged_sizes     = array_intersect_key( $registered_sizes, array_flip( $fallback_sizes ) );

		// Force thumbnails to be soft crops.
		if ( isset( $merged_sizes['thumbnail'] ) && is_array( $merged_sizes['thumbnail'] ) ) {
			$merged_sizes['thumbnail']['crop'] = false;
		}

		// Only load PDFs in an image editor if we're processing sizes.
		if ( ! empty( $merged_sizes ) ) {
			$editor = wp_get_image_editor( $file );

			if ( ! is_wp_error( $editor ) ) { // No support for this type of file.
				/*
				 * PDFs may have the same file filename as JPEGs.
				 * Ensure the PDF preview image does not overwrite any JPEG images that already exist.
				 */
				$dirname      = dirname( $file ) . '/';
				$ext          = '.' . pathinfo( $file, PATHINFO_EXTENSION );
				$preview_file = $dirname . wp_unique_filename( $dirname, wp_basename( $file, $ext ) . '-pdf.jpg' );

				$uploaded = $editor->save( $preview_file, 'image/jpeg' );
				unset( $editor );

				// Resize based on the full size image, rather than the source.
				if ( ! is_wp_error( $uploaded ) ) {
					$image_file = $uploaded['path'];
					unset( $uploaded['path'] );

					$metadata['sizes'] = array(
						'full' => $uploaded,
					);

					// Save the meta data before any image post-processing errors could happen.
					wp_update_attachment_metadata( $attachment_id, $metadata );

					// Create sub-sizes saving the image meta after each.
					$metadata = _wp_make_subsizes( $merged_sizes, $image_file, $metadata, $attachment_id );
				}
			}
		}
	}

	// Remove the blob of binary data from the array.
	unset( $metadata['image']['data'] );

	// Capture file size for cases where it has not been captured yet, such as PDFs.
	if ( ! isset( $metadata['filesize'] ) && file_exists( $file ) ) {
		$metadata['filesize'] = wp_filesize( $file );
	}

	/**
	 * Filters the generated attachment meta data.
	 *
	 * @since 2.1.0
	 * @since 5.3.0 The `$context` parameter was added.
	 *
	 * @param array  $metadata      An array of attachment meta data.
	 * @param int    $attachment_id Current attachment ID.
	 * @param string $context       Additional context. Can be 'create' when metadata was initially created for new attachment
	 *                              or 'update' when the metadata was updated.
	 */
	return apply_filters( 'wp_generate_attachment_metadata', $metadata, $attachment_id, 'create' );
}