function getFilesDataTransferItems(dataTransferItems) {
    function traverseFileTreePromise(item, path = "", folder) {
        return new Promise(resolve => {
            if (item === null || typeof item === 'undefined') {
                // nothing to do
            } else if (item.isFile) {
                item.file(file => {
                    file.filepath = path + file.name; //save full path
                    folder.push(file);
                    resolve(file);
                });
            } else if (item.isDirectory) {
                let dirReader = item.createReader();
                dirReader.readEntries(entries => {
                    let entriesPromises = [];
                    let subfolder = [];
                    folder.push({ name: item.name, subfolder: subfolder });
                    for (let entry of entries)
                        entriesPromises.push(
                            traverseFileTreePromise(entry, path + "/" + item.name + "/", subfolder)
                        );
                    resolve(Promise.all(entriesPromises));
                });
            }
        });
    }

    let files = [];
    return new Promise((resolve, reject) => {
        let entriesPromises = [];
        for (let it of dataTransferItems) {
            let entry;
            if (typeof it.webkitGetAsEntry === 'function') {
                entry = it.webkitGetAsEntry();
            } else if (typeof it.getAsEntry === 'function') {
                entry = it.getAsEntry();
            }
            entriesPromises.push(
                traverseFileTreePromise(entry, "", files)
            );
        }
        Promise.all(entriesPromises).then(entries => {
            resolve(files);
        });
    });
}

export function get_files_data_transfer_items(data_transfer_items) {
    return getFilesDataTransferItems(data_transfer_items);
}

export function read_file(file) {
    return new Promise((resolve, reject) => {
        let reader = new FileReader();
        reader.onloadend = () => {
            resolve(reader.result)
        };
        reader.onerror = reject;
        reader.readAsArrayBuffer(file);
    });
}

export function upload(content, content_type, url) {
    return new Promise((resolve, reject) => {
        fetch(url, {
            method: 'PUT',
            headers: {
                'Accept': 'application/json',
                'Content-Type': 'multipart/form-data',
            },
            body: new Blob([content], { type: content_type }),
        }).then((resp) => {
            if (resp.status >= 200 && resp.status < 300) {
                resolve(resp);
            } else {
                reject(resp);
            }
        });
    });
}