问如何使用angular2 http API跟踪上载/下载进度

EN

upload(file: File): Observable<string | number> {

    let fd: FormData = new FormData();

    fd.append("file", file);

    let xhr = new XMLHttpRequest;

    return Observable.create(observer => {

        xhr.addEventListener("progress", (progress) => {

            let percentCompleted;

            // Checks if we can really track the progress
            if (progress.lengthComputable) {

                // progress.loaded is a number between 0 and 1, so we'll multiple it by 100
                percentCompleted = Math.round(progress.loaded / progress.total * 100);

                if (percentCompleted < 1) {
                    observer.next(0);
                } else {
                    // Emit the progress percentage
                    observer.next(percentCompleted);
                }
            }
        });

        xhr.addEventListener("load", (e) => {

            if (e.target['status'] !== 200) observer.error(e.target['responseText']);

            else observer.complete(e.target['responseText']);
        });

        xhr.addEventListener("error", (err) => {

            console.log('upload error', err);

            observer.error('Upload error');
        });

        xhr.addEventListener("abort", (abort) => {

            console.log('upload abort', abort);

            observer.error('Transfer aborted by the user');
        });

        xhr.open('POST', 'http://some-dummy-url.com/v1/media/files');

        // Add any headers if necessary
        xhr.setRequestHeader("Authorization", `Bearer rqrwrewrqe`);

        // Send off the file
        xhr.send(fd);

        // This function will get executed once the subscription
        // has been unsubscribed
        return () => xhr.abort()
    });
}

// file is an instance of File that you need to retrieve from input[type="file"] element
const uploadSubscription = this.upload(file).subscribe(progress => {
    if (typeof progress === Number) {
        console.log("upload progress:", progress);
    }
});

// To abort the upload
// we should check whether the subscription is still active
if (uploadSubscription) uploadSubscription.unsubscribe();