在 Angular ngOnInit 方法中加载图像和用户的短列表失败，图像始终未定义

Question

I have a page containing few bootstrap cards (roughly 2 to 5). They present student info. I also want to load images of these students (from the server) together with their details but I can't figure out how. Here is what I got:

students: { student: Student, hasImage: boolean, image: any }[] = [];
loadingData: boolean;
imageToShow: any;

constructor(private userService: UsersService, private fileService: FilesService) {
}

createImageFromBlob(image: Blob) {
    const reader = new FileReader();
    reader.addEventListener('load', () => {
        this.imageToShow = reader.result;
    }, false);
    if (image) {
        reader.readAsDataURL(image);
    }
}

ngOnInit() {
    this.userService.getStudentsOfLoggedUser().subscribe(res => {
        this.loadingData = true;
        res.forEach(std => {
            if (std.imageUrl == null) {
                this.students.push({student: std, hasImage: false, image: undefined});
            } else {
                this.fileService.getImage(std.imageUrl).subscribe(data => {
                    this.createImageFromBlob(data);
                });
                this.students.push({student: std, hasImage: true, image: this.imageToShow});
            }
            this.loadingData = false;
        });
    });
}

this code fails to get the images from the server, they are always missing (undefined). How it should look like to make it right?

Answer 1

You must put this.students.push({student: std, hasImage: true, image: this.imageToShow}); inside this.fileService.getImage subscription because its execution is async.

On the other hand you must avoid subscriptions nested. Check this link https://coryrylan.com/blog/angular-multiple-http-requests-with-rxjs

Also if you use subscriptions you must unsubscribe them in ngOnDestroy method to avoid memory leaks. Check this other link https://brianflove.com/2016/12/11/anguar-2-unsubscribe-observables/

I hope that it will be useful to you

Regards

Answer 2

From my understanding, the reason why this is happening is because forEach is synchronous in nature, but this particular line is asynchronous:

this.fileService.getImage(std.imageUrl).subscribe(data => {
                    this.createImageFromBlob(data);
});

Instead you can Promises like this:

  this.userService.getStudentsOfLoggedUser().subscribe(res => {
  this.loadingData = true;
  let count = 0;
  new Promise((resolve, reject) => {
    res.forEach(std => {
      count++;
      if (std.imageUrl == null) {
        this.students.push({ student: std, hasImage: false, image: undefined });
      } else {
        this.fileService.getImage(std.imageUrl).subscribe(data => {
          this.createImageFromBlob(data);
        });
        this.students.push({ student: std, hasImage: true, image: this.imageToShow });
      }
      if (count > res.length -1) {
        resolve(); //Resolve condition
      }
    });
  }).then(() => {
    this.loadingData = false;
  })
});

This way, your code will be resolved only when all the async calls are completed, preventing you to get undefined data.

Answer 3

After combining all the suggestions and answers together the working code look like this:

createImageFromBlob(studentCard: StudentCard, image: Blob) {
    const reader = new FileReader();
    reader.addEventListener('load', () => {
        studentCard.image = reader.result
    }, false);
    if (image) {
        reader.readAsDataURL(image);
    }
}

ngOnInit() {
    this.userService.getStudentsOfLoggedUser().subscribe(res => {
        this.loadingData = true;
        res.forEach(std => {
            const studentCard = new StudentCard();
            studentCard.student = std;
            if (!std.imageUrl || std.imageUrl == null) {
                studentCard.hasImage = false;
            } else {
                studentCard.hasImage = true;
            }
            this.students.push(studentCard);
        });
        let count = 0;
        this.students.forEach(std => {
            if (std.hasImage) {
                count++;
                new Promise((resolve, reject) => {
                    this.fileService.getImage(std.student.imageUrl).subscribe(data => {
                        this.createImageFromBlob(std, data);
                        console.log(std);
                    });
                    if (count > this.students.length - 1) {
                        resolve(); //Resolve condition
                    }
                }).then(() => {
                    // there is nothing to do here 
                });
            }
        });
        this.loadingData = false;
    });
}

also except anonymous type I declared StudentCard used to exchange data:

export class StudentCard {
student: Student;
hasImage: boolean;
image: any;

}

and now all images appear on the screen as expected