How to implement item reorder/shuffle animations with Angular's ngFor?

Question

Vue users are easy to implement such item shuffle animations, see their official docs:

Answer 1

Once the animated elements are not in the view the animation breaks. I fixed it by editing refreshPosition function:

refreshPosition(prop: string) {
  this.items.forEach(item => {
    item[prop] = {
      top: item.el.offsetTop,
      left: item.el.offsetLeft
    }
  });
}

Originally @yurzui used el.getBoundingClientRect() to get the positions but this method returns positions relative to the viewport.

I changed it so it gets the positions using el.offsetTop and el.offsetLeft which are relative to the first ancestor that isn't positioned 'static'.

Answer 2

Here is simple implementation such functionality Plunker Example

1) Build directives

@Directive({
  selector: '[transition-group-item]'
})
export class TransitionGroupItemDirective {
  prevPos: any;

  newPos: any;

  el: HTMLElement;

  moved: boolean;

  moveCallback: any;

  constructor(elRef: ElementRef) {
    this.el = elRef.nativeElement;
  }
}


@Component({
  selector: '[transition-group]',
  template: '<ng-content></ng-content>'
})
export class TransitionGroupComponent {
  @Input('transition-group') class;

  @ContentChildren(TransitionGroupItemDirective) items: QueryList<TransitionGroupItemDirective>;

  ngAfterContentInit() {
    this.refreshPosition('prevPos');
    this.items.changes.subscribe(items => {
      items.forEach(item => {
        item.prevPos = item.newPos || item.prevPos;
      });

      items.forEach(this.runCallback);
      this.refreshPosition('newPos');
      items.forEach(this.applyTranslation);

      // force reflow to put everything in position
      const offSet = document.body.offsetHeight;
      this.items.forEach(this.runTransition.bind(this));
    })
  }

  runCallback(item: TransitionGroupItemDirective) {
    if(item.moveCallback) {
      item.moveCallback();
    }
  }

  runTransition(item: TransitionGroupItemDirective) {
    if (!item.moved) {
      return;
    }
    const cssClass = this.class + '-move';
    let el = item.el;
    let style: any = el.style;
    el.classList.add(cssClass);
    style.transform = style.WebkitTransform = style.transitionDuration = '';
    el.addEventListener('transitionend', item.moveCallback = (e: any) => {
      if (!e || /transform$/.test(e.propertyName)) {
        el.removeEventListener('transitionend', item.moveCallback);
        item.moveCallback = null;
        el.classList.remove(cssClass);
      }
    });
  }

  refreshPosition(prop: string) {
    this.items.forEach(item => {
      item[prop] = item.el.getBoundingClientRect();
    });
  }

  applyTranslation(item: TransitionGroupItemDirective) {
    item.moved = false;
    const dx = item.prevPos.left - item.newPos.left;
    const dy = item.prevPos.top - item.newPos.top;
    if (dx || dy) {
      item.moved = true;
      let style: any = item.el.style;
      style.transform = style.WebkitTransform = 'translate(' + dx + 'px,' + dy + 'px)';
      style.transitionDuration = '0s';
    }
  }
}

2) Use it as follows

<ul [transition-group]="'flip-list'">
  <li *ngFor="let item of items" transition-group-item>
    {{ item }}
  </li>
</ul>

Answer 3

More correct (and TSLint-compliant) would be to use a different Directive name, as:

@Directive({
    selector: '[appTransitionGroupItem]'
})

and using a component as an element and not overloading the input name:

@Component({
    selector: 'app-transition-group',
    template: '<ng-content></ng-content>'
})
export class TransitionGroupComponent implements AfterViewInit {
    @Input() className;

Which gives the code better Angular structure, my compliant, better read (YMMV) code, being:

<app-transition-group [className]="'flip-list'">
  <div class="list-items" *ngFor="let item of items" appTransitionGroupItem>
  etc

Also, if you're wondering why the transition animation isn't working, don't forget the CSS required:

.flip-list-move {
  transition: transform 1s;
}

Answer 4

Here is my version of @yurzui code. Changes:

• supports insert and deletion of items
• a forced reflow survives webpack optimizations

import { Component, ContentChildren, Directive, ElementRef, Input, QueryList } from '@angular/core';

@Directive({
    selector: '[transition-group-item]'
})
export class TransitionGroupItemDirective {
    prevPos: any;
    newPos: any;
    el: HTMLElement;
    moved: boolean;
    moveCallback: any;

    constructor(elRef: ElementRef) {
        this.el = elRef.nativeElement;
    }
}


@Component({
    selector: '[transition-group]',
    template: '<ng-content></ng-content>'
})
export class TransitionGroupComponent {
    @Input('transition-group') class;

    @ContentChildren(TransitionGroupItemDirective) items: QueryList<TransitionGroupItemDirective>;

    ngAfterViewInit() {
        setTimeout(() => this.refreshPosition('prevPos'), 0); // save init positions on next 'tick'

        this.items.changes.subscribe(items => {
            items.forEach(item => item.prevPos = item.newPos || item.prevPos);
            items.forEach(this.runCallback);
            this.refreshPosition('newPos');
            items.forEach(item => item.prevPos = item.prevPos || item.newPos); // for new items

            const animate = () => {
                items.forEach(this.applyTranslation);
                this['_forceReflow'] = document.body.offsetHeight; // force reflow to put everything in position
                this.items.forEach(this.runTransition.bind(this));
            }

            const willMoveSome = items.some((item) => {
                const dx = item.prevPos.left - item.newPos.left;
                const dy = item.prevPos.top - item.newPos.top;
                return dx || dy;
            });

            if (willMoveSome) {
                animate();
            } else {
                setTimeout(() => { // for removed items
                    this.refreshPosition('newPos');
                    animate();
                }, 0);
            }
        })
    }

    runCallback(item: TransitionGroupItemDirective) {
        if (item.moveCallback) {
            item.moveCallback();
        }
    }

    runTransition(item: TransitionGroupItemDirective) {
        if (!item.moved) {
            return;
        }
        const cssClass = this.class + '-move';
        let el = item.el;
        let style: any = el.style;
        el.classList.add(cssClass);
        style.transform = style.WebkitTransform = style.transitionDuration = '';
        el.addEventListener('transitionend', item.moveCallback = (e: any) => {
            if (!e || /transform$/.test(e.propertyName)) {
                el.removeEventListener('transitionend', item.moveCallback);
                item.moveCallback = null;
                el.classList.remove(cssClass);
            }
        });
    }

    refreshPosition(prop: string) {
        this.items.forEach(item => {
            item[prop] = item.el.getBoundingClientRect();
        });
    }

    applyTranslation(item: TransitionGroupItemDirective) {
        item.moved = false;
        const dx = item.prevPos.left - item.newPos.left;
        const dy = item.prevPos.top - item.newPos.top;
        if (dx || dy) {
            item.moved = true;
            let style: any = item.el.style;
            style.transform = style.WebkitTransform = 'translate(' + dx + 'px,' + dy + 'px)';
            style.transitionDuration = '0s';
        }
    }
}