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Question

我正在创建一个问答，其中每个问题都是一张卡片。答案开始显示第一行，但当点击它时，它应该展开以显示完整的答案。

当答案展开/折叠时，RecyclerView 的其余部分应进行动画处理，以便为展开或折叠腾出空间以避免显示空白。

我在 RecyclerView animations 上观看了演讲，并相信我想要一个自定义的 ItemAnimator，我在其中覆盖 animateChange。那时我应该创建一个 ObjectAnimator 来为 View 的 LayoutParams 的高度设置动画。不幸的是，我很难将它们联系在一起。我在覆盖 canReuseUpdatedViewHolder 时也返回了 true，所以我们重用了同一个 viewholder。

@Override
public boolean canReuseUpdatedViewHolder(RecyclerView.ViewHolder viewHolder) {
    return true;
}


@Override
public boolean animateChange(@NonNull RecyclerView.ViewHolder oldHolder,
                             @NonNull final RecyclerView.ViewHolder newHolder,
                             @NonNull ItemHolderInfo preInfo,
                             @NonNull ItemHolderInfo postInfo) {
    Log.d("test", "Run custom animation.");

    final ColorsAdapter.ColorViewHolder holder = (ColorsAdapter.ColorViewHolder) newHolder;

    FrameLayout.LayoutParams params = (FrameLayout.LayoutParams) holder.tvColor.getLayoutParams();
    ObjectAnimator halfSize = ObjectAnimator.ofInt(holder.tvColor.getLayoutParams(), "height", params.height, 0);
    halfSize.start();
    return super.animateChange(oldHolder, newHolder, preInfo, postInfo);
}

现在我只是想制作一些动画，但没有任何反应......有什么想法吗？

Answer 1

我认为您的动画无法正常工作，因为您无法以那种方式为 LayoutParams 设置动画，尽管如果可以的话，它会很整洁。我尝试了你的代码，它所做的只是让我的观点跳到了新的高度。我发现让它工作的唯一方法是使用 ValueAnimator，如下例所示。

在使用 DefaultItemAnimator 通过更新其可见性来显示/隐藏 View 时，我注意到了一些缺点。尽管它确实为新 View 腾出了空间并根据可展开 View 的可见性上下动画其余项目，但我注意到它没有为可展开 View 的高度设置动画。它只是使用 alpha 值简单地淡入淡出。

下面是一个自定义 ItemAnimator，它具有基于在 ViewHolder 布局中隐藏/显示 LinearLayout 的大小和 alpha 动画。它还允许重用相同的 ViewHolder 并在用户快速点击标题时尝试正确处理部分动画:

public static class MyAnimator extends DefaultItemAnimator {
    @Override
    public boolean canReuseUpdatedViewHolder(RecyclerView.ViewHolder viewHolder) {
        return true;
    }

    private HashMap<RecyclerView.ViewHolder, AnimatorState> animatorMap = new HashMap<>();

    @Override
    public boolean animateChange(@NonNull RecyclerView.ViewHolder oldHolder, @NonNull final RecyclerView.ViewHolder newHolder, @NonNull ItemHolderInfo preInfo, @NonNull ItemHolderInfo postInfo) {
        final ValueAnimator heightAnim;
        final ObjectAnimator alphaAnim;

        final CustomAdapter.ViewHolder vh = (CustomAdapter.ViewHolder) newHolder;
        final View expandableView = vh.getExpandableView();
        final int toHeight; // save height for later in case reversing animation

        if(vh.isExpanded()) {
            expandableView.setVisibility(View.VISIBLE);

            // measure expandable view to get correct height
            expandableView.measure(View.MeasureSpec.UNSPECIFIED, View.MeasureSpec.UNSPECIFIED);
            toHeight = expandableView.getMeasuredHeight();
            alphaAnim = ObjectAnimator.ofFloat(expandableView, "alpha", 1f);
        } else {
            toHeight = 0;
            alphaAnim = ObjectAnimator.ofFloat(expandableView, "alpha", 0f);
        }

        heightAnim = ValueAnimator.ofInt(expandableView.getHeight(), toHeight);
        heightAnim.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                expandableView.getLayoutParams().height = (Integer) heightAnim.getAnimatedValue();
                expandableView.requestLayout();
            }
        });

        AnimatorSet animSet = new AnimatorSet()
                .setDuration(getChangeDuration());
        animSet.playTogether(heightAnim, alphaAnim);
        animSet.addListener(new Animator.AnimatorListener() {
            private boolean isCanceled;

            @Override
            public void onAnimationStart(Animator animation) { }

            @Override
            public void onAnimationEnd(Animator animation) {
                if(!vh.isExpanded() && !isCanceled) {
                    expandableView.setVisibility(View.GONE);
                }

                dispatchChangeFinished(vh, false);
                animatorMap.remove(newHolder);
            }

            @Override
            public void onAnimationCancel(Animator animation) {
                isCanceled = true;
            }

            @Override
            public void onAnimationRepeat(Animator animation) { }
        });

        AnimatorState animatorState = animatorMap.get(newHolder);
        if(animatorState != null) {
            animatorState.animSet.cancel();

            // animation already running. Set start current play time of
            // new animations to keep them smooth for reverse animation
            alphaAnim.setCurrentPlayTime(animatorState.alphaAnim.getCurrentPlayTime());
            heightAnim.setCurrentPlayTime(animatorState.heightAnim.getCurrentPlayTime());

            animatorMap.remove(newHolder);
        }

        animatorMap.put(newHolder, new AnimatorState(alphaAnim, heightAnim, animSet));

        dispatchChangeStarting(newHolder, false);
        animSet.start();

        return false;
    }

    public static class AnimatorState {
        final ValueAnimator alphaAnim, heightAnim;
        final AnimatorSet animSet;

        public AnimatorState(ValueAnimator alphaAnim, ValueAnimator heightAnim, AnimatorSet animSet) {
            this.alphaAnim = alphaAnim;
            this.heightAnim = heightAnim;
            this.animSet = animSet;
        }
    }
}

这是使用稍微修改的 RecyclerView 演示的结果。

更新:

刚刚注意到你的用例在重新阅读问题后实际上有点不同。您有一个 TextView ，只想显示其中的一行，然后将其展开以显示所有行。幸运的是，这简化了自定义动画师:

public static class MyAnimator extends DefaultItemAnimator {
    @Override
    public boolean canReuseUpdatedViewHolder(RecyclerView.ViewHolder viewHolder) {
        return true;
    }

    private HashMap<RecyclerView.ViewHolder, ValueAnimator> animatorMap = new HashMap<>();

    @Override
    public boolean animateChange(@NonNull RecyclerView.ViewHolder oldHolder, @NonNull final RecyclerView.ViewHolder newHolder, @NonNull ItemHolderInfo preInfo, @NonNull ItemHolderInfo postInfo) {
        ValueAnimator prevAnim = animatorMap.get(newHolder);
        if(prevAnim != null) {
            prevAnim.reverse();
            return false;
        }

        final ValueAnimator heightAnim;
        final CustomAdapter.ViewHolder vh = (CustomAdapter.ViewHolder) newHolder;
        final TextView tv = vh.getExpandableTextView();

        if(vh.isExpanded()) {
            tv.measure(View.MeasureSpec.makeMeasureSpec(((View) tv.getParent()).getWidth(), View.MeasureSpec.AT_MOST), View.MeasureSpec.UNSPECIFIED);
            heightAnim = ValueAnimator.ofInt(tv.getHeight(), tv.getMeasuredHeight());
        } else {
            Paint.FontMetrics fm = tv.getPaint().getFontMetrics();
            heightAnim = ValueAnimator.ofInt(tv.getHeight(), (int)(Math.abs(fm.top) + Math.abs(fm.bottom)));
        }

        heightAnim.setDuration(getChangeDuration());
        heightAnim.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                tv.getLayoutParams().height = (Integer) heightAnim.getAnimatedValue();
                tv.requestLayout();
            }
        });

        heightAnim.addListener(new Animator.AnimatorListener() {
            @Override
            public void onAnimationEnd(Animator animation) {
                dispatchChangeFinished(vh, false);
                animatorMap.remove(newHolder);
            }

            @Override
            public void onAnimationCancel(Animator animation) { }

            @Override
            public void onAnimationStart(Animator animation) { }

            @Override
            public void onAnimationRepeat(Animator animation) { }
        });

        animatorMap.put(newHolder, heightAnim);

        dispatchChangeStarting(newHolder, false);
        heightAnim.start();

        return false;
    }
}

还有新的演示: