RecyclerView使用onBindViewHolder时，同一个Item总是有两个viewholder对象

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/lylddingHFFW/article/details/77897775

RecyclerView中有两个Api来绑定数据：onBindViewHolder( RecyclerView.ViewHolder holder, int position)和带参数onBindViewHolder(RecyclerView.ViewHolder holder, int position, List payloads)局部绑定/刷新数据。

场景：RecyclerView使用viewHolder保存数据时，同一个Item总是有两个viewHolder对象。比如保存上一次的下载时间，百分比等
比如需要在viewHolder中保存上次刷新的数据时，同一个item有两个viewHolder对象，肯定时不行的，这时只需要在mRecycler中设置setSupportsChangeAnimations()为false即可。

原因： RecyclerView默认情况下会创建ViewHolder的另一个副本，以便将视图相互淡化。

另外，当时用带参数的 onBindViewHolder时，因为旧的ViewHolder获取payloads，但新的ViewHolder没有获取到payloads，也需要配置如下代码。

((SimpleItemAnimator)mRecycler.getItemAnimator()).setSupportsChangeAnimations(false);

RecyclerView源码：ItemAnimator 决定使用同一个ViewHolder 还是重新创建ViewHolder，canReuseUpdatedViewHolder()默认返回为true。若为true，则使用同一个ViewHolder。
SimpleItemAnimator继承RecyclerView，通过setSupportsChangeAnimations(),给全局变量mSupportsChangeAnimations （默认为true）设置为false；那么canReuseUpdatedViewHolder()返回为true。
DefaultItemAnimator继承SimpleItemAnimator：canReuseUpdatedViewHolder()中调用super.canReuseUpdatedViewHolder(viewHolder, payloads);即SimpleItemAnimator中的canReuseUpdatedViewHolder()。

RecyclerView源码：

/**
         * When an item is changed, ItemAnimator can decide whether it wants to re-use
         * the same ViewHolder for animations or RecyclerView should create a copy of the
         * item and ItemAnimator will use both to run the animation (e.g. cross-fade).
         * <p>
         * Note that this method will only be called if the {@link ViewHolder} still has the same
         * type ({@link Adapter#getItemViewType(int)}). Otherwise, ItemAnimator will always receive
         * both {@link ViewHolder}s in the
         * {@link #animateChange(ViewHolder, ViewHolder, ItemHolderInfo, ItemHolderInfo)} method.
         *
         * @param viewHolder The ViewHolder which represents the changed item's old content.
         * @param payloads A non-null list of merged payloads that were sent with change
         *                 notifications. Can be empty if the adapter is invalidated via
         *                 {@link RecyclerView.Adapter#notifyDataSetChanged()}. The same list of
         *                 payloads will be passed into
         *                 {@link RecyclerView.Adapter#onBindViewHolder(ViewHolder, int, List)}
         *                 method <b>if</b> this method returns <code>true</code>.
         *
         * @return True if RecyclerView should just rebind to the same ViewHolder or false if
         *         RecyclerView should create a new ViewHolder and pass this ViewHolder to the
         *         ItemAnimator to animate. Default implementation calls
         *         {@link #canReuseUpdatedViewHolder(ViewHolder)}.
         *
         * @see #canReuseUpdatedViewHolder(ViewHolder)
         */
        public boolean canReuseUpdatedViewHolder(@NonNull ViewHolder viewHolder,
                @NonNull List<Object> payloads) {
            return canReuseUpdatedViewHolder(viewHolder);
        }


     public boolean canReuseUpdatedViewHolder(@NonNull ViewHolder            viewHolder) {
            return true;
        }

SimpleItemAnimator源码：

boolean mSupportsChangeAnimations = true;

    /**
     * Returns whether this ItemAnimator supports animations of change events.
     *
     * @return true if change animations are supported, false otherwise
     */
    @SuppressWarnings("unused")
    public boolean getSupportsChangeAnimations() {
        return mSupportsChangeAnimations;
    }

    /**
     * Sets whether this ItemAnimator supports animations of item change events.
     * If you set this property to false, actions on the data set which change the
     * contents of items will not be animated. What those animations do is left
     * up to the discretion of the ItemAnimator subclass, in its
     * {@link #animateChange(ViewHolder, ViewHolder, int, int, int, int)} implementation.
     * The value of this property is true by default.
     *
     * @param supportsChangeAnimations true if change animations are supported by
     *                                 this ItemAnimator, false otherwise. If the property is false,
     *                                 the ItemAnimator
     *                                 will not receive a call to
     *                                 {@link #animateChange(ViewHolder, ViewHolder, int, int, int,
     *                                 int)} when changes occur.
     * @see Adapter#notifyItemChanged(int)
     * @see Adapter#notifyItemRangeChanged(int, int)
     */
    public void setSupportsChangeAnimations(boolean supportsChangeAnimations) {
        mSupportsChangeAnimations = supportsChangeAnimations;
    }

    /**
     * {@inheritDoc}
     *
     * @return True if change animations are not supported or the ViewHolder is invalid,
     * false otherwise.
     *
     * @see #setSupportsChangeAnimations(boolean)
     */
    @Override
    public boolean canReuseUpdatedViewHolder(@NonNull RecyclerView.ViewHolder viewHolder) {
        return !mSupportsChangeAnimations || viewHolder.isInvalid();
    }

DefaultItemAnimator源码：

 /**
     * {@inheritDoc}
     * <p>
     * If the payload list is not empty, DefaultItemAnimator returns <code>true</code>.
     * When this is the case:
     * <ul>
     * <li>If you override {@link #animateChange(ViewHolder, ViewHolder, int, int, int, int)}, both
     * ViewHolder arguments will be the same instance.
     * </li>
     * <li>
     * If you are not overriding {@link #animateChange(ViewHolder, ViewHolder, int, int, int, int)},
     * then DefaultItemAnimator will call {@link #animateMove(ViewHolder, int, int, int, int)} and
     * run a move animation instead.
     * </li>
     * </ul>
     */
    @Override
    public boolean canReuseUpdatedViewHolder(@NonNull ViewHolder viewHolder,
            @NonNull List<Object> payloads) {
        return !payloads.isEmpty() || super.canReuseUpdatedViewHolder(viewHolder, payloads);
    }