RecyclerView多选的实现(非长按弹ActionMode

集成的步骤：

• 在使用rv的页面定义一个ItemSelectionSupport mSelectionSupport的成员变量。
• 在取到rv显示用的列表数据后，new 这个ItemSelectionSupport，然后调用mSelectionSupport.setChoiceModeMultiple(onAllSelectedListener)，参数可以为null。
• 将rv要使用的Adapter扩展一下。假设这个继承自你原来给rv用的Adapter的类的名字叫SelectableAdapter。其内部多了一个ItemSelectionSupport的成员。其值就是在第2步new出来的值，传进去的。接下来是关键的一步。
• 将你原有给rv用的ViewHolder也扩展一下。这回里面多了2个成员，一是CheckBox控件（这个可选，当然你的item的布局xml文件里面也多塞这么个CheckBox）; 二是多持有了相关联的Adapter(SelectableAdapter)的引用。再加上如下一点点代码，就可基本实现功能了：

public void bind(int position) {
    boolean checked = mAdapter.getSelectionSupport().isItemChecked(position);
    mCheckbox.setChecked(checked);
    itemView.setActivated(checked);
}

@OnClick(R.id.list_item_container)
void onItemClick() {
    mAdapter.notifyItemChanged(getPosition()); // 局部更新
}

上面调用了mAdapter.notifyItemChanged()之后，最终会调用上面的bind方法。更新点击的那条记录的选中与否的UI。说了这么多，还是需要贴上相关代码，才能表达清楚：

    @Override
    public void onLoadFinished(Loader<List<String>> loader, List<String> data) {
        mSelectionSupport = new ItemSelectionSupport(mRvDemo);
        handleAllSelectedOrNot(false);
        mSelectionSupport.setChoiceModeMultiple(allSelected -> handleAllSelectedOrNot(allSelected));
//        mAdapter = new BaseAdapter(data);  // You can enable(uncomment) this line, and disable next line
        mAdapter = new SelectableAdapter(data, mSelectionSupport);
        mRvDemo.setAdapter(mAdapter);
    }
    
    public static class SelectableAdapter extends BaseAdapter {
        private ItemSelectionSupport mSelectionSupport;

        public SelectableAdapter(List<String> data, ItemSelectionSupport selectionSupport) {
            super(data);
            mSelectionSupport = selectionSupport;
        }

        public ItemSelectionSupport getSelectionSupport() {
            return mSelectionSupport;
        }

        @Override
        public BaseViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            return new SelectableViewHolder(
                    LayoutInflater.from(parent.getContext()).inflate(R.layout.list_item, parent, false),
                    this);
        }

        @Override
        public void onBindViewHolder(BaseViewHolder holder, int position) {
            super.onBindViewHolder(holder, position);
            ((SelectableViewHolder) holder).bind(position);
        }

        /////////////////////////////////////////////////////////////////////////////////////////////////////////

        public static class SelectableViewHolder extends BaseViewHolder {
            @Bind(R.id.cb_demo)
            CheckBox mCheckbox;
            private SelectableAdapter mAdapter;

            public SelectableViewHolder(View v, SelectableAdapter adapter) {
                super(v);
                mAdapter = adapter;
                ButterKnife.bind(this, v);
            }

            public void bind(int position) {
                boolean checked = mAdapter.getSelectionSupport().isItemChecked(position);
                mCheckbox.setChecked(checked);
                itemView.setActivated(checked);
            }

            @OnClick(R.id.list_item_container)
            void onItemClick() {
                mAdapter.notifyItemChanged(getPosition()); // 局部更新
            }
        }
    }