RecyclerView、Adapter和ViewHolder

RecyclerView是ListView的升级版，强化了ListView的某些不足。比如listView如果不使用一些技巧的话，listView的性能就会非常差；ListView只能实现纵向滚动的效果等。

顾名思义，ViewHolder只做一件事：容纳View视图

RecyclerView自身是不会创建视图，它创建的是ViewHolder，而ViewHolder引用这一个个itemView

如下图：

RecyclerView自己不会创建ViewHolder。这个任务实际是由adapter来完成的。adapter是个控制器对象，从模型层获取数据，然后提供给RecyclerView显示，起到桥梁的作用。

adapter负责：

1：创建必要的ViewHolder

2：绑定ViewHolder至模型层数据

recyclerView需要显示视图时，就会去找它的adapter。下图展示了一个recyclerView可能发起的会话。

首先，通过调用adapter的getItemCount()方法，RrecyclerView询问数组列表中包含多少个对象

接着，RecyclerView调用adapter的CreateViewHolder()方法创建ViewHolder以及ViewHolder要显示的视图。

最后，RecyclerView会传入ViewHolder及其位置，调用onBindViewHolder（ViewHolder，int position）方法。adapter会找到目标位置的数据并更新到ViewHolder的视图上。

具体代码如下;

private class CrimeHolder extends RecyclerView.ViewHolder 
        implements View.OnClickListener {

    private TextView mTitleTextView;
    private TextView mDateTextView;
    private CheckBox mSolvedCheckBox;

    private Crime mCrime;

    public CrimeHolder(View itemView) {
        super(itemView);
        itemView.setOnClickListener(this);

        mTitleTextView = (TextView) itemView.findViewById(R.id.list_item_crime_title_text_view);
        mDateTextView = (TextView) itemView.findViewById(R.id.list_item_crime_date_text_view);
        mSolvedCheckBox = (CheckBox) itemView.findViewById(R.id.list_item_crime_solved_check_box);
    }

    public void bindCrime(Crime crime) {   //在viewholder中实现的，所以还是可以说viewholder只是容纳视图
        mCrime = crime;
        mTitleTextView.setText(mCrime.getTitle());
        mDateTextView.setText(mCrime.getDate().toString());
        mSolvedCheckBox.setChecked(mCrime.isSolved());
    }

    @Override
    public void onClick(View v) {
        mCallbacks.onCrimeSelected(mCrime);
    }
}

private class CrimeAdapter extends RecyclerView.Adapter<CrimeHolder> {

    private List<Crime> mCrimes;

    public CrimeAdapter(List<Crime> crimes) {
        mCrimes = crimes;
    }

    @Override
    /*
    锁定视图R.layout.list_item_crime，绑定到ViewHolder（CrimeHolder继承ViewHolder）中
     */
    public CrimeHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        LayoutInflater layoutInflater = LayoutInflater.from(getActivity());
        View view = layoutInflater.inflate(R.layout.list_item_crime, parent, false);
        return new CrimeHolder(view);
    }

    @Override
    /*
    向viewHolder中的布局添加数据c
     */
    public void onBindViewHolder(CrimeHolder holder, int position) {
        Crime crime = mCrimes.get(position);
        holder.bindCrime(crime);
    }

    @Override
    public int getItemCount() {
        return mCrimes.size();
    }

    public void setCrimes(List<Crime> crimes) {
        mCrimes = crimes;
    }
}

搞定了adapter，最后要做的就是将它和RecyclerView关联起来，该方法创建adapter，然后设置给RecyclerView。代码如下所示

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_crime_list, container, false);

    mCrimeRecyclerView = (RecyclerView) view
            .findViewById(R.id.crime_recycler_view);
    mCrimeRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));

    if (savedInstanceState != null) {
        mSubtitleVisible = savedInstanceState.getBoolean(SAVED_SUBTITLE_VISIBLE);//得到saveInstanceState的值
    }

    updateUI();

    return view;
}

public void updateUI() {
    CrimeLab crimeLab = CrimeLab.get(getActivity());//得到CrimeLab对象
    List<Crime> crimes = crimeLab.getCrimes();//得到list对象

  
        mAdapter = new CrimeAdapter(crimes);//将list对象添加到Adapter中
        mCrimeRecyclerView.setAdapter(mAdapter);//启用这个adapter


}