Android之RecyclerView

RecyclerView是ViewGroup的子类，每一个列表项都是作为一个View子对象展示的，而View子对象是可以自定义的。

先来个简易版的列表项显示，每个列表项只显示一个标题，并且View对象是一个简单的TextView，我们通过一个例子来理解和知道如何使用RecyclerView类。

RecyclerView意思为可回收视图，所做的就是回收再利用，反复循环。

贴图参考自：《Android编程权威指南第二版第九章》。

数据层Crime类：

package com.example.lu.criminalintent.Model;

import java.util.Date;
import java.util.UUID;

public class Crime {
    private UUID mId;
    private String mTitle;
    private Date mDate;
    private Boolean mSolved;

    public Date getmDate() {
        return mDate;
    }

    public void setmDate(Date mDate) {
        this.mDate = mDate;
    }

    public Boolean getmSolved() {
        return mSolved;
    }

    public void setmSolved(Boolean mSolved) {
        this.mSolved = mSolved;
    }

    public Crime() {
        mId = UUID.randomUUID();
        mDate = new Date();
    }

    public UUID getmId() {
        return mId;
    }

    public String getmTitle() {
        return mTitle;
    }

    public void setmTitle(String mTitle) {
        this.mTitle = mTitle;
    }
}

数据层CrimeLab类：

package com.example.lu.criminalintent.Model;

import android.content.Context;

import java.util.ArrayList;
import java.util.List;
import java.util.UUID;

public class CrimeLab {
    private static CrimeLab sCrimeLab;
    private List<Crime> mCrimes;//存储crime对象


    private CrimeLab(Context context) {
        mCrimes = new ArrayList<>();

        for (int i = 0;i < 100; i++) {
            Crime crime = new Crime();
            crime.setmTitle("Crime #" + i);
            crime.setmSolved(i % 2 == 0);
            mCrimes.add(crime);
        }
    }

    public static CrimeLab get(Context context) {
        if (sCrimeLab == null) {
            sCrimeLab = new CrimeLab(context);
        }
        return sCrimeLab;
    }
    //返回整个所有crime对象
    public List<Crime> getmCrimes() {
        return mCrimes;
    }
    //返回相对应位置id的crime对象
    public Crime getCrime(UUID id) {
        for (Crime crime : mCrimes) {
            if (crime.getmId().equals(id)) {
                return crime;
            }
        }
        return null;
    }
}

Activity类：

package com.example.lu.criminalintent.Controller;

import android.support.v4.app.Fragment;

import com.example.lu.criminalintent.Father.SingleFragmentActivity;
import com.example.lu.criminalintent.UI.CrimeListFragment;

public class CrimeListActivity extends SingleFragmentActivity {
    @Override
    protected Fragment createFragment() {
        return new CrimeListFragment();
    }
}

这里为了代码复用，把复用的代码做成了一个抽象类。

SingleFragmentActivity类：

package com.example.lu.criminalintent.Father;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;

import com.example.lu.criminalintent.R;


public abstract class SingleFragmentActivity extends FragmentActivity {

    protected abstract Fragment createFragment();

    /*
    *以下代码复用。
     */
    private Fragment fragment;
    private FragmentManager fm;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initLayout();

        if (fragment == null) {
            fragment = createFragment();
            fm.beginTransaction()
                    .add(R.id.fragment_container, fragment)
                    .commit();
        }
    }

    private void initLayout() {
        fm = getSupportFragmentManager();
        fragment = fm.findFragmentById(R.id.fragment_container);
    }
}

绿色代码可以解读为：

创建一个新的fragment事务，加入一个添加操作，然后提交该事务。

add（）方法是整个事务的核心，包含两个参数，分别为：容器视图资源id和新创建的fragment。

容器视图资源id的作用是

1.告诉FragmentManager，fragment视图应该出现在activity视图的什么位置

2.用作FragmentManager队列中fragment唯一的标识符

我们来看下标红的代码的方法，FragmentManager是如何实现的，

红色的代码：先获取到一个FragmentManager管理器，然后再找到一个Fragment，存进fragment队列中。

布局CrimeListFragment类：

package com.example.lu.criminalintent.UI;

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import com.example.lu.criminalintent.Model.Crime;
import com.example.lu.criminalintent.Model.CrimeLab;
import com.example.lu.criminalintent.R;

import java.util.List;

public class CrimeListFragment extends Fragment{

    private RecyclerView mCrimeRecyclerView;
    private CrimeAdapter mAdapter;//适配器，这里用内部类的形式实现适配器

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_crime_list, container, false);
        initLayout(view);
        initController();
        updateUI();
        return view;
    }

    private void initLayout(View view) {
        mCrimeRecyclerView = view.findViewById(R.id.crime_recycler_view);
    }
    private void initController() {
        //LinearLayoutManager负责定位屏幕上的列表项和屏幕滚动行为
        mCrimeRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
    }
    //实现ViewHolder
    private class CrimeHolder extends RecyclerView.ViewHolder{

        public TextView mTitleTextView;

        public CrimeHolder(View itemView) {
            super(itemView);
            mTitleTextView = (TextView) itemView;
        }
    }
    //实现Adapter
    private class CrimeAdapter extends RecyclerView.Adapter<CrimeHolder> {

        private List<Crime> mCrimes;

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

        @NonNull
        @Override
        public CrimeHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            LayoutInflater layoutInflater = LayoutInflater.from(getActivity());
            View view = layoutInflater.inflate(android.R.layout.simple_list_item_1, parent, false);
            return new CrimeHolder(view);
        }

        @Override
        public void onBindViewHolder(@NonNull CrimeHolder holder, int position) {
            Crime crime = mCrimes.get(position);
            holder.mTitleTextView.setText(crime.getmTitle());
        }

        @Override
        public int getItemCount() {
            return mCrimes.size();
        }
    }
    private void updateUI() {
        CrimeLab crimeLab = CrimeLab.get(getActivity());
        List<Crime> crimes = crimeLab.getmCrimes();

        mAdapter = new CrimeAdapter(crimes);
        mCrimeRecyclerView.setAdapter(mAdapter);
    }
}

接下来解读下标红代码：

布局文件：

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/fragment_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent">


</FrameLayout>
<!--
    通用布局文件
-->

fragment_crime_list.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.RecyclerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/crime_recycler_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

</android.support.v7.widget.RecyclerView>

效果图：

如果你能理解上述代码是如何实现的，那么相信你已经基本掌握了如何使用RecyclerView类了。

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