Material Design之RecyclerView基本講解與瀑布流的實現

LeiHolmes發表於2019-03-02

RecyclerView簡介

　　RecyclerView是一種新的檢視組，目標是為任何基於介面卡的檢視提供相似的渲染方式。它被作為ListView和GridView控制元件的繼承者，具有更優的靈活性與可替代性。在最新的support-v7版本中提供支援。本文將講解RecyclerView的簡單實現，新增刪除條目，點選事件新增與瀑布流的實現。

相關原理與簡單實現

新增依賴

　　在AndroidStudio的build.gradle中新增依賴：

dependencies {
    ...
    compile `com.android.support:recyclerview-v7:25.3.0`
}複製程式碼

在佈局中使用

　　新增完依賴後就可以在佈局中使用RecyclerView了：

<android.support.v7.widget.RecyclerView
    android:id="@+id/main_recyclerview"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />複製程式碼

RecyclerView.Adapter

　　RecyclerView封裝了一種新型的介面卡，與現在使用的介面卡大同小異。它強制使用者使用RecyclerView提供的ViewHolder，使用時主要需要重寫onCreateViewHolder與onBindViewHolder方法。前者用來展現檢視及其持有者，且只有真正需要一個新view時才會被回撥，不需要檢查是否已經被回收。後者用來繫結資料到View上。

import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import java.util.List;

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {
    private Context context;
    private List<String> list;
    private LayoutInflater inflater;

    public MyAdapter(Context context, List<String> list) {
        this.context = context;
        this.list = list;
        inflater = LayoutInflater.from(context);
    }

    @Override
    public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = inflater.inflate(R.layout.item_recyclerview, parent, false);
        MyViewHolder holder = new MyViewHolder(view);
        return holder;
    }

    @Override
    public void onBindViewHolder(MyViewHolder holder, int position) {
        holder.textView.setText(list.get(position));
    }

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

    class MyViewHolder extends RecyclerView.ViewHolder {
        TextView textView;
        public MyViewHolder(View itemView) {
            super(itemView);
            textView = (TextView) itemView.findViewById(R.id.item_textview);
        }
    }
}複製程式碼

LayoutManager

　　RecyclerView通過佈局管理器LayoutManager控制每一個item如何進行排列擺放，何時展示和隱藏。回收或重用一個View時LayoutManager會向介面卡請求新的資料來替換舊的資料，這種機制避免了建立過多的View和頻繁的呼叫findViewById方法，目前其自帶的主要有以下三種：

LinearLayoutManager：ListView樣式
GridLayoutManager：GridView樣式
StaggeredGridLayoutManager：瀑布流樣式

ItemDecoration

　　RecyclerView並不能像ListView一樣直接在xml佈局中修改item分割線樣式。需要在Activity動態設定，當然更推薦在單個條目佈局中設定margin或者padding來實現分割線效果，這裡提供一個分割線。
“`java
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.View;

public class DividerItemDecoration extends RecyclerView.ItemDecoration {
private static final int[] ATTRS = new int[]{
android.R.attr.listDivider
};
public static final int HORIZONTAL_LIST = LinearLayoutManager.HORIZONTAL;
public static final int VERTICAL_LIST = LinearLayoutManager.VERTICAL;
private Drawable mDivider;
private int mOrientation;

public DividerItemDecoration(Context context, int orientation) {
    final TypedArray a = context.obtainStyledAttributes(ATTRS);
    mDivider = a.getDrawable(0);
    a.recycle();
    setOrientation(orientation);
}

public void setOrientation(int orientation) {
    if (orientation != HORIZONTAL_LIST && orientation != VERTICAL_LIST) {
        throw new IllegalArgumentException("invalid orientation");
    }
    mOrientation = orientation;
}

@Override
public void onDraw(Canvas c, RecyclerView parent) {
    if (mOrientation == VERTICAL_LIST) {
        drawVertical(c, parent);
    } else {
        drawHorizontal(c, parent);
    }
}

public void drawVertical(Canvas c, RecyclerView parent) {
    final int left = parent.getPaddingLeft();
    final int right = parent.getWidth() - parent.getPaddingRight();
    final int childCount = parent.getChildCount();
    for (int i = 0; i < childCount; i++) {
        final View child = parent.getChildAt(i);
        final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child                 .getLayoutParams();
        final int top = child.getBottom() + params.bottomMargin;
        final int bottom = top + mDivider.getIntrinsicHeight();
        mDivider.setBounds(left, top, right, bottom);
        mDivider.draw(c);
    }
}

public void drawHorizontal(Canvas c, RecyclerView parent) {
    final int top = parent.getPaddingTop();
    final int bottom = parent.getHeight() - parent.getPaddingBottom();
    final int childCount = parent.getChildCount();
    for (int i = 0; i < childCount; i++) {
        final View child = parent.getChildAt(i);
        final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child                 .getLayoutParams();
        final int left = child.getRight() + params.rightMargin;
        final int right = left + mDivider.getIntrinsicHeight();
        mDivider.setBounds(left, top, right, bottom);
        mDivider.draw(c);
    }
}

@Override
public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {
    if (mOrientation == VERTICAL_LIST) {
        outRect.set(0, 0, 0, mDivider.getIntrinsicHeight());
    } else {
        outRect.set(0, 0, mDivider.getIntrinsicWidth(), 0);
        }
}複製程式碼

}

#### ItemAnimator
　　ItemAnimator會根據介面卡上收到的通知來動畫顯示檢視組的修改，比如item的新增與刪除。DefaultItemAnimator已經能很好的展現動畫效果了。  
#### RecyclerView初始化
　　若想初始化一個RecyclerView使其進入工作狀態，你需要在Activity中做以下的操作：  
```java
RecyclerView recyclerView = (RecyclerView)findViewById(R.id.recycler_view_main);
MyAdapter adapter = new MyAdapter(this, list);
//設定RecyclerView保持固定的大小
recyclerView.setHasFixedSize(true);
//設定介面卡
recyclerView.setAdapter(adapter);
//設定RecyclerView ListView樣式佈局管理
recyclerView.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false));
//設定RecyclerView的Item分割線
recyclerView.addItemDecoration(new DividerItemDecoration(this, DividerItemDecoration.VERTICAL_LIST));
//設定RecyclerView的動畫
recyclerView.setItemAnimator(new DefaultItemAnimator());

//設定RecyclerView GridView樣式
//recyclerView.setLayoutManager(new GridLayoutManager(MainActivity.this, 3));
//設定RecyclerView 水平GridView樣式
//recyclerView.setLayoutManager(new StaggeredGridLayoutManager(5, StaggeredGridLayoutManager.HORIZONTAL));
//設定RecyclerView 瀑布流樣式
//recyclerView.setLayoutManager(new StaggeredGridLayoutManager(3,StaggeredGridLayoutManager.VERTICAL));複製程式碼

點選事件與新增刪除Item

　　美中不足的是RecyclerView並沒有提供像ListView一樣的Item點選與Item長點選事件，不提供我們們就自己造，通過介面回撥來實現。

Adapter中

import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import java.util.ArrayList;

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {
    private Context context;
    private LayoutInflater inflater;
    protected ArrayList<String> datas;
    private onItemClickedListener onItemClickedListener;

    public MyAdapter(Context context, ArrayList<String> datas) {
        this.context = context;
        this.datas = datas;
        inflater = LayoutInflater.from(context);
    }

    public void setOnItemClickedListener(MyAdapter.onItemClickedListener onItemClickedListener) {
        this.onItemClickedListener = onItemClickedListener;
    }

    @Override
    public void onBindViewHolder(MyViewHolder holder, int position) {
        holder.textView.setText(datas.get(position));
    }

    @Override
    public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View view = inflater.inflate(R.layout.item_recycler_view, parent, false);
        MyViewHolder myViewHolder = new MyViewHolder(view);
        return myViewHolder;
    }

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

   /**
    * 新增條目
    */
    public void addItem(int position) {
        datas.add(position, "xulei");
    //        notifyDataSetChanged();
        notifyItemInserted(position);//呼叫這個才有動畫效果
    }

   /**
    * 移除條目
    */
    public void removeItem(int position) {
        datas.remove(position);
        notifyItemRemoved(position);
    }

    class MyViewHolder extends RecyclerView.ViewHolder {
        TextView textView;

        public MyViewHolder(View itemView) {
            super(itemView);
            //初始化控制元件
            textView = (TextView) itemView.findViewById(R.id.item_textview);
            //設定當前條目單擊監聽
            itemView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    if (onItemClickedListener != null)
                        onItemClickedListener.onClick(view, getAdapterPosition());//可立刻獲取到當前position
//                        onItemClickedListener.onClick(view, getLayoutPosition());//需等當前檢視更新完才能獲取到當前position，<16ms。
                }
            });
            //設定當前條目長按監聽
            itemView.setOnLongClickListener(new View.OnLongClickListener() {
                @Override
                public boolean onLongClick(View view) {
                    if (onItemClickedListener != null)
                        onItemClickedListener.onLongClick(view, getAdapterPosition());
                    return false;
                }
            });
        }
    }

   /**
    * 點選回撥的介面
    */
    interface onItemClickedListener {
        void onClick(View view, int position);

        void onLongClick(View view, int position);
    }
}複製程式碼

Activity中

　　在Activity中例項化Adapter之後新增如下程式碼：

adapter.setOnItemClickedListener(new MyAdapter.onItemClickedListener() {
    @Override
    public void onClick(View view, int position) {
        adapter.addItem(position);
        Toast.makeText(MainActivity.this, "點選click:" + position, Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onLongClick(View view, int position) {
        adapter.removeItem(position);
        Toast.makeText(MainActivity.this, "長按click:" + position, Toast.LENGTH_SHORT).show();
    }
});複製程式碼

瀑布流的實現

　　想實現瀑布流的樣式通過使用RecyclerView也很容易就能實現。

首先在Activity中設定LayoutManager時選擇：

recyclerView.setLayoutManager(new StaggeredGridLayoutManager(3,StaggeredGridLayoutManager.VERTICAL));複製程式碼

修改Adapter

　　瀑布流自然是每個條目的高度不同才能出現瀑布的效果（水平佈局則是寬度不同），那麼只需在每個條目繫結資料時動態改變下其高度即可，貼出瀑布流Adapter程式碼：

import android.content.Context;
import android.view.ViewGroup;

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

public class StaggerAdapter extends MyAdapter {
    private List<Integer> heights;
    public StaggerAdapter(Context context, ArrayList<String> datas) {
        super(context, datas);
        heights = new ArrayList<>();
        for (int i = 0; i < datas.size(); i++) {
            heights.add((int) (100 + Math.random() * 300));
        }
    }

    @Override
    public void onBindViewHolder(MyViewHolder holder, int position) {
        ViewGroup.LayoutParams layoutParams = holder.itemView.getLayoutParams();
        layoutParams.height = heights.get(position);
//        layoutParams.width = heights.get(position);
        holder.itemView.setLayoutParams(layoutParams);
        holder.textView.setText(datas.get(position));
    }
}複製程式碼

總結

　　通過實踐發現RecyclerView相較於ListView與GridView確實強大很多，更加的靈活與方便，提高了開發效率。但也有不足之處，如並未封裝點選事件的回撥，確實是比較頭疼。期待Google的完善。
附上GitHub原始碼：
RecyclerViewDemo
RecyclerViewDevelop

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