【RecyclerView】 八、RecyclerView.ItemDecoration 条目装饰 ( onDraw 和 onDrawOver 绘制要点 )（二）

package kim.hsl.recyclerview;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.view.View;
import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
public class ItemDecoration extends RecyclerView.ItemDecoration {
    @Override
    public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent,
                       @NonNull RecyclerView.State state) {
        // 1. 获取当前的组件个数
        int itemCount = parent.getChildCount();
        // 2. 遍历当前的所有组件
        for (int i = 0; i < itemCount; i++) {
            // 3. 获取组件 View 对象
            View view = parent.getChildAt(i);
            // 4. 获取 item 组件相对于父容器的坐标
            int left = view.getLeft();
            int top = view.getTop();
            int right = view.getRight();
            int bottom = view.getBottom();
            // 5. 根据上述坐标进行绘图
            if (i % 4 == 0){
                // 给每一行的第一个元素绘制红色矩形背景, 向外延展 5 像素
                Paint paint = new Paint();
                paint.setColor(Color.RED);
                c.drawRect(left - 5, top - 5, right + 5, bottom + 5, paint);
            }
        }
    }
    @Override
    public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent,
                           @NonNull RecyclerView.State state) {
        // 1. 获取当前的组件个数
        int itemCount = parent.getChildCount();
        // 2. 遍历当前的所有组件
        for (int i = 0; i < itemCount; i++) {
            // 3. 获取组件 View 对象
            View view = parent.getChildAt(i);
            // 4. 获取 item 组件相对于父容器的坐标
            int left = view.getLeft();
            int top = view.getTop();
            int right = view.getRight();
            int bottom = view.getBottom();
            // 5. 根据上述坐标进行绘图
            if (i % 2 == 0){
                // 偶数序号的元素绘制蓝色圆形遮罩
                Paint paint = new Paint();
                paint.setColor(Color.BLUE);
                paint.setStyle(Paint.Style.STROKE);
                paint.setStrokeWidth(5F);
                c.drawCircle((left + right) / 2F, (top + bottom) / 2F, (right - left) / 4F, paint);
            }else{
                // 奇数序号的元素绘制红色矩形遮罩
                Paint paint = new Paint();
                paint.setColor(Color.RED);
                paint.setStyle(Paint.Style.STROKE);
                paint.setStrokeWidth(5F);
                c.drawRect(left + 20, top + 20, right - 20, bottom - 20, paint);
            }
        }
    }
    @Override
    public void getItemOffsets(@NonNull Rect outRect, @NonNull View view,
                               @NonNull RecyclerView parent,
                               @NonNull RecyclerView.State state) {
        // 1. 获取当前设置边距的位置
        int currentPosition = parent.getChildAdapterPosition(view);
        // 2. 针对不同的位置设置不同的边距
        // 每排最左侧和最右侧的左右边距设置成 20 像素, 其余 4 个边距一律设置成 5
        if (currentPosition % 4 == 0){
            // 每排最左侧的边距
            outRect.left = 40;
            outRect.top = 20;
            outRect.right = 20;
            outRect.bottom = 20;
        }else if (currentPosition %4 == 3){
            // 每排最右侧的边距
            outRect.left = 20;
            outRect.top = 20;
            outRect.right = 40;
            outRect.bottom = 20;
        }else{
            // 普通元素的边距都是 5
            outRect.left = 20;
            outRect.top = 20;
            outRect.right = 20;
            outRect.bottom = 20;
        }
    }
}

package kim.hsl.recyclerview;
import android.graphics.Color;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.GridLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import androidx.recyclerview.widget.StaggeredGridLayoutManager;
public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //1 . 从布局中获取 RecyclerView
        RecyclerView recycler_view = findViewById(R.id.recycler_view);
        //2 . 创建并设置布局管理器
        //创建布局管理器
        StaggeredGridLayoutManager layoutManager = new StaggeredGridLayoutManager(
                4,
                RecyclerView.VERTICAL);
        //设置布局管理器
        recycler_view.setLayoutManager(layoutManager);
        // 设置边距
        recycler_view.addItemDecoration(new ItemDecoration());
        //3 . 创建并设置列表适配器
        Adapter adapter = new Adapter();
        recycler_view.setAdapter(adapter);
    }
    /**
     * RecyclerView 适配器
     */
    public class Adapter extends RecyclerView.Adapter<Adapter.ViewHolder> {
        @Override
        public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            View root_view = LayoutInflater.from(MainActivity.this)
                    .inflate(R.layout.item_recyclerview, parent, false);
            return new ViewHolder(root_view);
        }
        @Override
        public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
            holder.text.setText("" + position);
        }
        @Override
        public int getItemCount() {
            return 100;
        }
        public class ViewHolder extends RecyclerView.ViewHolder {
            TextView text;
            public ViewHolder(@NonNull View itemView) {
                super(itemView);
                text = itemView.findViewById(R.id.text);
            }
        }
    }
}