import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.LinearGradient;
import android.graphics.Paint;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.graphics.Shader;
import android.graphics.Xfermode;

import androidx.recyclerview.widget.RecyclerView;

/**
 * RecyclerView右側渐变的itemDecoration
 * 渐变参考文档：https://blog.csdn.net/u010126792/article/details/85237085
 * 半透明则是由0→黑的渐变效果
 * 使用方法： RecyclerView.addItemDecoration(new RightGradual());
 */

public class RightGradual extends RecyclerView.ItemDecoration {

    private Paint mPaint;
    private Xfermode mXfermode;
    private LinearGradient mLinearGradient;
    private int mLayerId;

    public RightGradual() {
        mPaint = new Paint();
        mXfermode = new PorterDuffXfermode(PorterDuff.Mode.DST_IN);
        mLinearGradient = null;
    }

    @Override
    public void onDrawOver(Canvas canvas, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(canvas, parent, state);
        float left=parent.getRight()-parent.getRight()/0.1f;
        float top=0f;
        float right=parent.getRight();
        float bottom=parent.getBottom();
        if(mLinearGradient==null){
            mLinearGradient=new LinearGradient(parent.getMeasuredWidth(), 0.0f, 0.0f, 0.0f, new int[]{0, Color.BLACK}, new float[]{0.0f,0.2f}, Shader.TileMode.CLAMP);
        }
        mPaint.setXfermode(mXfermode);
        mPaint.setShader(mLinearGradient);
        //mPaint.setColor(Color.BLUE);
        canvas.drawRect(left, top,right, bottom, mPaint);
        mPaint.setXfermode(null);
        canvas.restoreToCount(mLayerId);
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDraw(c, parent, state);
        mLayerId = c.saveLayer(0.0f, 0.0f, (float) parent.getWidth(), (float) parent.getHeight(), mPaint, Canvas.ALL_SAVE_FLAG);
    }
}