长图片自动循环滚动效果 （仿小红书）

• 先看小红书的效果

小红书.GIF

• 说说思路

滚动效果用RecyclerView实现。RecyclerView有个smoothScrollToPosition方法，可以滚动到指定位置（有滚动效果，不是直接到指定位置），不了解的看这里RecycleView4种定位滚动方式演示。每一个Item是一张长图，这样首尾相接滚动起来（滚到无限远）就是无限循环的效果，然后再改变滚动的速度，完成。

public class MainActivity extends AppCompatActivity {    private RecyclerView mRecyclerView;    @Override
    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        //全屏
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);

        setContentView(R.layout.activity_main);

        mRecyclerView = findViewById(R.id.mRecyclerView);
        mRecyclerView.setAdapter(new SplashAdapter(MainActivity.this));
        mRecyclerView.setLayoutManager(new ScollLinearLayoutManager(MainActivity.this));        //smoothScrollToPosition滚动到某个位置（有滚动效果）
        mRecyclerView.smoothScrollToPosition(Integer.MAX_VALUE / 2);
    }


}

• 无限循环

将RecyclerView的Item数量设置成很大的值，用smoothScrollToPosition方法滚到很远的位置，就能实现这样的效果，很多banner轮播图的实现也是如此；

public class SplashAdapter extends RecyclerView.Adapter<SplashAdapter.ViewHolder> {    private int imgWidth;    public SplashAdapter(Context context) {
        imgWidth = EasyUtil.getScreenWidth(context);
    }    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_splash, parent, false);        return new ViewHolder(itemView);
    }    @Override
    public void onBindViewHolder(final ViewHolder holder, final int position) {       /* ViewGroup.LayoutParams lp = holder.item_bg.getLayoutParams();
        lp.width = imgWidth;
        lp.height =imgWidth*5;
        holder.item_bg.setLayoutParams(lp);*/
    }    @Override
    public int getItemCount() {        return Integer.MAX_VALUE;
    }    public class ViewHolder extends RecyclerView.ViewHolder {

        ImageView item_bg;        public ViewHolder(final View itemView) {            super(itemView);
            item_bg = itemView.findViewById(R.id.item_bg);
        }

    }

}

• 控制smoothScrollToPosition的滑动速度

参考RecyclerView调用smoothScrollToPosition() 控制滑动速度，修改MILLISECONDS_PER_INCH的值即可

/**
 * 更改RecyclerView滚动的速度
 */public class ScollLinearLayoutManager extends LinearLayoutManager {    private float MILLISECONDS_PER_INCH = 25f;  //修改可以改变数据,越大速度越慢
    private Context contxt;    public ScollLinearLayoutManager(Context context) {        super(context);        this.contxt = context;
    }    @Override
    public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position) {
        LinearSmoothScroller linearSmoothScroller =                new LinearSmoothScroller(recyclerView.getContext()) {                    @Override
                    public PointF computeScrollVectorForPosition(int targetPosition) {                        return ScollLinearLayoutManager.this
                                .computeScrollVectorForPosition(targetPosition);
                    }                    @Override
                    protected float calculateSpeedPerPixel
                            (DisplayMetrics displayMetrics) {                        return MILLISECONDS_PER_INCH / displayMetrics.density;                        //返回滑动一个pixel需要多少毫秒
                    }

                };
        linearSmoothScroller.setTargetPosition(position);
        startSmoothScroll(linearSmoothScroller);
    }    //可以用来设置速度
    public void setSpeedSlow(float x) {        //自己在这里用density去乘，希望不同分辨率设备上滑动速度相同
        //0.3f是自己估摸的一个值，可以根据不同需求自己修改
        MILLISECONDS_PER_INCH = contxt.getResources().getDisplayMetrics().density * 0.3f + (x);
    }

}

• 图片宽度充满屏幕、高度按图片原始宽高比例自适应

@SuppressLint("AppCompatCustomView")public class FitImageView extends ImageView {    public FitImageView(Context context) {        super(context);
    }    public FitImageView(Context context, AttributeSet attrs) {        super(context, attrs);
    }    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec){
        Drawable drawable = getDrawable();        if(drawable!=null){            int width = MeasureSpec.getSize(widthMeasureSpec);            int height = (int) Math.ceil((float) width * (float) drawable.getIntrinsicHeight() / (float) drawable.getIntrinsicWidth());
            setMeasuredDimension(width, height);
        }else{            super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        }
    }

}

这里需要注意的是、Item的根布局  android:layout_height="wrap_content"，否则图片高度会受限

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <com.next.scrollimagedemo.view.FitImageView
        android:id="@+id/item_bg"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:class="lazyload" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC" data-original="@mipmap/ww1" />


    <!--  <ImageView
          android:id="@+id/item_bg"
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:class="lazyload" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC" data-original="@mipmap/ww2"
          android:scaleType="centerCrop"/>--></android.support.constraint.ConstraintLayout>

• 使RecyclerView不能手指触碰滑动

加层View屏蔽掉事件就好了

<android.support.constraint.ConstraintLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/mRecyclerView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"></android.support.v7.widget.RecyclerView>

    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#80000000"
        android:clickable="true" />

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="80dp"
        android:layout_marginTop="80dp"
        app:layout_constraintTop_toTopOf="parent"
        android:scaleType="centerInside"
        android:class="lazyload" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC" data-original="@mipmap/slogan"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent" /></android.support.constraint.ConstraintLayout>

• 完成效果

WW.GIF

作者：丶E
链接：https://www.jianshu.com/p/48a89e678d6c