最近在開發(fā)的時候步势，遇到了需要通過代碼使得RecyclerView能夠滑到指定item頂部位置的需求丹诀，在查看源碼之后喝滞，發(fā)現(xiàn)RecyclerView已經(jīng)提供了實現(xiàn)滑動到指定位置的方法,下面是可實現(xiàn)方法:

//平滑滾動
recyclerView.smoothScrollToPosition(position);
//非平滑滾動
recyclerView.scrollToPosition(position);

LinearLayoutManager manager = (LinearLayoutManager)recyclerView.getLayoutManager();
//平滑滾動
manager.scrollToPosition(position)

當然异旧，除了上述方法以外涵卵，RecyclerView還有scrollBy莱褒、smoothScrollBy這兩個方法(RecyclerView不支持scrollTo)击困，可以實現(xiàn)滑動到指定位置，但是使用這三個方法滑動到對應(yīng)item位置广凸，需要計算item的高度或?qū)挾仍牟瑁瑢崿F(xiàn)起來過于復(fù)雜。
在LayoutManager中谅海，也包含了滾動的方法脸哀，且調(diào)用對應(yīng)的方法也能實現(xiàn)滾動:

調(diào)用manager中的滾動方法.png

實際上，RecyclerView的滑動方法扭吁，都是通過代理manager的滑動方法實現(xiàn)的撞蜂，下面是RecyclerView中smoothScrollToPosition(int position)的源碼:

/**
 * Starts a smooth scroll to an adapter position.
 * <p>
 * To support smooth scrolling, you must override
 * {@link LayoutManager#smoothScrollToPosition(RecyclerView, State, int)} and create a
 * {@link SmoothScroller}.
 * <p>
 * {@link LayoutManager} is responsible for creating the actual scroll action. If you want to
 * provide a custom smooth scroll logic, override
 * {@link LayoutManager#smoothScrollToPosition(RecyclerView, State, int)} in your
 * LayoutManager.
 *
 * @param position The adapter position to scroll to
 * @see LayoutManager#smoothScrollToPosition(RecyclerView, State, int)
 */
public void smoothScrollToPosition(int position) {
        if (mLayoutFrozen) {
            return;
        }
        if (mLayout == null) {
            Log.e(TAG, "Cannot smooth scroll without a LayoutManager set. "
                    + "Call setLayoutManager with a non-null argument.");
            return;
        }
        layoutManger.smoothScrollToPosition(this, mState, position);
    }

雖然使用LayoutManager中的滾動方法，一樣可以實現(xiàn)滑動侥袜，但是一般情況下蝌诡，如果RecyclerView包含與LayoutManager同樣的方法，要優(yōu)先選擇使用RecyclerView中的方法枫吧。
好了浦旱，廢話不多說，下面以smoothScrollToPosition為例九杂，看看調(diào)用滑動方法的效果:

調(diào)用smoothScroll方法滾動到指定item.gif

可以發(fā)現(xiàn)颁湖，點擊對應(yīng)按鈕調(diào)用滑動方法時代兵，有時會無效。這是因為默認情況下爷狈，如果item可見植影，調(diào)用滑動到該item的方法時，該方法將不執(zhí)行滑動涎永。這就說明思币，調(diào)用scrollToPosition或者smoothScrollToPosition并不能保證能夠滑到item的頂部。這就尷尬了羡微，這個問題該怎么解決呢谷饿？
查閱資料后，發(fā)現(xiàn)可以通過下面兩種方法解決:

1.LinearLayoutManger.scrollToPositionWithOffset(int position, int offset)##

該方法是在LinearLayoutManager中妈倔，其中offset為滑動偏移量博投，當設(shè)置offset為0時，會滑動position對應(yīng)的item的頂部盯蝴，此方法與scrollToPosition一樣毅哗，為非平滑滾動:

LinearLayoutManager manager = (LinearLayoutManager)recyclerView.getLayoutManager();
manager.scrollToPositionWithOffset(position, 0);

效果如下：

調(diào)用scrollToPositionWithOffset方法.gif

2.復(fù)寫LinearSmoothScoller的getVerticalSnapPreference()##

此解決辦法來源于stackOverFlow，只要復(fù)寫LinearSmoothScroller的getVerticalSnapPreference(),即可實現(xiàn)平滑滾動到指定item頂部捧挺。首先我們看看getVerticalSnapPreference()方法:

    /**
     * When scrolling towards a child view, this method defines whether we should align the top
     * or the bottom edge of the child with the parent RecyclerView.
     *
     * @return SNAP_TO_START, SNAP_TO_END or SNAP_TO_ANY; depending on the current target vector
     * @see #SNAP_TO_START
     * @see #SNAP_TO_END
     * @see #SNAP_TO_ANY
     */
    protected int getVerticalSnapPreference() {
        return mTargetVector == null || mTargetVector.y == 0 ? SNAP_TO_ANY :
                mTargetVector.y > 0 ? SNAP_TO_END : SNAP_TO_START;
    }

看上面的注釋虑绵，可以知道，SNAP_TO_START代表滑動到item頂部闽烙，因此只要使該方法固定返回SNAP_TO_START翅睛，就能實現(xiàn)滑動到item頂部:

package cn.axen.mvp.scroller

import android.content.Context
import android.support.v7.widget.LinearSmoothScroller

public class TopLinearSmoothScroller extends LinearSmoothScroller {
    public TopLinearSmoothScroller(Context context) {
        super(context)
    }
    
    @Override
    public int getVerticalSnapPreference(): Int {
        return SNAP_TO_START
    }
}

最后，復(fù)寫LinearLayoutManager的smoothScrollToPosition方法黑竞，調(diào)用RecyclerView的smoothScrollToPosition方法：

LinearLayoutManager manager = new LinearLayoutManager(this) {
    @Override
    public void smoothScrollToPosition(RecyclerView view, RecyclerView.State state, int position) {
        TopLinearSmoothScroller scroller = new TopLinearSmoothScroller(view.getContext());
        scroller.setTargetPosition(position);
        startSmoothScroll(smoothScroller);
    }
};
recyclerView.smoothScrollToPosition(position);

效果如下:

重寫后的效果.gif