解決RecycleView滑動到指定位置的問題:

更改前,滑動位置不固定

更改后,滑動位置固定在條目頂部

相信好多人都用過自定義LinearLayoutManager,如下:

import android.content.Context;
import android.graphics.PointF;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.LinearSmoothScroller;
import android.support.v7.widget.RecyclerView;
import android.util.DisplayMetrics;

/**
 * Created by gjl on 2018/3/2.
 */

public class ScrollSpeedLinearLayoutManger extends LinearLayoutManager {
    private float MILLISECONDS_PER_INCH = 0.3f;

    public ScrollSpeedLinearLayoutManger(Context context) {
        super(context);
        //根據(jù)不同的屏幕分辨率設(shè)置速度
        MILLISECONDS_PER_INCH = context.getResources().getDisplayMetrics().density * 0.1f;
    }

    @Override
    public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position) {
        LinearSmoothScroller linearSmoothScroller =
                new LinearSmoothScroller(recyclerView.getContext()) {
                    @Override
                    public PointF computeScrollVectorForPosition(int targetPosition) {
                        return ScrollSpeedLinearLayoutManger.this
                                .computeScrollVectorForPosition(targetPosition);
                    }

                    //This returns the milliseconds it takes to
                    //scroll one pixel.
                    @Override
                    protected float calculateSpeedPerPixel
                    (DisplayMetrics displayMetrics) {
                        //返回滑動一個pixel需要多少毫秒
                        return MILLISECONDS_PER_INCH / displayMetrics.density;                      
                    }
                };
        linearSmoothScroller.setTargetPosition(position);
        startSmoothScroll(linearSmoothScroller);
    }
}

使用時:

ScrollSpeedLinearLayoutManger  linearLayoutManger  = new ScrollSpeedLinearLayoutManger(MainActivity.this);
recycleView.setLayoutManager(linearLayoutManger);
//滑動方法
linearLayoutManger.smoothScrollToPosition(recycleView,null,position);

結(jié)果發(fā)現(xiàn)滑動方法不好用了,使用smoothScrollToPosition時凉翻，移動到前面的項時，它默認(rèn)會將要顯示的項底部前计，但是移動到后面的項時垃杖，它默認(rèn)會將要顯示的項頂部，當(dāng)前移動項在屏幕上顯示時伶棒，不會移動，用過的應(yīng)該都有所了解肤无。

解決問題

從上面的代碼看到,我們只是設(shè)置了LinearSmoothScroller,那么查看一下LinearSmoothScroller的源碼發(fā)現(xiàn):

 /**
     * Align child view's left or top with parent view's left or top
     *
     * @see #calculateDtToFit(int, int, int, int, int)
     * @see #calculateDxToMakeVisible(android.view.View, int)
     * @see #calculateDyToMakeVisible(android.view.View, int)
     */
    public static final int SNAP_TO_START = -1;

    /**
     * Align child view's right or bottom with parent view's right or bottom
     *
     * @see #calculateDtToFit(int, int, int, int, int)
     * @see #calculateDxToMakeVisible(android.view.View, int)
     * @see #calculateDyToMakeVisible(android.view.View, int)
     */
    public static final int SNAP_TO_END = 1;

    /**
     * <p>Decides if the child should be snapped from start or end, depending on where it
     * currently is in relation to its parent.</p>
     * <p>For instance, if the view is virtually on the left of RecyclerView, using
     * {@code SNAP_TO_ANY} is the same as using {@code SNAP_TO_START}</p>
     *
     * @see #calculateDtToFit(int, int, int, int, int)
     * @see #calculateDxToMakeVisible(android.view.View, int)
     * @see #calculateDyToMakeVisible(android.view.View, int)
     */
    public static final int SNAP_TO_ANY = 0;

 /**
     * 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;
    }

LinearSmoothScroller里面3個狀態(tài),這里我們只需要在設(shè)置LinearSmoothScroller的時候重寫 getVerticalSnapPreference()方法,讓滑動位置始終滑動到條目開始位置就好了.

/**
 * Created by gjl on 2018/3/2.
 */

public class ScrollSpeedLinearLayoutManger extends LinearLayoutManager {
    private float MILLISECONDS_PER_INCH = 0.3f;

    public ScrollSpeedLinearLayoutManger(Context context) {
        super(context);
        MILLISECONDS_PER_INCH = context.getResources().getDisplayMetrics().density * 0.1f;
    }

    @Override
    public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position) {
        LinearSmoothScroller linearSmoothScroller =
                new LinearSmoothScroller(recyclerView.getContext()) {
                    @Override
                    public PointF computeScrollVectorForPosition(int targetPosition) {
                        return ScrollSpeedLinearLayoutManger.this
                                .computeScrollVectorForPosition(targetPosition);
                    }

                    //This returns the milliseconds it takes to
                    //scroll one pixel.
                    @Override
                    protected float calculateSpeedPerPixel
                    (DisplayMetrics displayMetrics) {
                        return MILLISECONDS_PER_INCH / displayMetrics.density;
                        //返回滑動一個pixel需要多少毫秒
                    }
                    
                    //重寫這個方法,保證滑動到指定條目的頂部
                    @Override
                    protected int getVerticalSnapPreference() {
                        return SNAP_TO_START;
                    }

                };
        linearSmoothScroller.setTargetPosition(position);
        startSmoothScroll(linearSmoothScroller);
    }

}