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字体中国设计网,百度优化,建筑企业资质,网站名称设置目录一、滑动到指定位置（target position）并且置顶 1. RecycleView默认的几个实现方法及缺陷 2. 优化源码实现置顶方案二、调整平移滑动速率三、其他方案：置顶、置顶加偏移、居中 1. 其他置顶方案 2. 置顶加偏移 3. 滚动居中在实…

一、滑动到指定位置（target position）并且置顶

1. RecycleView默认的几个实现方法及缺陷

2. 优化源码实现置顶方案

二、调整平移滑动速率

三、其他方案：置顶、置顶加偏移、居中

1. 其他置顶方案

2. 置顶加偏移

3. 滚动居中

在实际项目里，RecycleView 可以说是我们最常用到的组件，作为绑定并展示LIST数据的组件，经常需要实现平滑滚动到列表里的某个目标ITEM，并且将其置顶在屏幕最上方，而且在特殊情形下，我们需要控制滑动速度，来控制滚动的时长。

一、滑动到指定位置（target position）并且置顶

1. RecycleView默认的几个实现方法及缺陷

((LinearLayoutManager)recycleView.getLayoutManager()).scrollToPositionWithOffset(int position, int offset);

如果你没有滑动过程动画的要求，那上面这行代码将offset的值设置为0，就一步到位地满足需求了。

recycleView.scrollToPosition(int position);
recycleView.smoothScrollToPosition(int position);

以上两个方法遵循的是最少滑动原则，只要target position那项item已经完全可见了，就马上停止滑动；要是target position已经可见了，那根本不会滑动。所以按不同的滑动方向，会出现不同的结果，如果target position在屏幕可视范围的上方，则它默认会将target position置顶；反之，target position在屏幕可视范围的下方，则滚动完成后，target postion会处于屏幕的最下方，无法实现我们的置顶需求。所以缺陷很明显：要么不动，要么无法置顶。

2. 优化源码实现置顶方案

我们看下recycleview提供的方法的源代码，看看是否可以进行改进：

    public void smoothScrollToPosition(int position) {if (mLayoutSuppressed) {return;}if (mLayout == null) {Log.e(TAG, "Cannot smooth scroll without a LayoutManager set. "+ "Call setLayoutManager with a non-null argument.");return;}mLayout.smoothScrollToPosition(this, mState, position);}

由代码可以看出，RecyclerView的滑动方法是调用LayoutManager的smoothScrollToPosition方法：

    @Overridepublic void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state,int position) {LinearSmoothScroller linearSmoothScroller =new LinearSmoothScroller(recyclerView.getContext());linearSmoothScroller.setTargetPosition(position);startSmoothScroll(linearSmoothScroller);}

其中LinearSmoothScroller提供了三个滑动策略：

    /*** 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;

LinearSmoothScroller确定滑动方案的方法：

    /*** 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;}

重写LinearSmoothScroller的getVerticalSnapPreference方法：

class LinearTopSmoothScroller extends LinearSmoothScroller {public LinearTopSmoothScroller(Context context) {super(context);}@Overrideprotected int getVerticalSnapPreference() {return SNAP_TO_START;}}

这里为什么返回 SNAP_TO_START？可以看到LinearSmoothScrollerl的方法calculateDtToFit()根据不同滚动策略获取到需要滚动的距离，SNAP_TO_START是按置顶的方案来计算的。所以我们在getVerticalSnapPreference方法里固定返回SNAP_TO_START就可以实现目的。

public int calculateDtToFit(int viewStart, int viewEnd, int boxStart, int boxEnd, intsnapPreference) {switch (snapPreference) {case SNAP_TO_START:return boxStart - viewStart;case SNAP_TO_END:return boxEnd - viewEnd;case SNAP_TO_ANY:final int dtStart = boxStart - viewStart;if (dtStart > 0) {return dtStart;}final int dtEnd = boxEnd - viewEnd;if (dtEnd < 0) {return dtEnd;}break;default:throw new IllegalArgumentException("snap preference should be one of the"+ " constants defined in SmoothScroller, starting with SNAP_");}return 0;}

调用方式一：

    void scrollItemToTop(int position) {LinearSmoothScroller smoothScroller = new LinearTopSmoothScroller(this);smoothScroller.setTargetPosition(position);linearLayoutManager.startSmoothScroll(smoothScroller);}

调用方式二：

自定义一个类继承自 LinearLayoutManager：

private class TopLayoutManager extends LinearLayoutManager {public TopLayoutManager(Context context) {super(context);}public TopLayoutManager(Context context, int orientation, boolean reverseLayout) {super(context, orientation, reverseLayout);}@Overridepublic void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position) {RecyclerView.SmoothScroller smoothScroller = new LinearTopSmoothScroller(recyclerView.getContext());smoothScroller.setTargetPosition(position);startSmoothScroll(smoothScroller);}private class LinearTopSmoothScroller extends LinearSmoothScroller {public LinearTopSmoothScroller(Context context) {super(context);}@Overrideprotected int getVerticalSnapPreference() {return SNAP_TO_START;}}
}

调用代码：

TopLayoutManager topLayoutManager = new TopLayoutManager(this);
recycleview.setLayoutManager(topLayoutManager);
recycleview.smoothScrollToPosition(position);

二、调整平移滑动速率

同理，可以在LinearSmoothScroller类找到决定滚动速度的方法并修改。

    /*** Calculates the scroll speed.** @param displayMetrics DisplayMetrics to be used for real dimension calculations* @return The time (in ms) it should take for each pixel. For instance, if returned value is* 2 ms, it means scrolling 1000 pixels with LinearInterpolation should take 2 seconds.*/protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;}

上面的MILLISECONDS_PER_INCH的值为25F，如果希望更快可以将值改小，这个方法的返回值表示滚动一个像素需要的时间，单位ms，如果返回值为2ms，表示滚动1000个像素需要花费2秒时长。

平滑滚动到target position，【置顶+调速】的调用方式：

        RecyclerView.SmoothScroller smoothScroller = new LinearSmoothScroller(this) {@Override protected int getVerticalSnapPreference() {return LinearSmoothScroller.SNAP_TO_START;}@Overrideprotected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {//默认值是25F(MILLISECONDS_PER_INCH)，值越小滑动速度越快,值越大则越慢return 100F / displayMetrics.densityDpi;}};smoothScroller.setTargetPosition(position);linearLayoutManager.startSmoothScroll(smoothScroller);

目前还有一个问题，虽然我们可以调整速度，但是这里始终是一个固定的滚动速度，试想如果滚动的距离特别远，仍然需要滚动很长的时间；又或者滚动距离太近，那么滚动动画一瞬间就结束了，缺少了流畅感。

所以我们可以根据需要滚动的远或近来设置不同的滚动速度：

        RecyclerView.SmoothScroller smoothScroller = new LinearSmoothScroller(this) {@Override protected int getVerticalSnapPreference() {return LinearSmoothScroller.SNAP_TO_START;}@Overrideprotected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {// 第一个可见位置int firstItem = linearLayoutManager.findFirstVisibleItemPosition();int diff = Math.abs(position - firstItem);// 将 diff 作分母：滚动距离越远，速度越快。 (100f/diff) 数值如果过小会导致速度过快, 可以再乘一个速度因子变量(speedFactor)来调整int speedFactor = 5;float speed = (100f / diff) * speedFactor;return speed / displayMetrics.densityDpi;}};