RecyclerView实现吸顶效果---ItemDecoration

引入依赖库：

implementation 'androidx.recyclerview:recyclerview:1.1.0'

一、ItemDecoration概念

给recyclerView的item设置分割线的。

1、itemDecoration允许应用给具体的View添加具体的图画或者layout的偏移，对于绘制View之间的分割线，视觉分组边界等等是非常有用的。

2、当我们调用addItemDecoration()方法添加itemDecoration的时候，RecyclerView就会调用该类的onDraw方法去绘制分割线，也就是说，分割线是绘制出来的。

3、RecyclerView.ItemDocoration，该类为抽象类，官方目前只提供了一个实现类DivideItemDecoration。

二、ItemDocoration的源码，里面提供了三个废弃的方法和三个可用的方法（onDraw、onDrawOver、getItemOffsets）

public abstract static class ItemDecoration {/*** Draw any appropriate decorations into the Canvas supplied to the RecyclerView.* Any content drawn by this method will be drawn before the item views are drawn,* and will thus appear underneath the views.** @param c Canvas to draw into* @param parent RecyclerView this ItemDecoration is drawing into* @param state The current state of RecyclerView*/public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull State state) {onDraw(c, parent);}/*** @deprecated* Override {@link #onDraw(Canvas, RecyclerView, RecyclerView.State)}*/@Deprecatedpublic void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent) {}/*** Draw any appropriate decorations into the Canvas supplied to the RecyclerView.* Any content drawn by this method will be drawn after the item views are drawn* and will thus appear over the views.** @param c Canvas to draw into* @param parent RecyclerView this ItemDecoration is drawing into* @param state The current state of RecyclerView.*/public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent,@NonNull State state) {onDrawOver(c, parent);}/*** @deprecated* Override {@link #onDrawOver(Canvas, RecyclerView, RecyclerView.State)}*/@Deprecatedpublic void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent) {}/*** @deprecated* Use {@link #getItemOffsets(Rect, View, RecyclerView, State)}*/@Deprecatedpublic void getItemOffsets(@NonNull Rect outRect, int itemPosition,@NonNull RecyclerView parent) {outRect.set(0, 0, 0, 0);}/*** Retrieve any offsets for the given item. Each field of <code>outRect</code> specifies* the number of pixels that the item view should be inset by, similar to padding or margin.* The default implementation sets the bounds of outRect to 0 and returns.** <p>* If this ItemDecoration does not affect the positioning of item views, it should set* all four fields of <code>outRect</code> (left, top, right, bottom) to zero* before returning.** <p>* If you need to access Adapter for additional data, you can call* {@link RecyclerView#getChildAdapterPosition(View)} to get the adapter position of the* View.** @param outRect Rect to receive the output.* @param view    The child view to decorate* @param parent  RecyclerView this ItemDecoration is decorating* @param state   The current state of RecyclerView.*/public void getItemOffsets(@NonNull Rect outRect, @NonNull View view,@NonNull RecyclerView parent, @NonNull State state) {getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),parent);}}

有6个方法，其中三个有注解@Deprecated表示不能用的，其中OnDraw和onDrawOver代表的是绘制，getItemOffsets设置item的偏移量，通过outRect.set(0, 0, 0, 0);来设置的。

1、getItemOffsets方法

getItemOffsets方法，主要就是给itemView设置偏移量，比如RecyclerView的设置LinearLayoutManger,使用Vertical垂直方向，那上下item之间的分隔线的空间，就可以在这设置。
outRect.set(0, 5, 0, 0)，就代表top方向偏移5像素点，然后就预留出了5像素点高度的空间，给你绘制分隔线，而不会影响itemView（即垂直的时候设置了上下不会影响item高度，设置了左右会影响item宽度）。
看注释，设置偏移量要在后面，就是super要么不写，要么写在前面，看源码super把全部设置为0。

2、onDraw方法

这个方法绘制的东西，会在RecyclerView的item绘制之前绘制，所以onDraw绘制的东西可能会被压在item的下面，如果设置了getItemOffsets间距，绘制的距离在这个间距内就不会被遮住，如果onDraw绘制的东西的边距超过了Offsets设置的间距，就会被遮住。

3、onDrawOver方法

onDrawOver跟onDraw刚好相反，他是在RecyclerView绘制之后绘制，会盖在RecyclerView上面（如果设置了getItemOffsets间距，onDrawOver绘制的距离在这个间距内就不会遮住itemView，如果onDrawOver绘制的东西的边距超过了Offsets设置的间距，就会遮住ItemView。）

二、RecyclerView利用ItemDecoration实现吸顶效果

1.继承RecyclerView.ItemDecoration，重写getItemOffsets方法

@Overridepublic void getItemOffsets(@NonNull Rect outRect, @NonNull View view, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {super.getItemOffsets(outRect, view, parent, state);if (parent.getAdapter() instanceof StarAdapter) {StarAdapter adapter = (StarAdapter) parent.getAdapter();//RecyclerView的LayoutParams，是有viewHolder的，所以可以通过View 获取LayoutParams,再拿到ViewHolder//获取当前view对应的positionint childLayoutPosition = parent.getChildLayoutPosition(view);//判断是否是头部boolean isGroupHeader = adapter.isGroupHeader(childLayoutPosition);if (isGroupHeader) {//如果当前item是头部，则预留更大的空间outRect.set(0, groupHeaderHeight, 0, 0);} else {//不是头部隔开：1像素outRect.set(0, 1, 0, 0);}}}

如果此时给RecyclerView设置个背景，就可以看到间隔处有颜色了。记住这是recyclerView的背景，并不是我们画的分割线，我还没重写onDraw和onDrawOver呢。（分割线有清晰和不清晰应该是模拟器的问题，我在真机上都是清晰的）

2.重写onDraw方法，开始在上一步item之间分割出来的间距进行绘制。把画笔的颜色改成BLUE，即分割线绘制成蓝色，并绘制相关字符串。

@Overridepublic void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {super.onDraw(c, parent, state);if (parent.getAdapter() instanceof StarAdapter) {StarAdapter adapter = (StarAdapter) parent.getAdapter();//当前屏幕的item个数int childCount = parent.getChildCount();//recyclerView的左边padding值作为绘制分割线的左int left = parent.getPaddingLeft();//分割线的右边int right = parent.getWidth() - parent.getPaddingRight();for (int i = 0; i < childCount; i++) {//获取对应i的ViewView childAt = parent.getChildAt(i);int childLayoutPosition = parent.getChildLayoutPosition(childAt);boolean isGroupHeader = adapter.isGroupHeader(childLayoutPosition);//是否为头部if (isGroupHeader && childAt.getTop() - groupHeaderHeight - parent.getPaddingTop() >= 0) {c.drawRect(left, childAt.getTop() - groupHeaderHeight, right, childAt.getTop(), headerPaint);String groupName = adapter.getGroupName(childLayoutPosition);Log.i("BK", groupName + " " + childAt.getTop());textPaint.getTextBounds(groupName, 0, groupName.length(), textRect);c.drawText(groupName, left + 20, childAt.getTop() - groupHeaderHeight / 2+ textRect.height() / 2, textPaint);} else if (childAt.getTop() - groupHeaderHeight - parent.getPaddingTop() >= 0){//分割线c.drawRect(left, childAt.getTop() - 1, right, childAt.getTop(), headerPaint);}}}}

3.重写OnDrawOver方法，实现吸顶的效果：因为吸顶效果是覆盖在itemView上的，所以只能用个这个方法来实现。

@Overridepublic void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {super.onDrawOver(c, parent, state);if (parent.getAdapter() instanceof StarAdapter) {StarAdapter adapter = (StarAdapter) parent.getAdapter();//屏幕可视的第一个itemView的位置int firstVisibleItemPosition = ((LinearLayoutManager) parent.getLayoutManager()).findFirstVisibleItemPosition();//获取position对应的viewView itemView = parent.findViewHolderForAdapterPosition(firstVisibleItemPosition).itemView;int left = parent.getPaddingLeft();int right = parent.getWidth() - parent.getPaddingRight();int top = parent.getPaddingTop();//当屏幕可视范围内，第二个itemView是下一组的头部的时候boolean isGroupHeader = adapter.isGroupHeader(firstVisibleItemPosition + 1);if (isGroupHeader) {//这种情况就要将上一个吸顶的慢慢往上顶的效果Log.i("BK", "onDrawOver1: " + firstVisibleItemPosition);//bottom会随着上滑越来越小int bottom = Math.min(groupHeaderHeight, itemView.getBottom()-parent.getPaddingTop());c.drawRect(left, top, right, top + bottom, headerPaint);String groupName = adapter.getGroupName(firstVisibleItemPosition);textPaint.getTextBounds(groupName, 0, groupName.length(), textRect);c.drawText(groupName, left + 20, top + bottom - groupHeaderHeight / 2+ textRect.height() / 2, textPaint);} else {//固定在顶部的效果Log.i("BK", "onDrawOver2: " + firstVisibleItemPosition);c.drawRect(left, top, right, top + groupHeaderHeight, headerPaint);String groupName = adapter.getGroupName(firstVisibleItemPosition);textPaint.getTextBounds(groupName, 0, groupName.length(), textRect);c.drawText(groupName, left + 20, top + groupHeaderHeight / 2+ textRect.height() / 2, textPaint);}}}

每行代码的注释也清楚了，可以自己运行体会体会。其实在getItemOffsets把控件预留出来了，剩下的分割线绘制就和自定义View的绘制差不多了。

Github完整项目代码