ItemDecoration实现recyclerView实现头部吸顶

package com.example.itemdecoration; import android.content.Context; import android.graphics.Canvas; import android.graphics.Color; import android.graphics.Paint; import android.graphics.Rect; import android.view.View; import androidx.annotation.NonNull; import androidx.recyclerview.widget.LinearLayoutManager; import androidx.recyclerview.widget.RecyclerView; public class hahItemDecoration extends RecyclerView.ItemDecoration { private final int groupHeaderHeight; private final Paint headPaint; private final Paint textPaint; private final Rect textRect; public hahItemDecoration(Context context) { groupHeaderHeight = dp2px(context, 100); headPaint = new Paint(); headPaint.setColor(Color.GREEN); textPaint = new Paint(); textPaint.setTextSize(50); textPaint.setColor(Color.WHITE); textRect = new Rect(); } @Override public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) { super.onDraw(c, parent, state); if (parent.getAdapter() instanceof hahAdapter) { hahAdapter adapter = (hahAdapter) parent.getAdapter(); //当前屏幕中item的个数 int count = parent.getChildCount(); // 获取当前RecyclerView距离屏幕左边的padding int left = parent.getPaddingLeft(); // right == RecyclerView的宽度 - RecyclerView的右padding int right = parent.getWidth() - parent.getPaddingRight(); for (int i = 0; i < count; i++) { //获取对应 i 的 view View view = parent.getChildAt(i); // 获取 i 的 view 的布局位置 int position = parent.getChildLayoutPosition(view); // 判断是否是头部 boolean isGroupHeader = adapter.isGroupHeader(position); // i 的 view 是头部并且 当前view的top位置距离屏幕的顶部还有距离 if(isGroupHeader && view.getTop() - groupHeaderHeight - parent.getPaddingTop() >=0) { c.drawRect(left, view.getTop() - groupHeaderHeight, right, view.getTop(), headPaint); String groupName = adapter.getGroupName(position); textPaint.getTextBounds(groupName, 0, groupName.length(), textRect); c.drawText(groupName, left + 20, view.getTop() - groupHeaderHeight / 2f + textRect.height() / 2f, textPaint); } else if(view.getTop() - groupHeaderHeight - parent.getPaddingTop() >=0){ //分割线 c.drawRect(left, view.getTop() - 4, right, view.getTop(), headPaint); } } } } @Override public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) { super.onDrawOver(c, parent, state); if (parent.getAdapter() instanceof hahAdapter) { hahAdapter adapter = (hahAdapter) parent.getAdapter(); //TODO 返回可见区域内的第一个item的position int position =((LinearLayoutManager) parent.getLayoutManager()).findFirstVisibleItemPosition(); // 获取第一个item的view View itemView = parent.findViewHolderForAdapterPosition(position).itemView; int left = parent.getPaddingLeft(); int right = parent.getWidth() - parent.getPaddingRight(); int top = parent.getPaddingTop(); //TODO 当第二个是头部时, boolean isGroupHeader = adapter.isGroupHeader(position + 1); if (isGroupHeader) { int bottom = Math.min(groupHeaderHeight, itemView.getBottom() - parent.getPaddingTop()); c.drawRect(left, top, right, top + bottom, headPaint); String groupName = adapter.getGroupName(position); textPaint.getTextBounds(groupName, 0, groupName.length(), textRect); c.drawText(groupName, left + 20, top + bottom - groupHeaderHeight / 2f + textRect.height() / 2f, textPaint); }else { // 如果不是头部, 即普通的itemView，则当前的头部一直固定在顶部 c.drawRect(left, top, right, top + groupHeaderHeight, headPaint);; String groupName = adapter.getGroupName(position); textPaint.getTextBounds(groupName, 0, groupName.length(), textRect); c.drawText(groupName, left + 20, top + groupHeaderHeight / 2f + textRect.height() / 2f, textPaint); } } } @Override public 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 hahAdapter){ // parent.getAdapter()获取到当前RecyclerView的Adapter hahAdapter adapter = (hahAdapter) parent.getAdapter(); // 获取当前view的位置 int position = parent.getChildLayoutPosition(view); if (adapter.isGroupHeader(position)) { // 如果是头部，则预留更大的地方 outRect.set(0, groupHeaderHeight, 0, 0); }else { outRect.set(0, 4, 0, 0); } } } private int dp2px(Context context, float dpValue){ float scale = context.getResources().getDisplayMetrics().density; return (int) (dpValue * scale * .5f); } }