ItemDecoration 是 RecyclerView 组件的一个非常有用的功能，用于添加自定义的装饰项（如分隔线、边距、背景等）到 RecyclerView 的每个 item 之间或周围。
recyclerView.addItemDecoration()ItemDecoration主要的三个方法：
- onDraw(Canvas c, RecyclerView parent, RecyclerView.State state): 在 RecyclerView 的 canvas 上绘制自定义的装饰项，通常用于绘制分隔线或背景。
- onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state): 与 onDraw 类似，但绘制的内容会出现在 item 的视图之上。在 item 视图上方绘制内容（如高亮或选择效果），可以使用这个方法。
- getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state): 设置 item 的边距。outRect 参数是一个 Rect 对象，可以设置它的 left、top、right 和 bottom 属性来定义 item 的额外空间。这些额外的空间会用于绘制分隔线或边距。
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- 图1：代表了getItemOffsets()，可以实现类似padding的效果。
- 图2：代表了onDraw()，可以实现类似绘制背景的效果，内容在上面。
- 图3：代表了onDrawOver()，可以绘制在内容的上面，覆盖内容。
分割线
实现分割线效果需要 getItemOffsets()和 onDraw()2个方法，首先用 getItemOffsets给item下方空出一定高度的空间（例子中是1dp），然后用onDraw绘制这个空间。
public class SimpleDividerDecoration extends RecyclerView.ItemDecoration {
private int dividerHeight;
private Paint dividerPaint;
public SimpleDividerDecoration(Context context) {
dividerPaint = new Paint();
dividerPaint.setColor(context.getResources().getColor(R.color.colorAccent));
dividerHeight = context.getResources().getDimensionPixelSize(R.dimen.divider_height);
}
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
super.getItemOffsets(outRect, view, parent, state);
outRect.bottom = dividerHeight;
}
@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
int childCount = parent.getChildCount();
int left = parent.getPaddingLeft();
int right = parent.getWidth() - parent.getPaddingRight();
for (int i = 0; i < childCount - 1; i++) {
View view = parent.getChildAt(i);
float top = view.getBottom();
float bottom = view.getBottom() + dividerHeight;
c.drawRect(left, top, right, bottom, dividerPaint);
}
}
}
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标签
标签都是覆盖在内容之上的，可以用onDrawOver()来实现，这里简单实现一个颜色标签。
public class LeftAndRightTagDecoration extends RecyclerView.ItemDecoration {
private int tagWidth;
private Paint leftPaint;
private Paint rightPaint;
public LeftAndRightTagDecoration(Context context) {
leftPaint = new Paint();
leftPaint.setColor(context.getResources().getColor(R.color.colorAccent));
rightPaint = new Paint();
rightPaint.setColor(context.getResources().getColor(R.color.colorPrimary));
tagWidth = context.getResources().getDimensionPixelSize(R.dimen.tag_width);
}
@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDrawOver(c, parent, state);
int childCount = parent.getChildCount();
for (int i = 0; i < childCount; i++) {
View child = parent.getChildAt(i);
int pos = parent.getChildAdapterPosition(child);
boolean isLeft = pos % 2 == 0;
if (isLeft) {
float left = child.getLeft();
float right = left + tagWidth;
float top = child.getTop();
float bottom = child.getBottom();
c.drawRect(left, top, right, bottom, leftPaint);
} else {
float right = child.getRight();
float left = right - tagWidth;
float top = child.getTop();
float bottom = child.getBottom();
c.drawRect(left, top, right, bottom, rightPaint);
}
}
}
}
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ItemDecoration组合
ItemDecoration是可以叠加的，可以将多个效果通过addItemDecoration方法叠加，将上面两种效果叠加。
recyclerView.addItemDecoration(new LeftAndRightTagDecoration(this));
recyclerView.addItemDecoration(new SimpleDividerDecoration(this));
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Section分组
定义接口用来进行数据分组和获取首字母，重写getItemOffsets()和onDraw()方法，并根据数据进行分组处理。
public interface DecorationCallback {
long getGroupId(int position);
String getGroupFirstLine(int position);
}public class SectionDecoration extends RecyclerView.ItemDecoration {
private static final String TAG = "SectionDecoration";
private DecorationCallback callback;
private TextPaint textPaint;
private Paint paint;
private int topGap;
private Paint.FontMetrics fontMetrics;
public SectionDecoration(Context context, DecorationCallback decorationCallback) {
Resources res = context.getResources();
this.callback = decorationCallback;
paint = new Paint();
paint.setColor(res.getColor(R.color.colorAccent));
textPaint = new TextPaint();
textPaint.setTypeface(Typeface.DEFAULT_BOLD);
textPaint.setAntiAlias(true);
textPaint.setTextSize(80);
textPaint.setColor(Color.BLACK);
textPaint.getFontMetrics(fontMetrics);
textPaint.setTextAlign(Paint.Align.LEFT);
fontMetrics = new Paint.FontMetrics();
topGap = res.getDimensionPixelSize(R.dimen.sectioned_top);//32dp
}
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
super.getItemOffsets(outRect, view, parent, state);
int pos = parent.getChildAdapterPosition(view);
Log.i(TAG, "getItemOffsets：" + pos);
long groupId = callback.getGroupId(pos);
if (groupId < 0) return;
if (pos == 0 || isFirstInGroup(pos)) {//同组的第一个才添加padding
outRect.top = topGap;
} else {
outRect.top = 0;
}
}
@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDraw(c, parent, state);
int left = parent.getPaddingLeft();
int right = parent.getWidth() - parent.getPaddingRight();
int childCount = parent.getChildCount();
for (int i = 0; i < childCount; i++) {
View view = parent.getChildAt(i);
int position = parent.getChildAdapterPosition(view);
long groupId = callback.getGroupId(position);
if (groupId < 0) return;
String textLine = callback.getGroupFirstLine(position).toUpperCase();
if (position == 0 || isFirstInGroup(position)) {
float top = view.getTop() - topGap;
float bottom = view.getTop();
c.drawRect(left, top, right, bottom, paint);//绘制红色矩形
c.drawText(textLine, left, bottom, textPaint);//绘制文本
}
}
}
private boolean isFirstInGroup(int pos) {
if (pos == 0) {
return true;
} else {
long prevGroupId = callback.getGroupId(pos - 1);
long groupId = callback.getGroupId(pos);
return prevGroupId != groupId;
}
}
public interface DecorationCallback {
long getGroupId(int position);
String getGroupFirstLine(int position);
}
}recyclerView.addItemDecoration(new SectionDecoration(this, new SectionDecoration.DecorationCallback() {
@Override
public long getGroupId(int position) {
return Character.toUpperCase(dataList.get(position).getName().charAt(0));
}
@Override
public String getGroupFirstLine(int position) {
return dataList.get(position).getName().substring(0, 1).toUpperCase();
}
}));StickyHeader
头部吸顶效果，header不动肯定是要绘制item内容之上，需要重写onDrawOver()方法，其和Section实现一样。
public class PinnedSectionDecoration extends RecyclerView.ItemDecoration {
private static final String TAG = "PinnedSectionDecoration";
private DecorationCallback callback;
private TextPaint textPaint;
private Paint paint;
private int topGap;
private Paint.FontMetrics fontMetrics;
public PinnedSectionDecoration(Context context, DecorationCallback decorationCallback) {
Resources res = context.getResources();
this.callback = decorationCallback;
paint = new Paint();
paint.setColor(res.getColor(R.color.colorAccent));
textPaint = new TextPaint();
textPaint.setTypeface(Typeface.DEFAULT_BOLD);
textPaint.setAntiAlias(true);
textPaint.setTextSize(80);
textPaint.setColor(Color.BLACK);
textPaint.getFontMetrics(fontMetrics);
textPaint.setTextAlign(Paint.Align.LEFT);
fontMetrics = new Paint.FontMetrics();
topGap = res.getDimensionPixelSize(R.dimen.sectioned_top);
}
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
super.getItemOffsets(outRect, view, parent, state);
int pos = parent.getChildAdapterPosition(view);
long groupId = callback.getGroupId(pos);
if (groupId < 0) return;
if (pos == 0 || isFirstInGroup(pos)) {
outRect.top = topGap;
} else {
outRect.top = 0;
}
}
@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDrawOver(c, parent, state);
int itemCount = state.getItemCount();
int childCount = parent.getChildCount();
int left = parent.getPaddingLeft();
int right = parent.getWidth() - parent.getPaddingRight();
float lineHeight = textPaint.getTextSize() + fontMetrics.descent;
long preGroupId, groupId = -1;
for (int i = 0; i < childCount; i++) {
View view = parent.getChildAt(i);
int position = parent.getChildAdapterPosition(view);
preGroupId = groupId;
groupId = callback.getGroupId(position);
if (groupId < 0 || groupId == preGroupId) continue;
String textLine = callback.getGroupFirstLine(position).toUpperCase();
if (TextUtils.isEmpty(textLine)) continue;
int viewBottom = view.getBottom();
float textY = Math.max(topGap, view.getTop());
if (position + 1 < itemCount) { //下一个和当前不一样移动当前
long nextGroupId = callback.getGroupId(position + 1);
if (nextGroupId != groupId && viewBottom < textY ) {//组内最后一个view进入了header
textY = viewBottom;
}
}
c.drawRect(left, textY - topGap, right, textY, paint);
c.drawText(textLine, left, textY, textPaint);
}
}
}
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