ItemDecoration
RecyclerView.ItemDecoration注释是这样写的
/**
* An ItemDecoration allows the application to add a special drawing and layout offset
* to specific item views from the adapter's data set. This can be useful for drawing dividers
* between items, highlights, visual grouping boundaries and more.
*
* <p>All ItemDecorations are drawn in the order they were added, before the item
* views (in {@link ItemDecoration#onDraw(Canvas, RecyclerView, RecyclerView.State) onDraw()}
* and after the items (in {@link ItemDecoration#onDrawOver(Canvas, RecyclerView,
* RecyclerView.State)}.</p>
*/
允许程序添加特殊的图形和布局偏移量到适配器中指定的项目视图，可以用于项目视图之间绘制分割线、高亮等等。还指出了在项目之前调用onDraw() 之后调用onDrawOver;
三个重要方法的重写
public class ItemDecoration extends RecyclerView.ItemDecoration{
//通过该方法，在Canvas上绘制内容，在绘制Item之前调用。（如果没有通过getItemOffsets设置偏移的话，Item的内容会将其覆盖）
@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDraw(c, parent, state);
}
//通过该方法，在RecyclerView的Canvas上绘制内容,在Item之后调用。(画的内容会覆盖在item的上层)
@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDrawOver(c, parent, state);
}
//设置偏移量
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
super.getItemOffsets(outRect, view, parent, state);
}
}
常见的悬浮粘性头部
ItemDecoration.png
public abstract class GroupHeadItemDecoration<T> extends RecyclerView.ItemDecoration {
private Context mContext;
private List<T> tags;
private int groupHeaderHeight;
private int groupHeaderLeftPadding;
private Paint mPaint;
private TextPaint mTextPaint;
private boolean isStickHead = true;//是否是粘性头部
public GroupHeadItemDecoration(Context context, List<T> tags) {
mContext = context;
this.tags = tags;
groupHeaderHeight = dp2px(context, 20);
groupHeaderLeftPadding = dp2px(context,10);
mPaint = new Paint();
mPaint.setAntiAlias(true);
mPaint.setColor(Color.parseColor("#FFEEEEEE"));
mTextPaint = new TextPaint();
mTextPaint.setAntiAlias(true);
mTextPaint.setColor(Color.parseColor("#FF999999"));
mTextPaint.setTextSize(sp2px(context, 14));
}
public abstract String getTag(T t);//实现该抽象方法，获得分类的头部
@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDraw(c, parent, state);
for (int i = 0;i<parent.getChildCount();i++){
View view = parent.getChildAt(i);
int position = parent.getChildAdapterPosition(view);
String tag = getTag(tags.get(position));
//如果位置等于0 或者头部的tag不等于上一个tag，绘制分组头部
if(position == 0 || !tag.equals(getTag(tags.get(position-1)))){
drawGroupHeader(c,parent,view,tag);
}
}
}
/*
*该方法同样也是用来绘制的，但是它在ItemDecoration的onDraw()方法和ItemView的onDraw()完成后才执行。
* 所以其绘制的内容会遮挡在RecyclerView上，因此我们可以在该方法中绘制分组索引列表中悬浮的GroupHeader，
* 也就是在列表顶部随着列表滚动切换的GroupHeader
*/
@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDrawOver(c, parent, state);
if (!isStickHead) {
return;
}
if(tags.size()==0){
return;
}
int position = ((LinearLayoutManager)(parent.getLayoutManager())).findFirstVisibleItemPosition();
String tag = getTag(tags.get(position));
View view = parent.findViewHolderForAdapterPosition(position).itemView;
boolean flag = false;
if((position+1)<tags.size()&&!tag.equals(getTag(tags.get(position+1)))){
if (view.getBottom() <= groupHeaderHeight) {
c.save();
flag = true;
c.translate(0, view.getHeight() + view.getTop() - groupHeaderHeight);
}
}
drawSuspensionGroupHeader(c, parent, tag);
if(flag){
c.restore();
}
}
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
super.getItemOffsets(outRect, view, parent, state);
RecyclerView.LayoutManager manager = parent.getLayoutManager();
//只处理线性垂直类型的列表
if ((manager instanceof LinearLayoutManager)
&& LinearLayoutManager.VERTICAL != ((LinearLayoutManager) manager).getOrientation()) {
return;
}
if(tags == null || tags.size() == 0){
return;
}
int position = parent.getChildAdapterPosition(view);
if(position == 0||!getTag(tags.get(position)).equals(getTag(tags.get(position-1)))){
outRect.set(0,groupHeaderHeight,0,0);
}
private void drawGroupHeader(Canvas c, RecyclerView parent, View view, String tag){
int[] params = getGroupHeaderCoordinate(parent, view);
c.drawRect(params[0], params[1], params[2], params[3], mPaint);
int x = params[0] + groupHeaderLeftPadding;
int y = params[1] + (groupHeaderHeight + getTextHeight(mTextPaint, tag)) / 2;
c.drawText(tag,x,y,mTextPaint);
}
public int[] getGroupHeaderCoordinate(RecyclerView parent, View view) {
RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) view.getLayoutParams();
int left = parent.getPaddingLeft();
int right = parent.getWidth()-parent.getPaddingRight();
int bottom = view.getTop() - params.topMargin;
int top = bottom - groupHeaderHeight;
return new int[]{left,top,right,bottom};
}
public int[] getSuspensionGroupHeaderCoordinate(RecyclerView parent) {
int left = parent.getPaddingLeft();
int right = parent.getWidth() - parent.getPaddingRight();
int bottom = groupHeaderHeight;
int top = 0;
return new int[]{left, top, right, bottom};
}
private void drawSuspensionGroupHeader(Canvas c, RecyclerView parent, String tag) {
int[] params = getSuspensionGroupHeaderCoordinate(parent);
c.drawRect(params[0], params[1], params[2], params[3], mPaint);
int x = params[0] + groupHeaderLeftPadding;
int y = params[1] + (groupHeaderHeight + getTextHeight(mTextPaint, tag)) / 2;
c.drawText(tag, x, y, mTextPaint);
}
public static int getTextHeight(TextPaint textPaint, String text) {
Rect bounds = new Rect();
textPaint.getTextBounds(text, 0, text.length(), bounds);
return bounds.height();
}
源码分析
RecyclerView的itemView的一些测量小细节，会通过getItemDecorInsetsForChild(child)调用装饰物的getItemOffsets，获得区域大小，累加宽高数值，然后完成测量
public void measureChildWithMargins(View child, int widthUsed, int heightUsed) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);
// 累加当前ItemDecoration 4个属性值
widthUsed += insets.left + insets.right;
heightUsed += insets.top + insets.bottom;
final int widthSpec = getChildMeasureSpec(getWidth(), getWidthMode(),
getPaddingLeft() + getPaddingRight()
+ lp.leftMargin + lp.rightMargin + widthUsed, lp.width,
canScrollHorizontally());
final int heightSpec = getChildMeasureSpec(getHeight(), getHeightMode(),
getPaddingTop() + getPaddingBottom()
+ lp.topMargin + lp.bottomMargin + heightUsed, lp.height,
canScrollVertically());
if (shouldMeasureChild(child, widthSpec, heightSpec, lp)) {
child.measure(widthSpec, heightSpec);
}
}
Rect getItemDecorInsetsForChild(View child) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
if (!lp.mInsetsDirty) {
return lp.mDecorInsets;
}
if (mState.isPreLayout() && (lp.isItemChanged() || lp.isViewInvalid())) {
// changed/invalid items should not be updated until they are rebound.
return lp.mDecorInsets;
}
final Rect insets = lp.mDecorInsets;
insets.set(0, 0, 0, 0);
final int decorCount = mItemDecorations.size();
for (int i = 0; i < decorCount; i++) {
mTempRect.set(0, 0, 0, 0);
// 获取getItemOffsets（） 中设置的值
mItemDecorations.get(i).getItemOffsets(mTempRect, child, this, mState);
insets.left += mTempRect.left;
insets.top += mTempRect.top;
insets.right += mTempRect.right;
insets.bottom += mTempRect.bottom;
}
lp.mInsetsDirty = false;
return insets;
}
ItemTouchHelper
RecyclerView通过系统的API就可以实现拖拽排序或者删除的效果
ItemTouchHelper helper = new ItemTouchHelper(new ItemTouchHelper.Callback() {
@Override
public int getMovementFlags(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
return 0;
}
@Override
public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
return false;
}
@Override
public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction) {
}
});
helper.attachToRecyclerView(rv);
实现 ItemTouchHelper.Callback 接口后有三个方法需要重写
1.getMovementFlags:设置滑动类型的标记
需要设置两种类型的 flag ，即 dragFlags 和 swipeFlags，拖拽标记和滑动标记
最后需要调用 makeMovementFlags(dragFlags, swipeFlags) 方法来合成返回
2.onMove: 当用户拖拽列表某个 item 时会回调。很明显，拖拽排序的代码应该在这个方法中实现。
3.onSwiped:当用户滑动列表某个 item 时会回调。所以侧滑删除的代码应该在这个方法中实现。
ItemTouchHelper helper = new ItemTouchHelper(new ItemTouchHelper.Callback() {
//通过返回值来设置是否处理某次拖曳或者滑动事件
@Override
public int getMovementFlags(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
if(recyclerView.getLayoutManager() instanceof GridLayoutManager){
int dragFlags = ItemTouchHelper.UP | ItemTouchHelper.DOWN
| ItemTouchHelper.LEFT | ItemTouchHelper.RIGHT;
int swipeFlags = 0;
return makeMovementFlags(dragFlags,swipeFlags);
}else {
int dragFlags = ItemTouchHelper.UP|ItemTouchHelper.DOWN;
int swipeFlags = ItemTouchHelper.START | ItemTouchHelper.END;
return makeMovementFlags(dragFlags,swipeFlags);
}
}
@Override
public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
int fromposition = viewHolder.getAdapterPosition();
int toposition = target.getAdapterPosition();
if(fromposition < toposition){
for(int i = fromposition;i<toposition;i++){
Collections.swap(list,i,i+1);
}
}else {
for(int i = fromposition;i > toposition;i--){
Collections.swap(list,i,i-1);
}
}
mAdapter.notifyItemMoved(fromposition,toposition);
return true;
}
@Override
public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction) {
//滑动删除的回调 只要是linearlayoutmanager的时候
int adapterPosition = viewHolder.getAdapterPosition();
mAdapter.notifyItemRemoved(adapterPosition);
list.remove(adapterPosition);
//同时也不要忘了修改一下 getMovementFlags() 方法，以便能够相应滑动事件
//int swipeFlags = ItemTouchHelper.START | ItemTouchHelper.END;
}
//我们发现还有一些不完美的地方：比如当用户在拖拽排序的时候，可以改变当前拖拽 item 的透明度，这样就可以和其他 item 区分开来了。
// 那么，我们需要去重写 onSelectedChanged
@Override
public void onSelectedChanged(RecyclerView.ViewHolder viewHolder, int actionState) {
//当长按 item 刚开始拖曳的时候调用
if(actionState!=ItemTouchHelper.ACTION_STATE_IDLE){//拖拽或删除结束
viewHolder.itemView.setBackgroundColor(Color.YELLOW);
}
super.onSelectedChanged(viewHolder, actionState);
}
@Override
public void clearView(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
//当完成拖曳手指松开的时候调用
super.clearView(recyclerView, viewHolder);
viewHolder.itemView.setBackgroundColor(getResources().getColor(R.color.colorAccent));
}
}
SnapHelper
RecyclerView在24.2.0版本中新增了SnapHelper这个辅助类，用于辅助RecyclerView在滚动结束时将Item对齐到某个位置。特别是列表横向滑动时，很多时候不会让列表滑到任意位置，而是会有一定的规则限制，这时候就可以通过SnapHelper来定义对齐规则了。
LinearSnapHelper&PagerSnapHelper是抽象类SnapHelper的具体实现。
LinearSnapHelper 可以滑动多页
PagerSnapHelper 每次只能滑动一页
代码比较简单
LinearSnapHelper linearSnapHelper = new LinearSnapHelper();
linearSnapHelper.attachToRecyclerView(mRv)
源码分析
SnapHelper是一个抽象类
public abstract class SnapHelper extends RecyclerView.OnFlingListener{
/**
* Override this method to snap to a particular point within the target view or the container
* view on any axis.
* <p>
* This method is called when the {@link SnapHelper} has intercepted a fling and it needs
* to know the exact distance required to scroll by in order to snap to the target view.
*
* @param layoutManager the {@link RecyclerView.LayoutManager} associated with the attached
* {@link RecyclerView}
* @param targetView the target view that is chosen as the view to snap
*
* @return the output coordinates the put the result into. out[0] is the distance
* on horizontal axis and out[1] is the distance on vertical axis.
*/
@SuppressWarnings("WeakerAccess")
@Nullable
public abstract int[] calculateDistanceToFinalSnap(@NonNull LayoutManager layoutManager,
@NonNull View targetView);
/**
* Override this method to provide a particular target view for snapping.
* <p>
* This method is called when the {@link SnapHelper} is ready to start snapping and requires
* a target view to snap to. It will be explicitly called when the scroll state becomes idle
* after a scroll. It will also be called when the {@link SnapHelper} is preparing to snap
* after a fling and requires a reference view from the current set of child views.
* <p>
* If this method returns {@code null}, SnapHelper will not snap to any view.
*
* @param layoutManager the {@link RecyclerView.LayoutManager} associated with the attached
* {@link RecyclerView}
*
* @return the target view to which to snap on fling or end of scroll
*/
@SuppressWarnings("WeakerAccess")
@Nullable
public abstract View findSnapView(LayoutManager layoutManager);
/**
* Override to provide a particular adapter target position for snapping.
*
* @param layoutManager the {@link RecyclerView.LayoutManager} associated with the attached
* {@link RecyclerView}
* @param velocityX fling velocity on the horizontal axis
* @param velocityY fling velocity on the vertical axis
*
* @return the target adapter position to you want to snap or {@link RecyclerView#NO_POSITION}
* if no snapping should happen
*/
public abstract int findTargetSnapPosition(LayoutManager layoutManager, int velocityX,
int velocityY);
}
三个主要的抽象方法
findTargetSnapPosition
该方法会根据触发Fling操作的速率（参数velocityX和参数velocityY）来找到RecyclerView需要滚动到哪个位置，该位置对应的ItemView就是那个需要进行对齐的列表项。我们把这个位置称为targetSnapPosition，对应的View称为targetSnapView。如果找不到targetSnapPosition，就返回RecyclerView.NO_POSITION。
findSnapView
该方法会找到当前layoutManager上最接近对齐位置的那个view，该view称为SanpView，对应的position称为SnapPosition。如果返回null，就表示没有需要对齐的View，也就不会做滚动对齐调整。
calculateDistanceToFinalSnap
这个方法会计算第二个参数对应的ItemView当前的坐标与需要对齐的坐标之间的距离。该方法返回一个大小为2的int数组，分别对应x轴和y轴方向上的距离。
SnapHelper实现了OnFlingListener这个接口，该接口中的onFling()方法会在RecyclerView触发Fling操作时调用。在onFling()方法中判断当前方向上的速率是否足够做滚动操作，如果速率足够大就调用snapFromFling()方法实现滚动相关的逻辑。在snapFromFling()方法中会创建一个SmoothScroller，并且根据速率计算出滚动停止时的位置，将该位置设置给SmoothScroller并启动滚动。而滚动的操作都是由SmoothScroller全权负责，它可以控制Item的滚动速度（刚开始是匀速），并且在滚动到targetSnapView被layout时变换滚动速度（转换成减速），以让滚动效果更加真实。
让你明明白白的使用RecyclerView——SnapHelper详解
DiffUtil
它主要是为了配合 RecyclerView 使用，通过比对新、旧两个数据集的差异，生成旧数据到新数据的最小变动，然后对有变动的数据项，进行局部刷新。
需要关注的是两个类DiffUtil.Callback,DiffUtil.DiffResult
基本的使用方法是
DiffUtil.DiffResult diffResult = DiffUtil.calculateDiff(mDiffCallback);
diffResult.dispatchUpdatesTo(mAdapter);
代码上可已看出更多的是diffcallback的实现,四个重写方法
@Override
public int getOldListSize() {
return 0;//旧数据集的size
}
@Override
public int getNewListSize() {
return 0;//新数据集的size
}
@Override
public boolean areItemsTheSame(int i, int i1) {
return false;//同一个Item
}
@Override
public boolean areContentsTheSame(int i, int i1) {
return false;//如果是通一个Item，此方法用于判断是否同一个 Item 的内容也相同。
}
不过,需要注意是子线程中去计算oldlist与newlist的差异
