ItemDecoration
RecyclerView.ItemDecoration注釋是這樣寫(xiě)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>
*/
允許程序添加特殊的圖形和布局偏移量到適配器中指定的項(xiàng)目視圖，可以用于項(xiàng)目視圖之間繪制分割線折砸、高亮等等。還指出了在項(xiàng)目之前調(diào)用onDraw() 之后調(diào)用onDrawOver;
三個(gè)重要方法的重寫(xiě)
public class ItemDecoration extends RecyclerView.ItemDecoration{
//通過(guò)該方法愕贡，在Canvas上繪制內(nèi)容济舆，在繪制Item之前調(diào)用抵怎。（如果沒(méi)有通過(guò)getItemOffsets設(shè)置偏移的話，Item的內(nèi)容會(huì)將其覆蓋）
@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDraw(c, parent, state);
}
//通過(guò)該方法玫坛，在RecyclerView的Canvas上繪制內(nèi)容,在Item之后調(diào)用结笨。(畫(huà)的內(nèi)容會(huì)覆蓋在item的上層)
@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDrawOver(c, parent, state);
}
//設(shè)置偏移量
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
super.getItemOffsets(outRect, view, parent, state);
}
}
常見(jiàn)的懸浮粘性頭部
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;//是否是粘性頭部
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);//實(shí)現(xiàn)該抽象方法，獲得分類的頭部
@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 或者頭部的tag不等于上一個(gè)tag湿镀，繪制分組頭部
if(position == 0 || !tag.equals(getTag(tags.get(position-1)))){
drawGroupHeader(c,parent,view,tag);
}
}
}
/*
*該方法同樣也是用來(lái)繪制的炕吸，但是它在ItemDecoration的onDraw()方法和ItemView的onDraw()完成后才執(zhí)行。
* 所以其繪制的內(nèi)容會(huì)遮擋在RecyclerView上肠骆，因此我們可以在該方法中繪制分組索引列表中懸浮的GroupHeader算途，
* 也就是在列表頂部隨著列表滾動(dòng)切換的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();
//只處理線性垂直類型的列表
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();
}
源碼分析
RecyclerView的itemView的一些測(cè)量小細(xì)節(jié)，會(huì)通過(guò)getItemDecorInsetsForChild(child)調(diào)用裝飾物的getItemOffsets蚀腿，獲得區(qū)域大小嘴瓤，累加寬高數(shù)值，然后完成測(cè)量
public void measureChildWithMargins(View child, int widthUsed, int heightUsed) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);
// 累加當(dāng)前ItemDecoration 4個(gè)屬性值
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);
// 獲取getItemOffsets（） 中設(shè)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通過(guò)系統(tǒng)的API就可以實(shí)現(xiàn)拖拽排序或者刪除的效果
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);
實(shí)現(xiàn) ItemTouchHelper.Callback 接口后有三個(gè)方法需要重寫(xiě)
1.getMovementFlags:設(shè)置滑動(dòng)類型的標(biāo)記
需要設(shè)置兩種類型的 flag 莉钙，即 dragFlags 和 swipeFlags廓脆，拖拽標(biāo)記和滑動(dòng)標(biāo)記
最后需要調(diào)用 makeMovementFlags(dragFlags, swipeFlags) 方法來(lái)合成返回
2.onMove: 當(dāng)用戶拖拽列表某個(gè) item 時(shí)會(huì)回調(diào)。很明顯磁玉，拖拽排序的代碼應(yīng)該在這個(gè)方法中實(shí)現(xiàn)停忿。
3.onSwiped:當(dāng)用戶滑動(dòng)列表某個(gè) item 時(shí)會(huì)回調(diào)。所以側(cè)滑刪除的代碼應(yīng)該在這個(gè)方法中實(shí)現(xiàn)蚊伞。
ItemTouchHelper helper = new ItemTouchHelper(new ItemTouchHelper.Callback() {
//通過(guò)返回值來(lái)設(shè)置是否處理某次拖曳或者滑動(dòng)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) {
//滑動(dòng)刪除的回調(diào) 只要是linearlayoutmanager的時(shí)候
int adapterPosition = viewHolder.getAdapterPosition();
mAdapter.notifyItemRemoved(adapterPosition);
list.remove(adapterPosition);
//同時(shí)也不要忘了修改一下 getMovementFlags() 方法席赂，以便能夠相應(yīng)滑動(dòng)事件
//int swipeFlags = ItemTouchHelper.START | ItemTouchHelper.END;
}
//我們發(fā)現(xiàn)還有一些不完美的地方：比如當(dāng)用戶在拖拽排序的時(shí)候，可以改變當(dāng)前拖拽 item 的透明度时迫，這樣就可以和其他 item 區(qū)分開(kāi)來(lái)了颅停。
// 那么，我們需要去重寫(xiě) onSelectedChanged
@Override
public void onSelectedChanged(RecyclerView.ViewHolder viewHolder, int actionState) {
//當(dāng)長(zhǎng)按 item 剛開(kāi)始拖曳的時(shí)候調(diào)用
if(actionState!=ItemTouchHelper.ACTION_STATE_IDLE){//拖拽或刪除結(jié)束
viewHolder.itemView.setBackgroundColor(Color.YELLOW);
}
super.onSelectedChanged(viewHolder, actionState);
}
@Override
public void clearView(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
//當(dāng)完成拖曳手指松開(kāi)的時(shí)候調(diào)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這個(gè)輔助類掠拳，用于輔助RecyclerView在滾動(dòng)結(jié)束時(shí)將Item對(duì)齊到某個(gè)位置癞揉。特別是列表橫向滑動(dòng)時(shí)，很多時(shí)候不會(huì)讓列表滑到任意位置溺欧，而是會(huì)有一定的規(guī)則限制喊熟，這時(shí)候就可以通過(guò)SnapHelper來(lái)定義對(duì)齊規(guī)則了。
LinearSnapHelper&PagerSnapHelper是抽象類SnapHelper的具體實(shí)現(xiàn)姐刁。
LinearSnapHelper 可以滑動(dòng)多頁(yè)
PagerSnapHelper 每次只能滑動(dòng)一頁(yè)
代碼比較簡(jiǎn)單
LinearSnapHelper linearSnapHelper = new LinearSnapHelper();
linearSnapHelper.attachToRecyclerView(mRv)
源碼分析
SnapHelper是一個(gè)抽象類
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);
}
三個(gè)主要的抽象方法
findTargetSnapPosition
該方法會(huì)根據(jù)觸發(fā)Fling操作的速率（參數(shù)velocityX和參數(shù)velocityY）來(lái)找到RecyclerView需要滾動(dòng)到哪個(gè)位置芥牌，該位置對(duì)應(yīng)的ItemView就是那個(gè)需要進(jìn)行對(duì)齊的列表項(xiàng)。我們把這個(gè)位置稱為targetSnapPosition聂使，對(duì)應(yīng)的View稱為targetSnapView壁拉。如果找不到targetSnapPosition拐叉，就返回RecyclerView.NO_POSITION。
findSnapView
該方法會(huì)找到當(dāng)前l(fā)ayoutManager上最接近對(duì)齊位置的那個(gè)view扇商，該view稱為SanpView，對(duì)應(yīng)的position稱為SnapPosition宿礁。如果返回null案铺，就表示沒(méi)有需要對(duì)齊的View，也就不會(huì)做滾動(dòng)對(duì)齊調(diào)整梆靖。
calculateDistanceToFinalSnap
這個(gè)方法會(huì)計(jì)算第二個(gè)參數(shù)對(duì)應(yīng)的ItemView當(dāng)前的坐標(biāo)與需要對(duì)齊的坐標(biāo)之間的距離控汉。該方法返回一個(gè)大小為2的int數(shù)組，分別對(duì)應(yīng)x軸和y軸方向上的距離返吻。
SnapHelper實(shí)現(xiàn)了OnFlingListener這個(gè)接口姑子，該接口中的onFling()方法會(huì)在RecyclerView觸發(fā)Fling操作時(shí)調(diào)用。在onFling()方法中判斷當(dāng)前方向上的速率是否足夠做滾動(dòng)操作测僵，如果速率足夠大就調(diào)用snapFromFling()方法實(shí)現(xiàn)滾動(dòng)相關(guān)的邏輯街佑。在snapFromFling()方法中會(huì)創(chuàng)建一個(gè)SmoothScroller，并且根據(jù)速率計(jì)算出滾動(dòng)停止時(shí)的位置捍靠，將該位置設(shè)置給SmoothScroller并啟動(dòng)滾動(dòng)沐旨。而滾動(dòng)的操作都是由SmoothScroller全權(quán)負(fù)責(zé)，它可以控制Item的滾動(dòng)速度（剛開(kāi)始是勻速）榨婆，并且在滾動(dòng)到targetSnapView被layout時(shí)變換滾動(dòng)速度（轉(zhuǎn)換成減速）磁携，以讓滾動(dòng)效果更加真實(shí)。
讓你明明白白的使用RecyclerView——SnapHelper詳解
DiffUtil
它主要是為了配合 RecyclerView 使用良风，通過(guò)比對(duì)新谊迄、舊兩個(gè)數(shù)據(jù)集的差異，生成舊數(shù)據(jù)到新數(shù)據(jù)的最小變動(dòng)烟央，然后對(duì)有變動(dòng)的數(shù)據(jù)項(xiàng)统诺，進(jìn)行局部刷新。
需要關(guān)注的是兩個(gè)類DiffUtil.Callback,DiffUtil.DiffResult
基本的使用方法是
DiffUtil.DiffResult diffResult = DiffUtil.calculateDiff(mDiffCallback);
diffResult.dispatchUpdatesTo(mAdapter);
代碼上可已看出更多的是diffcallback的實(shí)現(xiàn),四個(gè)重寫(xiě)方法
@Override
public int getOldListSize() {
return 0;//舊數(shù)據(jù)集的size
}
@Override
public int getNewListSize() {
return 0;//新數(shù)據(jù)集的size
}
@Override
public boolean areItemsTheSame(int i, int i1) {
return false;//同一個(gè)Item
}
@Override
public boolean areContentsTheSame(int i, int i1) {
return false;//如果是通一個(gè)Item吊档，此方法用于判斷是否同一個(gè) Item 的內(nèi)容也相同篙议。
}
不過(guò),需要注意是子線程中去計(jì)算oldlist與newlist的差異
