上篇文章介紹了利用ItemDecoration的getItemOffsets方法來(lái)設(shè)置每個(gè)ItemView的間隔背零，這篇文章繼續(xù)介紹下ItemDecoration剩下的兩個(gè)方法（未過(guò)時(shí)的）——onDraw和onDrawOver晃危。
在官方的開發(fā)文檔中有指出细睡，onDraw是在itemview繪制之前晰绎，onDrawOver是在itemview繪制之后彼硫。
All ItemDecorations are drawn in the order they were added, before the item views (in onDraw() and after the items (in onDrawOver(Canvas, RecyclerView, RecyclerView.State).
相信稍微了解過(guò)Android中View的繪制流程的都知道吃溅，View先會(huì)調(diào)用draw方法溶诞，在draw中又會(huì)調(diào)用onDraw方法。 而在RecyclerView的draw方法中會(huì)先通過(guò)super.draw() 調(diào)用父類也就是View的draw方法决侈，進(jìn)而繼續(xù)調(diào)用RecyclerView的OnDraw方法螺垢，ItemDecorations的onDraw方法就在此時(shí)會(huì)被調(diào)用，RecyclerView執(zhí)行完super.draw()之后赖歌，ItemDecorations的onDrawOver方法也被調(diào)用枉圃，這也就解釋了為什么說(shuō)onDraw會(huì)繪制在itemview之前，表現(xiàn)形式是在最底層（抽象的說(shuō)法庐冯，最底層應(yīng)該是background）孽亲，onDrawOver是在itemview繪制之后，表現(xiàn)形式在最上層展父。如果你覺(jué)得有點(diǎn)繞的話墨林，可以看看下面的部分源碼：
/**
* RecyclerView的draw方法
* @param c
*/
@Override
public void draw(Canvas c) {
// 調(diào)用父類也就是View的draw方法
super.draw(c);
final int count = mItemDecorations.size();
for (int i = 0; i < count; i++) {
// 執(zhí)行ItemDecorations的onDrawOver方法
mItemDecorations.get(i).onDrawOver(c, this, mState);
}
}
/**
* View的draw方法
* @param canvas
*/
@CallSuper
public void draw(Canvas canvas) {
....
// View會(huì)繼續(xù)調(diào)用onDraw
if (!dirtyOpaque) onDraw(canvas);
....
}
/**
* RecyclerView的onDraw方法
* @param c
*/
@Override
public void onDraw(Canvas c) {
super.onDraw(c);
final int count = mItemDecorations.size();
for (int i = 0; i < count; i++) {
// 執(zhí)行ItemDecorations的onDraw方法
mItemDecorations.get(i).onDraw(c, this, mState);
}
}
通過(guò)上面的分析，我們大概的了解了它的原理犯祠，接下來(lái)再來(lái)看看怎么用吧旭等。
一般情況下，一個(gè)列表中的item除了上篇文章中有間隔那種表現(xiàn)形式衡载，還有另一種帶分割線的表現(xiàn)是形式搔耕。如下圖所示:
之前在使用ListView的時(shí)候，可以通過(guò)在xml中定義中divider dividerHeight來(lái)實(shí)現(xiàn)，不過(guò)RecyclerView并沒(méi)有這些屬性弃榨，在未使用ItemDecoration時(shí)菩收，一般都是在Item的xml布局的底部設(shè)置一個(gè)View來(lái)充當(dāng)Divider，然后在ViewHolder根據(jù)position來(lái)控制鲸睛，這樣同樣顯得ViewHolder中有很一些看上去很不爽的代碼娜饵。
那么問(wèn)題來(lái)了，用ItemDecoration怎么實(shí)現(xiàn)呢官辈？其實(shí)很簡(jiǎn)單箱舞，只需要在上篇文章的基礎(chǔ)上，把間隔處繪制相應(yīng)顏色就行了拳亿，廢話不多說(shuō)晴股，我們來(lái)實(shí)現(xiàn)吧。
-
通過(guò)構(gòu)造方法傳入divider的高度(
orientation為HORIZONTAL時(shí)表示寬度)肺魁，還有顏色电湘。public DividerDecoration(int height, Context ctx) { this(height, Color.GRAY, ctx); } public DividerDecoration(int height, @ColorInt int color, Context ctx) { this.mDividerHeight = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, height, ctx.getResources().getDisplayMetrics()); mPaint = new Paint(); mPaint.setAntiAlias(true); mPaint.setColor(color); } -
設(shè)置每個(gè)Item間的間隔，留出空間畫
divider@Override public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) { RecyclerView.LayoutManager layoutManager = parent.getLayoutManager(); if (layoutManager instanceof LinearLayoutManager) { if (((LinearLayoutManager) layoutManager).getOrientation() == LinearLayoutManager.HORIZONTAL) { if (parent.getChildAdapterPosition(view) != parent.getAdapter().getItemCount() - 1) outRect.set(0, 0, (int) mDividerHeight, 0); } else { if (parent.getChildAdapterPosition(view) != parent.getAdapter().getItemCount() - 1) outRect.set(0, 0, 0, (int) mDividerHeight); } } } -
畫
divider@Override public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) { RecyclerView.LayoutManager manager = parent.getLayoutManager(); if (manager instanceof LinearLayoutManager) { if (((LinearLayoutManager) manager).getOrientation() == LinearLayoutManager.VERTICAL) { drawVertical(c, parent); } else { drawHorizontal(c, parent); } } } /** * 畫divider (orientation為vertical) * * @param c * @param parent */ private void drawVertical(Canvas c, RecyclerView parent) { // recyclerView是否設(shè)置了paddingLeft和paddingRight final int left = parent.getPaddingLeft(); final int right = parent.getWidth() - parent.getPaddingRight(); final int childCount = parent.getChildCount(); for (int i = 0; i < childCount; i++) { final View child = parent.getChildAt(i); final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child .getLayoutParams(); // divider的top 應(yīng)該是 item的bottom 加上 marginBottom 再加上 Y方向上的位移 final int top = child.getBottom() + params.bottomMargin + Math.round(ViewCompat.getTranslationY(child)); // divider的bottom就是top加上divider的高度了 final int bottom = (int) (top + mDividerHeight); c.drawRect(left, top, right, bottom, mPaint); } } /** * 畫divider (當(dāng)orientation為horizontal) * * @param c * @param parent */ private void drawHorizontal(Canvas c, RecyclerView parent) { // 和drawVertical差不多 left right 與 top和bottom對(duì)調(diào)一下 final int top = parent.getPaddingTop(); final int bottom = parent.getHeight() - parent.getPaddingBottom(); final int childCount = parent.getChildCount(); for (int i = 0; i < childCount; i++) { final View child = parent.getChildAt(i); final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child .getLayoutParams(); final int left = child.getRight() + params.rightMargin + Math.round(ViewCompat.getTranslationX(child)); final int right = (int) (left + mDividerHeight); c.drawRect(left, top, right, bottom, mPaint); } }
其實(shí)重寫onDraw或者onDrawOver都能實(shí)現(xiàn)上圖的效果鹅经，因?yàn)檫@里并沒(méi)有層級(jí)之分寂呛，當(dāng)有其他的需求時(shí)，我們只需要記住onDraw在繪制ItemView之前繪制瘾晃，onDrawOver會(huì)在繪制ItemView之后繪制昧谊，然后根據(jù)實(shí)際情況處理就行了。下一篇,我們?cè)儆?code>ItemDecoration實(shí)現(xiàn)更好玩的——stickHeader的效果酗捌，先來(lái)個(gè)圖呢诬，預(yù)告下。
