自从RecyclerView出来之后就得到广泛的应用，但是由于其使用和定制化比较复杂，所以很多时候都停留在使用阶段上。这片文章简单讲述一个自定义一个简单的LayoutManager.
翻译
/**
* A <code>LayoutManager</code> is responsible for measuring and positioning item views
* within a <code>RecyclerView</code> as well as determining the policy for when to recycle
* item views that are no longer visible to the user. By changing the <code>LayoutManager</code>
* a <code>RecyclerView</code> can be used to implement a standard vertically scrolling list,
* a uniform grid, staggered grids, horizontally scrolling collections and more. Several stock
* layout managers are provided for general use.
* <p/>
* If the LayoutManager specifies a default constructor or one with the signature
* ({@link Context}, {@link AttributeSet}, {@code int}, {@code int}), RecyclerView will
* instantiate and set the LayoutManager when being inflated. Most used properties can
* be then obtained from {@link #getProperties(Context, AttributeSet, int, int)}. In case
* a LayoutManager specifies both constructors, the non-default constructor will take
* precedence.
*
*/
翻译：
一个LayoutManager负责测量和定位RecyclerView的item views, 同时需要处理在item views不再可见时的回收工作。通过设置不同的LayoutManager, RecyclerView可以用来实现标准的纵向滑动列表，通用的网格、交错型、横向滑动等效果。这几种LayoutManager都已经提供了。
如果一个LayoutManager设定了默认的构造器或者一个参数分别为Context, AttributeSet, int, int的构造器，RecyclerView可以通过xml来进行实例化该LayoutManager, 不用在代码中设置。可以在LayoutManager构造器中使用方法getProperties(Context, AttributeSet, int, int)来获取自定义的属性。如果两种构造器都制定了，那么优先使用默认的构造器。
功能
从官方文档中可以看出，一个LayoutManager的工作是：测量、定位和回收RecyclerView的item views. 简而言之，LayoutManager实际上就是让item views合理的显示在RecyclerView, 同时需要进行回收。这些工作和定义一个使用了回收机制的ViewGroup非常相似，就像ListView一样，不过它并不考虑数据的绑定。
定义一个有View回收机制的ViewGroup需要处理的工作：
- 测量（measure）
- 布局（定位, layout）
- 考虑滑动
- 回收
View
这里我们先讨论一下在LayoutManager需要怎么处理这些工作。
- 测量：获取到
View后，需要对其进行测量，因为复用的原因，所以这一步必须做 - 布局：对获取到的
View根据要求放置在界面上，同时需要考虑滑动 - 滑动：touch的处理是由
RecyclerView来完成的，LayoutManager需要处理的是决定滑动方向和修正滑动距离 - 回收
View：关于如何回收View都是放在Recycler中的，LayoutManager需要告诉那些View需要被回收
接下来说明一下LayoutManager做这些工作可能使用到的方法。
测量
获取到View的过程完全是交由Recycler来完成，LinearLayoutManager并不关心它是重新构造，还是从缓存中获取的，然后对其测量即可。不过另外需要注意的是View的margin尺寸。
下面是测量方法和另外两个计算一个child view布局时所占用的空间尺寸的方法，考虑到了margin:
/**
* Measure a child view using standard measurement policy, taking the padding
* of the parent RecyclerView, any added item decorations and the child margins
* into account.
*
* <p>If the RecyclerView can be scrolled in either dimension the caller may
* pass 0 as the widthUsed or heightUsed parameters as they will be irrelevant.</p>
*
* @param child Child view to measure
* @param widthUsed Width in pixels currently consumed by other views, if relevant
* @param heightUsed Height in pixels currently consumed by other views, if relevant
*/
public void measureChildWithMargins(View child, int widthUsed, int heightUsed) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);
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);
}
}
/**
* 获取 child view 横向上需要占用的空间，margin计算在内
*
* @param view item view
* @return child view 横向占用的空间
*/
private int getDecoratedMeasurementHorizontal(View view) {
final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams)
view.getLayoutParams();
return getDecoratedMeasuredWidth(view) + params.leftMargin
+ params.rightMargin;
}
/**
* 获取 child view 纵向上需要占用的空间，margin计算在内
*
* @param view item view
* @return child view 纵向占用的空间
*/
private int getDecoratedMeasurementVertical(View view) {
final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) view.getLayoutParams();
return getDecoratedMeasuredHeight(view) + params.topMargin + params.bottomMargin;
}
因为ItemDecoration的存在，所以获取尺寸时使用的是getDecoratedMeasuredWidth(View)和getDecoratedMeasuredHeight(View), 将ItemDecoration考虑在内了。
布局
布局是LayoutManager最基本的功能，同时有比较复杂的尺寸结算。如果对计算不那么排斥或有比较好的计算方法的话，实际上也很简单。
下面是LayoutManager对一个child view进行布局（定位）的方法：
/**
* Lay out the given child view within the RecyclerView using coordinates that
* include any current {@link ItemDecoration ItemDecorations} and margins.
*
* <p>LayoutManagers should prefer working in sizes and coordinates that include
* item decoration insets whenever possible. This allows the LayoutManager to effectively
* ignore decoration insets within measurement and layout code. See the following
* methods:</p>
* <ul>
* <li>{@link #layoutDecorated(View, int, int, int, int)}</li>
* <li>{@link #measureChild(View, int, int)}</li>
* <li>{@link #measureChildWithMargins(View, int, int)}</li>
* <li>{@link #getDecoratedLeft(View)}</li>
* <li>{@link #getDecoratedTop(View)}</li>
* <li>{@link #getDecoratedRight(View)}</li>
* <li>{@link #getDecoratedBottom(View)}</li>
* <li>{@link #getDecoratedMeasuredWidth(View)}</li>
* <li>{@link #getDecoratedMeasuredHeight(View)}</li>
* </ul>
*
* @param child Child to lay out
* @param left Left edge, with item decoration insets and left margin included
* @param top Top edge, with item decoration insets and top margin included
* @param right Right edge, with item decoration insets and right margin included
* @param bottom Bottom edge, with item decoration insets and bottom margin included
*
* @see View#layout(int, int, int, int)
* @see #layoutDecorated(View, int, int, int, int)
*/
public void layoutDecoratedWithMargins(View child, int left, int top, int right,
int bottom) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final Rect insets = lp.mDecorInsets;
child.layout(left + insets.left + lp.leftMargin, top + insets.top + lp.topMargin,
right - insets.right - lp.rightMargin,
bottom - insets.bottom - lp.bottomMargin);
}
注释翻译：
在RecyclerView中定位一个child view, 考虑了ItemDecoration和margin的影响。
LayoutManager应该尽可能地更加关注包含ItemDecoration嵌入时的尺寸和定位。这个方法允许LayoutManager不用考虑受ItemDecoration影响的尺寸和布局代码，获取的直接是最终结果。
参考的方法：忽略。
参数
- child: child view
- left, 左侧边距，包括
ItemDecoration和margin - top, 上边距
- right, 右边距
- bottom, 下边距
可以看出，这个方法布局时需要考虑ItemDecoration的影响和margin, 而进行测量的时候其实已经包括了ItemDecoration的影响和margin，我们就不再做过多的计算工作。
滑动
滑动有两个方向，一般允许一个方向的滑动，实际上，也可以同时允许两个方向上的滑动，不过一般没有这个需求。
下面是考虑滑动时可能需要处理的几个方法：
/**
* Scroll horizontally by dx pixels in screen coordinates and return the distance traveled.
* The default implementation does nothing and returns 0.
*
* @param dx distance to scroll by in pixels. X increases as scroll position
* approaches the right.
* @param recycler Recycler to use for fetching potentially cached views for a
* position
* @param state Transient state of RecyclerView
* @return The actual distance scrolled. The return value will be negative if dx was
* negative and scrolling proceeeded in that direction.
* <code>Math.abs(result)</code> may be less than dx if a boundary was reached.
*/
public int scrollHorizontallyBy(int dx, Recycler recycler, State state) {
return 0;
}
/**
* Scroll vertically by dy pixels in screen coordinates and return the distance traveled.
* The default implementation does nothing and returns 0.
*
* @param dy distance to scroll in pixels. Y increases as scroll position
* approaches the bottom.
* @param recycler Recycler to use for fetching potentially cached views for a
* position
* @param state Transient state of RecyclerView
* @return The actual distance scrolled. The return value will be negative if dy was
* negative and scrolling proceeeded in that direction.
* <code>Math.abs(result)</code> may be less than dy if a boundary was reached.
*/
public int scrollVerticallyBy(int dy, Recycler recycler, State state) {
return 0;
}
/**
* Query if horizontal scrolling is currently supported. The default implementation
* returns false.
*
* @return True if this LayoutManager can scroll the current contents horizontally
*/
public boolean canScrollHorizontally() {
return false;
}
/**
* Query if vertical scrolling is currently supported. The default implementation
* returns false.
*
* @return True if this LayoutManager can scroll the current contents vertically
*/
public boolean canScrollVertically() {
return false;
}
注释翻译：
- scrollHorizontallyBy(int, Recycler, State): 根据屏幕横向滑动的距离dx计算实际滑动的距离。默认不进行滑动，返回值为0
- scrollVerticallyBy(int, Recycler, State): 根据屏幕纵向滑动的距离dy计算实际滑动的距离。默认不进行滑动，返回值为0
- canScrollHorizontally(): 查询目前是否支持横向滑动。默认返回false
- canScrollVertically(): 查询目前是否支持横向滑动。默认返回false
方法canScrollHorizontally()和canScrollVertically()是判断是否处理RecyclerView上的滑动，这个根据需求自行实现。真正处理滑动的是在scrollHorizontallyBy(int, Recycler, State)和scrollVerticallyBy(int, Recycler, State)中，手指滑动距离作为参数传入这两个方法中，而根据逻辑，处理滑动，最后返回处理后的滑动距离，一般情况下，如果没有到达边界，那么处理后的滑动距离和实际滑动距离是一样的，到达边界时对滑动距离进行修正。
另外需要注意的是在scrollHorizontallyBy(int, Recycler, State)和scrollVerticallyBy(int, Recycler, State)中需要进行滑动时的布局问题，即滑动一定距离之后，实际上是重新进行了布局的。
回收View
回收View原因和原理是，在根据逻辑布局View时，它超出了用户的可视范围，所以为了性能考虑，我们应该及时进行回收，避免耗费过多的内存。而通常的处理方法是，在用户的可视范围上进行布局，查看超出边界的child view, 然后进行回收，当然，及时发现不可能显示在界面上的child view, 在布局过程中就可以决定是否需要对余下的child view进行布局。
概念
在RecyclerView的概念中，有几种对回收的概念
- attach/detach: 将item view添加到
RecyclerView中，和add/remove不同的是不会触发layout - scrap: 标识一个item view表示其已经从
RecyclerView中移除，但是实际上还是在RecyclerView中 - recycle: 表示一个没有parent的
View的回收处理工作，可以销毁，也可以用来复用
/**
* Temporarily detach and scrap all currently attached child views. Views will be scrapped
* into the given Recycler. The Recycler may prefer to reuse scrap views before
* other views that were previously recycled.
*
* @param recycler Recycler to scrap views into
*/
public void detachAndScrapAttachedViews(Recycler recycler) {
final int childCount = getChildCount();
for (int i = childCount - 1; i >= 0; i--) {
final View v = getChildAt(i);
scrapOrRecycleView(recycler, i, v);
}
}
注释翻译：
临时detach和scrap所有的child view. 这些view将被放置到Recycler中处理。相对于之前回收的那些view, Recycler会首先使用利用这些views.
除了回收所有的child view之外，还有很多其他的处理child view的方法，全部列举在下面，就不再一一翻译，可能会用到的一个方法removeAndRecycleView(View, Recycler), 表示移除一个child view, 并交由指定的Recycler处理。
/**
* Temporarily detach a child view.
*
* <p>LayoutManagers may want to perform a lightweight detach operation to rearrange
* views currently attached to the RecyclerView. Generally LayoutManager implementations
* will want to use {@link #detachAndScrapView(android.view.View, RecyclerView.Recycler)}
* so that the detached view may be rebound and reused.</p>
*
* <p>If a LayoutManager uses this method to detach a view, it <em>must</em>
* {@link #attachView(android.view.View, int, RecyclerView.LayoutParams) reattach}
* or {@link #removeDetachedView(android.view.View) fully remove} the detached view
* before the LayoutManager entry point method called by RecyclerView returns.</p>
*
* @param child Child to detach
*/
public void detachView(View child) {
final int ind = mChildHelper.indexOfChild(child);
if (ind >= 0) {
detachViewInternal(ind, child);
}
}
/**
* Temporarily detach a child view.
*
* <p>LayoutManagers may want to perform a lightweight detach operation to rearrange
* views currently attached to the RecyclerView. Generally LayoutManager implementations
* will want to use {@link #detachAndScrapView(android.view.View, RecyclerView.Recycler)}
* so that the detached view may be rebound and reused.</p>
*
* <p>If a LayoutManager uses this method to detach a view, it <em>must</em>
* {@link #attachView(android.view.View, int, RecyclerView.LayoutParams) reattach}
* or {@link #removeDetachedView(android.view.View) fully remove} the detached view
* before the LayoutManager entry point method called by RecyclerView returns.</p>
*
* @param index Index of the child to detach
*/
public void detachViewAt(int index) {
detachViewInternal(index, getChildAt(index));
}
private void detachViewInternal(int index, View view) {
if (DISPATCH_TEMP_DETACH) {
ViewCompat.dispatchStartTemporaryDetach(view);
}
mChildHelper.detachViewFromParent(index);
}
/**
* Reattach a previously {@link #detachView(android.view.View) detached} view.
* This method should not be used to reattach views that were previously
* {@link #detachAndScrapView(android.view.View, RecyclerView.Recycler)} scrapped}.
*
* @param child Child to reattach
* @param index Intended child index for child
* @param lp LayoutParams for child
*/
public void attachView(View child, int index, LayoutParams lp) {
ViewHolder vh = getChildViewHolderInt(child);
if (vh.isRemoved()) {
mRecyclerView.mViewInfoStore.addToDisappearedInLayout(vh);
} else {
mRecyclerView.mViewInfoStore.removeFromDisappearedInLayout(vh);
}
mChildHelper.attachViewToParent(child, index, lp, vh.isRemoved());
if (DISPATCH_TEMP_DETACH) {
ViewCompat.dispatchFinishTemporaryDetach(child);
}
}
/**
* Reattach a previously {@link #detachView(android.view.View) detached} view.
* This method should not be used to reattach views that were previously
* {@link #detachAndScrapView(android.view.View, RecyclerView.Recycler)} scrapped}.
*
* @param child Child to reattach
* @param index Intended child index for child
*/
public void attachView(View child, int index) {
attachView(child, index, (LayoutParams) child.getLayoutParams());
}
/**
* Reattach a previously {@link #detachView(android.view.View) detached} view.
* This method should not be used to reattach views that were previously
* {@link #detachAndScrapView(android.view.View, RecyclerView.Recycler)} scrapped}.
*
* @param child Child to reattach
*/
public void attachView(View child) {
attachView(child, -1);
}
/**
* Finish removing a view that was previously temporarily
* {@link #detachView(android.view.View) detached}.
*
* @param child Detached child to remove
*/
public void removeDetachedView(View child) {
mRecyclerView.removeDetachedView(child, false);
}
/**
* Moves a View from one position to another.
*
* @param fromIndex The View's initial index
* @param toIndex The View's target index
*/
public void moveView(int fromIndex, int toIndex) {
View view = getChildAt(fromIndex);
if (view == null) {
throw new IllegalArgumentException("Cannot move a child from non-existing index:"
+ fromIndex);
}
detachViewAt(fromIndex);
attachView(view, toIndex);
}
/**
* Detach a child view and add it to a {@link Recycler Recycler's} scrap heap.
*
* <p>Scrapping a view allows it to be rebound and reused to show updated or
* different data.</p>
*
* @param child Child to detach and scrap
* @param recycler Recycler to deposit the new scrap view into
*/
public void detachAndScrapView(View child, Recycler recycler) {
int index = mChildHelper.indexOfChild(child);
scrapOrRecycleView(recycler, index, child);
}
/**
* Detach a child view and add it to a {@link Recycler Recycler's} scrap heap.
*
* <p>Scrapping a view allows it to be rebound and reused to show updated or
* different data.</p>
*
* @param index Index of child to detach and scrap
* @param recycler Recycler to deposit the new scrap view into
*/
public void detachAndScrapViewAt(int index, Recycler recycler) {
final View child = getChildAt(index);
scrapOrRecycleView(recycler, index, child);
}
/**
* Remove a child view and recycle it using the given Recycler.
*
* @param child Child to remove and recycle
* @param recycler Recycler to use to recycle child
*/
public void removeAndRecycleView(View child, Recycler recycler) {
removeView(child);
recycler.recycleView(child);
}
/**
* Remove a child view and recycle it using the given Recycler.
*
* @param index Index of child to remove and recycle
* @param recycler Recycler to use to recycle child
*/
public void removeAndRecycleViewAt(int index, Recycler recycler) {
final View view = getChildAt(index);
removeViewAt(index);
recycler.recycleView(view);
}
实现FlowLayoutManger
看过很多例子，在学习LayoutManager的时候通常都是以FlowLayoutManager来作为实践的，因为我们对其比较了解，而且也没有更好的示例来作为练习。
FlowLayoutManager的功能
- 每个item view从第一行开始进行横向排列，当一行不足显示下一个item的时候，将其布局在下一行
- 支持纵向滑动（也可以支持横向滑动，不过目前不在考虑范围内）
- 每个item高度不一致的考虑
构造器
根据前面的阐述，我们实现两个构造器，分别用于代码构造和xml中使用。
public FlowLayoutManager() {
setAutoMeasureEnabled(true);
}
public FlowLayoutManager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
setAutoMeasureEnabled(true);
}
注意setAutoMeasureEnabled(boolean)是表明RecyclerView的布局是否交由LayoutManager进行处理，否则应该重写LayoutManager#onMeasure(int, int)来自定义测量的实现。一般传true, 除非有特殊需求。
实现默认方法
在LayoutManager必须实现这个方法。
@Override
public RecyclerView.LayoutParams generateDefaultLayoutParams() {
return new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
}
就是生成默认的LayoutParams, 参考LinearLayoutManager, 除非有特殊需要再改变。
暂时不考虑滑动进行布局
只考虑第一次进行布局时
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
if (getItemCount() == 0) {
detachAndScrapAttachedViews(recycler);
return;
}
// 如果正在进行动画，则不进行布局
if (getChildCount() == 0 && state.isPreLayout()) {
return;
}
detachAndScrapAttachedViews(recycler);
// 进行布局
layout(recycler, state);
}
/**
* 布局操作
*
* @param recycler
* @param state
*/
private void layout(RecyclerView.Recycler recycler, RecyclerView.State state) {
// 纵向计算偏移量，考虑padding
int topOffset = getPaddingTop();
// 横向计算偏移量，考虑padding
int leftOffset = getPaddingLeft();
// 行高，以最高的item作为参考
int maxLineHeight = 0;
final int childCount = getChildCount();
// 当第一次进行布局时
if (childCount == 0) {
for (int i = 0; i < getItemCount(); i++) {
// 获取一个item view, 添加到RecyclerView中，进行测量、布局
final View itemView = recycler.getViewForPosition(i);
addView(itemView);
// 测量，获取尺寸
measureChildWithMargins(itemView, 0, 0);
final int sizeHorizontal = getDecoratedMeasurementHorizontal(itemView);
final int sizeVertical = getDecoratedMeasurementVertical(itemView);
// 进行布局
if (leftOffset + sizeHorizontal <= getHorizontalSpace()) {
// 如果这行能够布局，则往后排
// layout
layoutDecoratedWithMargins(itemView, leftOffset, topOffset, leftOffset + sizeHorizontal, topOffset + sizeVertical);
// 修正横向计算偏移量
leftOffset += sizeHorizontal;
maxLineHeight = Math.max(maxLineHeight, sizeVertical);
} else {
// 如果当前行不够，则往下一行挪
// 修正计算偏移量、行高
topOffset += maxLineHeight;
maxLineHeight = 0;
leftOffset = getPaddingLeft();
// layout
if (topOffset > getHeight() - getPaddingBottom()) {
// 如果超出下边界
// 移除并回收该item view
removeAndRecycleView(itemView, recycler);
} else {
// 如果没有超出下边界，则继续布局
layoutDecoratedWithMargins(itemView, leftOffset, topOffset, leftOffset + sizeHorizontal, topOffset + sizeVertical);
// 修正计算偏移量、行高
leftOffset += sizeHorizontal;
maxLineHeight = Math.max(maxLineHeight, sizeVertical);
}
}
}
} else {
// nothing
}
}
//...
/**
* @return 横向的可布局的空间
*/
private int getHorizontalSpace() {
return getWidth() - getPaddingLeft() - getPaddingRight();
}
考虑滑动
因为是纵向滑动，我们将RecyclerView想象成一个宽度一定，长度可变的纸张，长度有其中的item布局来决定。当手指滑动屏幕时，实际上是让纸张在上面移动，同时保证纸张顶部不能低于RecyclerView顶部，纸张底部不能高于RecyclerView底部。
布局亦是这样，我们假定所有的item view都已经布局在张纸上面（不考虑回收），我们滑动时，只是改变了这张纸相对于屏幕的滑动距离。
不考虑边界
/**
* @return 可以纵向滑动
*/
@Override
public boolean canScrollVertically() {
return true;
}
// 纵向偏移量
private int mVerticalOffset = 0;
@Override
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
Log.e(TAG, String.valueOf(dy));
// 如果滑动距离为0, 或是没有任何item view, 则不移动
if (dy == 0 || getChildCount() == 0) {
return 0;
}
mVerticalOffset += dy;
detachAndScrapAttachedViews(recycler);
layout(recycler, state, mVerticalOffset);
return dy;
}
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
if (getItemCount() == 0) {
detachAndScrapAttachedViews(recycler);
return;
}
// 如果正在进行动画，则不进行布局
if (getChildCount() == 0 && state.isPreLayout()) {
return;
}
detachAndScrapAttachedViews(recycler);
// 进行布局
layout(recycler, state, 0);
}
/**
* 布局操作
*
* @param recycler
* @param state
*/
private void layout(RecyclerView.Recycler recycler, RecyclerView.State state, int verticalOffset) {
// 纵向计算偏移量，考虑padding
int topOffset = getPaddingTop();
// 横向计算偏移量，考虑padding
int leftOffset = getPaddingLeft();
// 行高，以最高的item作为参考
int maxLineHeight = 0;
for (int i = 0; i < getItemCount(); i++) {
// 获取一个item view, 添加到RecyclerView中，进行测量、布局
final View itemView = recycler.getViewForPosition(i);
addView(itemView);
// 测量，获取尺寸
measureChildWithMargins(itemView, 0, 0);
final int sizeHorizontal = getDecoratedMeasurementHorizontal(itemView);
final int sizeVertical = getDecoratedMeasurementVertical(itemView);
// 进行布局
if (leftOffset + sizeHorizontal <= getHorizontalSpace()) {
// 如果这行能够布局，则往后排
// layout
layoutDecoratedWithMargins(itemView, leftOffset, topOffset - verticalOffset, leftOffset + sizeHorizontal, topOffset + sizeVertical - verticalOffset);
// 修正横向计算偏移量
leftOffset += sizeHorizontal;
maxLineHeight = Math.max(maxLineHeight, sizeVertical);
} else {
// 如果当前行不够，则往下一行挪
// 修正计算偏移量、行高
topOffset += maxLineHeight;
maxLineHeight = 0;
leftOffset = getPaddingLeft();
// layout
// 不考虑边界
layoutDecoratedWithMargins(itemView, leftOffset, topOffset - verticalOffset, leftOffset + sizeHorizontal, topOffset + sizeVertical - verticalOffset);
// 修正计算偏移量、行高
leftOffset += sizeHorizontal;
maxLineHeight = Math.max(maxLineHeight, sizeVertical);
}
}
}
上面示例，对滑动的处理比较简单，记录总共滑动的距离，并在定位时将滑动距离计算上。
考虑边界
对于FlowLayoutManager来说，上边界比较容易处理，下边界的处理则需要稍加处理。
上边界：因为每一行的顶部都是相同的，所以手指下滑时，我们考虑第一行的顶部是否已经到达上边界，取第一个item即可。
下边界：每个item的高度不一定相同，为每一个item进行布局时，这样不会出现问题，但是在手指上滑时，判断是否到达下边界，需要知道最后一行中最高的item是多少，所以在计算的时候，要将最后一行的所有item考虑在内。
注：我们认为，一旦一个item构造完成，那么它的尺寸是不应该发生变化的，如果在滑动过程中，尺寸发生变化，会影响到布局的计算。
一个概念和tip: RecyclerView就是一个ViewGroup, 而通常显示的顺序，也是FlowLayoutManagerlayout的顺序都是从第一个开始的，所以可以这样认为，在position较大的item view显示之前，较小position的item view都是经过测量和布局过的。我们可以将布局过的item view的位置保存，在手指下滑，即加载position较小的item view时，不再对其进行测量和布局，只需要取出其位置，使用滑动修正当前的位置即可。
但是会如果数据改变，item view的尺寸和布局则会发生改变，原先的记录则会被认为是脏数据，所以当任何数据改变时，需要对其修正。不过一个好的修正方案需要有好的策略，在实现中，我只采用了清理的方法，暂时不能提供更加理想化的策略。
下面是完整代码：
import android.content.Context;
import android.graphics.Rect;
import android.support.v7.widget.RecyclerView;
import android.util.AttributeSet;
import android.util.SparseArray;
import android.view.View;
import android.view.ViewGroup;
/**
* Created by Mycroft on 2017/1/11.
*/
public final class FlowLayoutManager extends RecyclerView.LayoutManager {
private static final String TAG = FlowLayoutManager.class.getSimpleName();
public FlowLayoutManager() {
setAutoMeasureEnabled(true);
}
public FlowLayoutManager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
setAutoMeasureEnabled(true);
}
@Override
public RecyclerView.LayoutParams generateDefaultLayoutParams() {
return new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
}
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
if (getItemCount() == 0) {
detachAndScrapAttachedViews(recycler);
return;
}
// 如果正在进行动画，则不进行布局
if (getChildCount() == 0 && state.isPreLayout()) {
return;
}
detachAndScrapAttachedViews(recycler);
// 进行布局
layout(recycler, state, 0);
}
/**
* @return 可以纵向滑动
*/
@Override
public boolean canScrollVertically() {
return true;
}
// 纵向偏移量
private int mVerticalOffset = 0;
@Override
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
// 如果滑动距离为0, 或是没有任何item view, 则不移动
if (dy == 0 || getChildCount() == 0) {
return 0;
}
// 实际滑动的距离，到达边界时需要进行修正
int realOffset = dy;
if (mVerticalOffset + realOffset < 0) {
realOffset = -mVerticalOffset;
} else if (realOffset > 0) {
// 手指上滑，判断是否到达下边界
final View lastChildView = getChildAt(getChildCount() - 1);
if (getPosition(lastChildView) == getItemCount() - 1) {
int maxBottom = getDecoratedBottom(lastChildView);
int lastChildTop = getDecoratedTop(lastChildView);
for (int i = getChildCount() - 2; i >= 0; i--) {
final View child = getChildAt(i);
if (getDecoratedTop(child) == lastChildTop) {
maxBottom = Math.max(maxBottom, getDecoratedBottom(getChildAt(i)));
} else {
break;
}
}
int gap = getHeight() - getPaddingBottom() - maxBottom;
if (gap > 0) {
realOffset = -gap;
} else if (gap == 0) {
realOffset = 0;
} else {
realOffset = Math.min(realOffset, -gap);
}
}
}
realOffset = layout(recycler, state, realOffset);
mVerticalOffset += realOffset;
offsetChildrenVertical(-realOffset);
return realOffset;
}
private final SparseArray<Rect> mItemRects = new SparseArray<>();
/**
* 布局操作
*
* @param recycler
* @param state
* @param dy 用于判断回收、显示item, 对布局/定位本身没有影响
* @return
*/
private int layout(RecyclerView.Recycler recycler, RecyclerView.State state, int dy) {
int firstVisiblePos = 0;
// 纵向计算偏移量，考虑padding
int topOffset = getPaddingTop();
// 横向计算偏移量，考虑padding
int leftOffset = getPaddingLeft();
// 行高，以最高的item作为参考
int maxLineHeight = 0;
int childCount = getChildCount();
// 当是滑动进入时（在onLayoutChildren方法里面，我们移除了所有的child view, 所以只有可能从scrollVerticalBy方法里面进入这个方法）
if (childCount > 0) {
// 计算滑动后，需要被回收的child view
if (dy > 0) {
// 手指上滑，可能需要回收顶部的view
for (int i = 0; i < childCount; i++) {
final View child = getChildAt(i);
if (getDecoratedBottom(child) - dy < topOffset) {
// 超出顶部的item
removeAndRecycleView(child, recycler);
i--;
childCount--;
} else {
firstVisiblePos = i;
break;
}
}
} else if (dy < 0) {
// 手指下滑，可能需要回收底部的view
for (int i = childCount - 1; i >= 0; i--) {
final View child = getChildAt(i);
if (getDecoratedTop(child) - dy > getHeight() - getPaddingBottom()) {
// 超出底部的item
removeAndRecycleView(child, recycler);
} else {
break;
}
}
}
}
// 进行布局
if (dy >= 0) {
// 手指上滑，按顺序布局item
int minPosition = firstVisiblePos;
if (getChildCount() > 0) {
final View lastVisibleChild = getChildAt(getChildCount() - 1);
// 修正当前偏移量
topOffset = getDecoratedTop(lastVisibleChild);
leftOffset = getDecoratedRight(lastVisibleChild);
// 修正第一个应该进行布局的item view
minPosition = getPosition(lastVisibleChild) + 1;
// 使用排在最后一行的所有的child view进行高度修正
maxLineHeight = Math.max(maxLineHeight, getDecoratedMeasurementVertical(lastVisibleChild));
for (int i = getChildCount() - 2; i >= 0; i--) {
final View child = getChildAt(i);
if (getDecoratedTop(child) == topOffset) {
maxLineHeight = Math.max(maxLineHeight, getDecoratedMeasurementVertical(child));
} else {
break;
}
}
}
// 布局新的 item view
for (int i = minPosition; i < getItemCount(); i++) {
// 获取item view, 添加、测量、获取尺寸
final View itemView = recycler.getViewForPosition(i);
addView(itemView);
measureChildWithMargins(itemView, 0, 0);
final int sizeHorizontal = getDecoratedMeasurementHorizontal(itemView);
final int sizeVertical = getDecoratedMeasurementVertical(itemView);
// 进行布局
if (leftOffset + sizeHorizontal <= getHorizontalSpace()) {
// 如果这行能够布局，则往后排
// layout
layoutDecoratedWithMargins(itemView, leftOffset, topOffset, leftOffset + sizeHorizontal, topOffset + sizeVertical);
final Rect rect = new Rect(leftOffset, topOffset + mVerticalOffset, leftOffset + sizeHorizontal, topOffset + sizeVertical + mVerticalOffset);
// 保存布局信息
mItemRects.put(i, rect);
// 修正横向计算偏移量
leftOffset += sizeHorizontal;
maxLineHeight = Math.max(maxLineHeight, sizeVertical);
} else {
// 如果当前行不够，则往下一行挪
// 修正计算偏移量、行高
topOffset += maxLineHeight;
maxLineHeight = 0;
leftOffset = getPaddingLeft();
// layout
if (topOffset - dy > getHeight() - getPaddingBottom()) {
// 如果超出下边界
// 移除并回收该item view
removeAndRecycleView(itemView, recycler);
break;
} else {
// 如果没有超出下边界，则继续布局
layoutDecoratedWithMargins(itemView, leftOffset, topOffset, leftOffset + sizeHorizontal, topOffset + sizeVertical);
final Rect rect = new Rect(leftOffset, topOffset + mVerticalOffset, leftOffset + sizeHorizontal, topOffset + sizeVertical + mVerticalOffset);
// 保存布局信息
mItemRects.put(i, rect);
// 修正计算偏移量、行高
leftOffset += sizeHorizontal;
maxLineHeight = Math.max(maxLineHeight, sizeVertical);
}
}
}
} else {
// 手指下滑，逆序布局新的child
int maxPos = getItemCount() - 1;
if (getChildCount() > 0) {
maxPos = getPosition(getChildAt(0)) - 1;
}
for (int i = maxPos; i >= 0; i--) {
Rect rect = mItemRects.get(i);
// 判断底部是否在上边界下面
if (rect.bottom - mVerticalOffset - dy >= getPaddingTop()) {
// 获取item view, 添加、设置尺寸、布局
final View itemView = recycler.getViewForPosition(i);
addView(itemView, 0);
measureChildWithMargins(itemView, 0, 0);
layoutDecoratedWithMargins(itemView, rect.left, rect.top - mVerticalOffset, rect.right, rect.bottom - mVerticalOffset);
}
}
}
return dy;
}
/* 对数据改变时的一些修正 */
@Override
public void onItemsChanged(RecyclerView recyclerView) {
mVerticalOffset = 0;
mItemRects.clear();
}
@Override
public void onItemsAdded(RecyclerView recyclerView, int positionStart, int itemCount) {
mVerticalOffset = 0;
mItemRects.clear();
}
@Override
public void onItemsMoved(RecyclerView recyclerView, int from, int to, int itemCount) {
mVerticalOffset = 0;
mItemRects.clear();
}
@Override
public void onItemsRemoved(RecyclerView recyclerView, int positionStart, int itemCount) {
mVerticalOffset = 0;
mItemRects.clear();
}
@Override
public void onItemsUpdated(RecyclerView recyclerView, int positionStart, int itemCount) {
mVerticalOffset = 0;
mItemRects.clear();
}
@Override
public void onItemsUpdated(RecyclerView recyclerView, int positionStart, int itemCount, Object payload) {
mVerticalOffset = 0;
mItemRects.clear();
}
@Override
public void onAdapterChanged(RecyclerView.Adapter oldAdapter, RecyclerView.Adapter newAdapter) {
mVerticalOffset = 0;
mItemRects.clear();
}
/**
* 获取 child view 横向上需要占用的空间，margin计算在内
*
* @param view item view
* @return child view 横向占用的空间
*/
private int getDecoratedMeasurementHorizontal(View view) {
final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams)
view.getLayoutParams();
return getDecoratedMeasuredWidth(view) + params.leftMargin
+ params.rightMargin;
}
/**
* 获取 child view 纵向上需要占用的空间，margin计算在内
*
* @param view item view
* @return child view 纵向占用的空间
*/
private int getDecoratedMeasurementVertical(View view) {
final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) view.getLayoutParams();
return getDecoratedMeasuredHeight(view) + params.topMargin + params.bottomMargin;
}
/**
* @return 横向的可布局的空间
*/
private int getHorizontalSpace() {
return getWidth() - getPaddingLeft() - getPaddingRight();
}
}
源代码地址：FlowLayoutManager
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