AndroidICS4.0的文件夹和2.3的文件夹区别比较大，主要区别有：
一、android2.3的文件夹大小是固定的，4.0的文件夹大小是按照里面的元素大小决定的。
二、android2.3的文件夹图标是固定的文件夹的形式展示的，而4.0是从文件中取前3个的缩略图垂直展示在屏幕上的。估计谷歌怕侵犯苹果文件夹的知识产权，所以没有做成和苹果一样的效果。
三、android2.3的文件中可以放多于16的应用程序的快捷方式，而4.0最多只能放16个快捷方式。
四、android2.3的文件夹中的图标不可以交换位置，而4.0的文件夹中的图标可以相互交换位置。
转载请标明出处：http://blog.csdn.net/wdaming1986/article/details/7748738
对比图如下：
android2.3的文件夹 android4.0的文件夹
下面来看看4.0的代码怎么实现文件夹的：
Step 1：如果系统一开始有fold，一启动launcher的时候，在Launcher.java类中bindFolders回调方法中：
/** * Implementation of the method from LauncherModel.Callbacks. */ public void bindFolders(HashMap<Long, FolderInfo> folders) { setLoadOnResume(); sFolders.clear(); sFolders.putAll(folders); }绑定所有fold的对象交给sFolders，去处理。
private static HashMap<Long, FolderInfo> sFolders = new HashMap<Long, FolderInfo>();
Step 2：如果是把一个图标拖放到另一图标上面，也形成folder。具体流程如下：
1、首先在workspace中的onDrop（）方法中会判断是否会形成一个fold。代码如下：
public void onDrop(DragObject d) { ... ... // If the item being dropped is a shortcut and the nearest drop // cell also contains a shortcut, then create a folder with the two shortcuts. if (!mInScrollArea && createUserFolderIfNecessary(cell, container, dropTargetLayout, mTargetCell, false, d.dragView, null)) { return; } ... ...}
2、在Workspace.java类的createUserFolderIfNecessary（）方法中来增加fold，具体代码如下：
boolean createUserFolderIfNecessary(View newView, long container, CellLayout target, int[] targetCell, boolean external, DragView dragView, Runnable postAnimationRunnable) { 。。。 。。。 FolderIcon fi = mLauncher.addFolder(target, container, screen, targetCell[0], targetCell[1]); destInfo.cellX = -1; destInfo.cellY = -1; sourceInfo.cellX = -1; sourceInfo.cellY = -1; 。。。 。。。}通过mLauncher.addFolder来传递folder的信息，包含一些位置信息绑定哪个屏幕的。
3、在Launcher.java类的addFolder（）这个方法是真正形成folder的，以及在launcher的数据库中插入一条信息，代码如下：
FolderIcon addFolder(CellLayout layout, long container, final int screen, int cellX, int cellY) { final FolderInfo folderInfo = new FolderInfo(); folderInfo.title = getText(R.string.folder_name); // Update the model LauncherModel.addItemToDatabase(Launcher.this, folderInfo, container, screen, cellX, cellY, false); sFolders.put(folderInfo.id, folderInfo); // Create the view FolderIcon newFolder = FolderIcon.fromXml(R.layout.folder_icon, this, layout, folderInfo, mIconCache); mWorkspace.addInScreen(newFolder, container, screen, cellX, cellY, 1, 1, isWorkspaceLocked()); return newFolder; }FolderIcon.fromXml（）这个方法是从xml中形成folder，addInScreen（），把相应的信息插入数据库。
4、在FolderIcon.java中fromXml（）方法中的代码如下：
static FolderIcon fromXml(int resId, Launcher launcher, ViewGroup group, FolderInfo folderInfo, IconCache iconCache) { if (INITIAL_ITEM_ANIMATION_DURATION >= DROP_IN_ANIMATION_DURATION) { throw new IllegalStateException("DROP_IN_ANIMATION_DURATION must be greater than " + "INITIAL_ITEM_ANIMATION_DURATION, as sequencing of adding first two items " + "is dependent on this"); } FolderIcon icon = (FolderIcon) LayoutInflater.from(launcher).inflate(resId, group, false); icon.mFolderName = (BubbleTextView) icon.findViewById(R.id.folder_icon_name); icon.mFolderName.setText(folderInfo.title); icon.mPreviewBackground = (ImageView) icon.findViewById(R.id.preview_background); icon.setTag(folderInfo); icon.setOnClickListener(launcher); icon.mInfo = folderInfo; icon.mLauncher = launcher; icon.setContentDescription(String.format(launcher.getString(R.string.folder_name_format), folderInfo.title)); Folder folder = Folder.fromXml(launcher); folder.setDragController(launcher.getDragController()); folder.setFolderIcon(icon); folder.bind(folderInfo); icon.mFolder = folder; icon.mFolderRingAnimator = new FolderRingAnimator(launcher, icon); folderInfo.addListener(icon); return icon; }Folder folder = Folder.fromXml(launcher);是真正产生了一个folder对象。代码如下：
/** * Creates a new UserFolder, inflated from R.layout.user_folder. * * @param context The application's context. * * @return A new UserFolder. */ static Folder fromXml(Context context) { return (Folder) LayoutInflater.from(context).inflate(R.layout.user_folder, null); }
并且给folder设置拖拽的控制器，绑定folderInfo设置folderInfo.addListener(icon)图标改变的监听。
这个接口 interface FolderListener定义了一个方法---->如下：
interface FolderListener { public void onAdd(ShortcutInfo item); public void onRemove(ShortcutInfo item); public void onTitleChanged(CharSequence title); public void onItemsChanged(); }
在folder.bind(folderInfo);方法中的操作如下：
void bind(FolderInfo info) { mInfo = info; ArrayList<ShortcutInfo> children = info.contents; ArrayList<ShortcutInfo> overflow = new ArrayList<ShortcutInfo>(); setupContentForNumItems(children.size()); int count = 0; for (int i = 0; i < children.size(); i++) { ShortcutInfo child = (ShortcutInfo) children.get(i); if (!createAndAddShortcut(child)) { overflow.add(child); } else { count++; } } // We rearrange the items in case there are any empty gaps setupContentForNumItems(count); // If our folder has too many items we prune them from the list. This is an issue // when upgrading from the old Folders implementation which could contain an unlimited // number of items. for (ShortcutInfo item: overflow) { mInfo.remove(item); LauncherModel.deleteItemFromDatabase(mLauncher, item); } mItemsInvalidated = true; updateTextViewFocus(); mInfo.addListener(this); if (!sDefaultFolderName.contentEquals(mInfo.title)) { mFolderName.setText(mInfo.title); } else { mFolderName.setText(""); } }主要的操作是：给拖拽进来的快捷方式安排位置，判断Folder是否已经放满，设置监听，设置folder的Name；
5、folder桌面的缩略图怎么形成的，是在第2步Workspace.java的createUserFolderIfNecessary（）方法中
// If the dragView is null, we can't animate boolean animate = dragView != null; if (animate) { fi.performCreateAnimation(destInfo, v, sourceInfo, dragView, folderLocation, scale, postAnimationRunnable); } else { fi.addItem(destInfo); fi.addItem(sourceInfo); }fi.performCreateAnimation（）这个方法是给folder添加个动画。
6、在FolderIcon.java中的performCreateAnimation（）方法中：
public void performCreateAnimation(final ShortcutInfo destInfo, final View destView, final ShortcutInfo srcInfo, final View srcView, Rect dstRect, float scaleRelativeToDragLayer, Runnable postAnimationRunnable) { Drawable animateDrawable = ((TextView) destView).getCompoundDrawables()[1]; computePreviewDrawingParams(animateDrawable.getIntrinsicWidth(), destView.getMeasuredWidth()); // This will animate the dragView (srcView) into the new folder onDrop(srcInfo, srcView, dstRect, scaleRelativeToDragLayer, 1, postAnimationRunnable); // This will animate the first item from it's position as an icon into its // position as the first item in the preview animateFirstItem(animateDrawable, INITIAL_ITEM_ANIMATION_DURATION); postDelayed(new Runnable() { public void run() { addItem(destInfo); } }, INITIAL_ITEM_ANIMATION_DURATION); }computePreviewDrawingParams（）这个方法是计算绘制folder图标的方法；
7、在FolderIcon.java类中的computePreviewItemDrawingParams（）方法中:
private PreviewItemDrawingParams computePreviewItemDrawingParams(int index, PreviewItemDrawingParams params) { index = NUM_ITEMS_IN_PREVIEW - index - 1; float r = (index * 1.0f) / (NUM_ITEMS_IN_PREVIEW - 1); float scale = (1 - PERSPECTIVE_SCALE_FACTOR * (1 - r)); float offset = (1 - r) * mMaxPerspectiveShift; float scaledSize = scale * mBaselineIconSize; float scaleOffsetCorrection = (1 - scale) * mBaselineIconSize; // We want to imagine our coordinates from the bottom left, growing up and to the // right. This is natural for the x-axis, but for the y-axis, we have to invert things. float transY = mAvailableSpaceInPreview - (offset + scaledSize + scaleOffsetCorrection); float transX = offset + scaleOffsetCorrection; float totalScale = mBaselineIconScale * scale; final int overlayAlpha = (int) (80 * (1 - r)); if (params == null) { params = new PreviewItemDrawingParams(transX, transY, totalScale, overlayAlpha); } else { params.transX = transX; params.transY = transY; params.scale = totalScale; params.overlayAlpha = overlayAlpha; } return params; }主要工作是：计算图标的排列，每一个相对上一个有点偏移的距离；效果图如下：
以上基本是把Step 2流程大致过了一遍。
Step 3：folder类中的长按事件的传递，以及Fold中长按交换位置的流程过一下：
1、先来说给Folder设置长按监听的地方，因为Folder.java类是继承了View.OnClickListener,
View.OnLongClickListener事件，所以，长按事件就交给自己的onLongClick（）事件来处理。
2、所以Folder中的长按事件，被自己的public boolean onLongClick(View v) {}时间捕获，代码如下：
public boolean onLongClick(View v) { ... ... mLauncher.getWorkspace().onDragStartedWithItem(v); mLauncher.getWorkspace().beginDragShared(v, this); mIconDrawable = ((TextView) v).getCompoundDrawables()[1]; mCurrentDragInfo = item; mEmptyCell[0] = item.cellX; mEmptyCell[1] = item.cellY; mCurrentDragView = v; mContent.removeView(mCurrentDragView); mInfo.remove(mCurrentDragInfo); mDragInProgress = true; mItemAddedBackToSelfViaIcon = false; ... ...}同样拖拽事件是交给Workspace来处理，最后也是统一交给DragController.java类处理和分发相应的事件。这个过程在
Android-->Launcher拖拽事件详解【androidICS4.0--Launcher系列二】中做了详细的介绍，这里就不做赘述了。
3、主要看Folder.java类中的onDragOver（）这个方法，当在文件夹中拖拽到另一个快捷方式的上面的时候，发生交换，
来看代码如下：
public void onDragOver(DragObject d) { float[] r = getDragViewVisualCenter(d.x, d.y, d.xOffset, d.yOffset, d.dragView, null); mTargetCell = mContent.findNearestArea((int) r[0], (int) r[1], 1, 1, mTargetCell); if (mTargetCell[0] != mPreviousTargetCell[0] || mTargetCell[1] != mPreviousTargetCell[1]) { mReorderAlarm.cancelAlarm(); mReorderAlarm.setOnAlarmListener(mReorderAlarmListener); mReorderAlarm.setAlarm(150); mPreviousTargetCell[0] = mTargetCell[0]; mPreviousTargetCell[1] = mTargetCell[1]; } }这个方法主要做的操作是：判断拖拽的是哪个对象mContent.findNearestArea((int) r[0], (int) r[1], 1, 1, mTargetCell);判断在
哪个目标的附近，然后判断和是否是正在拖拽的对象的坐标，设置mReorderAlarmListener来进行交换，设置150毫秒用来处理动画的。
4、在ReorderAlarmListener内部类的代码如下：
OnAlarmListener mReorderAlarmListener = new OnAlarmListener() { public void onAlarm(Alarm alarm) { realTimeReorder(mEmptyCell, mTargetCell); } };
5、在realTimeReorder（）方法中传递这个快捷方式在屏幕x轴，y轴上的为止，进行交换,代码如下：
private void realTimeReorder(int[] empty, int[] target) { boolean wrap; int startX; int endX; int startY; int delay = 0; float delayAmount = 30; if (readingOrderGreaterThan(target, empty)) { wrap = empty[0] >= mContent.getCountX() - 1; startY = wrap ? empty[1] + 1 : empty[1]; for (int y = startY; y <= target[1]; y++) { startX = y == empty[1] ? empty[0] + 1 : 0; endX = y < target[1] ? mContent.getCountX() - 1 : target[0]; for (int x = startX; x <= endX; x++) { View v = mContent.getChildAt(x,y); if (mContent.animateChildToPosition(v, empty[0], empty[1], REORDER_ANIMATION_DURATION, delay)) { empty[0] = x; empty[1] = y; delay += delayAmount; delayAmount *= 0.9; } } } } else { wrap = empty[0] == 0; startY = wrap ? empty[1] - 1 : empty[1]; for (int y = startY; y >= target[1]; y--) { startX = y == empty[1] ? empty[0] - 1 : mContent.getCountX() - 1; endX = y > target[1] ? 0 : target[0]; for (int x = startX; x >= endX; x--) { View v = mContent.getChildAt(x,y); if (mContent.animateChildToPosition(v, empty[0], empty[1], REORDER_ANIMATION_DURATION, delay)) { empty[0] = x; empty[1] = y; delay += delayAmount; delayAmount *= 0.9; } } } } }readingOrderGreaterThan（）这个方法的作用是判断是从上往下拖动，还是从下往上拖动，这两种情况的交换方式不一样。循环也就不一样。交换的过程中通过animateChildToPosition（）；这个方法设置了一个动画。
6、在放下的时候会触发Folder.java的onDrop（）方法，
public void onDrop(DragObject d) { ShortcutInfo item; 。。。 。。。if (d.dragView.hasDrawn()) { mLauncher.getDragLayer().animateViewIntoPosition(d.dragView, mCurrentDragView); } else { mCurrentDragView.setVisibility(VISIBLE); } mInfo.add(item); 。。。 。。。 }作用是设置放下的view可见，把当前的这个快捷方式添加到mInfo中。
7、当执行完onDrop（）方法后会走onDropCompleted（）方法:
public void onDropCompleted(View target, DragObject d, boolean success) { ... ... // Reordering may have occured, and we need to save the new item locations. We do this once // at the end to prevent unnecessary database operations. updateItemLocationsInDatabase(); ... ...}这个方法的作用是更新item的位置信息在数据库中。
private void updateItemLocationsInDatabase() { ArrayList<View> list = getItemsInReadingOrder(); for (int i = 0; i < list.size(); i++) { View v = list.get(i); ItemInfo info = (ItemInfo) v.getTag(); LauncherModel.moveItemInDatabase(mLauncher, info, mInfo.id, 0, info.cellX, info.cellY); } }好了，folder的大致流程就是这些，更详细的请参考launcher源代码。
写的仓促，欢迎大家指出里面的错误，如果有不解的欢迎留言！