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前言

如果我们需要固定住布局，不给用户拖拽，可以通过修改长按点击监听事件来达到禁止拖拽的目的

二、代码追踪

1、src/com/android/launcher3/touch/ItemLongClickListener.java
在这个类开头注册了两种类型的监听，一个是在桌面拖拽应用，一个是在所有应用里面长按拖拽

/*** Class to handle long-clicks on workspace items and start drag as a result.*/
public class ItemLongClickListener {public static final OnLongClickListener INSTANCE_WORKSPACE =ItemLongClickListener::onWorkspaceItemLongClick;public static final OnLongClickListener INSTANCE_ALL_APPS =ItemLongClickListener::onAllAppsItemLongClick;

接着分析桌面的长按事件

    private static boolean onWorkspaceItemLongClick(View v) {if (v instanceof LauncherAppWidgetHostView) {TestLogging.recordEvent(TestProtocol.SEQUENCE_MAIN, "Widgets.onLongClick");} else {TestLogging.recordEvent(TestProtocol.SEQUENCE_MAIN, "onWorkspaceItemLongClick");}Launcher launcher = Launcher.getLauncher(v.getContext());if (!canStartDrag(launcher)) return false;//请注意这里，这里判断是否允许拖拽，如果不允许拖拽，则用户长按的时候是没有任何反应的，包括长按卸载和快捷方式啥的都没有了。所以不可以在这里修改if (!launcher.isInState(NORMAL) && !launcher.isInState(OVERVIEW)) return false;if (!(v.getTag() instanceof ItemInfo)) return false;launcher.setWaitingForResult(null);beginDrag(v, launcher, (ItemInfo) v.getTag(), launcher.getDefaultWorkspaceDragOptions());return true;}

我们看到有**beginDrag(v, launcher, (ItemInfo) v.getTag(), launcher.getDefaultWorkspaceDragOptions());**这个方法，这个方法也是不能注释掉的,点进去这个方法

    public static void beginDrag(View v, Launcher launcher, ItemInfo info,DragOptions dragOptions) {if (info.container >= 0) {Folder folder = Folder.getOpen(launcher);if (folder != null) {if (!folder.getIconsInReadingOrder().contains(v)) {folder.close(true);} else {folder.startDrag(v, dragOptions);return;}}}CellLayout.CellInfo longClickCellInfo = new CellLayout.CellInfo(v, info);launcher.getWorkspace().startDrag(longClickCellInfo, dragOptions);}

重点关注最后一行，发现走到了我们的主桌面workspace，点进去继续看

    public void startDrag(CellLayout.CellInfo cellInfo, DragOptions options) {View child = cellInfo.cell;mDragInfo = cellInfo;//child.setVisibility(INVISIBLE);//需要把这一行注释掉，不然长按的时候，图标会消失不见if (options.isAccessibleDrag) {mDragController.addDragListener(new AccessibleDragListenerAdapter(this, WorkspaceAccessibilityHelper::new) {@Overrideprotected void enableAccessibleDrag(boolean enable) {super.enableAccessibleDrag(enable);setEnableForLayout(mLauncher.getHotseat(), enable);}});}beginDragShared(child, this, options);}

重点关注最后一行，这一行也不能注释掉，点击去到这里

    public void beginDragShared(View child, DragSource source, DragOptions options) {Object dragObject = child.getTag();if (!(dragObject instanceof ItemInfo)) {String msg = "Drag started with a view that has no tag set. This "+ "will cause a crash (issue 11627249) down the line. "+ "View: " + child + "  tag: " + child.getTag();throw new IllegalStateException(msg);}beginDragShared(child, null, source, (ItemInfo) dragObject,new DragPreviewProvider(child), options);}

还是最后一行代码，点进去才到重点

    /*** Core functionality for beginning a drag operation for an item that will be dropped within* the workspace*/public DragView beginDragShared(View child, DraggableView draggableView, DragSource source,ItemInfo dragObject, DragPreviewProvider previewProvider, DragOptions dragOptions) {float iconScale = 1f;if (child instanceof BubbleTextView) {Drawable icon = ((BubbleTextView) child).getIcon();if (icon instanceof FastBitmapDrawable) {iconScale = ((FastBitmapDrawable) icon).getAnimatedScale();}}// Clear the pressed state if necessarychild.clearFocus();child.setPressed(false);if (child instanceof BubbleTextView) {BubbleTextView icon = (BubbleTextView) child;icon.clearPressedBackground();}if (draggableView == null && child instanceof DraggableView) {draggableView = (DraggableView) child;}final View contentView = previewProvider.getContentView();final float scale;// The draggable drawable follows the touch point around on the screenfinal Drawable drawable;if (contentView == null) {drawable = previewProvider.createDrawable();scale = previewProvider.getScaleAndPosition(drawable, mTempXY);} else {drawable = null;scale = previewProvider.getScaleAndPosition(contentView, mTempXY);}int halfPadding = previewProvider.previewPadding / 2;int dragLayerX = mTempXY[0];int dragLayerY = mTempXY[1];Point dragVisualizeOffset = null;Rect dragRect = new Rect();if (draggableView != null) {draggableView.getSourceVisualDragBounds(dragRect);dragLayerY += dragRect.top;dragVisualizeOffset = new Point(- halfPadding, halfPadding);}if (child.getParent() instanceof ShortcutAndWidgetContainer) {mDragSourceInternal = (ShortcutAndWidgetContainer) child.getParent();}if (child instanceof BubbleTextView) {BubbleTextView btv = (BubbleTextView) child;if (!dragOptions.isAccessibleDrag) {dragOptions.preDragCondition = btv.startLongPressAction();}if (btv.isDisplaySearchResult()) {dragOptions.preDragEndScale = (float) mAllAppsIconSize / btv.getIconSize();}}final DragView dv;//start change that do not to drop the workspace and allApps icon./*if (contentView instanceof View) {if (contentView instanceof LauncherAppWidgetHostView) {mDragController.addDragListener(new AppWidgetHostViewDragListener(mLauncher));}dv = mDragController.startDrag(contentView,draggableView,dragLayerX,dragLayerY,source,dragObject,dragVisualizeOffset,dragRect,scale * iconScale,scale,dragOptions);} else {dv = mDragController.startDrag(drawable,draggableView,dragLayerX,dragLayerY,source,dragObject,dragVisualizeOffset,dragRect,scale * iconScale,scale,dragOptions);}return dv;*///end change that do not to drop the workspace and allApps icon.return null;}

这个方法是拖拽的核心方法，在开始拖拽之前执行，改方法前面都是一些计算，不用看，重点看final DragView dv;后面这个if语句，这里构造一个拖拽对象，我们将该对象构造过程给注释掉，返回一个null，这样子，后续的所有拖拽流程都无法进行了，因为没有拖拽对象了，这样子我们就把禁止拖拽禁掉了，同时长按桌面添加微件的功能还是正常的，不影响其他长按功能。
因为所有应用里面的拖拽最终也是走到了最后一个方法，所以所有应用的拖拽也被禁掉了