前言

以前只知道Launcher是桌面，也就是显示已安装的应用图标，其它的一概不知。当自己要去学习它，要去修改它时，却发现Launcher涉及到的东西很多，此次则将自己对Launcher布局的理解做个记录。

一

通过了解，Launcher和普通的APP其实没啥两样，它可以单独编译，打包成APK，安装到手机上，所以要学习它，我还是从AndroidManifest文件开始，使用的源码是一位大佬分享到Github上，Launcher源码。

<?xml version="1.0" encoding="utf-8"?>
<!--
/*
**
** Copyright 2008, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
**     http://www.apache.org/licenses/LICENSE-2.0
**
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/
-->
<manifest
    xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.android.launcher3">
    <uses-sdk android:targetSdkVersion="23" android:minSdkVersion="21"/>
    <!--
    Manifest entries specific to Launcher3. This is merged with AndroidManifest-common.xml.
    Refer comments around specific entries on how to extend individual components.
    -->

    <!--
    Permissions required for read/write access to the workspace data. These permission name
    should not conflict with that defined in other apps, as such an app should embed its package
    name in the permissions. eq com.mypackage.permission.READ_SETTINGS
    -->
    <permission
        android:name="com.android.launcher3.permission.READ_SETTINGS"
        android:permissionGroup="android.permission-group.SYSTEM_TOOLS"
        android:protectionLevel="signatureOrSystem"
        android:label="@string/permlab_read_settings"
        android:description="@string/permdesc_read_settings"/>
    <permission
        android:name="com.android.launcher3.permission.WRITE_SETTINGS"
        android:permissionGroup="android.permission-group.SYSTEM_TOOLS"
        android:protectionLevel="signatureOrSystem"
        android:label="@string/permlab_write_settings"
        android:description="@string/permdesc_write_settings"/>

    <uses-permission android:name="com.android.launcher.permission.READ_SETTINGS" />
    <uses-permission android:name="com.android.launcher.permission.WRITE_SETTINGS" />
    <uses-permission android:name="com.android.launcher3.permission.READ_SETTINGS" />
    <uses-permission android:name="com.android.launcher3.permission.WRITE_SETTINGS" />

    <application
        android:backupAgent="com.android.launcher3.LauncherBackupAgent"
        android:fullBackupOnly="true"
        android:fullBackupContent="@xml/backupscheme"
        android:hardwareAccelerated="true"
        android:icon="@drawable/ic_launcher_home"
        android:label="@string/derived_app_name"
        android:theme="@style/AppTheme"
        android:largeHeap="@bool/config_largeHeap"
        android:restoreAnyVersion="true"
        android:supportsRtl="true" >

        <!--
        Main launcher activity. When extending only change the name, and keep all the
        attributes and intent filters the same
        -->
        <activity
            android:name="com.android.launcher3.Launcher"
            android:launchMode="singleTask"
            android:clearTaskOnLaunch="true"
            android:stateNotNeeded="true"
            android:windowSoftInputMode="adjustPan"
            android:screenOrientation="unspecified"
            android:configChanges="keyboard|keyboardHidden|mcc|mnc|navigation|orientation|screenSize|screenLayout|smallestScreenSize"
            android:resizeableActivity="true"
            android:resumeWhilePausing="true"
            android:taskAffinity=""
            android:enabled="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.HOME" />
                <category android:name="android.intent.category.DEFAULT" />
                <category android:name="android.intent.category.MONKEY"/>
                <category android:name="android.intent.category.LAUNCHER_APP" />
            </intent-filter>
        </activity>

        <!--
        The settings activity. When extending keep the intent filter present
        -->
        <activity
            android:name="com.android.launcher3.SettingsActivity"
            android:label="@string/settings_button_text"
            android:theme="@android:style/Theme.DeviceDefault.Settings"
            android:autoRemoveFromRecents="true">
            <intent-filter>
                <action android:name="android.intent.action.APPLICATION_PREFERENCES" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>

        <!--
        The settings provider contains Home's data, like the workspace favorites. The permissions
        should be changed to what is defined above. The authorities should also be changed to
        represent the package name.
        -->
        <provider
            android:name="com.android.launcher3.LauncherProvider"
            android:authorities="com.android.launcher3.settings"
            android:exported="true"
            android:writePermission="com.android.launcher3.permission.WRITE_SETTINGS"
            android:readPermission="com.android.launcher3.permission.READ_SETTINGS" />

    </application>
</manifest>

看AndroidManifest.xml文件,很明显并不复杂，在Application的标签下，有两个activity和一个provider，它们分别是：Launcher、SettingsActivity和LauncherProvider；

• Launcher即我们手机看到的桌面；
• SettingsActivity是桌面的设置页面；
• LauncherProvider是用来存储页面布局信息的。

二

本想着逐行去看源码，奈何细节过多，拘泥于一处，反而影响对项目的理解；所以借助于前人总结文档，结合自己理解，翻看源码，以学习Launcher布局。

---

首先介绍Launcher的一些界面，让自己有个认识，而不是凭空想象。

此为Launcher首页，桌面有快捷方式、小部件和Google搜索框等

此为Launcher滑动后的第二页。

通过上滑即可进入Launcher的抽屉页，里面包括所有的已安装应用。

此为长按桌面后的一些操作，有壁纸的更换，小部件的添加，以及桌面设置。

此为更换桌面和锁屏壁纸选择页。

此为添加桌面小部件选择页。

此为桌面设置页面。

此为长按桌面上的图标或者小部件时，显示移除按钮。

此为桌面布局的一个示意图。（如有侵权，立即删除。）

• DragLayer是根布局，用来处理子View的拖动的布局；
• Workspace是用来装载桌面上其它所有布局的控件；
• SearchDropTargetBar是用来显示移除和卸载的控件；
• CellLayout是用来显示应用和桌面小工具的，一个Workspace中可以显示多个CellLayout；
• PageIndicator是用来指示当前为第几页的CellLayout；
• Hotseat是常用应用显示控件。

三

接下来可以看看桌面的布局具体是怎么加载的；
翻看Launcher.java的代码，在onCreate()方法中，有如下代码。

      //将xml文件加载为View对象
      mLauncherView = LayoutInflater.from(this).inflate(R.layout.launcher, null);
      //将View设置到Activity上
      setContentView(mLauncherView);

很简单，就是将launcher.xml加载到Activity中；

<?xml version="1.0" encoding="utf-8"?><!-- Copyright (C) 2007 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->
<com.android.launcher3.LauncherRootView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:launcher="http://schemas.android.com/apk/res-auto"
    android:id="@+id/launcher"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true">

    <com.android.launcher3.dragndrop.DragLayer
        android:id="@+id/drag_layer"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:clipChildren="false"
        android:clipToPadding="false"
        android:importantForAccessibility="no">

        <!-- The workspace contains 5 screens of cells -->
        <!-- DO NOT CHANGE THE ID -->
        <com.android.launcher3.Workspace
            android:id="@+id/workspace"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_gravity="center"
            android:theme="@style/HomeScreenElementTheme"
            launcher:pageIndicator="@+id/page_indicator" />

        <include
            android:id="@+id/overview_panel"
            layout="@layout/overview_panel"
            android:visibility="gone" />

        <!-- Keep these behind the workspace so that they are not visible when
         we go into AllApps -->
        <com.android.launcher3.pageindicators.WorkspacePageIndicator
            android:id="@+id/page_indicator"
            android:layout_width="match_parent"
            android:layout_height="@dimen/vertical_drag_handle_size"
            android:layout_gravity="bottom|center_horizontal"
            android:theme="@style/HomeScreenElementTheme" />

        <include
            android:id="@+id/drop_target_bar"
            layout="@layout/drop_target_bar" />

        <include android:id="@+id/scrim_view"
            layout="@layout/scrim_view" />

        <include
            android:id="@+id/apps_view"
            layout="@layout/all_apps"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:visibility="invisible" />

        <!-- DO NOT CHANGE THE ID -->
        <include
            android:id="@+id/hotseat"
            layout="@layout/hotseat"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />
    </com.android.launcher3.dragndrop.DragLayer>

</com.android.launcher3.LauncherRootView>

查看launcher.xml文件，可以清晰的看到它包含的View有如下几个：

• LauncherRootView
• DragLayer
• Workspace
• WorkspacePageIndicator
• overview_panel
• drop_target_bar
• scrim_view
• all_apps
• hotseat

其中前四个是通过类名直接引入到xml文件中，后五个则是通过include形式引入到xml文件中的；
有几个前面有提及，例如：DragLayer、Workspace、WorkspacePageIndicator和hotseat；
有几个是第一次看到，包括LauncherRootView、overview_panel、drop_target_bar、scrim_view、以及all_apps；
根据我们学到的Android xml布局知识可以判断，桌面布局示意图和launcher.xml还是有差异的。

差异点有如下：

• 在xml文件中，根布局是LauncherRootView，在示意图中，我们是没有看到它；
• 同样在xml文件中有，在示意图中没有的布局还有overview_panel、scrim_view、all_apps。
• 在示意图中，Workspace是SearchDropTargetBar、CellLayout、PageIndicator和Hotseat的父控件，而在xml文件中，我们看到Workspace是一个单独控件；
• 在示意图中，有看到CellLayout布局，在xml文件中没有看到它；

我们学习Launcher当然是以源码为基础，所以如果示意图和xml有差异，当然是以xml为准，接下来就逐一看一下它们的差异点。

四

在xml文件中，根布局是LauncherRootView，在示意图中，我们是没有看到它；
通过翻看源码可知，LauncherRootView是InsettableFrameLayout的子类，InsettableFrameLayout又是FrameLayout的子类，且它实现了Insettable接口；由此可知LauncherRootView就是一个FrameLayout，且是具有Insettable功能的FrameLayout，那Insettable接口的功能又是什么呢？通过查资料可知，该接口是让实现它的类的子布局不被状态栏或者导航栏所遮挡，使用的方法是给子View设置Margin值，关键代码看下方；LauncherRootView的子类只有一个，它就是DragLayer，所以DragLayer肯定不会被状态栏和导航栏所遮挡。

    @Override
    public void setInsets(Rect insets) {
        final int n = getChildCount();
        for (int i = 0; i < n; i++) {
            final View child = getChildAt(i);
            setFrameLayoutChildInsets(child, insets, mInsets);
        }
        mInsets.set(insets);
    }

    public void setFrameLayoutChildInsets(View child, Rect newInsets, Rect oldInsets) {
        final LayoutParams lp = (LayoutParams) child.getLayoutParams();

        if (child instanceof Insettable) {
            ((Insettable) child).setInsets(newInsets);
        } else if (!lp.ignoreInsets) {
            lp.topMargin += (newInsets.top - oldInsets.top);
            lp.leftMargin += (newInsets.left - oldInsets.left);
            lp.rightMargin += (newInsets.right - oldInsets.right);
            lp.bottomMargin += (newInsets.bottom - oldInsets.bottom);
        }
        child.setLayoutParams(lp);
    }

五

同样在xml文件中有，在示意图中没有的布局还有overview_panel、scrim_view、all_apps。

---

overview_panel

通过查看overview_panel.xml文件，发现它就是一个Space控件，Space控件是一个轻量级视图子类，可用于在通用布局中的组件之间创建间隙，这里它的宽高设置为0dp，可见它并不占用空间；

<?xml version="1.0" encoding="utf-8"?>
<!--
     Copyright (C) 2016 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->
<Space
      xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="0dp"
      android:layout_height="0dp" />

在代码中，也只是在DragLayer中确认了一下它的坐标，其它地方均无调用。

mLauncher.getDragLayer().getDescendantRectRelativeToSelf(mLauncher.getOverviewPanel(),
                    sTempRect);

scrim_view

查看scrim_view.xml文件，它就是一个ScrimView，它的作用是在滑出抽屉页面时，桌面背景过渡的View

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2018 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->
<com.android.launcher3.views.ScrimView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/scrim_view" />

all_apps

查看all_apps.xml文件，它的根布局是AllAppsContainerView，在AllAppsContainerView下通过include又引入了四个布局，它们分别是：all_apps_rv_layout、all_apps_floating_header、search_container_all_apps和all_apps_fast_scroller；

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2016 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->
<!-- The top and bottom paddings are defined in this container, but since we want
     the list view to span the full width (for touch interception purposes), we
     will bake the left/right padding into that view's background itself. -->
<com.android.launcher3.allapps.AllAppsContainerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/apps_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:clipChildren="true"
    android:clipToPadding="false"
    android:focusable="false"
    android:saveEnabled="false" >

    <include layout="@layout/all_apps_rv_layout" />

    <include layout="@layout/all_apps_floating_header" />

    <include
        android:id="@id/search_container_all_apps"
        layout="@layout/search_container_all_apps"/>

    <include layout="@layout/all_apps_fast_scroller" />
</com.android.launcher3.allapps.AllAppsContainerView>

all_apps_rv_layout 是用来显示所有已安装app的控件，翻看源码可知它的父类的父类就是我们常用的RecyclerView；

public class AllAppsRecyclerView extends BaseRecyclerView
public abstract class BaseRecyclerView extends RecyclerView

all_apps_floating_header 是头部悬停布局，翻看源码和测试，发现其内部的控件会被隐藏；

        //1.通过打印得知mUsingTabs的值为false
        if (mUsingTabs) {
            mAH[AdapterHolder.MAIN].setup(mViewPager.getChildAt(0), mPersonalMatcher);
            mAH[AdapterHolder.WORK].setup(mViewPager.getChildAt(1), mWorkMatcher);
            onTabChanged(mViewPager.getNextPage());
        } else {
            mAH[AdapterHolder.MAIN].setup(findViewById(R.id.apps_list_view), null);
            //2.mAH[AdapterHolder.WORK].recyclerView设置为null
            mAH[AdapterHolder.WORK].recyclerView = null;
        }
        setupHeader();

    public void setupHeader() {
        mHeader.setVisibility(View.VISIBLE);
        //3. 第二个参数为true
        mHeader.setup(mAH, mAH[AllAppsContainerView.AdapterHolder.WORK].recyclerView == null);

        int padding = mHeader.getMaxTranslation();
        for (int i = 0; i < mAH.length; i++) {
            mAH[i].padding.top = padding;
            mAH[i].applyPadding();
        }
    }

    public void setup(AllAppsContainerView.AdapterHolder[] mAH, boolean tabsHidden) {
        mTabsHidden = tabsHidden;
        //4. tabsHidden为true时，mTabLayout就会被隐藏。
        mTabLayout.setVisibility(tabsHidden ? View.GONE : View.VISIBLE);
        mMainRV = setupRV(mMainRV, mAH[AllAppsContainerView.AdapterHolder.MAIN].recyclerView);
        mWorkRV = setupRV(mWorkRV, mAH[AllAppsContainerView.AdapterHolder.WORK].recyclerView);
        mParent = (ViewGroup) mMainRV.getParent();
        setMainActive(mMainRVActive || mWorkRV == null);
        reset(false);
    }

六

在示意图中，Workspace是SearchDropTargetBar、CellLayout、PageIndicator和Hotseat的父控件，而在xml文件中，我们看到Workspace是一个单独控件；

Workspace

Workspace它是一个单独的控件，如果它有子View，那么子View肯定就是通过代码的形式添加，翻看源码，确实如此，而且可以确定的是，它有子View，且它的子View是CellLayout。

public CellLayout insertNewWorkspaceScreen(long screenId, int insertIndex) {
        if (mWorkspaceScreens.containsKey(screenId)) {
            throw new RuntimeException("Screen id " + screenId + " already exists!");
        }

        // Inflate the cell layout, but do not add it automatically so that we can get the newly
        // created CellLayout.
        CellLayout newScreen = (CellLayout) LayoutInflater.from(getContext()).inflate(
                R.layout.workspace_screen, this, false /* attachToRoot */);
        newScreen.getShortcutsAndWidgets().setId(R.id.workspace_page_container);
        int paddingLeftRight = mLauncher.getDeviceProfile().cellLayoutPaddingLeftRightPx;
        int paddingBottom = mLauncher.getDeviceProfile().cellLayoutBottomPaddingPx;
        newScreen.setPadding(paddingLeftRight, 0, paddingLeftRight, paddingBottom);

        mWorkspaceScreens.put(screenId, newScreen);
        mScreenOrder.add(insertIndex, screenId);
        addView(newScreen, insertIndex);
        mStateTransitionAnimation.applyChildState(
                mLauncher.getStateManager().getState(), newScreen, insertIndex);

        if (mLauncher.getAccessibilityDelegate().isInAccessibleDrag()) {
            newScreen.enableAccessibleDrag(true, CellLayout.WORKSPACE_ACCESSIBILITY_DRAG);
        }

        return newScreen;
    }

经过搜索，在Workspace的代码中，只有insertNewWorkspaceScreen（）这块代码中有调用addView（）的方法，可以看到它添加的View便是newScreen，而它又是通过LayoutInflater加载的View，通过查看workspace_screen.xml可以知道，它就是一个CellLayout。

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2008 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->

<com.android.launcher3.CellLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:launcher="http://schemas.android.com/apk/res-auto"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:hapticFeedbackEnabled="false"
    launcher:containerType="workspace" />

进一步查看，可知有三个位置调用了insertNewWorkspaceScreen（）方法：

/**
     * Initializes and binds the first page
     * @param qsb an existing qsb to recycle or null.
     */
    public void bindAndInitFirstWorkspaceScreen(View qsb) {
        if (!FeatureFlags.QSB_ON_FIRST_SCREEN) {
            return;
        }
        // Add the first page
        CellLayout firstPage = insertNewWorkspaceScreen(Workspace.FIRST_SCREEN_ID, 0);
        // Always add a QSB on the first screen.
        if (qsb == null) {
            // In transposed layout, we add the QSB in the Grid. As workspace does not touch the
            // edges, we do not need a full width QSB.
            qsb = LayoutInflater.from(getContext())
                    .inflate(R.layout.search_container_workspace,firstPage, false);
        }

        CellLayout.LayoutParams lp = new CellLayout.LayoutParams(0, 0, firstPage.getCountX(), 1);
        lp.canReorder = false;
        if (!firstPage.addViewToCellLayout(qsb, 0, R.id.search_container_workspace, lp, true)) {
            Log.e(TAG, "Failed to add to item at (0, 0) to CellLayout");
        }
    }

    //插入新的CellLayout到后面
    public void insertNewWorkspaceScreenBeforeEmptyScreen(long screenId) {
        /**
         * Find the index to insert this view into.
         * If the empty screen exists, then insert it before that.
          */
        int insertIndex = mScreenOrder.indexOf(EXTRA_EMPTY_SCREEN_ID);
        if (insertIndex < 0) {
            insertIndex = mScreenOrder.size();
        }
        insertNewWorkspaceScreen(screenId, insertIndex);
    }

    /**
     * 当拖拽应用需要添加一屏时或者当前没有任何一屏时，
     * 调用此方法在Workspace中添加一个CellLayout
     * @param screenId
     */
    public void insertNewWorkspaceScreen(long screenId) {
        insertNewWorkspaceScreen(screenId, getChildCount());
    }

由此可知，Workspace是一个或多个CellLayout的父控件。

---

其它三个控件，在此也进行 一 一 说明下：

SearchDropTargetBar

SearchDropTargetBar它对应的就是launcher.xml中的drop_target_bar，它是用来显示当用户拖拽应用快捷图标时，屏幕上方会显示移除或者卸载按钮的控件；

<?xml version="1.0" encoding="utf-8"?><!--
     Copyright (C) 2018 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->
<com.android.launcher3.DropTargetBar xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="@dimen/dynamic_grid_drop_target_size"
    android:layout_gravity="center_horizontal|top"
    android:focusable="false"
    android:alpha="0"
    android:theme="@style/HomeScreenElementTheme"
    android:visibility="invisible">

    <!-- Delete target -->
    <com.android.launcher3.DeleteDropTarget
        android:id="@+id/delete_target_text"
        style="@style/DropTargetButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:gravity="center"
        android:text="@string/remove_drop_target_label" />

    <!-- Uninstall target -->
    <com.android.launcher3.SecondaryDropTarget
        android:id="@+id/uninstall_target_text"
        style="@style/DropTargetButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:gravity="center"
        android:text="@string/uninstall_drop_target_label" />

</com.android.launcher3.DropTargetBar>

PageIndicator

PageIndicator对应launcher.xml文件中的WorkspacePageIndicator，它是Workspace中CellLayout也没的指示器，它们通过launcher:pageIndicator属性关联起来；

        <com.android.launcher3.pageindicators.WorkspacePageIndicator
            android:id="@+id/page_indicator"
            android:layout_width="match_parent"
            android:layout_height="@dimen/vertical_drag_handle_size"
            android:layout_gravity="bottom|center_horizontal"
            android:theme="@style/HomeScreenElementTheme" />

        <com.android.launcher3.Workspace
            android:id="@+id/workspace"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_gravity="center"
            android:theme="@style/HomeScreenElementTheme"
            launcher:pageIndicator="@+id/page_indicator" />

Hotseat

Hotseat对应launcher.xml文件中的hotseat，它是桌面上常驻底部图标栏，经查看它里面也是一个CellLayout，查看源码可知，它确实只显示一行或者一列。

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2011 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->
<com.android.launcher3.Hotseat
    android:theme="@style/HomeScreenElementTheme"
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:launcher="http://schemas.android.com/apk/res-auto">

    <com.android.launcher3.CellLayout
        android:id="@+id/layout"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="center"
        launcher:containerType="hotseat"
        android:importantForAccessibility="no" />
</com.android.launcher3.Hotseat>

        if (hasVerticalHotseat) {
            mContent.setGridSize(1, idp.numHotseatIcons);
        } else {
            mContent.setGridSize(idp.numHotseatIcons, 1);
        }

七

在示意图中，有看到CellLayout布局，在xml文件中没有看到它；
从前面的介绍可知，CellLayout是Workspace或者Hotseat的子View,它在Workspace中时是用来显示桌面上的快捷方式或者小部件的，在Hotseat中是用来显示常驻的快捷方式的。
翻看源码可知，CellLayout是根据配置将页面分割成同样的矩形，在矩形中显示快捷方式，或者小部件，每个应用的快捷方式占一个矩形，而如果是小部件，则需要根据小部件的大小来确定需要占几个矩形。

//celllayout宽高减掉padding值之后所剩余的大小
int childWidthSize = widthSize - (getPaddingLeft() + getPaddingRight());
int childHeightSize = heightSize - (getPaddingTop() + getPaddingBottom());

//均宽
int cw = DeviceProfile.calculateCellWidth(childWidthSize, mCountX);
//均高
int ch = DeviceProfile.calculateCellHeight(childHeightSize, mCountY);

    public static int calculateCellWidth(int width, int countX) {
        return width / countX;
    }
    public static int calculateCellHeight(int height, int countY) {
        return height / countY;
    }

      	 //从配置文件中获取行数和列数
        DeviceProfile grid = mLauncher.getDeviceProfile();
        mCountX = grid.inv.numColumns;
        mCountY = grid.inv.numRows;

总结

本篇文章其实只是对launcher.xml文件的介绍，通过查看源码方式，对每个控件都有了新的认识；在其它地方看到了launcher布局的示意图，它帮助我理解launche布局，同时它也有不足之处，促使我也画一个自己理解的launcher布局示意图，如下：

此图为个人理解，不喜勿喷。

参考资料

墨香带你学Launcher
Android 9.0 Launcher源码分析（三）——Launcher的布局与多设备适配
Android M Launcher3主流程源码浅析
还有许多已查看，但找不到的资料了，感谢各位的无私分享，谢谢。