本文对Android O（8.0） Car Launcher进行全面解析。

Car Launcher的界面

  下图为Car Launcher的界面，

Car Launcher 界面

  跟普通的Launcher不同，Car Launcher没有应用抽屉，也没有左右滑动的应用页面；导航按钮也不是home，back，recent按钮，而是不同类型的应用按钮。下面先来看看导航按钮是如何构造的。

Car status bar / navigation bar 构造流程

  下图为Navigation bar的构造流程，它是 SystemUI 的一部分。

Navigation bar 构造流程

  Car status bar / navigation bar 构造是在 SystemUI 模块完成的，最终的执行是在 CarNavigationBarController 类的bind方法中，代码如下：

139     /**
140      * Iterates through the items in arrays_car.xml and sets up the facet bar buttons to
141      * perform the task in that configuration file when clicked or long-pressed.
142      */
143     private void bind() {
144         Resources res = mContext.getResources();
145 
146         TypedArray icons = res.obtainTypedArray(R.array.car_facet_icons);
147         TypedArray intents = res.obtainTypedArray(R.array.car_facet_intent_uris);
148         TypedArray longPressIntents = res.obtainTypedArray(R.array.car_facet_longpress_intent_uris);
149         TypedArray facetPackageNames = res.obtainTypedArray(R.array.car_facet_package_filters);
150         TypedArray facetCategories = res.obtainTypedArray(R.array.car_facet_category_filters);
151 
152         try {
153             if (icons.length() != intents.length()
154                     || icons.length() != longPressIntents.length()
155                     || icons.length() != facetPackageNames.length()
156                     || icons.length() != facetCategories.length()) {
157                 throw new RuntimeException("car_facet array lengths do not match");
158             }
159 
160             for (int i = 0, size = icons.length(); i < size; i++) {
161                 Drawable icon = icons.getDrawable(i);
162                 CarNavigationButton button = createNavButton(icon);
163                 initClickListeners(button, i, intents.getString(i), longPressIntents.getString(i));
164 
165                 mNavButtons.add(button);
166                 mNavBar.addButton(button, createNavButton(icon) /* lightsOutButton */);
167 
168                 initFacetFilterMaps(i, facetPackageNames.getString(i).split(FACET_FILTER_DELIMITER),
169                         facetCategories.getString(i).split(FACET_FILTER_DELIMITER));
170                 mFacetHasMultipleAppsCache.put(i, facetHasMultiplePackages(i));
171             }
172         } finally {
173             // Clean up all the TypedArrays.
174             icons.recycle();
175             intents.recycle();
176             longPressIntents.recycle();
177             facetPackageNames.recycle();
178             facetCategories.recycle();
179         }
180     }

  该方法遍历 arrays_car.xml文件的内容来构建导航栏按钮。我们来看 arrays_car.xml的定义，路径是packages/services/Car/car_product/overlay/frameworks/base/packages/SystemUI/
res/values/arrays_car.xml：

20 <resources>
21     <!-- There needs to be correspondence per index between these arrays, which means that if there
22          isn't a longpress action associated with a shortcut item, put in an empty item to make
23          sure everything lines up.
24     -->
25     <array name="car_facet_icons">
26         <item>@drawable/car_ic_navigation</item>
27         <item>@drawable/car_ic_phone</item>
28         <item>@drawable/car_ic_overview</item>
29         <item>@drawable/car_ic_music</item>
30         <item>@drawable/car_ic_car</item>
31     </array>
32     <array name="car_facet_intent_uris">
33         <!-- Launch the lenspicker for all the facets. The lens picker will trampoline into the last run app or display a list of valid apps -->
34         <item>intent:#Intent;action=android.intent.action.MAIN;category=android.intent.category.LAUNCHER;launchFlags=0x14000000;package=com.android.support.car.lenspicker;end</item>
35         <item>intent:#Intent;action=android.intent.action.MAIN;category=android.intent.category.LAUNCHER;launchFlags=0x14000000;package=com.android.support.car.lenspicker;end</item>
36         <item>intent:#Intent;action=android.intent.action.MAIN;category=android.intent.category.LAUNCHER;launchFlags=0x14000000;package=com.android.support.car.lenspicker;S.system_command=show_notifications;end</item>
37         <item>intent:#Intent;action=android.intent.action.MAIN;category=android.intent.category.LAUNCHER;launchFlags=0x14000000;package=com.android.support.car.lenspicker;end</item>
38         <item>intent:#Intent;action=android.intent.action.MAIN;category=android.intent.category.LAUNCHER;launchFlags=0x14000000;package=com.android.support.car.lenspicker;end</item>
39     </array>
40     <array name="car_facet_longpress_intent_uris">
41         <item></item>
42         <item></item>
43         <item></item>
44         <item></item>
45         <!-- Long pressing the overflow triggers a bug report -->
46         <item>intent:#Intent;component=com.google.android.car.bugreport/.BugReportActivity;end</item>
47     </array>
48     <array name="car_facet_category_filters">
49         <item>android.intent.category.APP_MAPS</item>
50         <item>android.intent.category.APP_MESSAGING</item>
51         <item></item>
52         <item>android.intent.category.APP_MUSIC</item>
53         <item></item>
54     </array>
55     <array name="car_facet_package_filters">
56         <item>com.android.car.mapsplaceholder</item>
57         <item>com.android.car.dialer</item>
58         <item>com.android.car.overview</item>
59         <item></item>
60         <item>com.android.car.hvac;com.android.settings;com.android.car.settings;com.android.vending;com.google.android.car.bugreport;com.google.android.car.kitchensink;com.android.car.systemupdater;org.chromium.webview_shell;com.android.contacts;org.codeaurora.bluetooth.bttestapp;com.google.android.projection.sink</item>
61     </array>
62 </resources>

  在 arrays_car.xml 中定义了一系列数组，它们对应的含义如下：

  有些 item 定义了多个值，用英文分号将每个值分开。

导航按钮单击响应

  我们来看导航按钮单击响应方法 CarNavigationBarController.onFacetClicked：

356     /**
357      * Handles a click on a facet. A click will trigger the given Intent.
358      *
359      * @param index The index of the facet that was clicked.
360      */
361     private void onFacetClicked(Intent intent, int index) {
362         String packageName = intent.getPackage();
363 
364         if (packageName == null) {
365             return;
366         }
367 
368         intent.putExtra(EXTRA_FACET_CATEGORIES, mFacetCategories.get(index));
369         intent.putExtra(EXTRA_FACET_PACKAGES, mFacetPackages.get(index));
370         // The facet is identified by the index in which it was added to the nav bar.
371         // This value can be used to determine which facet was selected
372         intent.putExtra(EXTRA_FACET_ID, Integer.toString(index));
373 
374         // If the current facet is clicked, we want to launch the picker by default
375         // rather than the "preferred/last run" app.
376         intent.putExtra(EXTRA_FACET_LAUNCH_PICKER, index == mCurrentFacetIndex);
377 
378         int stackId = StackId.FULLSCREEN_WORKSPACE_STACK_ID;
379         if (intent.getCategories().contains(Intent.CATEGORY_HOME)) {
380             stackId = StackId.HOME_STACK_ID;
381         }
382 
383         setCurrentFacet(index);
384         mStatusBar.startActivityOnStack(intent, stackId);
385     }

  参数 intent 是从 car_facet_intent_uris 数组解析而来，我们发现数组所有的条目对应的 package 都是 com.android.support.car.lenspicker， action 和 category 也都是一致的，可以推测出所有的条目都对应着同一个 activity，也就是 com.android.support.car.lenspicker 包中的 main activity。在LensPicker工程中，我们找到这个activity是 LensPickerTrampolineActivity。

LensPickerTrampolineActivity 顺序图如下：

LensPickerTrampolineActivity 顺序图

我们先来看看从 SystemUI 传过来的数据有哪些。

  可以发现除了限定要展示的应用类型，最后两个还指明了是否执行系统命令，目前只发现触发系统栏下拉这一个系统命令。对应的执行代码在上面序列图的第二步executeSystemCommand。

  LensPickerTrampolineActivity 的 onCreate 方法主要执行了下面的逻辑：

LensPickerTrampolineActivity构造逻辑图

  LensPickerTrampolineActivity 会触发五种操作：

• 执行 SystemCommand；
• 启动LensPickerActivity；
• 启动StreamOverviewActivity；
• 启动保存的app；
• 启动上次的intent；

应用列表展示

  我们再来看看 LensPickerActivity 是如何展示应用列表的。使用PagedListView展示列表，adapter为LensPickerAdapter，数据在getComponent方法中构造，

133     private ArrayList<ResolveInfo> getComponents(String[] packages, String[] categories) {
134         List<ResolveInfo> packageList = new ArrayList<>();
135         if (packages != null) {
136             for (int i = 0; i < packages.length; i++) {
137                 packageList.addAll(resolvePackage(packages[i]));
138             }
139         }
140 
141         if (categories != null) {
142             for (int i = 0; i < categories.length; i++) {
143                 packageList.addAll(resolveCategory(categories[i]));
144 
145                 if (categories[i].equals(Intent.CATEGORY_APP_MUSIC)) {
146                     packageList.addAll(resolveMediaBrowserServices());
147                 }
148             }
149         }
150 
151         // De-dupe the list based on package names
152         HashMap<String, ResolveInfo> dedupeList = new HashMap<>();
153         for (ResolveInfo pkg : packageList) {
154             String packageName = LensPickerUtils.getPackageName(pkg);
155             if (!dedupeList.containsKey(packageName) || LensPickerUtils.isMediaService(pkg)) {
156                 dedupeList.put(packageName, pkg);
157             }
158         }
159 
160         ArrayList<ResolveInfo> filteredPackageList = new ArrayList<>(dedupeList.values());
161         if (Log.isLoggable(TAG, Log.DEBUG)) {
162             printResolveInfo("before dedupe", packageList);
163             printResolveInfo("after dedupe", filteredPackageList);
164         }
165 
166         return filteredPackageList;
167     }

  其中resolveCategory 使用PackageManager 的 queryIntentActivities 方法查询对应 category 的 intent。

总结

  Android O Car Launcher 跟手机等设备的Launcher 实现差别较大，并没有 CarLauncher这个应用，而是由 SystemUI 的 CarStatusBar 构造的导航栏来完成 Launcher 的功能。每个导航按钮对应一个应用列表（由LensPickerActivity实现）。除了应用列表，也可以导航到单个的应用，比如默认应用Overview，以及上次导航到的应用。此外导航按钮还可以触发系统行为，比如系统菜单栏的下拉。