??本文對Android O（8.0） Car Launcher進行全面解析。

Car Launcher的界面

??下圖為Car Launcher的界面，

Car Launcher 界面

??跟普通的Launcher不同，Car Launcher沒有應用抽屜，也沒有左右滑動的應用頁面；導航按鈕也不是home，back，recent按鈕，而是不同類型的應用按鈕。下面先來看看導航按鈕是如何構造的。

Car status bar / navigation bar 構造流程

??下圖為Navigation bar的構造流程，它是 SystemUI 的一部分。

Navigation bar 構造流程

??Car status bar / navigation bar 構造是在 SystemUI 模塊完成的，最終的執行是在 CarNavigationBarController 類的bind方法中，代碼如下：

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     }

??該方法遍歷 arrays_car.xml文件的內容來構建導航欄按鈕。我們來看 arrays_car.xml的定義，路徑是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 中定義了一系列數組，它們對應的含義如下：

??有些 item 定義了多個值，用英文分號將每個值分開。

導航按鈕單擊響應

??我們來看導航按鈕單擊響應方法 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     }

??參數 intent 是從 car_facet_intent_uris 數組解析而來，我們發現數組所有的條目對應的 package 都是 com.android.support.car.lenspicker， action 和 category 也都是一致的，可以推測出所有的條目都對應著同一個 activity，也就是 com.android.support.car.lenspicker 包中的 main activity。在LensPicker工程中，我們找到這個activity是 LensPickerTrampolineActivity。

LensPickerTrampolineActivity 順序圖如下：

LensPickerTrampolineActivity 順序圖

我們先來看看從 SystemUI 傳過來的數據有哪些。

??可以發現除了限定要展示的應用類型，最后兩個還指明了是否執行系統命令，目前只發現觸發系統欄下拉這一個系統命令。對應的執行代碼在上面序列圖的第二步executeSystemCommand。

??LensPickerTrampolineActivity 的 onCreate 方法主要執行了下面的邏輯：

LensPickerTrampolineActivity構造邏輯圖

??LensPickerTrampolineActivity 會觸發五種操作：

• 執行 SystemCommand；
• 啟動LensPickerActivity；
• 啟動StreamOverviewActivity；
• 啟動保存的app；
• 啟動上次的intent；

應用列表展示

??我們再來看看 LensPickerActivity 是如何展示應用列表的。使用PagedListView展示列表，adapter為LensPickerAdapter，數據在getComponent方法中構造，

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 方法查詢對應 category 的 intent。

總結

??Android O Car Launcher 跟手機等設備的Launcher 實現差別較大，并沒有 CarLauncher這個應用，而是由 SystemUI 的 CarStatusBar 構造的導航欄來完成 Launcher 的功能。每個導航按鈕對應一個應用列表（由LensPickerActivity實現）。除了應用列表，也可以導航到單個的應用，比如默認應用Overview，以及上次導航到的應用。此外導航按鈕還可以觸發系統行為，比如系統菜單欄的下拉。