對Android近期任務列表（Recent Applications）的簡單分析

本文轉載自查看原文 2013-06-05 13:02 28252 task/ android/ 近期任務/ 源碼/ recent/ Android學習手記

轉載請注明出處，謝謝！

這里的近期任務列表就是長按Home鍵出來的那個Dialog，里面放着近期打開過的應用，當然3.0以上系統的多任務切換鍵也是。

這個Dialog的實現在Android源碼的/frameworks/base/policy/src/com/android/internal/policy/impl/RecentApplicationsDialog.java中。

接下來就對這個源碼分析一下。

先把整個源碼貼出來：

  1 public class RecentApplicationsDialog extends Dialog implements OnClickListener {
  2     // Elements for debugging support
  3 //  private static final String LOG_TAG = "RecentApplicationsDialog";
  4     private static final boolean DBG_FORCE_EMPTY_LIST = false;
  5 
  6     static private StatusBarManager sStatusBar;
  7 
  8     private static final int NUM_BUTTONS = 8;
  9     private static final int MAX_RECENT_TASKS = NUM_BUTTONS * 2;    // allow for some discards
 10 
 11     final TextView[] mIcons = new TextView[NUM_BUTTONS];
 12     View mNoAppsText;
 13     IntentFilter mBroadcastIntentFilter = new IntentFilter(Intent.ACTION_CLOSE_SYSTEM_DIALOGS);
 14 
 15     class RecentTag {
 16         ActivityManager.RecentTaskInfo info;
 17         Intent intent;
 18     }
 19 
 20     Handler mHandler = new Handler();
 21     Runnable mCleanup = new Runnable() {
 22         public void run() {
 23             // dump extra memory we're hanging on to
 24             for (TextView icon: mIcons) {
 25                 icon.setCompoundDrawables(null, null, null, null);
 26                 icon.setTag(null);
 27             }
 28         }
 29     };
 30 
 31     public RecentApplicationsDialog(Context context) {
 32         super(context, com.android.internal.R.style.Theme_Dialog_RecentApplications);
 33 
 34     }
 35 
 36     /**
 37      * We create the recent applications dialog just once, and it stays around (hidden)
 38      * until activated by the user.
 39      *
 40      * @see PhoneWindowManager#showRecentAppsDialog
 41      */
 42     @Override
 43     protected void onCreate(Bundle savedInstanceState) {
 44         super.onCreate(savedInstanceState);
 45 
 46         Context context = getContext();
 47 
 48         if (sStatusBar == null) {
 49             sStatusBar = (StatusBarManager)context.getSystemService(Context.STATUS_BAR_SERVICE);
 50         }
 51 
 52         Window window = getWindow();
 53         window.requestFeature(Window.FEATURE_NO_TITLE);
 54         window.setType(WindowManager.LayoutParams.TYPE_SYSTEM_DIALOG);
 55         window.setFlags(WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM,
 56                 WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM);
 57         window.setTitle("Recents");
 58 
 59         setContentView(com.android.internal.R.layout.recent_apps_dialog);
 60 
 61         final WindowManager.LayoutParams params = window.getAttributes();
 62         params.width = WindowManager.LayoutParams.MATCH_PARENT;
 63         params.height = WindowManager.LayoutParams.MATCH_PARENT;
 64         window.setAttributes(params);
 65         window.setFlags(0, WindowManager.LayoutParams.FLAG_DIM_BEHIND);
 66 
 67         //默認顯示8個
 68         mIcons[0] = (TextView)findViewById(com.android.internal.R.id.button0);
 69         mIcons[1] = (TextView)findViewById(com.android.internal.R.id.button1);
 70         mIcons[2] = (TextView)findViewById(com.android.internal.R.id.button2);
 71         mIcons[3] = (TextView)findViewById(com.android.internal.R.id.button3);
 72         mIcons[4] = (TextView)findViewById(com.android.internal.R.id.button4);
 73         mIcons[5] = (TextView)findViewById(com.android.internal.R.id.button5);
 74         mIcons[6] = (TextView)findViewById(com.android.internal.R.id.button6);
 75         mIcons[7] = (TextView)findViewById(com.android.internal.R.id.button7);
 76         mNoAppsText = findViewById(com.android.internal.R.id.no_applications_message);
 77 
 78         //關鍵在哪，你懂得...
 79         for (TextView b: mIcons) {
 80             b.setOnClickListener(this);
 81         }
 82     }
 83 
 84     @Override
 85     public boolean onKeyDown(int keyCode, KeyEvent event) {
 86         if (keyCode == KeyEvent.KEYCODE_TAB) {
 87             // Ignore all meta keys other than SHIFT.  The app switch key could be a
 88             // fallback action chorded with ALT, META or even CTRL depending on the key map.
 89             // DPad navigation is handled by the ViewRoot elsewhere.
 90             final boolean backward = event.isShiftPressed();
 91             final int numIcons = mIcons.length;
 92             int numButtons = 0;
 93             while (numButtons < numIcons && mIcons[numButtons].getVisibility() == View.VISIBLE) {
 94                 numButtons += 1;
 95             }
 96             if (numButtons != 0) {
 97                 int nextFocus = backward ? numButtons - 1 : 0;
 98                 for (int i = 0; i < numButtons; i++) {
 99                     if (mIcons[i].hasFocus()) {
100                         if (backward) {
101                             nextFocus = (i + numButtons - 1) % numButtons;
102                         } else {
103                             nextFocus = (i + 1) % numButtons;
104                         }
105                         break;
106                     }
107                 }
108                 final int direction = backward ? View.FOCUS_BACKWARD : View.FOCUS_FORWARD;
109                 if (mIcons[nextFocus].requestFocus(direction)) {
110                     mIcons[nextFocus].playSoundEffect(
111                             SoundEffectConstants.getContantForFocusDirection(direction));
112                 }
113             }
114 
115             // The dialog always handles the key to prevent the ViewRoot from
116             // performing the default navigation itself.
117             return true;
118         }
119 
120         return super.onKeyDown(keyCode, event);
121     }
122 
123     /**
124      * Dismiss the dialog and switch to the selected application.
125      */
126     public void dismissAndSwitch() {
127         final int numIcons = mIcons.length;
128         RecentTag tag = null;
129         for (int i = 0; i < numIcons; i++) {
130             if (mIcons[i].getVisibility() != View.VISIBLE) {
131                 break;
132             }
133             if (i == 0 || mIcons[i].hasFocus()) {
134                 tag = (RecentTag) mIcons[i].getTag();
135                 if (mIcons[i].hasFocus()) {
136                     break;
137                 }
138             }
139         }
140         if (tag != null) {
141             switchTo(tag);
142         }
143         dismiss();
144     }
145 
146     /**
147      * Handler for user clicks.  If a button was clicked, launch the corresponding activity.
148      */
149     public void onClick(View v) {
150         for (TextView b: mIcons) {
151             if (b == v) {
152                 RecentTag tag = (RecentTag)b.getTag();
153                 switchTo(tag);
154                 break;
155             }
156         }
157         dismiss();
158     }
159 
160     //
161     private void switchTo(RecentTag tag) {
162         if (tag.info.id >= 0) {
163             // This is an active task; it should just go to the foreground.
164             final ActivityManager am = (ActivityManager)
165                     getContext().getSystemService(Context.ACTIVITY_SERVICE);
166             am.moveTaskToFront(tag.info.id, ActivityManager.MOVE_TASK_WITH_HOME);
167         } else if (tag.intent != null) {
168             tag.intent.addFlags(Intent.FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY
169                     | Intent.FLAG_ACTIVITY_TASK_ON_HOME);
170             try {
171                 getContext().startActivity(tag.intent);
172             } catch (ActivityNotFoundException e) {
173                 Log.w("Recent", "Unable to launch recent task", e);
174             }
175         }
176     }
177 
178     /**
179      * Set up and show the recent activities dialog.
180      */
181     @Override
182     public void onStart() {
183         super.onStart();
184         reloadButtons();
185         if (sStatusBar != null) {
186             sStatusBar.disable(StatusBarManager.DISABLE_EXPAND);
187         }
188 
189         // receive broadcasts
190         getContext().registerReceiver(mBroadcastReceiver, mBroadcastIntentFilter);
191 
192         mHandler.removeCallbacks(mCleanup);
193     }
194 
195     /**
196      * Dismiss the recent activities dialog.
197      */
198     @Override
199     public void onStop() {
200         super.onStop();
201 
202         if (sStatusBar != null) {
203             sStatusBar.disable(StatusBarManager.DISABLE_NONE);
204         }
205 
206         // stop receiving broadcasts
207         getContext().unregisterReceiver(mBroadcastReceiver);
208 
209         mHandler.postDelayed(mCleanup, 100);
210      }
211 
212     /**
213      * Reload the 6 buttons with recent activities
214      */
215     private void reloadButtons() {
216 
217         final Context context = getContext();
218         final PackageManager pm = context.getPackageManager();
219         final ActivityManager am = (ActivityManager)
220                 context.getSystemService(Context.ACTIVITY_SERVICE);
221         final List<ActivityManager.RecentTaskInfo> recentTasks =
222                 am.getRecentTasks(MAX_RECENT_TASKS, ActivityManager.RECENT_IGNORE_UNAVAILABLE);
223 
224         ActivityInfo homeInfo = 
225             new Intent(Intent.ACTION_MAIN).addCategory(Intent.CATEGORY_HOME)
226                     .resolveActivityInfo(pm, 0);
227 
228         IconUtilities iconUtilities = new IconUtilities(getContext());
229 
230         // Performance note:  Our android performance guide says to prefer Iterator when
231         // using a List class, but because we know that getRecentTasks() always returns
232         // an ArrayList<>, we'll use a simple index instead.
233         int index = 0;
234         int numTasks = recentTasks.size();
235         for (int i = 0; i < numTasks && (index < NUM_BUTTONS); ++i) {
236             final ActivityManager.RecentTaskInfo info = recentTasks.get(i);
237 
238             // for debug purposes only, disallow first result to create empty lists
239             if (DBG_FORCE_EMPTY_LIST && (i == 0)) continue;
240 
241             Intent intent = new Intent(info.baseIntent);
242             if (info.origActivity != null) {
243                 intent.setComponent(info.origActivity);
244             }
245 
246             // Skip the current home activity.
247             if (homeInfo != null) {
248                 if (homeInfo.packageName.equals(
249                         intent.getComponent().getPackageName())
250                         && homeInfo.name.equals(
251                                 intent.getComponent().getClassName())) {
252                     continue;
253                 }
254             }
255 
256             intent.setFlags((intent.getFlags()&~Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED)
257                     | Intent.FLAG_ACTIVITY_NEW_TASK);
258             final ResolveInfo resolveInfo = pm.resolveActivity(intent, 0);
259             if (resolveInfo != null) {
260                 final ActivityInfo activityInfo = resolveInfo.activityInfo;
261                 final String title = activityInfo.loadLabel(pm).toString();
262                 Drawable icon = activityInfo.loadIcon(pm);
263 
264                 if (title != null && title.length() > 0 && icon != null) {
265                     final TextView tv = mIcons[index];
266                     tv.setText(title);
267                     icon = iconUtilities.createIconDrawable(icon);
268                     tv.setCompoundDrawables(null, icon, null, null);
269                     RecentTag tag = new RecentTag();
270                     tag.info = info;
271                     tag.intent = intent;
272                     tv.setTag(tag);
273                     tv.setVisibility(View.VISIBLE);
274                     tv.setPressed(false);
275                     tv.clearFocus();
276                     ++index;
277                 }
278             }
279         }
280 
281         // handle the case of "no icons to show"
282         mNoAppsText.setVisibility((index == 0) ? View.VISIBLE : View.GONE);
283 
284         // hide the rest
285         for (; index < NUM_BUTTONS; ++index) {
286             mIcons[index].setVisibility(View.GONE);
287         }
288     }
289 
290     /**
291      * This is the listener for the ACTION_CLOSE_SYSTEM_DIALOGS intent.  It's an indication that
292      * we should close ourselves immediately, in order to allow a higher-priority UI to take over
293      * (e.g. phone call received).
294      */
295     private BroadcastReceiver mBroadcastReceiver = new BroadcastReceiver() {
296         @Override
297         public void onReceive(Context context, Intent intent) {
298             String action = intent.getAction();
299             if (Intent.ACTION_CLOSE_SYSTEM_DIALOGS.equals(action)) {
300                 String reason = intent.getStringExtra(PhoneWindowManager.SYSTEM_DIALOG_REASON_KEY);
301                 if (! PhoneWindowManager.SYSTEM_DIALOG_REASON_RECENT_APPS.equals(reason)) {
302                     dismiss();
303                 }
304             }
305         }
306     };
307 }

RecentApplicationsDialog.java完整源碼

從源碼可以看出，關鍵部分有三處。

一個很關鍵的內部類：

// 每個任務都包含一個Tag，這個Tag保存着這個App的一些非常有用的信息
    class RecentTag {
        ActivityManager.RecentTaskInfo info;
        Intent intent;
    }

這個RecentTag保存在每個近期任務的圖標里，並且保存着這個任務的原始信息。

剛啟動Dialog時對每個任務的初始化：

 1     private void reloadButtons() {
 2 
 3         final Context context = getContext();
 4         final PackageManager pm = context.getPackageManager();
 5         final ActivityManager am = (ActivityManager)
 6                 context.getSystemService(Context.ACTIVITY_SERVICE);
 7                 
 8         //拿到最近使用的應用的信息列表
 9         final List<ActivityManager.RecentTaskInfo> recentTasks =
10                 am.getRecentTasks(MAX_RECENT_TASKS, ActivityManager.RECENT_IGNORE_UNAVAILABLE);
11 
12         //自制一個home activity info，用來區分
13         ActivityInfo homeInfo = 
14             new Intent(Intent.ACTION_MAIN).addCategory(Intent.CATEGORY_HOME)
15                     .resolveActivityInfo(pm, 0);
16 
17         IconUtilities iconUtilities = new IconUtilities(getContext());
18 
19         int index = 0;
20         int numTasks = recentTasks.size();
21         //開始初始化每個任務的信息
22         for (int i = 0; i < numTasks && (index < NUM_BUTTONS); ++i) {
23             final ActivityManager.RecentTaskInfo info = recentTasks.get(i);
24 
25             //復制一個任務的原始Intent
26             Intent intent = new Intent(info.baseIntent);
27             if (info.origActivity != null) {
28                 intent.setComponent(info.origActivity);
29             }
30 
31             //跳過home activity
32             if (homeInfo != null) {
33                 if (homeInfo.packageName.equals(
34                         intent.getComponent().getPackageName())
35                         && homeInfo.name.equals(
36                                 intent.getComponent().getClassName())) {
37                     continue;
38                 }
39             }
40 
41             intent.setFlags((intent.getFlags()&~Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED)
42                     | Intent.FLAG_ACTIVITY_NEW_TASK);
43             final ResolveInfo resolveInfo = pm.resolveActivity(intent, 0);
44             if (resolveInfo != null) {
45                 final ActivityInfo activityInfo = resolveInfo.activityInfo;
46                 final String title = activityInfo.loadLabel(pm).toString();
47                 Drawable icon = activityInfo.loadIcon(pm);
48 
49                 if (title != null && title.length() > 0 && icon != null) {
50                     final TextView tv = mIcons[index];
51                     tv.setText(title);
52                     icon = iconUtilities.createIconDrawable(icon);
53                     tv.setCompoundDrawables(null, icon, null, null);
54                     //new一個Tag，保存這個任務的RecentTaskInfo和Intent
55                     RecentTag tag = new RecentTag();
56                     tag.info = info;
57                     tag.intent = intent;
58                     tv.setTag(tag);
59                     tv.setVisibility(View.VISIBLE);
60                     tv.setPressed(false);
61                     tv.clearFocus();
62                     ++index;
63                 }
64             }
65         }
66 
67        ...//無關緊要的代碼
68     }

這里的過程是：先用ActivityManager獲取RecentTasks並生成一個List，然后利用這個List為Dialog中的每個任務初始化，並生成對應的信息RecentTag。

需要注意的是，RecentTag中的Intent是從對應任務的原始Intent復制過來的，這意味着那個原始Intent的一些Extra參數將會一並復制過來，

我來舉個例子：比如我的App支持從第三方啟動，並且第三方要提供一個token，當然這個token就以Extra參數的形式放進Intent里，然后通過startActivity()啟動我的App；然后我的App根據這個token來處理，注意這里，當我的App退出后，再從近期任務里啟動這個App，之前的那個token還會傳遞給我的App，這里就會出現錯誤了，原因就是上面分析的。這就是為什么從第三方跳轉的應用不會出現在近期任務的列表里（比如點擊短信里的url啟動一個瀏覽器，之后近期任務里只有短信app，不會出現瀏覽器app）。要想不出現在近期任務里，可以給Intent設置FLAG_ACTIVITY_NO_HISTORY標志。

響應每個任務的點擊事件：

 1     private void switchTo(RecentTag tag) {
 2         if (tag.info.id >= 0) {
 3             // 這個Task沒有退出，直接移動到前台
 4             final ActivityManager am = (ActivityManager)
 5                     getContext().getSystemService(Context.ACTIVITY_SERVICE);
 6             am.moveTaskToFront(tag.info.id, ActivityManager.MOVE_TASK_WITH_HOME);
 7         } else if (tag.intent != null) {
 8             //task退出了的話，id為-1，則使用RecentTag中的Intent重新啟動
 9             tag.intent.addFlags(Intent.FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY
10                     | Intent.FLAG_ACTIVITY_TASK_ON_HOME);
11             try {
12                 getContext().startActivity(tag.intent);
13             } catch (ActivityNotFoundException e) {
14                 Log.w("Recent", "Unable to launch recent task", e);
15             }
16         }
17     }

如果該Task沒有退出，只是切到后台，則切換到前台；如果已經退出，就要重新啟動了。

這里的Intent就是之前說的，重復使用的舊Intent了，這里注意，系統添加了FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY和FLAG_ACTIVITY_TASK_ON_HOME標志，所以我們可以在App中通過判斷Intent的flag是否包含這兩個來判斷是否是從近期任務里啟動的。注意FLAG_ACTIVITY_TASK_ON_HOME標志是Api 11添加的，所以11一下的之判斷FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY就行了。

免責聲明！

本站轉載的文章為個人學習借鑒使用，本站對版權不負任何法律責任。如果侵犯了您的隱私權益，請聯系本站郵箱yoyou2525@163.com刪除。

猜您在找 Android AsyncTask 深度理解、簡單封裝、任務隊列分析、自定義線程池 Android Launcher 3 簡單分析 Android如何屏蔽home鍵和recent鍵 Android.mk簡單分析 Android Studio no debuggable applications解決方案2 Android Monitor no debuggable Applications 解決方案近期找工作的一點總結 Android Android Studio中的Gradle面板沒有Task任務列表如何找回？木馬APP的簡單分析(Android Killer分析) Android：在任務列表隱藏最近打開的app