对Android近期任务列表（Recent Applications）的简单分析
转载请注明出处，谢谢！
这里的近期任务列表就是长按Home键出来的那个Dialog，里面放着近期打开过的应用，当然3.0以上系统的多任务切换键也是。
这个Dialog的实现在Android源码的/frameworks/base/policy/src/com/android/internal/policy/impl/RecentApplicationsDialog.java中。
接下来就对这个源码分析一下。
先把整个源码贴出来：
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 //默认显示8个 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 //关键在哪，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 }
从源码可以看出，关键部分有三处。
一个很关键的内部类：
// 每个任务都包含一个Tag，这个Tag保存着这个App的一些非常有用的信息 class RecentTag { ActivityManager.RecentTaskInfo info; Intent intent; }
这个RecentTag保存在每个近期任务的图标里，并且保存着这个任务的原始信息。
刚启动Dialog时对每个任务的初始化：
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 //拿到最近使用的应用的信息列表 9 final List<ActivityManager.RecentTaskInfo> recentTasks = 10 am.getRecentTasks(MAX_RECENT_TASKS, ActivityManager.RECENT_IGNORE_UNAVAILABLE); 11 12 //自制一个home activity info，用来区分 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 //开始初始化每个任务的信息 22 for (int i = 0; i < numTasks && (index < NUM_BUTTONS); ++i) { 23 final ActivityManager.RecentTaskInfo info = recentTasks.get(i); 24 25 //复制一个任务的原始Intent 26 Intent intent = new Intent(info.baseIntent); 27 if (info.origActivity != null) { 28 intent.setComponent(info.origActivity); 29 } 30 31 //跳过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一个Tag，保存这个任务的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 ...//无关紧要的代码 68 }
这里的过程是：先用ActivityManager获取RecentTasks并生成一个List，然后利用这个List为Dialog中的每个任务初始化，并生成对应的信息RecentTag。
需要注意的是，RecentTag中的Intent是从对应任务的原始Intent复制过来的，这意味着那个原始Intent的一些Extra参数将会一并复制过来，
我来举个例子：比如我的App支持从第三方启动，并且第三方要提供一个token，当然这个token就以Extra参数的形式放进Intent里，然后通过startActivity()启动我的App；然后我的App根据这个token来处理，注意这里，当我的App退出后，再从近期任务里启动这个App，之前的那个token还会传递给我的App，这里就会出现错误了，原因就是上面分析的。这就是为什么从第三方跳转的应用不会出现在近期任务的列表里（比如点击短信里的url启动一个浏览器，之后近期任务里只有短信app，不会出现浏览器app）。要想不出现在近期任务里，可以给Intent设置FLAG_ACTIVITY_NO_HISTORY标志。
响应每个任务的点击事件：
1 private void switchTo(RecentTag tag) { 2 if (tag.info.id >= 0) { 3 // 这个Task没有退出，直接移动到前台 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退出了的话，id为-1，则使用RecentTag中的Intent重新启动 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 }
如果该Task没有退出，只是切到后台，则切换到前台；如果已经退出，就要重新启动了。
这里的Intent就是之前说的，重复使用的旧Intent了，这里注意，系统添加了FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY和FLAG_ACTIVITY_TASK_ON_HOME标志，所以我们可以在App中通过判断Intent的flag是否包含这两个来判断是否是从近期任务里启动的。注意FLAG_ACTIVITY_TASK_ON_HOME标志是Api 11添加的，所以11一下的之判断FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY就行了。
