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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。
来源：https://www.cnblogs.com/coding-way/archive/2013/06/05/3118732.html