Android Home鍵詳解

當我們從Home點擊ShortCut圖標啟動一個應用程序後，這個應用程序打開了很多個Activity，假設順序為A,B,C，然後我們按Home鍵，再次從桌面用圖標啟動這個應用程序，我們會發現顯示的是剛才的C,而不是A。這裡我們普遍的想法是按Home鍵是讓程序退到後台，然後讓桌面顯示出來。那麼我們就來看看Home鍵到底是怎麼回事。

在Framework中我們找到源碼，我們首先在interceptKeyBeforeDispatching這個方法中找到Home按鍵代碼如下

[java]

1. // If the HOME button is currently being held, then we do special
2. // chording with it.
3. if (mHomePressed) {
5. // If we have released the home key, and didn't do anything else
6. // while it was pressed, then it is time to go home!
7. if (keyCode == KeyEvent.KEYCODE_HOME) {
8. if (!down) {
9. mHomePressed = false;
11. if (!canceled) {
12. // If an incoming call is ringing, HOME is totally disabled.
13. // (The user is already on the InCallScreen at this point,
14. // and his ONLY options are to answer or reject the call.)
15. boolean incomingRinging = false;
16. try {
17. ITelephony phoneServ = getPhoneInterface();
18. if (phoneServ != null) {
19. incomingRinging = phoneServ.isRinging();
20. } else {
21. Log.w(TAG, "Unable to find ITelephony interface");
22. }
23. } catch (RemoteException ex) {
24. Log.w(TAG, "RemoteException from getPhoneInterface()", ex);
25. }
27. if (incomingRinging) {
28. Log.i(TAG, "Ignoring HOME; there's a ringing incoming call.");
29. } else {
30. launchHomeFromHotKey();
31. }
32. } else {
33. Log.i(TAG, "Ignoring HOME; event canceled.");
34. }
35. }
36. }
38. return true;
39. }

這裡就是按Home鍵時執行的方法我們找到 [java]

1. launchHomeFromHotKey();
2. /**
3. * A home key -> launch home action was detected. Take the appropriate action
4. * given the situation with the keyguard.
5. */
6. void launchHomeFromHotKey() {
7. if (mKeyguardMediator.isShowingAndNotHidden()) {
8. // don't launch home if keyguard showing
9. } else if (!mHideLockScreen && mKeyguardMediator.isInputRestricted()) {
10. // when in keyguard restricted mode, must first verify unlock
11. // before launching home
12. mKeyguardMediator.verifyUnlock(new OnKeyguardExitResult() {
13. public void onKeyguardExitResult(boolean success) {
14. if (success) {
15. try {
16. ActivityManagerNative.getDefault().stopAppSwitches();
17. } catch (RemoteException e) {
18. }
19. sendCloseSystemWindows(SYSTEM_DIALOG_REASON_HOME_KEY);
20. startDockOrHome();
21. }
22. }
23. });
24. } else {
25. // no keyguard stuff to worry about, just launch home!
26. try {
27. ActivityManagerNative.getDefault().stopAppSwitches();
28. } catch (RemoteException e) {
29. }
30. sendCloseSystemWindows(SYSTEM_DIALOG_REASON_HOME_KEY);
31. startDockOrHome();
32. }
33. }

再進入startDockOrHome();

[java]

1. void startDockOrHome() {
2. Intent dock = createHomeDockIntent();
3. if (dock != null) {
4. try {
5. mContext.startActivity(dock);
6. return;
7. } catch (ActivityNotFoundException e) {
8. }
9. }
10. mContext.startActivity(mHomeIntent);
11. }

這裡我們發現，源碼中做了車載模式和普通模式Home的區別，在Android2.3原生系統中有車載模式這個應用程式，打開後按Home鍵我們返回的是車載模式的桌面，而普通情況下按Home返回正常桌面,再來看看mContext.startActivity(mHomeIntent)這個方法，我們發現，Home鍵也是打開一個activity，不同的是下面的代碼

[java]

1. mHomeIntent = new Intent(Intent.ACTION_MAIN, null);
2. mHomeIntent.addCategory(Intent.CATEGORY_HOME);
3. mHomeIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK
4. | Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED);

這段代碼是在init()方法中，對mHomeIntent的初始化，這個Intent就是跳轉到Launcher程序中的配置了 <category android:name="android.intent.category.HOME" />這個屬性的Launcher Activity,另外我們還看到兩個flag [java]

1. FLAG_ACTIVITY_NEW_TASK和FLAG_ACTIVITY_RESET_TASK_IF_NEEDED

newTask保證了我們回到之前Launcher所在的棧,reset task if need是設置清理棧標志，保證清除掉該棧除Launcher以外其他activity(在Launcher清單文件中我們還看到android:clearTaskOnLaunch="true"這個屬性，後面會描述),這樣我們的Launcher當然會在自己單獨的Task中,而且android:launchMode="singleTask"這個屬性保證不會啟動一個新的Launcher。

通過Launcher啟動的其他Activity不會跑到Launcher所在的Task中(後面的文章會提到Launcher啟動activity的過程)。所以按Home鍵是打開了Launcher這個activity並且保證他在自己單獨的Task中，其他的activity進入stop狀態。

另外Launcher中打開一個activity直接進入之前打開的Task 而不是新打開一個activity.