提供向上的導航 · Android Training官方培訓課程中文版

> 編寫:[Lin-H](https://github.com/Lin-H) - 原文:[http://developer.android.com/training/implementing-navigation/ancestral.html](http://developer.android.com/training/implementing-navigation/ancestral.html) 所有不是從主屏幕("home"屏幕)進入app的，都應該給用戶提供一種方法，通過點擊[action bar](http://developer.android.com/guide/topics/ui/actionbar.html)中的Up按鈕。可以回到app的結構層次中邏輯父屏幕。本課程向你說明如何正確地實現這一操作。 > **Up Navigation 設計** > [Designing Effective Navigation](http://developer.android.com/training/design-navigation/ancestral-temporal.html)和the [Navigation](http://developer.android.com/training/design-navigation/ancestral-temporal.html) design guide中描述了向上導航的概念和設計準則。 ![Figure 1. action bar中的Up按鈕.](https://box.kancloud.cn/2015-07-28_55b72476a881a.png) **Figure 1**. action bar中的Up按鈕. ### 指定父[Activity](# "An activity represents a single screen with a user interface.") 要實現向上導航，第一步就是為每一個[activity](# "An activity represents a single screen with a user interface.")聲明合適的父[activity](# "An activity represents a single screen with a user interface.")。這么做可以使系統簡化導航模式，例如向上導航，因為系統可以從manifest文件中判斷它的邏輯父(logical parent)[activity](# "An activity represents a single screen with a user interface.")。從Android 4.1 (API level 16)開始，你可以通過指定[`<activity>`](http://developer.android.com/guide/topics/manifest/activity-element.html)元素中的[android:parentActivityName](http://developer.android.com/guide/topics/manifest/activity-element.html#parent)屬性來聲明每一個[activity](# "An activity represents a single screen with a user interface.")的邏輯父[activity](# "An activity represents a single screen with a user interface.")。如果你的app需要支持Android 4.0以下版本，在你的app中包含[Support Library](http://developer.android.com/tools/support-library/index.html)并添加[`<meta-data>`](http://developer.android.com/guide/topics/manifest/meta-data-element.html)元素到[`<activity>`](http://developer.android.com/guide/topics/manifest/activity-element.html)中。然后指定父[activity](# "An activity represents a single screen with a user interface.")的值為`android.support.PARENT_ACTIVITY`，并匹配[android:parentActivityName](http://developer.android.com/guide/topics/manifest/activity-element.html#parent)的值。例如: ~~~ <application ... > ... <activity android:name="com.example.myfirstapp.MainActivity" ...> ... </activity> <activity android:name="com.example.myfirstapp.DisplayMessageActivity" android:label="@string/title_activity_display_message" android:parentActivityName="com.example.myfirstapp.MainActivity" > <meta-data android:name="android.support.PARENT_ACTIVITY" android:value="com.example.myfirstapp.MainActivity" /> </activity> </application> ~~~ 在父[activity](# "An activity represents a single screen with a user interface.")這樣聲明后，你可以使用[NavUtils](http://developer.android.com/reference/android/support/v4/app/NavUtils.html) API進行向上導航操作，就像下一面這節。 ### 添加向上操作(Up Action) 要使用action bar的app圖標來完成向上導航，需要調用[setDisplayHomeAsUpEnabled()](http://developer.android.com/reference/android/app/ActionBar.html#setDisplayHomeAsUpEnabled%28boolean%29): ~~~ @Override public void onCreate(Bundle savedInstanceState) { ... getActionBar().setDisplayHomeAsUpEnabled(true); } ~~~ 這樣，在app旁添加了一個左向符號，并用作操作按鈕。當用戶點擊它時，你的[activity](# "An activity represents a single screen with a user interface.")會接收一個對[onOptionsItemSelected()](http://developer.android.com/reference/android/app/Activity.html#onOptionsItemSelected%28android.view.MenuItem%29)的調用。操作的ID是`android.R.id.home`。 ### 向上導航至父[activity](# "An activity represents a single screen with a user interface.") 要在用戶點擊app圖標時向上導航，你可以使用[NavUtils](http://developer.android.com/reference/android/support/v4/app/NavUtils.html)類中的靜態方法[navigateUpFromSameTask()](http://developer.android.com/reference/android/support/v4/app/NavUtils.html#navigateUpFromSameTask%28android.app.Activity%29)。當你調用這一方法時，系統會結束當前的[activity](# "An activity represents a single screen with a user interface.")并啟動(或恢復)相應的父[activity](# "An activity represents a single screen with a user interface.")。如果目標[activity](# "An activity represents a single screen with a user interface.")在任務的后退棧中(back stack)，則目標[activity](# "An activity represents a single screen with a user interface.")會像[FLAG_ACTIVITY_CLEAR_TOP](http://developer.android.com/reference/android/content/Intent.html#FLAG_ACTIVITY_CLEAR_TOP)定義的那樣，提到棧頂。提到棧頂的方式取決于父[activity](# "An activity represents a single screen with a user interface.")是否處理了對[onNewIntent()](http://developer.android.com/reference/android/app/Activity.html#onNewIntent(android.content.Intent))的調用。例如: ~~~ @Override public boolean onOptionsItemSelected(MenuItem item) { switch (item.getItemId()) { // 對action bar的Up/Home按鈕做出反應 case android.R.id.home: NavUtils.navigateUpFromSameTask(this); return true; } return super.onOptionsItemSelected(item); } ~~~ 但是，**只能是當你的app擁有當前任務(current task)**(用戶從你的app中發起這一任務)時[navigateUpFromSameTask()](http://developer.android.com/reference/android/support/v4/app/NavUtils.html#navigateUpFromSameTask%28android.app.Activity%29)才有用。如果你的[activity](# "An activity represents a single screen with a user interface.")是從別的app的任務中啟動的話，向上導航操作就應該創建一個屬于你的app的新任務，并需要你創建一個新的后退棧。 ### 用新的后退棧來向上導航如果你的[activity](# "An activity represents a single screen with a user interface.")提供了任何允許被別的app啟動的[intent filters](http://developer.android.com/guide/components/intents-filters.html#ifs)，那么你應該實現[onOptionsItemSelected()](http://developer.android.com/reference/android/app/Activity.html#onOptionsItemSelected%28android.view.MenuItem%29)回調，在用戶從別的app任務進入你的[activity](# "An activity represents a single screen with a user interface.")后，點擊Up按鈕，在向上導航之前你的app用相應的后退棧開啟一個新的任務。在這么做之前，你可以先調用[shouldUpRecreateTask()](http://developer.android.com/reference/android/support/v4/app/NavUtils.html#shouldUpRecreateTask%28android.app.Activity,%20android.content.Intent%29)來檢查當前的[activity](# "An activity represents a single screen with a user interface.")實例是否在另一個不同的app任務中。如果返回true，就使用[TaskStackBuilder](http://developer.android.com/reference/android/support/v4/app/TaskStackBuilder.html)創建一個新任務。或者，你可以向上面那樣使用[navigateUpFromSameTask()](http://developer.android.com/reference/android/support/v4/app/NavUtils.html#navigateUpFromSameTask%28android.app.Activity%29)方法。例如: ~~~ @Override public boolean onOptionsItemSelected(MenuItem item) { switch (item.getItemId()) { // 對action bar的Up/Home按鈕做出反應 case android.R.id.home: Intent upIntent = NavUtils.getParentActivityIntent(this); if (NavUtils.shouldUpRecreateTask(this, upIntent)) { // 這個activity不是這個app任務的一部分, 所以當向上導航時創建 // 用合成后退棧(synthesized back stack)創建一個新任務。 TaskStackBuilder.create(this) // 添加這個activity的所有父activity到后退棧中 .addNextIntentWithParentStack(upIntent) // 向上導航到最近的一個父activity .startActivities(); } else { // 這個activity是這個app任務的一部分, 所以 // 向上導航至邏輯父activity. NavUtils.navigateUpTo(this, upIntent); } return true; } return super.onOptionsItemSelected(item); } ~~~ > **Note**:為了能使[addNextIntentWithParentStack()](http://developer.android.com/reference/android/support/v4/app/TaskStackBuilder.html#addNextIntentWithParentStack%28android.content.Intent%29)方法起作用，你必須像上面說的那樣，在你的manifest文件中使用[android:parentActivityName](http://developer.android.com/guide/topics/manifest/activity-element.html#parent)(和相應的[`<meta-data>`](http://developer.android.com/guide/topics/manifest/meta-data-element.html)元素)屬性聲明所有的[activity](# "An activity represents a single screen with a user interface.")的邏輯父[activity](# "An activity represents a single screen with a user interface.")。