提供向上的导航

> 编写:[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) 所有不是从主屏幕("home"屏幕)进入app的，都应该给用户提供一种方法，通过点击[action bar](http://developer.android.com/guide/topics/ui/actionbar.html)中的Up按钮。可以回到app的结构层次中逻辑父屏幕。本课程向你说明如何正确地实现这一操作。 > **Up Navigation 设计** > [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中描述了向上导航的概念和设计准则。  **Figure 1**. action bar中的Up按钮. ### 指定父[Activity](# "An activity represents a single screen with a user interface.") 要实现向上导航，第一步就是为每一个[activity](# "An activity represents a single screen with a user interface.")声明合适的父[activity](# "An activity represents a single screen with a user interface.")。这么做可以使系统简化导航模式，例如向上导航，因为系统可以从manifest文件中判断它的逻辑父(logical parent)[activity](# "An activity represents a single screen with a user interface.")。 从Android 4.1 (API level 16)开始，你可以通过指定[`<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)属性来声明每一个[activity](# "An activity represents a single screen with a user interface.")的逻辑父[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.")的值为`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.")这样声明后，你可以使用[NavUtils](http://developer.android.com/reference/android/support/v4/app/NavUtils.html) API进行向上导航操作，就像下一面这节。 ### 添加向上操作(Up Action) 要使用action bar的app图标来完成向上导航，需要调用[setDisplayHomeAsUpEnabled()](http://developer.android.com/reference/android/app/ActionBar.html#setDisplayHomeAsUpEnabled%28boolean%29): ~~~ @Override public void onCreate(Bundle savedInstanceState) { ... getActionBar().setDisplayHomeAsUpEnabled(true); } ~~~ 这样，在app旁添加了一个左向符号，并用作操作按钮。当用户点击它时，你的[activity](# "An activity represents a single screen with a user interface.")会接收一个对[onOptionsItemSelected()](http://developer.android.com/reference/android/app/Activity.html#onOptionsItemSelected%28android.view.MenuItem%29)的调用。操作的ID是`android.R.id.home`。 ### 向上导航至父[activity](# "An activity represents a single screen with a user interface.") 要在用户点击app图标时向上导航，你可以使用[NavUtils](http://developer.android.com/reference/android/support/v4/app/NavUtils.html)类中的静态方法[navigateUpFromSameTask()](http://developer.android.com/reference/android/support/v4/app/NavUtils.html#navigateUpFromSameTask%28android.app.Activity%29)。当你调用这一方法时，系统会结束当前的[activity](# "An activity represents a single screen with a user interface.")并启动(或恢复)相应的父[activity](# "An activity represents a single screen with a user interface.")。如果目标[activity](# "An activity represents a single screen with a user interface.")在任务的后退栈中(back stack)，则目标[activity](# "An activity represents a single screen with a user interface.")会像[FLAG_ACTIVITY_CLEAR_TOP](http://developer.android.com/reference/android/content/Intent.html#FLAG_ACTIVITY_CLEAR_TOP)定义的那样，提到栈顶。提到栈顶的方式取决于父[activity](# "An activity represents a single screen with a user interface.")是否处理了对[onNewIntent()](http://developer.android.com/reference/android/app/Activity.html#onNewIntent(android.content.Intent))的调用。 例如: ~~~ @Override public boolean onOptionsItemSelected(MenuItem item) { switch (item.getItemId()) { // 对action bar的Up/Home按钮做出反应 case android.R.id.home: NavUtils.navigateUpFromSameTask(this); return true; } return super.onOptionsItemSelected(item); } ~~~ 但是，**只能是当你的app拥有当前任务(current task)**(用户从你的app中发起这一任务)时[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.")是从别的app的任务中启动的话，向上导航操作就应该创建一个属于你的app的新任务，并需要你创建一个新的后退栈。 ### 用新的后退栈来向上导航 如果你的[activity](# "An activity represents a single screen with a user interface.")提供了任何允许被别的app启动的[intent filters](http://developer.android.com/guide/components/intents-filters.html#ifs)，那么你应该实现[onOptionsItemSelected()](http://developer.android.com/reference/android/app/Activity.html#onOptionsItemSelected%28android.view.MenuItem%29)回调，在用户从别的app任务进入你的[activity](# "An activity represents a single screen with a user interface.")后，点击Up按钮，在向上导航之前你的app用相应的后退栈开启一个新的任务。 在这么做之前，你可以先调用[shouldUpRecreateTask()](http://developer.android.com/reference/android/support/v4/app/NavUtils.html#shouldUpRecreateTask%28android.app.Activity,%20android.content.Intent%29)来检查当前的[activity](# "An activity represents a single screen with a user interface.")实例是否在另一个不同的app任务中。如果返回true，就使用[TaskStackBuilder](http://developer.android.com/reference/android/support/v4/app/TaskStackBuilder.html)创建一个新任务。或者，你可以向上面那样使用[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()) { // 对action bar的Up/Home按钮做出反应 case android.R.id.home: Intent upIntent = NavUtils.getParentActivityIntent(this); if (NavUtils.shouldUpRecreateTask(this, upIntent)) { // 这个activity不是这个app任务的一部分, 所以当向上导航时创建 // 用合成后退栈(synthesized back stack)创建一个新任务。 TaskStackBuilder.create(this) // 添加这个activity的所有父activity到后退栈中 .addNextIntentWithParentStack(upIntent) // 向上导航到最近的一个父activity .startActivities(); } else { // 这个activity是这个app任务的一部分, 所以 // 向上导航至逻辑父activity. NavUtils.navigateUpTo(this, upIntent); } return true; } return super.onOptionsItemSelected(item); } ~~~ > **Note**:为了能使[addNextIntentWithParentStack()](http://developer.android.com/reference/android/support/v4/app/TaskStackBuilder.html#addNextIntentWithParentStack%28android.content.Intent%29)方法起作用，你必须像上面说的那样，在你的manifest文件中使用[android:parentActivityName](http://developer.android.com/guide/topics/manifest/activity-element.html#parent)(和相应的[`<meta-data>`](http://developer.android.com/guide/topics/manifest/meta-data-element.html)元素)属性声明所有的[activity](# "An activity represents a single screen with a user interface.")的逻辑父[activity](# "An activity represents a single screen with a user interface.")。