Android 如何保持横竖屏切换不变以及Fragment调用setRetainInstance的原理

限定屏幕方向:
设置Manifest文件的activity的属性:
android:screenOrientation="portrait"(始终竖屏)
landscape(始终横屏)

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自己处理变更:
设置Manifest文件中的activity属性:
android:configChanges="orientation|screenSize|keyboardHidden"

视图状态会自动保存(适用于EditText不适用于TextView):
前提条件:
(1)拥有Id
(2)实现onSaveInstanceState回调
(3)实现onRestoreInstanceState回调

    protected void onSaveInstanceState(Bundle outState) {        super.onSaveInstanceState(outState);        Log.d(TAG, "onSaveInstanceState() called with: " + "outState = [" + outState + "]");        outState.putLong(CREATE_TIME, this.createTime);        Log.d(TAG, "onSaveInstanceState() called with: " + "outState = [" + outState + "]");    }    @Override    protected void onRestoreInstanceState(Bundle savedInstanceState) {        super.onRestoreInstanceState(savedInstanceState);        Log.d(TAG, "onRestoreInstanceState() called with: " + "savedInstanceState = [" + savedInstanceState + "]");    }    @Override    protected void onDestroy() {        super.onDestroy();        Log.d(TAG, "onDestroy: " + this);    }

保持Fragment对象:
(1)在onCreate函数里调用setRetainInstance(true);

setRetainInstance(true);

(2)findFragmentByTag无则创建并添加,有就什么都不做
(2)Fragment对象添加到Activity中，当Activity重启时，通过FragmentManager过去此Fragment对象

    protected void onCreate(Bundle savedInstanceState) {        private Fragment fragment;        super.onCreate(savedInstanceState);        FragmentManager fm = getSupportFragmentManager();        fragment = fm.findFragmentByTag("ScoreFragment");        if (fragment == null) {            fragment = new ScoreFragment();            fm.beginTransaction().replace(android.R.id.content, fragment, "ScoreFragment").commit();        }        Log.d(TAG, "onCreate: ");    }

保存和恢复Fragment的状态
onSaveInstanceState保持状态
onActivityCreated恢复状态

    public ScoreFragment() {        // Required empty public constructor    }    @Override    public void onSaveInstanceState(Bundle outState) {        super.onSaveInstanceState(outState);        outState.putInt(SCORE, this.score);        Log.d(TAG, "onSaveInstanceState() called with: " + "outState = [" + outState + "]");    }    @Override    public void onCreate(@Nullable Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        Log.d(TAG, "onCreate() called with: " + "savedInstanceState = [" + savedInstanceState + "]");//        setRetainInstance(true);        if (savedInstanceState != null) {            this.score = savedInstanceState.getInt(SCORE);        }    }    @Override    public void onDestroy() {        super.onDestroy();        Log.d(TAG, "onDestroy: " + this);    }

setRetainInstance的原理

只有调用了fragment的setRetainInstance(true)方法，并且因设备配置改变，托管Activity正在被销毁的条件下， fragment才会短暂的处于保留状态。

比较来说，onSaveInstanceState可以更长久的保持数据。
当Activity所在进程被关闭（非用户主动关闭），系统将会重新创建activity时，将恢复onSaveInstanceState中保留的数据。

当Fragment设置了setRetainInstance(true)后，在设备旋转等情况下，该Fragment可以暂时与Activity分离。

如果此时Fragment持有的后台线程，例如AsyncTask中的后台操作等，需要使用Fragment的Context等信息，就可能出现错误。

为此，在Fragment中定义了isAdded，用于判断Fragment是否已经绑定到了某个Activity。

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