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优雅的让Fragment监听返回键
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轉載請注明出處：http://write.blog.csdn.net/postedit/40507387
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Activity可以很容易的得到物理返回鍵的監聽事件，而Fragment卻不能。假設FragmentActivity有三個Fragment，一般安卓用戶期望點擊返回鍵會一層層返回到FragmentActivity。當然，我們可以將每個Fragment對應的Transaction放到BackStack中，但是如果每個Fragment有對返回事件的特殊消費，那么在FragmentActivity的onBackPressed()中的代碼就會比較混亂，例如：
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@Override??public?void?onBackPressed()?{??????if(selectedFragment.equals(fragmentA)?&&?fragmentA.hasExpandedRow())?{??????????fragmentA.collapseRow();??????}?else?if(selectedFragment.equals(fragmentA)?&&?fragmentA.isShowingLoginView())?{??????????fragmentA.hideLoginView();??????}?else?if(selectedFragment.equals(fragmentA))?{??????????popBackStack();??????}?else?if(selectedFragment.equals(fragmentB)?&&?fragmentB.hasCondition1())?{??????????fragmentB.reverseCondition1();??????}?else?if(selectedFragment.equals(fragmentB)?&&?fragmentB.hasCondition2())?{??????????fragmentB.reverseCondition2();??????}?else?if(selectedFragment.equals(fragmentB))?{??????????popBackStack();??????}?else?{??????????????????super.onBackPressed();??????}??}?? 這對于有代碼潔癖的程序猿顯然是不能容忍的，后來發現了一種優雅的解決方案。
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首先創建一個抽象類BackHandledFragment，該類有一個抽象方法onBackPressed()，所有BackHandledFragment的子類在onBackPressed方法中處理各自對Back事件的消費邏輯。onBackPressed返回布爾值，宿主FragmentActivity將會根據該方法的返回值判斷子Fragment是否有消費Back事件。此外，宿主FragmentActivity還會保持一份當前Fragment的引用，當用戶按下Back鍵時，宿主Activity會判斷當前Fragment是否需要消費該事件，如果沒有Fragment消費才會自己消費。
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public?abstract?class?BackHandledFragment?extends?Fragment?{????????protected?BackHandledInterface?mBackHandledInterface;????????????????protected?abstract?boolean?onBackPressed();????????????@Override??????public?void?onCreate(Bundle?savedInstanceState)?{??????????super.onCreate(savedInstanceState);??????????if(!(getActivity()?instanceof?BackHandledInterface)){??????????????throw?new?ClassCastException("Hosting?Activity?must?implement?BackHandledInterface");??????????}else{??????????????this.mBackHandledInterface?=?(BackHandledInterface)getActivity();??????????}??????}????????????@Override??????public?void?onStart()?{??????????super.onStart();??????????????????mBackHandledInterface.setSelectedFragment(this);??????}????????}?? 宿主FragmentActivity需要繼承BackHandledIntegerface，子Fragment會通過該接口告訴宿主FragmentActivity自己是當前屏幕可見的Fragment。
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public?interface?BackHandledInterface?{????????public?abstract?void?setSelectedFragment(BackHandledFragment?selectedFragment);??}?? 所以在Fragment的onCreate中會判斷宿主FragmentActivity是否已繼承了該接口。在Fragment的onStart()方法中就會調用該接口告訴宿主FragmentActivity自己是當前屏幕可見的Fragment。
宿主FragmentActivity就可以在onBackPressed()方法中對Back事件進行判斷處理了。
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public?class?MainActivity?extends?FragmentActivity?implements?BackHandledInterface{????????private?BackHandledFragment?mBackHandedFragment;??????private?boolean?hadIntercept;????????@Override??????public?void?setSelectedFragment(BackHandledFragment?selectedFragment)?{??????????this.mBackHandedFragment?=?selectedFragment;??????}????????????@Override??????public?void?onBackPressed()?{??????????if(mBackHandedFragment?==?null?||?!mBackHandedFragment.onBackPressed()){??????????????if(getSupportFragmentManager().getBackStackEntryCount()?==?0){??????????????????super.onBackPressed();??????????????}else{??????????????????getSupportFragmentManager().popBackStack();??????????????}??????????}??????}??}?? ?
轉載于:https://www.cnblogs.com/xiaorenwu702/p/4561393.html
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