最近，在做公司客製化專案的時候遇到一個問題：有些layout下面的xml佈局檔案需要進行動態的載入，而我使用的是ButterKnife去繫結的view，而且在類的全域性變數中已經宣告好了，如下所示：

@BindView(R.id.ll_display_a)
LinearLayout llDisplayA;
@BindView(R.id.ll_display_b)
LinearLayout llDisplayB;

上面兩個佈局在不同的場景下載入，因此對應的layout佈局也不同（llDisplayA對應的是佈局layout_a.xml，llDisplayB對應的是佈局layout_b.xml），而Butterknife並沒有這麼智慧，只要是需要載入的view，都可以通過@BindView的註解形式進行初始化，點選事件也可通過 layout_a.xml，而Butterknife並沒有這麼智慧，只要是需要載入的view，都可以通過@BindView的註解形式進行初始化，以及點選事件通過@onViewCliked的形式指定。那麼問題來了，如果我只初始化了layout_a.xml，

而變數llDisplayB在類中已經宣告，執行程式之後會報錯，報錯如下：

12-13 11:55:56.187  4158  4158 E AndroidRuntime: java.lang.IllegalStateException: Required view 'll_display_b' with ID 2131558712 for method 'onViewClicked' was not found. If this view is optional add '@Nullable' (fields) or '@Optional' (methods) annotation.
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at butterknife.internal.Utils.findRequiredView(Utils.java:92)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at com.xbh.systemsetting.advancedsetting.view.AdvancedFragment_ViewBinding.<init>(AdvancedFragment_ViewBinding.java:322)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at java.lang.reflect.Constructor.newInstance(Native Method)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at butterknife.ButterKnife.createBinding(ButterKnife.java:199)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at butterknife.ButterKnife.bind(ButterKnife.java:172)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at com.xbh.systemsetting.advancedsetting.view.AdvancedFragment.initView(AdvancedFragment.java:765)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at com.xbh.systemsetting.advancedsetting.view.AdvancedFragment.onCreateView(AdvancedFragment.java:299)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.Fragment.performCreateView(Fragment.java:2192)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1299)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1528)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1595)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:758)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2363)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2149)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.FragmentManagerImpl.optimizeAndExecuteOps(FragmentManager.java:2103)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:1984)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:626)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:143)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.view.ViewPager.populate(ViewPager.java:1268)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.view.ViewPager.populate(ViewPager.java:1116)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1642)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.View.measure(View.java:18788)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.View.measure(View.java:18788)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1112)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.onMeasure(LinearLayout.java:632)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.View.measure(View.java:18788)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1112)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.onMeasure(LinearLayout.java:632)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.View.measure(View.java:18788)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.View.measure(View.java:18788)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
12-13 11:55:56.187  4158  4158 E AndroidRuntime: 	at android.view.View.measure(View.java:1[  179.092576] init: Service 'cpudvfsscaling' is being killed...
8788)
 

檢視列印資訊，發現Required view ID was not found ，說有一個id為ll_display_b的view未找到。緊接著，我注意到了後面一句話：

If this view is optional add '@Nullable' (fields) or '@Optional' (methods) annotation.

它說如果這個view是可選的，那麼可以加上@Nullable（變數）或者@Optional（方法）註解。這時我才發現這才是解決問題的關鍵所在，而且ButterKnife也確實考慮到了這一點。於是我在成員變數的@BindView註解上面加上了@Nullable，在點選事件@onViewCliked上加上了@Optional註解：

//必須加上@Nullable註解，這樣即使控制元件id不存在，也不會報空指標異常
@Nullable
@BindView(R.id.ll_display)
LinearLayout llDisplay;

//必須加上@Optional註解，這樣即使控制元件id不存在，也不會報空指標異常
@Optional
@OnClick({R.id.ll_display_b})
public void onViewClicked(View view) {}

問題完美解決！