是 KeyguardSecurityView的子类 ，
在 getLayoutIdFor 中 LayoutInflater 一个 keyguard_pattern_view.xml ，
放入 mSecurityViewFlipper.addView(v)， 是 KeyguardSecurityContainer 的孩子 。
E FATAL EXCEPTION: main
Process: com.android.systemui, PID: 640
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
at com.android.keyguard.KeyguardPatternView.onVisibilityChanged(KeyguardPatternView.java:147)
at android.view.View.dispatchAttachedToWindow(View.java:20507)
at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:3489)
at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:3496)
at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:3496)
at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:3496)
at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:3496)
at android.view.ViewGroup.addViewInner(ViewGroup.java:5278)
at android.view.ViewGroup.addView(ViewGroup.java:5064)
at android.view.ViewGroup.addView(ViewGroup.java:5004)
at com.android.systemui.statusbar.phone.KeyguardBouncer.inflateView(KeyguardBouncer.java:452)
at com.android.systemui.statusbar.phone.KeyguardBouncer.ensureView(KeyguardBouncer.java:441)
at com.android.systemui.statusbar.phone.KeyguardBouncer.prepare(KeyguardBouncer.java:378)
at com.android.systemui.statusbar.phone.StatusBarKeyguardViewManager.showBouncerOrKeyguard(StatusBarKeyguardViewManager.java:366)
at com.android.systemui.statusbar.phone.StatusBarKeyguardViewManager.reset(StatusBarKeyguardViewManager.java:456)
at com.android.systemui.statusbar.phone.StatusBarKeyguardViewManager.show(StatusBarKeyguardViewManager.java:348)
at com.android.systemui.keyguard.KeyguardViewMediator.handleShow(KeyguardViewMediator.java:1865)
at com.android.systemui.keyguard.KeyguardViewMediator.access$3100(KeyguardViewMediator.java:151)
at com.android.systemui.keyguard.KeyguardViewMediator$6.handleMessage(KeyguardViewMediator.java:1611)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7677)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
