转自：http://stackoverflow.com/questions/1691569/findviewbyid-returns-null-for-custom-component-in-layout-xml-not-for-other-co

I have ares/layout/main.xmlincluding these elements and others:

<some.package.MyCustomView android:id="@+id/foo" (someotherparams) /> <TextView"@+id/boring"/>

In my Activity's onCreate,I do this:

setContentView(R.layout.main); (TextView) boring = findViewById(R.id.boring); // ...find other elements... MyCustomView foo = (MyCustomView) findViewById(R.id.foo); if (foo == null) { Log.d(TAG,"epic fail"); }

The other elements are found successfully,butfoocomes back null. MyCustomView has a constructorMyCustomView(Context c,AttributeSet a)and aLog.d(...)at the end of that constructor appears successfully in logcat just before the "epic fail".

Why isfoonull?

解决方法：

Because in the constructor,I hadsuper(context)instead ofsuper(context,attrs).

Makes sense,if you don't pass in the attributes,such as the id,then the view will have no id and therefore not be findable using that id. :-)