findViewById() returns null for custom component in layout XML, not for other components

最新推荐文章于 2024-03-05 19:59:45 发布

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最新推荐文章于 2024-03-05 19:59:45 发布

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本文介绍了一个关于在Android布局文件中使用自定义组件时遇到的问题：通过findViewById无法找到自定义视图的情况及解决方案。问题在于自定义视图构造函数中未正确传递属性集参数，导致视图ID丢失，最终无法通过ID找到该视图。

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

I have a res/layout/main.xml including these elements and others:

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

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, but foo comes back null. MyCustomView has a constructor MyCustomView(Context c, AttributeSet a) and a Log.d(...) at the end of that constructor appears successfully in logcat just before the "epic fail".

Why is foo null?

解决方法：

Because in the constructor, I had super(context) instead of super(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. :-)