Button按钮= findViewById(R.id.button)始终解析为NULL的Android工作室 [英] Button button = findViewById(R.id.button) always resolves to null in Android Studio
问题描述
我是新的Android开发和Android Studio的,所以请原谅我的无知。
一个按钮,我添加了findViewById始终解析为null。因此,如果我尝试setonClickListener失败了整个活动。请帮忙。谢谢你。
MainActivity:
公共类MainActivity扩展ActionBarActivity {
@覆盖
保护无效的onCreate(包savedInstanceState){
super.onCreate(savedInstanceState);
的setContentView(R.layout.activity_main);
如果(savedInstanceState == NULL){
getSupportFragmentManager()的BeginTransaction()
。新增(R.id.container,新PlaceholderFragment())
。承诺();
}
按钮buttonClick =(按钮)findViewById(R.id.button);
buttonClick.setOnClickListener(新View.OnClickListener(){
@覆盖
公共无效的onClick(视图查看){
onButtonClick((按钮)查看);
}
});
}
公共无效onButtonClick(按钮查看){
TextView的textHello =(TextView中)findViewById(R.id.textView);
textHello.setText(已点击!!!);
}
@覆盖
公共布尔onCreateOptionsMenu(功能菜单){
//充气菜单;这增加了项目操作栏,如果它是present。
。getMenuInflater()膨胀(R.menu.main,菜单);
返回true;
}
@覆盖
公共布尔onOptionsItemSelected(菜单项项){
//处理动作栏项目点击这里。将操作栏
//自动在主/向上按钮操作的点击,只要
//你在AndroidManifest.xml中指定一个父活动。
INT的id = item.getItemId();
如果(ID == R.id.action_settings){
返回true;
}
返回super.onOptionsItemSelected(项目);
}
/ **
*包含一个简单的视图的占位符片段。
* /
公共静态类PlaceholderFragment扩展片段{
公共PlaceholderFragment(){
}
@覆盖
公共查看onCreateView(LayoutInflater充气,容器的ViewGroup,
捆绑savedInstanceState){
查看rootView = inflater.inflate(R.layout.fragment_main,集装箱,假);
返回rootView;
}
}
}
activity_main.xml
<的FrameLayout的xmlns:机器人=http://schemas.android.com/apk/res/android
的xmlns:工具=http://schemas.android.com/tools
机器人:ID =@ + ID /容器
机器人:layout_width =match_parent
机器人:layout_height =match_parent
工具:上下文=com.pluralsight.activitylifecycle.MainActivity
工具:忽略=MergeRootFrame/>
fragment_main.xml
< RelativeLayout的的xmlns:机器人=http://schemas.android.com/apk/res/android
的xmlns:工具=http://schemas.android.com/tools
机器人:layout_width =match_parent
机器人:layout_height =match_parent
机器人:以下属性来=@扪/ activity_horizontal_margin
机器人:paddingRight =@扪/ activity_horizontal_margin
机器人:paddingTop =@扪/ activity_vertical_margin
机器人:paddingBottom会=@扪/ activity_vertical_margin
工具:上下文=com.pluralsight.activitylifecycle.MainActivity $ PlaceholderFragment>
<的TextView
机器人:文本=@字符串/参考hello world
机器人:layout_width =WRAP_CONTENT
机器人:layout_height =WRAP_CONTENT
机器人:ID =@ + ID / TextView的/>
<按钮
机器人:layout_width =WRAP_CONTENT
机器人:layout_height =WRAP_CONTENT
机器人:文本=@字符串/ buttonClick
机器人:ID =@ + ID /按钮
机器人:layout_below =@ + ID / TextView的
机器人:layout_alignParentRight =真
机器人:layout_alignParentEnd =真
机器人:layout_marginRight =54dp/>
< / RelativeLayout的>
这是因为 findViewById()
在 activity_main $ C搜索$ C>的布局,而按钮位于片段的布局
fragment_main
。
移动那块code在片段的 onCreateView()
方法:
// ...
查看rootView = inflater.inflate(R.layout.fragment_main,集装箱,假);
按钮buttonClick =(按钮)rootView.findViewById(R.id.button);
buttonClick.setOnClickListener(新View.OnClickListener(){
@覆盖
公共无效的onClick(视图查看){
onButtonClick((按钮)查看);
}
});
,现在你通过 rootView
访问通知视图:
按钮buttonClick =(按钮)rootView.findViewById(R.id.button);
否则你将再次得到NullPointerException异常。
I'm new to Android development and Android Studio, so pardon my ignorance.
findViewById of a button I added always resolves to null. Hence if i try to setonClickListener it fails the whole Activity. Please help. Thanks.
MainActivity:
public class MainActivity extends ActionBarActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment())
.commit();
}
Button buttonClick = (Button) findViewById(R.id.button);
buttonClick.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
onButtonClick((Button) view);
}
});
}
public void onButtonClick(Button view) {
TextView textHello = (TextView) findViewById(R.id.textView);
textHello.setText("Clicked !!!");
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
return rootView;
}
}
}
activity_main.xml
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.pluralsight.activitylifecycle.MainActivity"
tools:ignore="MergeRootFrame" />
fragment_main.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="com.pluralsight.activitylifecycle.MainActivity$PlaceholderFragment">
<TextView
android:text="@string/hello_world"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/textView" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/buttonClick"
android:id="@+id/button"
android:layout_below="@+id/textView"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"
android:layout_marginRight="54dp" />
</RelativeLayout>
This is because findViewById()
searches in the activity_main
layout, while the button is located in the fragment's layout fragment_main
.
Move that piece of code in the onCreateView()
method of the fragment:
//...
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
Button buttonClick = (Button)rootView.findViewById(R.id.button);
buttonClick.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
onButtonClick((Button) view);
}
});
Notice that now you access it through rootView
view:
Button buttonClick = (Button)rootView.findViewById(R.id.button);
otherwise you would get again NullPointerException.
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