为什么 findViewById 返回 null？答案

【问题标题】：Why does findViewById return null?为什么 findViewById 返回 null？
【发布时间】：2018-04-19 18:40:16
【问题描述】：

为什么我的 findViewById 在这里返回 null swipeRefreshLayout = findViewById(R.id.swipeRefreshLayoutDashboard);？

我已经尝试使用.getRootView() 并自己向下层级，但没有任何成功。你能帮帮我吗？

DashboardActivity.java

package eu.niehus.app;

import android.app.AlertDialog;
import android.app.DialogFragment;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v4.widget.SwipeRefreshLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import com.github.mikephil.charting.charts.LineChart;
import com.github.mikephil.charting.components.YAxis;
import com.github.mikephil.charting.data.Entry;
import com.github.mikephil.charting.data.LineData;
import com.github.mikephil.charting.data.LineDataSet;
import com.github.mikephil.charting.utils.EntryXComparator;

import java.util.ArrayList;
import java.util.Calendar;
import java.util.Collections;
import java.util.List;
public class DashboardActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    private SwipeRefreshLayout swipeRefreshLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_dashboard);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                DialogFragment fragment = new AddWeightDialog();
                fragment.show(getFragmentManager(), "fab");
            }
        });

        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);

        swipeRefreshLayout = findViewById(R.id.swipeRefreshLayoutDashboard);
        swipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
            @Override
            public void onRefresh() {
                Log.i("onRefreshListener", "refreshing");
                update();
            }
        });
    }
}

activity_dashboard.xml：

   <?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:openDrawer="start">

    <include
        layout="@layout/app_bar_dashboard"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <android.support.design.widget.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:fitsSystemWindows="true"
        app:headerLayout="@layout/nav_header_dashboard"
        app:menu="@menu/activity_dashboard_drawer" />

</android.support.v4.widget.DrawerLayout>

app_bar_dashboard.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".DashboardActivity"
        android:id="@+id/constraintLayout">

    <android.support.design.widget.AppBarLayout
            android:layout_width="0dp"
            android:theme="@style/AppTheme.AppBarOverlay"
            android:id="@+id/appBarLayout"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            android:layout_height="wrap_content">

        <android.support.v7.widget.Toolbar
                android:id="@+id/toolbar"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:background="?attr/colorPrimary"
                app:popupTheme="@style/AppTheme.PopupOverlay"
                />

    </android.support.design.widget.AppBarLayout>

    <include
            layout="@layout/content_dashboard"
            android:layout_height="0dp"
            android:layout_width="0dp"
            android:id="@+id/include"
            app:layout_constraintTop_toBottomOf="@+id/appBarLayout"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            />

    <android.support.design.widget.FloatingActionButton
            android:id="@+id/fab"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:srcCompat="@android:drawable/ic_input_add"
            app:layout_constraintEnd_toEndOf="parent"
            android:layout_marginEnd="8dp"
            app:layout_constraintBottom_toBottomOf="parent"
            android:layout_marginBottom="8dp"/>

</android.support.constraint.ConstraintLayout>

content_dashboard.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.SwipeRefreshLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/swipeRefreshLayoutDashboard">

    <ScrollView
            android:layout_width="match_parent"
            android:layout_height="match_parent">

        <TableLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content">

            <TableRow
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:id="@+id/tableRowWeighGraph"
                    android:minHeight="200dp">

                <com.github.mikephil.charting.charts.LineChart
                        android:id="@+id/lineChart"
                        android:layout_width="match_parent"
                        android:layout_height="match_parent"
                        android:layout_weight="999"/>
            </TableRow>

        </TableLayout>
    </ScrollView>

</android.support.v4.widget.SwipeRefreshLayout>

【问题讨论】：

你能在活动中分享你的导入吗，因为我猜你使用了错误的 R.layout 而不是你的应用程序。
findViewById not working for an include?的可能重复
@vikaskumar 我已经添加了导入
根据另一个用户设置的重复标志，这应该通过删除包含 @SvenNiehus 上的 id 来解决
上述评论可能有效，从包含标签中删除 id 参数

标签： java android xml android-layout

【解决方案1】：

如果您在另一个布局中包含布局，首先您需要使用 findViewById 为包含的布局创建具有其 Id 的对象。然后在其上调用 findViewById 以滑动刷新布局。

【讨论】：

【解决方案2】：

我已经通过在 app_bar_dashboard.xml 中的“包含”上添加一个 android:id 参数并使用它来获取我的 SwipeRefreshLayout 来解决它。

这里是更新的源代码：

DashboardActivity.java：

package eu.niehus.app;

import android.app.AlertDialog;
import android.app.DialogFragment;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.constraint.ConstraintLayout;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v4.widget.SwipeRefreshLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import com.github.mikephil.charting.charts.LineChart;
import com.github.mikephil.charting.components.YAxis;
import com.github.mikephil.charting.data.Entry;
import com.github.mikephil.charting.data.LineData;
import com.github.mikephil.charting.data.LineDataSet;
import com.github.mikephil.charting.utils.EntryXComparator;
import eu.niehus.app.common.DBHelper;

import java.util.ArrayList;
import java.util.Calendar;
import java.util.Collections;
import java.util.List;

public class DashboardActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    private ConstraintLayout outer;
    private SwipeRefreshLayout swipeRefreshLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_dashboard);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                DialogFragment fragment = new AddWeightDialog();
                fragment.show(getFragmentManager(), "fab");
            }
        });

        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);

        dbHelper = new DBHelper(this);
        outer = findViewById(R.id.outer);
        swipeRefreshLayout = outer.findViewById(R.id.swipeRefreshLayoutInclude);
        swipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
            @Override
            public void onRefresh() {
                Log.i("onRefreshListener", "refreshing");
                update();
            }
        });
        lineChart = swipeRefreshLayout.findViewById(R.id.lineChartWeight);
        lineChart.setTouchEnabled(false);
        update();
    }
}

activity_dashboard.xml：

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:openDrawer="start">

    <include
        android:id="@+id/outer"
        layout="@layout/app_bar_dashboard"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <android.support.design.widget.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:fitsSystemWindows="true"
        app:headerLayout="@layout/nav_header_dashboard"
        app:menu="@menu/activity_dashboard_drawer" />

</android.support.v4.widget.DrawerLayout>

app_bar_dashboard.xml：

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".DashboardActivity"
        android:id="@+id/constraintLayout">

    <android.support.design.widget.AppBarLayout
            android:layout_width="0dp"
            android:theme="@style/AppTheme.AppBarOverlay"
            android:id="@+id/appBarLayout"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            android:layout_height="wrap_content">

        <android.support.v7.widget.Toolbar
                android:id="@+id/toolbar"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:background="?attr/colorPrimary"
                app:popupTheme="@style/AppTheme.PopupOverlay"
                />

    </android.support.design.widget.AppBarLayout>

    <include
            layout="@layout/content_dashboard"
            android:layout_height="0dp"
            android:layout_width="0dp"
            android:id="@+id/swipeRefreshLayoutInclude"
            app:layout_constraintTop_toBottomOf="@+id/appBarLayout"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            />

    <android.support.design.widget.FloatingActionButton
            android:id="@+id/fab"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:srcCompat="@android:drawable/ic_input_add"
            app:layout_constraintEnd_toEndOf="parent"
            android:layout_marginEnd="8dp"
            app:layout_constraintBottom_toBottomOf="parent"
            android:layout_marginBottom="8dp"/>

</android.support.constraint.ConstraintLayout>

content_dashboard.xml：

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.SwipeRefreshLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/swipeRefreshLayoutDashboard">

    <ScrollView
            android:layout_width="match_parent"
            android:layout_height="match_parent">

        <TableLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content">

            <TableRow
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:id="@+id/tableRowWeighGraph"
                    android:minHeight="200dp">

                <com.github.mikephil.charting.charts.LineChart
                        android:id="@+id/lineChartWeight"
                        android:layout_width="match_parent"
                        android:layout_height="match_parent"
                        android:layout_weight="999"/>
            </TableRow>

        </TableLayout>
    </ScrollView>

</android.support.v4.widget.SwipeRefreshLayout>

【讨论】：

【解决方案3】：

尝试制作演员表：swipeRefreshLayout = (SwipeRefreshLayout) findViewById(R.id.swipeRefreshLayoutDashboard);

【讨论】：