java为什么findViewById返回null？

Question

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

我已经尝试过使用.getRootView()并自己沿着层次结构往下走，但没有成功。你能帮帮我吗

仪表板活动。爪哇

package eu.niehus.app;

import 安卓.app.AlertDialog;
import 安卓.app.DialogFragment;
import 安卓.content.DialogInterface;
import 安卓.content.Intent;
import 安卓.os.Bundle;
import 安卓.support.annotation.NonNull;
import 安卓.support.design.widget.FloatingActionButton;
import 安卓.support.design.widget.NavigationView;
import 安卓.support.v4.view.GravityCompat;
import 安卓.support.v4.widget.DrawerLayout;
import 安卓.support.v4.widget.SwipeRefreshLayout;
import 安卓.support.v7.app.ActionBarDrawerToggle;
import 安卓.support.v7.app.AppCompatActivity;
import 安卓.support.v7.widget.Toolbar;
import 安卓.util.Log;
import 安卓.view.Menu;
import 安卓.view.MenuItem;
import 安卓.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();
            }
        });
    }
}

活动面板。xml:

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

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

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

</安卓.support.v4.widget.DrawerLayout>

应用程序工具栏仪表板。xml

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

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

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

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

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

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

</安卓.support.constraint.ConstraintLayout>

内容面板。xml

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

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

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

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

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

        </TableLayout>
    </ScrollView>

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

Answer 1

尝试进行转换：swipeRefreshLayout = (SwipeRefreshLayout) findViewById(R.id.swipeRefreshLayoutDashboard);

Answer 2

我已经解决了这个问题，在我的app\u bar\u仪表板内部的“include”中添加了一个android:id参数。xml并使用它来获取我的布局

以下是更新的源代码：

仪表板活动。爪哇：

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();
    }
}

活动面板。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>

应用程序工具栏仪表板。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>

内容面板。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>

Answer 3

如果在另一个布局中包含布局，首先需要使用findViewById为包含的布局创建Id为的对象。然后在其上调用findViewById进行滑动以刷新布局