工具栏中的向上箭头没有后退

Question

我想将 return 代表我的箭头添加到我的父 class，主 activity，我添加了一个使用 material 设计的工具栏。它应该只在第二个 activity

中添加此代码才能工作

 getSupportActionBar().setDisplayHomeAsUpEnabled(true);

但是当我这样做时它没有打开应用程序并且我收到以下错误

06-30 15:04:57.869 11854-11854/com.niccode.pets W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0x41786930)
06-30 15:04:57.879 11854-11854/com.niccode.pets E/AndroidRuntime: FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.niccode.pets/com.niccode.pets.Favorites}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
        at android.app.ActivityThread.access0(ActivityThread.java:141)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:5041)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
        at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
        at com.niccode.pets.Favorites.onCreate(Favorites.java:14)
        at android.app.Activity.performCreate(Activity.java:5131)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1090)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)

所以我在 xml 和 java 文件中添加了工具栏。但是现在显示了日期，但它没有 return 我到我的父文件 main activity

谢谢

package com.niccode.pets;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;

import android.os.Bundle;

public class Favorites extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_favorites);

        Toolbar mybar = (Toolbar) findViewById(R.id.my_toolbar);
        setSupportActionBar(mybar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);




    }


}

xml秒activity

<androidx.constraintlayout.widget.ConstraintLayout android:layout_width="match_parent" android:layout_height="match_parent" tools:context=".Favorites">
<include android:id="@+id/my_toolbar" layout="@layout/toolbar"/>
</androidx.constraintlayout.widget.ConstraintLayout>

清单文件

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.niccode.pets">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".Favorites"
            android:parentActivityName=".MainActivity">

            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value=".MainActivity"
                >

            </meta-data>
        </activity>
    </application>

</manifest>

按箭头没有任何反应

Answer 1

这段代码之后：

setSupportActionBar(mybar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);

添加导航 onclick 侦听器：

mybar.setNavigationOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            onBackPressed();
        }
});

工具栏中的向上箭头没有后退

Arrow Up in toolbar no back

android

back

arrow-functions