如何使android中的不同活动正确通信并发送意图？

Question

我正在关注本指南：https://developer.android.com/training/basics/firstapp/starting-activity.html

并尝试在活动之间发送消息。我希望用户在主活动中输入文本，按发送并让它显示在不同的活动中。出于某种原因，在我在手机上播放应用程序并按下本应将意图和消息内容传输到下一个活动的“发送”按钮后 - 它只显示一个空白活动。

我认为我可能不需要更改 content_main，因为我对这个 xml 文件和 activity_main 有点困惑，并且不确定每个代码块应该放在哪里。

当前代码：

MainActivity.java

package com.example.myfirstapp;

import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {
public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

/** Called when the user clicks the Send button */
public void sendMessage(View view) {
    Intent intent = new Intent(this, DisplayMessageActivity.class);
    EditText editText = (EditText) findViewById(R.id.edit_message);
    String message = editText.getText().toString();
    intent.putExtra(EXTRA_MESSAGE, message);
    startActivity(intent);
}
}

DisplayMessageActivity.java：

package com.example.myfirstapp;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.TextView;


public class DisplayMessageActivity extends AppCompatActivity {

public class MainActivity extends AppCompatActivity {
    public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    /** Called when the user clicks the Send button */
    public void sendMessage(View view) {
        Intent intent = new Intent(this, DisplayMessageActivity.class);
        EditText editText = (EditText) findViewById(R.id.edit_message);
        String message = editText.getText().toString();
        intent.putExtra(EXTRA_MESSAGE, message);
        startActivity(intent);
    }
}
}

activity_main.xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent">
<EditText android:id="@+id/edit_message"
    android:layout_weight="1"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:hint="@string/edit_message" />
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage"
    />
</LinearLayout>

activity_display_message.xml：

<?xml version="1.0" encoding="utf-8"?>
<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.example.myfirstapp.DisplayMessageActivity"
android:id="@+id/activity_display_message"
>

</RelativeLayout>

content_main.xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent">
<EditText android:id="@+id/edit_message"
    android:layout_weight="1"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:hint="@string/edit_message" />
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage"
    />
</LinearLayout>

Answer 1

编辑

public class DisplayMessageActivity extends AppCompatActivity {

public class MainActivity extends AppCompatActivity { 

有问题。

Answer 2

在新的 Activity 中，即 DisplayMessageActivity.java 检索这些值：

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Bundle extras = getIntent().getExtras();
        if (extras != null) {
        String value = extras.getString(EXTRA_MESSAGE);
        //The key argument here must match that used in the other activity
        EditText editText = (EditText) findViewById(R.id.edit_message);
        editText .setText(value)

        }
    }

使用这种技术将变量从一个 Activity 传递到另一个。

Answer 3

你必须在你的activity_display_message.xml中声明一个TextView

<TextView 
android:id="@+id/show_message"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />

然后在你的 DisplayMessageActivity.java 中

public class DisplayMessageActivity extends AppCompatActivity {

    public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Bundle bundle = getIntent().getExtras();
        String message = bundle.getString(EXTRA_MESSAGE);

        TextView textView = (TextView) findViewById(R.id.show_message);
        textView.setText(message);
    }
}

Answer 4

如果您想在DisplayMessageActivity 中获取extraString，您已从之前的活动MainActivity 发送过来
试试这个

// try this in onCreate() of DisplayMessageActivity
 String myExtraString = getIntent.getStringExtra(EXTRA_MESSAGE);
 if(myExtraString == null )
 { // no extraString was attached to the intent 
 }