Question

我略微修改了MyFirstApp。我将它变成了一个方便，简单的应用程序，需要行驶数英里并加油，按下按钮后，开始新的活动，显示燃油使用量为升/ 100公里。

我面临的问题是，无论在按钮点击之前输入什么值，输出总是显示62,14 l / 100km。我猜测这是第一次计算的结果，而不是更新。

的strings.xml：

<resources>
    <string name="app_name">FuelEcoCalc</string>
    <string name="action_settings">Settings</string>
    <string name="edit_miles">Distance in miles</string>
    <string name="edit_litres">Fuel qty in litres</string>
    <string name="button_calculate">Calculate</string>
    <string name="title_activity_main">MainActivity</string>
    <string name="title_activity_display_message">My Message</string>
    <string name="hello_world">Hello World</string>
</resources>

menu_display_message.xml：

<menu 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"
    tools:context="com.example.wojtek.myfirstapp.DisplayMessageActivity">
    <item android:id="@+id/action_settings" android:title="@string/action_settings"
    android:orderInCategory="100" app:showAsAction="never" />
</menu>

menu_my.xml：

<menu 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" tools:context=".MyActivity">
    <item android:id="@+id/action_settings" android:title="@string/action_settings"
    android:orderInCategory="100" app:showAsAction="never" />
</menu>

activity_my.xml：

<LinearLayout 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:orientation="vertical">
    <EditText
        android:id="@+id/edit_miles"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="@string/edit_miles">
    </EditText>
    <EditText
        android:id="@+id/edit_litres"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="@string/edit_litres">
    </EditText>
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_calculate"
        android:onClick="sendMessage">
    </Button>
</LinearLayout>

activity_display_message.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.example.wojtek.myfirstapp.DisplayMessageActivity">

    <TextView android:text="@string/title_activity_display_message" android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</RelativeLayout>

MyActivity.java：

import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;

public class MyActivity extends ActionBarActivity {

    public final static String MILES_MESSAGE = "com.mycompany.myfirstapp.MESSAGE";
    public final static String FUEL_MESSAGE = "com.mycompany.myfirstapp.MESSAGE";


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

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_my, 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();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    /** Called when the user clicks the Send button */
    public void sendMessage(View view) {
        Intent intent = new Intent(this, DisplayMessageActivity.class);
        EditText editMiles = (EditText) findViewById(R.id.edit_miles);
        String miles = editMiles.getText().toString();
        EditText editFuel = (EditText) findViewById(R.id.edit_litres);
        String fuel = editFuel.getText().toString();
        intent.removeExtra(MILES_MESSAGE);
        intent.removeExtra(FUEL_MESSAGE);
        intent.putExtra(MILES_MESSAGE, miles);
        intent.putExtra(FUEL_MESSAGE, fuel);
        startActivity(intent);
    }
}

DisplayMessageActivity.java：

import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.MenuItem;
import android.widget.TextView;
import java.text.DecimalFormat;

public class DisplayMessageActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Intent intent = getIntent();
        String miles = intent.getStringExtra(MyActivity.MILES_MESSAGE);
        String fuel = intent.getStringExtra(MyActivity.FUEL_MESSAGE);
        TextView textView = new TextView(this);
        textView.setTextSize(40);
        String message = "";
        try {
            double milesTravelled = Double.parseDouble(miles);
            double fuelTaken = Double.parseDouble(fuel);
            double mileToKm = 1.609344;
            double kmTravelled = milesTravelled * mileToKm;
            double fuelUsagePer100Km = (fuelTaken * 100) / kmTravelled;
            DecimalFormat df = new DecimalFormat("#.##");
            String output = df.format(fuelUsagePer100Km).toString();
            message = output + " litres/100km";
        } catch (Exception e) {
            message = "Invalid input";
        }
        textView.setText(message);
        setContentView(textView);
    }


    @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();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

的AndroidManifest.xml：

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

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

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".DisplayMessageActivity"
            android:label="@string/title_activity_display_message"
        android:parentActivityName="com.example.wojtek.myfirstapp.MyActivity" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="com.example.wojtek.myfirstapp.MyActivity" />
        </activity>
    </application>
</manifest>

非常感谢您的帮助。

编辑：我想我也应该添加日志...... 单击按钮时出现：

05-09 05:54:12.917    2106-2141/com.example.wojtek.myfirstapp W/EGL_emulation﹕ eglSurfaceAttrib not implemented
05-09 05:54:12.917    2106-2141/com.example.wojtek.myfirstapp W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xa64ae220, error=EGL_SUCCESS

Answer 1

也许是因为在DisplayMessageActivity.java中，您没有在布局中获得对TextView的引用

TextView textView = (TextView)findViewById(R.id.textView);

您还需要在布局文件中为TextView提供一个ID。

<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.wojtek.myfirstapp.DisplayMessageActivity">

<TextView android:id="@+id/textView" android:text="@string/title_activity_display_message" android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

编辑：我弄清楚你的问题是什么。你不需要做任何其他事情。在MyActivity.java中，您已将两个字符串常量设置为相同的值。让它们与众不同。例如：

public final static String MILES_MESSAGE = "com.mycompany.myfirstapp.MESSAGEMILES";
public final static String FUEL_MESSAGE = "com.mycompany.myfirstapp.MESSAGEFUEL";

希望它现在适合你。

Answer 2

尝试调试一下，你在String miles，String fuel中获得不同的值吗？每次你去展示活动..我认为你应该使用textview的id

Android Studio TextView.text不会更新

2 个答案: