这是一个应用程序练习，我希望能得到一些帮助和编码示例。我正在尝试将背景颜色从我的 SettingsActivity.java 设置为我的 MainActivity.java。这将通过一个复选框发生，我单击它并触发更改。
我可以通过 Intent 访问 MainActivity，同时单击复选框。但是，如果我尝试设置 MainActivity 背景的颜色，Java 会抛出空指针错误，导致值为空。
这是日志:
01-05 16:28:58.612 22547-22547/com.smartdroidesign.fitnessassistant E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.smartdroidesign.fitnessassistant, PID: 22547
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.constraint.ConstraintLayout.setBackgroundColor(int)' on a null object reference
at com.smartdroidesign.fitnessassistant.SettingsActivity$1.onCheckedChanged(SettingsActivity.java:52)
at android.widget.CompoundButton.setChecked(CompoundButton.java:156)
at android.widget.CompoundButton.toggle(CompoundButton.java:115)
at android.widget.CompoundButton.performClick(CompoundButton.java:120)
at android.view.View$PerformClick.run(View.java:21147)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
我很确定这是导致它的行:
mainAcBackground.setBackgroundColor(getResources().getColor(R.color.colorAccent));
我不太确定如何从这里开始，并且对 Android 还是很陌生，所以我希望得到一些指导/最佳实践。
到目前为止，我已经尝试过:实例化布局(约束布局)——这似乎是个坏主意，因为它会泄漏 我曾尝试使用共享首选项，但未成功。
这是我的主 Activity
import android.app.Activity;
import android.content.Intent;
import android.content.SharedPreferences;
import android.graphics.Color;
import android.os.Bundle;
import android.support.constraint.ConstraintLayout;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.support.v7.widget.Toolbar;
public class MainActivity extends Activity {
public static final String SECOND_ACTIVITY_TITLE = "second.activity.title";
public static final String THIRD_ACTIVITY_COLOR = "third.activity.color";
public static final String EXCERCISE_WEIGHT = "Weight lifting";
public static final String EXCERCISE_YOGA = "Yoga routine";
public static final String EXCERCISE_CARDIO = "Cardio routine";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ConstraintLayout weightBtn = (ConstraintLayout) findViewById(R.id.weightLayout);
ConstraintLayout yogaBtn = (ConstraintLayout) findViewById(R.id.yogaLayout);
ConstraintLayout cardioBtn = (ConstraintLayout) findViewById(R.id.cardioLayout);
final ConstraintLayout mainAcBackground = findViewById(R.id.mainAcBackground);
weightBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
loadDetailActivity(MainActivity.EXCERCISE_WEIGHT);
}
});
yogaBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
loadDetailActivity(MainActivity.EXCERCISE_YOGA);
}
});
cardioBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
loadDetailActivity(MainActivity.EXCERCISE_CARDIO);
}
});
}
private void setSupportActionBar(Toolbar mToolbar) {
}
private void loadDetailActivity(String exerciseTitle) {
Intent intent = new Intent(MainActivity.this, DetailsActivity.class);
intent.putExtra(MainActivity.SECOND_ACTIVITY_TITLE, exerciseTitle);
startActivity(intent);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater mMenuInflater = getMenuInflater();
mMenuInflater.inflate(R.menu.settings, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.theme:
Intent intent = new Intent(MainActivity.this, SettingsActivity.class);
startActivity(intent);
}
return super.onOptionsItemSelected(item);
}
}
这是我的 SettingsActivity(应该更改 MainActivity 背景的那个)
import android.app.Activity;
import android.content.Intent;
import android.content.SharedPreferences;
import android.graphics.Color;
import android.os.Bundle;
import android.os.Parcelable;
import android.support.constraint.ConstraintLayout;
import android.support.v4.content.ContextCompat;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.CheckBox;
import android.widget.CompoundButton;
public class SettingsActivity extends Activity {
ConstraintLayout mainAcBackground;
public static final String ACTIVITY_COLOR = "color";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_settings);
Menu settings = (Menu) findViewById(R.id.action_setting);
Menu theme = (Menu) findViewById(R.id.theme);
final ConstraintLayout settingBG = findViewById(R.id.settingBG);
final ConstraintLayout mainAcBackground = findViewById(R.id.mainAcBackground);
final CheckBox nightMode = (CheckBox)findViewById(R.id.NightMode);
nightMode.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
@Override
public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
if (buttonView.isChecked()){
loadSettingsActivity(MainActivity.THIRD_ACTIVITY_COLOR);
mainAcBackground.setBackgroundColor(getResources().getColor(R.color.colorAccent));
}else {
mainAcBackground.setBackgroundColor(getResources().getColor(R.color.colorPrimaryDark));
}
}
});
}
private void loadSettingsActivity(String exerciseTitle) {
Intent intent = new Intent(SettingsActivity.this, MainActivity.class);
intent.putExtra("backgroundColor", getResources().getColor(R.color.colorAccent));
startActivity(intent);
}
}
编辑:这是 MainActivity 的 XLM
<?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:id="@+id/mainAcBackground"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.smartdroidesign.fitnessassistant.MainActivity"
tools:layout_editor_absoluteY="81dp">
<android.support.constraint.ConstraintLayout
android:id="@+id/weightLayout"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="16dp"
android:background="@drawable/rounded_corners"
android:backgroundTint="#455A64"
app:layout_constraintBottom_toTopOf="@+id/yogaLayout"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
tools:targetApi="lollipop">
<ImageView
android:id="@+id/imageView"
android:layout_width="245dp"
android:layout_height="75dp"
android:layout_marginBottom="40dp"
android:layout_marginTop="40dp"
android:src="@drawable/weight"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/textView"
android:layout_width="45dp"
android:layout_height="20dp"
android:layout_marginLeft="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:background="@drawable/rounded_corners"
android:backgroundTint="#2196F3"
android:paddingLeft="5dp"
android:text="Day 1"
android:textAllCaps="true"
android:textStyle="bold"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/textView7"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginRight="8dp"
android:text="Week 1"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent" />
</android.support.constraint.ConstraintLayout>
<android.support.constraint.ConstraintLayout
android:id="@+id/yogaLayout"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:background="@drawable/rounded_corners"
android:backgroundTint="#455A64"
app:layout_constraintBottom_toTopOf="@+id/cardioLayout"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/weightLayout">
<TextView
android:id="@+id/textView10"
android:layout_width="45dp"
android:layout_height="20dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:background="@drawable/rounded_corners"
android:backgroundTint="#673AB7"
android:paddingLeft="5dp"
android:text="Day 2"
android:textAllCaps="true"
android:textStyle="bold"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/textView8"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginRight="8dp"
android:text="Week 1"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent" />
<ImageView
android:id="@+id/imageView2"
android:layout_width="245dp"
android:layout_height="75dp"
android:layout_marginBottom="40dp"
android:layout_marginTop="40dp"
android:src="@drawable/lotus"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>
<android.support.constraint.ConstraintLayout
android:id="@+id/cardioLayout"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_marginBottom="16dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:background="@drawable/rounded_corners"
android:backgroundTint="#455A64"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/yogaLayout">
<TextView
android:id="@+id/textView11"
android:layout_width="45dp"
android:layout_height="20dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:background="@drawable/rounded_corners"
android:backgroundTint="#E53935"
android:paddingLeft="5dp"
android:text="Day 3"
android:textAllCaps="true"
android:textStyle="bold"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/textView9"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginRight="8dp"
android:text="Week 1"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent" />
<ImageView
android:id="@+id/imageView3"
android:layout_width="245dp"
android:layout_height="74dp"
android:layout_marginBottom="40dp"
android:layout_marginTop="40dp"
android:src="@drawable/heart"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>
</android.support.constraint.ConstraintLayout>
这是 SettingsActivity 的 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:id="@+id/settingBG"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.smartdroidesign.fitnessassistant.SettingsActivity">
<CheckBox
android:id="@+id/NightMode"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginTop="40dp"
android:paddingLeft="35dp"
android:text="Night mode"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>
希望我已经解释清楚了，如果您需要更多信息，请告诉我，在此先感谢您!
最佳答案
很明显，您不能通过简单地存储未初始化的 View 引用来更改另一个 Activity 的背景颜色。
在这里做这样的事情。
当你从主要 Activity 开始设置 Activity 时，开始这样的事情
private void pickColor() {
Intent settings = new Intent(this, SettingsActivity.class);
startActivityForResult(settings, 101);
}
然后您可以在设置 Activity 中更改颜色并将颜色代码存储在 Intent 中并传回调用 Activity 并像这样完成设置 Activity 。
public void colorChanged(int colorCode){
Intent color = new Intent();
color.putExtra("colorSelected",colorCode);
setResult(RESULT_OK,color);
finish();
}
并在主 Activity 中接收这样的结果，这也是您的调用 Activity。
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
// Check which request we're responding to
if(RESULT_OK==resultCode && requestCode==101){
//here get the color as follows.
int colorCode = data.getIntExtra("colorSelected");
//here change the background
mainAcBackground.setBackgroundColor(colorCode);
}
}
关于java - 尝试通过单击另一个 Activity 的复选框来更改 Activity 的背景颜色，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/48117613/