这是一个应用练习,我将非常感谢一些帮助和编码示例。我正在尝试将SettingsActivity.java中的背景颜色设置为MainActivity.java。这将通过一个复选框发生,我单击并触发更改。
我可以通过意图访问MainActivity,同时单击复选框。但是,如果我尝试设置MainActivity背景的颜色,Java会抛出一个空指针错误,导致该值为null。
这是logcat:
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一线:
mainAcBackground.setBackgroundColor(getResources().getColor(R.color.colorAccent));
我不太确定如何离开这里,对Android仍然相当新,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(Constraint布局) - 看起来很糟糕,因为它会泄漏 我试图使用共享首选项,但未成功。
这是我的MainActivity
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>
以下是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>
希望我已经清楚解释过,如果您需要更多信息,请告诉我,并提前感谢您!
答案 0 :(得分:1)
很明显,您可以通过简单存储未初始化的视图参数来更改某些活动的背景颜色。
这里做了类似的事。
当您从主要活动开始设置活动时,请启动此类
private void pickColor() {
Intent settings = new Intent(this, SettingsActivity.class);
startActivityForResult(settings, 101);
}
然后您可以更改设置活动中的颜色并将颜色代码存储在intent中并传回调用活动并完成类似的设置活动。
public void colorChanged(int colorCode){
Intent color = new Intent();
color.putExtra("colorSelected",colorCode);
setResult(RESULT_OK,color);
finish();
}
并在主Activity中收到这样的结果,这也是你的通话活动。
@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);
}
}
答案 1 :(得分:0)
我猜测标识为ConstraintLayout
的{{1}}位于mainAcBackground
而不是activity_main.xml
。如果是这样,
<{1}}中的activity_settings.xml
将返回null,因此findViewById(R.id.mainAcBackground);
被提升。
您要执行的操作是在复选框项目更改时将值存储在SettingsActivity.java
中,并使用NullPointerException
中的值加载正确的背景颜色。您必须在SharedPreference
中使用OnSharedPreferenceChangeListener
,以确保在恢复活动时,您可以加载正确的背景。
答案 2 :(得分:0)
您有一个settingsActivity
,您可以在其中决定主要活动中的颜色mainAcBackground
。
这是一个简单的方法,使用未经测试的代码。但是一旦有效,就不应该使用Intent.putExtra()
代替SharedPreferences
或<{1}}。
问题是:您无法从其他活动访问主Activity中的View,因为只有在创建了Activity后才会存在。 (作为Android的新手,您应该阅读Activity Lifecycle,这通常会引起混淆。)当您的活动第一次加载时,会调用onCreate()
。当它暂停并稍后恢复时,会调用onResume()
。在这两者之间,用户可能已更改背景设置,因此您需要在主要活动onResume()
中设置该背景。这看起来像这样:
@Override
public void onResume() {
super.onResume(); // Always call the superclass method first
if(myBackgroundColor!=null){
ConstraintLayout mainAcBackground = (ConstraintLayout) findViewById(R.id.mainAcBackground);
mainAcBackground.setBackgroundColor(myBackgroundColor);
}
}
但是如何将你的颜色变成myBackgroundColor
?
在这种方法中,我们使用全局变量,因为这很简单。请写入您的主要活动类
public static int myBackgroundColor;
您可以使用
从设置活动中访问此变量MainActivity.myBackgroundColor = getResources().getColor(R.color.colorAccent)
在onCheckedStateChangedListener
。