如何在类之间传递值和更新回收器视图时修复"System services not available to Activities before onCreate"？

Question

我正在下订单接受收银员申请。使用回收器视图显示订单队列。但是，我在类之间传递 ArrayList 值以及更新回收器视图时遇到问题。在这种情况下，我不会有大量的回收者视图，因此性能不会是一个大问题，但优化的方法将不胜感激。我只是不知道如何解决这个问题。

我尝试在 MainActivty 类中创建一个集合方法，我也尝试了意图 put extra，但我相信我还没有完全理解 put extra。

主活动类：

public class MainActivity extends AppCompatActivity implements MyRecyclerViewAdapter.ItemClickListener {
private RecyclerView recyclerView;
private RecyclerView.Adapter mAdapter;
private RecyclerView.LayoutManager layoutManager;
public  ArrayList<String> orderList = new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);

// set up the RecyclerView
recyclerView = findViewById(R.id.recyle_view_activity);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
mAdapter = new MyRecyclerViewAdapter(this, orderList);
((MyRecyclerViewAdapter) mAdapter).setClickListener(this);

recyclerView.addItemDecoration(new DividerItemDecoration(this,
DividerItemDecoration.VERTICAL));

recyclerView.setAdapter(mAdapter);

}
//This is the set method I attempted:
public void setOrderAdd(ArrayList<String> data){
if (data != null){
mAdapter = new MyRecyclerViewAdapter(this,data); //Error refrences here
}
}
}

其他类别：

public class regularCoffee extends AppCompatActivity implements AdapterView.OnItemSelectedListener, Serializable {
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_regular_coffee);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
addToOrder();
}
public void addToOrder(){
//Reg Addons:
final CheckBox leaveRoomReg = findViewById(R.id.leave_room_reg);
final CheckBox cinnamonReg = findViewById(R.id.cinnamon_reg);
final CheckBox chocolateSyrupReg = findViewById(R.id.chocolate_syrup_reg);
final CheckBox whiteChocolateSyrupReg = findViewById(R.id.white_chocolate_syrup_reg);
final CheckBox caramelReg = findViewById(R.id.caramelReg);
final CheckBox hazelnutReg = findViewById(R.id.hazel_nut_reg);
final CheckBox[] RegCoffeeAddOns = {leaveRoomReg,cinnamonReg,chocolateSyrupReg,whiteChocolateSyrupReg,caramelReg,hazelnutReg};
//Decaf Addons:
final Button leaveRoomDecaf = findViewById(R.id.leave_room_decaf);
final Button cinnamonDecaf = findViewById(R.id.cinnamon_decaf);
final Button chocolateSyrupDecaf = findViewById(R.id.chocolate_syrup_decaf);
final Button whiteChocolateDecaf = findViewById(R.id.white_chocolate_syrup_decaf);
final Button caramelDecaf = findViewById(R.id.caramel_decaf);
final Button hazelnutDecaf = findViewById(R.id.hazel_nut_decaf);
final Button[] DecafCoffeeAddOns = {leaveRoomDecaf,cinnamonDecaf,caramelDecaf,chocolateSyrupDecaf,whiteChocolateDecaf,hazelnutDecaf};
Button addToOrderButton = findViewById(R.id.addToOrderReg);

addToOrderButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
MainActivity addToOrderArea = new MainActivity();
String forOrder;
ArrayList<String> tempArray = new ArrayList<>();
if (regCoffeeQuantity > 0){
forOrder = "Regular Coffee (x" + regCoffeeQuantity + ") n   ";
for (int i = 0; i < RegCoffeeAddOns.length; i++){
if (RegCoffeeAddOns[i].isChecked()){
forOrder = forOrder + "| " + RegCoffeeAddOns[i].getText().toString() + " ";
System.out.println(forOrder);
}
}
addToOrderArea.setOrderAdd(tempArray); //Error refrences here
}
}
});
}

回收器查看适配器：

package com.example.holygroundsapplication;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.recyclerview.widget.RecyclerView;
import java.util.List;
public class MyRecyclerViewAdapter extends RecyclerView.Adapter<MyRecyclerViewAdapter.ViewHolder> {
private List<String> mData;
private LayoutInflater mInflater;
private ItemClickListener mClickListener;
// data is passed into the constructor
MyRecyclerViewAdapter(Context context, List<String> data) {
this.mInflater = LayoutInflater.from(context); //Error refrences here.
this.mData = data;
}

// inflates the row layout from xml when needed
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = mInflater.inflate(R.layout.recyclerview_row, parent, false);
return new ViewHolder(view);
}
// binds the data to the TextView in each row
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
String animal = mData.get(position);
holder.myTextView.setText(animal);
}
// total number of rows
@Override
public int getItemCount() {
return mData.size();
}

// stores and recycles views as they are scrolled off screen
public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
TextView myTextView;
ViewHolder(View itemView) {
super(itemView);
myTextView = itemView.findViewById(R.id.tvAnimalName);
itemView.setOnClickListener(this);
}
@Override
public void onClick(View view) {
if (mClickListener != null) mClickListener.onItemClick(view, getAdapterPosition());
}
}
// convenience method for getting data at click position
String getItem(int id) {
return mData.get(id);
}
// allows clicks events to be caught
void setClickListener(MainActivity itemClickListener) {
this.mClickListener = itemClickListener;
}
// parent activity will implement this method to respond to click events
public interface ItemClickListener {
void onItemClick(View view, int position);
}
}

这是错误：

java.lang.IllegalStateException: System services not available to Activities before onCreate()
at android.app.Activity.getSystemService(Activity.java:5774)
at android.view.LayoutInflater.from(LayoutInflater.java:233)
at com.example.holygroundsapplication.MyRecyclerViewAdapter.<init>(MyRecyclerViewAdapter.java:21)
at com.example.holygroundsapplication.MainActivity.setOrderAdd(MainActivity.java:89)
at com.example.holygroundsapplication.regularCoffee$8.onClick(regularCoffee.java:262)
...

Answer 1

在你的onClick函数中，你有

MainActivity addToOrderArea = new MainActivity();

不应自行创建活动实例。这是Android系统所做的。因为您自己创建活动，所以它没有初始化(即onCreate()不叫(。在setOrderAdd()中创建的适配器需要初始化活动，因此会引发异常。

为防止这些问题，您需要创建带有startActivity()的活动。您可以通过向起始意向添加额外内容来将数据传递给新活动。

Answer 2

我认为您在逻辑上需要替换

MainActivity addToOrderArea = new MainActivity();

跟

MainActivity addToOrderArea = MainActivity.this;

但是由于您不在同一个类中，因此需要相应地传递引用。 只要 MainActivity 仍然处于活动状态，而另一个类只是一个帮手，就不会有问题。只需将主活动添加到所有构造函数...有点像这样：

OtherClass{
private MainClass addToOrderArea;
public OtherClass(MainClass addToOrderArea){
this.addToOrderArea=addToOrderArea;
}
// ...
abstract class MyOnclicklistener extends View.OnClickListene{
private MainClass addToOrderArea;
public MyOnclicklistener(MainClass addToOrderArea){
this.addToOrderArea=addToOrderArea;
}
}
// ...
addToOrderButton.setOnClickListener(new MyOnclicklistener(this.addToOrderArea) {
@Override
public void onClick(View v) {
String forOrder;
ArrayList<String> tempArray = new ArrayList<>();
if (regCoffeeQuantity > 0){
forOrder = "Regular Coffee (x" + regCoffeeQuantity + ") n   ";
for (int i = 0; i < RegCoffeeAddOns.length; i++){
if (RegCoffeeAddOns[i].isChecked()){
forOrder = forOrder + "| " + RegCoffeeAddOns[i].getText().toString() + " ";
System.out.println(forOrder);
}
}
addToOrderArea.setOrderAdd(tempArray); //Error refrences here
}
}
});
}

注意：我想这仍然可能会让您遇到另一个错误，因为您尝试从另一个活动编辑一个活动......

可能只存储两个活动之间的变量，这似乎没问题。但是，直接从另一个活动更新回收器视图可能会使您进入一些Android视图或安全错误。我会说只是传递变量并稍后在 onResume 函数中更新回收器视图。像这样：

private ArrayList<String> _data;
public void setOrderAdd(ArrayList<String> data){
if (data != null){
// only store variable
_data = data;
}
}
@Override
protected void onResume() {
super.onResume();
// make update of your viewing components here
if(_data!=null){
mAdapter = new MyRecyclerViewAdapter(this,_data);
}
}

现在它应该工作了，我想