如何使用`onSaveInstanceState`？

Question

我想达到什么目标？

• 我想保存x、y 和score 的值，所以当用户旋转屏幕时，这些值会被检索而不是重新生成。

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我有什么？

1. 可能是正确的 onSaveInstanceState 代码无法正常工作，可能是因为某些 @Override 与其发生冲突

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我需要什么？

• 一些可以帮助我保存变量值的修复

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代码

MainActivity.java（没有onSaveInstanceState）

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import java.util.Random;

public class MainActivity extends AppCompatActivity {

    private EditText textBox;
    private int z;
    public int score = 0;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ((TextView) findViewById(R.id.scoreText)).setText(R.string.nullScore);
        textBox = findViewById(R.id.editText);
        textBox.requestFocus();
        setActual();
    }

    private void setActual() {
        Random random = new Random();
        int x = random.nextInt(10) + 1; //you get a random number between 0 and 9 then add 1 to get 1 - 10, no need to use array..
        int y = random.nextInt(10) + 1;
        z = x * y;
        String hintStr = x + " × " + y;
        ((TextView) findViewById(R.id.hintTextView)).setText(hintStr);
    }


            public void onCheck(View view) {

        int answer = Integer.parseInt(textBox.getText().toString());
        String res = "Incorrect";
        if (answer == z) {
            res = "Correct";
            score++;
            Log.d("TAG", "Correct."); // right answer
            textBox.setText("");
            setActual(); // let's regenerate values
        } else if (score == 0) {
            Log.d("TAG", "Incorrect."); // wrong answer
            textBox.setText("");
            setActual();
            ((TextView) findViewById(R.id.scoreText)).setText(R.string.nullScore);
        } else {
            Log.d("TAG", "Incorrect."); // wrong answer
            textBox.setText("");
            score--;
            setActual();
        }
        ((TextView) findViewById(R.id.scoreText)).setText("Score:" + ' ' + score);
        ((TextView) findViewById(R.id.result_text_view)).setText(res);
    }

}

MainActivity.java（带有onSaveInstanceState）

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import java.util.Random;

public class MainActivity extends AppCompatActivity {

    private EditText textBox;
    private int z;
    public int score = 0;
    private int x;
    private int y;

    public void onSaveInstanceState(Bundle savedInstanceState) {
        savedInstanceState.putInt("x", x);
        savedInstanceState.putInt("y", y);
        savedInstanceState.putInt("score", score);
    }


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textBox = findViewById(R.id.editText);
        textBox.requestFocus();
        setActual();
        if (savedInstanceState != null) {
            x = savedInstanceState.getInt("x");
            y = savedInstanceState.getInt("y");
            score = savedInstanceState.getInt("score");
        }
    }

    private void setActual() {
        Random random = new Random();
        x = random.nextInt(10) + 1; //you get a random number between 0 and 9 then add 1 to get 1 - 10, no need to use array..
        y = random.nextInt(10) + 1;
        z = x * y;
        String hintStr = x + " × " + y;
        ((TextView) findViewById(R.id.hintTextView)).setText(hintStr);
    }


            public void onCheck(View view) {

        int answer = Integer.parseInt(textBox.getText().toString());
        String res = "Incorrect";
        if (answer == z) {
            res = "Correct";
            score++;
            Log.d("TAG", "Correct."); // right answer
            textBox.setText("");
            setActual(); // let's regenerate values
        } else if (score == 0) {
            Log.d("TAG", "Incorrect."); // wrong answer
            textBox.setText("");
            setActual();
            ((TextView) findViewById(R.id.scoreText)).setText(R.string.nullScore);
        } else {
            Log.d("TAG", "Incorrect."); // wrong answer
            textBox.setText("");
            score--;
            setActual();
        }
        ((TextView) findViewById(R.id.scoreText)).setText("Score:" + ' ' + score);
        ((TextView) findViewById(R.id.result_text_view)).setText(res);
    }

}

Answer 1

你应该考虑调用 super.onSaveInstanceState()

@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
    // save your state
    super.onSaveInstanceState(outState);
}

此外，为避免每次屏幕旋转时都重新创建 Activity，您可以通过将其添加到应用清单中的 Activity 声明来自己处理配置更改：

android:configChanges="keyboardHidden|orientation|screenSize"

并覆盖：

@Override
public void onConfigurationChanged(Configuration newConfig) {
    super.onConfigurationChanged(newConfig);
}

如果您的应用在方向或屏幕尺寸更改后不需要重新创建，则可以为用户节省时间...

Answer 2

您确定没有检索到值吗？ 我相信这里的问题是您总是在 onCreate() 中调用 setActual() 来生成 x 和 y 值并将 z 值分配给生成的 x 和 y（即使 savedInstanceState != null）

更好的处理方式是

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    textBox = findViewById(R.id.editText);
    textBox.requestFocus();

    if (savedInstanceState != null) {
        x = savedInstanceState.getInt("x");
        y = savedInstanceState.getInt("y");
        score = savedInstanceState.getInt("score");
        z = x * y;
        String hintStr = x + " × " + y;
        ((TextView) findViewById(R.id.hintTextView)).setText(hintStr);
    } else { setActual(); }
}

你也应该像这样调用 super.onSaveInstanceState(savedInstanceState)

@Override
protected void onSaveInstanceState(Bundle savedInstanceState) {
    super.onSaveInstanceState(savedInstanceState);
    savedInstanceState.putInt("x", x);
    savedInstanceState.putInt("y", y);
    savedInstanceState.putInt("score", score);
}