Activity Lifecycle:startActivityForResult并按Back Button

// 导入所需库
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    private Button mButton;

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

        mButton = findViewById(R.id.button);
        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // 启动另一个 Activity
                startActivityForResult(new Intent(MainActivity.this, SecondActivity.class), 0);
            }
        });
    }

    @Override
    protected void onPause() {
        super.onPause();

        // 在按 Back Button 时执行操作
        if (mButton != null) {
            mButton.setText("Back");
        }
    }

    @Override
    protected void onResume() {
        super.onResume();

        // 在按 Back Button 时更新按钮文本
        if (mButton != null) {
            mButton.setText("Forward");
        }
    }
}