Android Activity 資料保存與恢復：Intent 傳值、SharedPreferences、ViewModel 全方案

場景描述

Activity A 包含 EditText、TextView，使用者輸入後跳轉至 Activity B，再從 B 返回 A 時需恢復原輸入內容。

方案一：Intent 暫存（適用單向跳轉、資料量小）

// A → B
Intent intent = new Intent(A.this, B.class);
intent.putExtra("et_text", editText.getText().toString());
intent.putExtra("tv_text", textView.getText().toString());
startActivity(intent);

// B → A (假設 B 以 finish() 返回)
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    if (resultCode == RESULT_OK) {
        editText.setText(data.getStringExtra("et_text"));
        textView.setText(data.getStringExtra("tv_text"));
    }
}

// 存檔 (A onPause/onStop)
SharedPreferences sp = getSharedPreferences("form_data", MODE_PRIVATE);
sp.edit()
  .putString("et_text", editText.getText().toString())
  .putString("tv_text", textView.getText().toString())
  .apply();

// 讀取 (A onCreate/onResume)
SharedPreferences sp = getSharedPreferences("form_data", MODE_PRIVATE);
editText.setText(sp.getString("et_text", ""));
textView.setText(sp.getString("tv_text", ""));

// ViewModel
public class FormViewModel extends ViewModel {
    private final SavedStateHandle handle;
    public FormViewModel(SavedStateHandle handle) { this.handle = handle; }
    public String getEtText() { return handle.get("et_text"); }
    public void setEtText(String v) { handle.set("et_text", v); }
    // tv_text 同理
}

// Activity A
FormViewModel vm = new ViewModelProvider(this).get(FormViewModel.class);
// 即時雙向綁定或 onPause 存、onResume 取