在 onActivityResult 之后设置 TextView 文本

Question

我有一个带有登录页面的应用程序。

首先，我有我的MainActivity一切开始的地方：

public class MainActivity extends AppCompatActivity {

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

    // here i launch LoginActivity
    startActivityForResult(new Intent(this, LoginActivity.class), REQUEST_CODE);

    emailText = findViewById(R.id.nav_text_email);
    usernameText = findViewById(R.id.nav_text_username);
}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data){
    if(requestCode == REQUEST_CODE){
        if(resultCode != Activity.RESULT_OK) {
            finish();
        }
        // if login succesful, i attach to variable instance of
        // logged user class.
        // that part works correct, in object vars exist
        loggedUser = loggedUser.getInstance();

        // i want to attach strings after succesful login
        emailText.setText(loggedUser.getEmail());
        usernameText.setText(loggedUser.getUsername());
    }
}

当运行带有上述代码的应用程序成功后 LoginActivity 应用程序崩溃并出现此异常：

2019-01-26 20:46:48.015 13071-13071/com.example.admin.keystroke_dynamics E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.admin.keystroke_dynamics, PID: 13071
java.lang.RuntimeException: Unable to resume activity {com.example.admin.keystroke_dynamics/com.example.admin.keystroke_dynamics.Activities.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3586)
    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3626)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2876)
    at android.app.ActivityThread.-wrap12(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1567)
    at android.os.Handler.dispatchMessage(Handler.java:105)
    at android.os.Looper.loop(Looper.java:156)
    at android.app.ActivityThread.main(ActivityThread.java:6523)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
    at com.example.admin.keystroke_dynamics.Activities.MainActivity.onResume(MainActivity.java:89)
    at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1279)
    at android.app.Activity.performResume(Activity.java:7022)
    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3561)
    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3626) 
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2876) 
    at android.app.ActivityThread.-wrap12(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1567) 
    at android.os.Handler.dispatchMessage(Handler.java:105) 
    at android.os.Looper.loop(Looper.java:156) 
    at android.app.ActivityThread.main(ActivityThread.java:6523) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832)

当我从 `onActivityResult() 中删除 setText() 方法时，我没有得到该异常。当我单击 Login 按钮并且应用程序崩溃时出现错误。

调试时，编译器甚至不符合 setText()

我的问题是，如何在我上面编码的场景中设置 textviews 中的文本？

更新

即使我在构造函数中将 loggedUser 中的字符串填充为空，移动 loggedUser = loggedUser.getInstance() 并将 setText() 移动到 onCreat，我仍然得到 NPE。

编辑：

我添加了onResume方法：

@Override
protected void onResume()
{
    if(isLogged) {
        emailText.setText(loggedUser.getEmail());
        usernameText.setText(loggedUser.getUsername());
    }
    super.onResume();
}

并编辑 onActivityResult():

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data){
    if(requestCode == REQUEST_CODE){
        if(resultCode != Activity.RESULT_OK) {
            finish();
        }
        isLogged = true; //var for if statement in onResume()
        loggedUser = loggedUser.getInstance();
    }
}

当我设置 TextViews 时，onResume() 方法仍然出现相同的错误。

2019-01-26 21:21:57.752 18262-18262/com.example.admin.keystroke_dynamics E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.admin.keystroke_dynamics, PID: 18262
java.lang.RuntimeException: Unable to resume activity {com.example.admin.keystroke_dynamics/com.example.admin.keystroke_dynamics.Activities.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3586)
    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3626)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1618)
    at android.os.Handler.dispatchMessage(Handler.java:105)
    at android.os.Looper.loop(Looper.java:156)
    at android.app.ActivityThread.main(ActivityThread.java:6523)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
    at com.example.admin.keystroke_dynamics.Activities.MainActivity.onResume(MainActivity.java:90)
    at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1279)
    at android.app.Activity.performResume(Activity.java:7022)
    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3561)
    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3626) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1618) 
    at android.os.Handler.dispatchMessage(Handler.java:105) 
    at android.os.Looper.loop(Looper.java:156) 
    at android.app.ActivityThread.main(ActivityThread.java:6523)

Answer 1

空视图通常是因为您的布局不包含具有您在 findViewById() 中指定的 ID 的视图。因此，当调用 onResume() 时，findViewById() returns null 和变量保持为 null。仔细检查 main_activity.xml.

中的视图 ID

如果您使用更复杂的布局，也有可能在 onCreate() 中无法使用完整内容视图。如果您发现您的视图在 onCreate() 中为 null，请尝试将 findViewById() 移到生命周期的后期。

Answer 2

尝试像这样重新排序代码

emailText = findViewById(R.id.nav_text_email);
usernameText = findViewById(R.id.nav_text_username);
startActivityForResult(new Intent(this, LoginActivity.class), REQUEST_CODE);

Answer 3

尝试在访问变量之前对其进行初始化。

public class MainActivity extends AppCompatActivity {
TextView emailText;
TextView usernameText;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main_activity);

    // here i launch LoginActivity
    startActivityForResult(new Intent(this, LoginActivity.class), REQUEST_CODE);

    emailText = findViewById(R.id.nav_text_email);
    usernameText = findViewById(R.id.nav_text_username);
}

可以放在评论中

您需要从 activity

访问 NavigationView

更新

NavigationView navigationView = findViewById(R.id.nav_id)
View header = navigationView.getHeaderView(0)
TextView usernameText = header.findViewById(R.id.username_text_view_id);

在 onActivityResult 之后设置 TextView 文本

Setting TextView text after onActivityResult

java

android

android-lifecycle