使用倒数计时器时应用程序停止答案

【问题标题】：App Stops when using countdown timer使用倒数计时器时应用程序停止
【发布时间】：2016-09-30 11:01:57
【问题描述】：

我正在开发一个测验应用程序，它有五个片段，每个片段都有一个问题。片段以两种方式移动。

当用户点击一个选项时，下一个片段会立即出现。
我有一个倒数计时器，如果用户不点击任何选项，那么 10 秒后片段将移动到新片段。

我已经实现了点击功能。当用户单击选项时，应用程序会将其带到下一个片段，但如果在该下一个片段中用户未单击任何选项，则应用程序会立即停止。请看一下，我正在共享代码。

Question.java (Fragment1)

 package com.example.sheikhspc.testapp;


import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.drawable.ShapeDrawable;
import android.os.Bundle;
import android.os.CountDownTimer;
import android.app.Fragment;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.RelativeLayout;
import android.widget.TextView;


public class Question1 extends Fragment implements View.OnClickListener {


public Question1() {
    // Required empty public constructor
}
int counter = 0;
View view;
Fragment frag = null;
TextView tv;
ImageView imageView1, imageViewR, imageView2;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    view = inflater.inflate(R.layout.fragment_question1, container, false);
    imageView1 = (ImageView) view.findViewById(R.id.wrong1);
    imageViewR = (ImageView) view.findViewById(R.id.right);
    imageView2 = (ImageView) view.findViewById(R.id.wrong2);
    tv = (TextView) view.findViewById(R.id.timer1);

    imageView1.setOnClickListener(this);
    imageViewR.setOnClickListener(this);
    imageView2.setOnClickListener(this);
    new CountDownTimer(10000, 1000) {

        public void onTick(long millisUntilFinished) {
            tv.setText("Remaining Time: " + millisUntilFinished / 1000);
            //here you can have your logic to set text to edittext
        }

        public void onFinish() {
            frag = new Question2();
            FragmentManager fmm = getFragmentManager();
            FragmentTransaction fragmentTransactionn =  fmm.beginTransaction();
            fragmentTransactionn.addToBackStack(null);

            fragmentTransactionn.replace(R.id.container, frag);

            fragmentTransactionn.commit();

        }


    }.start();


    return view;
}


@Override
public void onClick(View v) {
    switch (v.getId()) {
        case R.id.wrong1:
            frag = new Question2();
            FragmentManager fm = getFragmentManager();
            FragmentTransaction fragmentTransaction = fm.beginTransaction();

            fragmentTransaction.replace(R.id.container,frag);

            fragmentTransaction.commit();

            break;
        case R.id.right:
            frag = new Question2();
            FragmentManager fm1 = getFragmentManager();
            FragmentTransaction fragmentTransaction1 = fm1.beginTransaction();

            fragmentTransaction1.replace(R.id.container,frag);

            fragmentTransaction1.commit();





            break;
        case R.id.wrong2:
            frag = new Question2();
            FragmentManager fm2 = getFragmentManager();
            FragmentTransaction fragmentTransaction2 = fm2.beginTransaction();

            fragmentTransaction2.replace(R.id.container,frag);

            fragmentTransaction2.commit();

            break;
        default:
            new CountDownTimer(10000, 1000) {

                public void onTick(long millisUntilFinished) {
                    tv.setText("Remaining Time: " + millisUntilFinished / 1000);
                    //here you can have your logic to set text to edittext
                }

                public void onFinish() {
                    frag = new Question2();
                    FragmentManager fmm = getFragmentManager();
                    FragmentTransaction fragmentTransactionn = fmm.beginTransaction();
                    fragmentTransactionn.addToBackStack(null);

                    fragmentTransactionn.replace(R.id.container, frag);

                    fragmentTransactionn.commit();

                }


            }.start();
            break;

    }
}
}

Question2.java (Fragment2)

package com.example.sheikhspc.testapp;


import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.app.Fragment;
import android.os.CountDownTimer;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;


public class Question2 extends Fragment implements View.OnClickListener {


public Question2() {
    // Required empty public constructor
}


Fragment frag = null;
TextView tv;
View view;
ImageView imageView1, imageViewR, imageView2;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    view = inflater.inflate(R.layout.fragment_question2, container, false);

    tv = (TextView)view.findViewById(R.id.timer2);
    imageView1 = (ImageView) view.findViewById(R.id.wrong1);
    imageViewR = (ImageView) view.findViewById(R.id.right);
    imageView2 = (ImageView) view.findViewById(R.id.wrong2);
    imageView1.setOnClickListener(this);
    imageViewR.setOnClickListener(this);
    imageView2.setOnClickListener(this);
    new CountDownTimer(10000, 1000) {

        public void onTick(long millisUntilFinished) {
            tv.setText("seconds remaining: " + millisUntilFinished / 1000);
            //here you can have your logic to set text to edittext
        }

        public void onFinish() {
            tv.setText("done!");
            frag = new Question3();
            FragmentManager fm = getFragmentManager();
            FragmentTransaction fragmentTransaction = fm.beginTransaction();

            fragmentTransaction.replace(R.id.container,frag);

            fragmentTransaction.commit();

        }

    }.start();


    return view;
}


@Override
public void onClick(View v) {
    switch (v.getId()) {
        case R.id.wrong1:
            frag = new Question3();
            FragmentManager fm = getFragmentManager();
            FragmentTransaction fragmentTransaction = fm.beginTransaction();

            fragmentTransaction.replace(R.id.container,frag);

            fragmentTransaction.commit();

            break;
        case R.id.right:
            frag = new Question3();
            FragmentManager fm1 = getFragmentManager();
            FragmentTransaction fragmentTransaction1 = fm1.beginTransaction();

            fragmentTransaction1.replace(R.id.container,frag);

            fragmentTransaction1.commit();





            break;
        case R.id.wrong2:
            frag = new Question3();
            FragmentManager fm2 = getFragmentManager();
            FragmentTransaction fragmentTransaction2 = fm2.beginTransaction();

            fragmentTransaction2.replace(R.id.container,frag);

            fragmentTransaction2.commit();

            break;
        default:
            new CountDownTimer(10000, 1000) {

                public void onTick(long millisUntilFinished) {
                    tv.setText("seconds remaining: " + millisUntilFinished / 1000);
                    //here you can have your logic to set text to edittext
                }

                public void onFinish() {
                    tv.setText("done!");
                    frag = new Question3();
                    FragmentManager fm = getFragmentManager();
                    FragmentTransaction fragmentTransaction = fm.beginTransaction();

                    fragmentTransaction.replace(R.id.container,frag);

                    fragmentTransaction.commit();

                }

            }.start();

            break;

    }
}
}

错误日志

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.sheikhspc.testapp, PID: 32330
              java.lang.NullPointerException: Attempt to invoke virtual  method 'android.app.FragmentTransaction  android.app.FragmentManager.beginTransaction()' on a null object reference
                  at com.example.sheikhspc.testapp.Question1$1.onFinish(Question1.java:58)
                  at android.os.CountDownTimer$1.handleMessage(CountDownTimer.java:127)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:135)
                  at android.app.ActivityThread.main(ActivityThread.java:5343)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:372)
                  at  com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:905)
                  at  com.android.internal.os.ZygoteInit.main(ZygoteInit.java:700)
I/Process: Sending signal. PID: 32330 SIG: 9
Application terminated.

Second.java（显示所有片段的Activity）

package com.example.sheikhspc.testapp;

import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.app.Fragment;
import android.os.CountDownTimer;
import android.provider.DocumentsContract;
import android.support.v7.app.AppCompatActivity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.widget.Toast;

public class Second extends AppCompatActivity {

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

    FragmentManager fm = getFragmentManager();
    FragmentTransaction fragmentTransaction = fm.beginTransaction();

    frag = new Question1();
    fragmentTransaction.replace(R.id.container,frag);

    fragmentTransaction.commit();

}
}

【问题讨论】：

显然 getFragmentManager() 在某处返回 null ...我敢打赌，这是因为 Activity 不再存在
在移动到另一个片段后尝试终止倒计时。
问题出在您的活动类中。你能告诉我们吗？
我已经用我的活动更新了问题。请看一下

标签： java android android-fragments crash

【解决方案1】：

在点击替换片段之前，您需要取消计时器。

当您在 onCreate 方法中创建 CountDownTimer 的新实例时，将其分配给一个变量，例如

CountDownTimer timer = new CountDownTimer(10000, 1000) {...};
timer.start();

然后在onClick 方法中，即当用户单击任何选项时，例如停止计时器

@Override
public void onClick(View v) {
switch (v.getId()) {
    case R.id.btnNext:

        // Add this line to avoid crash
        timer.cancel();

        frag = new FragmentQuestion2();
        FragmentManager fm = getFragmentManager();
        FragmentTransaction fragmentTransaction = fm.beginTransaction();

        fragmentTransaction.replace(R.id.container,frag);

        fragmentTransaction.commit();

        break;
}

【讨论】：

谢谢。这对我有用:)