与FragmentManager.popBackStack相关的NullPointerException.怎么解决? [英] NullPointerException related to FragmentManager.popBackStack. How to resolve?

问题描述

我有一个Android应用程序，其中有一个活动和一堆片段.

I have an Android app where I have an activity and a stack of fragments.

使用Crashlytics，我收到了以下 Exception 的单个实例:

Using Crashlytics, I have received a single instance of the following Exception:

Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.FragmentManager.popBackStack(java.lang.String, int)' on a null object reference
       at com.company.app.Fragment$7$2.onClick(Fragment.java:397)
       at android.view.View.performClick(View.java:5197)
       at android.view.View$PerformClick.run(View.java:20926)
       at android.os.Handler.handleCallback(Handler.java:739)
       at android.os.Handler.dispatchMessage(Handler.java:95)
       at android.os.Looper.loop(Looper.java:145)
       at android.app.ActivityThread.main(ActivityThread.java:5942)
       at java.lang.reflect.Method.invoke(Method.java)
       at java.lang.reflect.Method.invoke(Method.java:372)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

产生此错误的代码是:

okButton.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        dialog.dismiss();
        // The next line produces the Exception
        getFragmentManager().popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {
                // Other code...
        }, 250);
    }
});

根据我对该应用所做的测试，发生此异常的用户以及仅发生过一次的事实，我认为当某些奇怪的合并时发生此 Exception 发生情况.(基本上，我无法在终端上重现此 Exception .)我认为这与用户为我的应用程序添加背景，然后在以后恢复它，然后是getFragmentManager()调用返回 null .

Based on the testing I have done with this app, the user for which this Exception occurred, and the fact that is has only occurred once, I am thinking that this Exception occurs when some strange consolidation of circumstances occurs. (Basically, I can't reproduce this Exception on my end.) I am assuming this has to do with the user backgrounding my app and then resuming it at a later time, and then the getFragmentManager() call returns a null.

因此，我知道我可以使用以下代码修复"此问题(将其作为对GitHub存储库的应用修复程序发现):

So, I know that I can "fix" this with code like the following (found this as an applied fix to a GitHub repository):

FragmentManager fm = getFragmentManager();
if (fm != null) fm.popBackStack();

虽然我意识到上面的代码将解决"该问题，因为它可以避免 NPE (从而避免应用崩溃)，但实际上并不能解决"该问题通过允许我的应用程序按需运行.所讨论的 Fragment 在片段堆中为#3，如下所示:

While I realize that the code above will "fix" the problem in that it will avoid the NPE, (thus stopping the app from crashing), it doesn't really "fix" the problem by allowing my app to function as desired. The Fragment in question is #3 in a stack of fragments, like this:

#1 --> #2 --> #3

应用所需的行为是通过弹出可见的 Fragment #1来响应此按钮单击.仅添加上面的代码似乎可以防止应用崩溃，但不能以期望的方式更改应用的用户界面.

The desired behavior for the app is to respond to this button click by popping back to Fragment #1 being visible. Merely adding in the code above seems like it would keep the app from crashing, but not change the app's UI in the desired manner.

我做错了什么，以便当我的应用恢复时，它的碎片状态"变得不正常了?

What am I doing wrong such that when my app resumes, it's "fragment state" is out of whack?

推荐答案

我从未遇到过getFragmentManager()为null的问题，因此我假设是导致问题的语句的popBack部分.这是防止崩溃和处理异常的一种可能的解决方案.如果仅发生一次，则可能是罕见的情况.

I have never faced an issue with the getFragmentManager() being null so I am assuming it is the popBack part of the statement that is causing the issue. Here is one possible solution to prevent a crash and handling the exceptions. If it only happened once, it may have been a rare instance.

okButton.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        if (getSupportFragmentManager() != null) {
            // Assuming the getFragmentManager() is not the Issue, rather the popBackStack is the issue
            try {
                getFragmentManager().popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
            } catch (Exception e) {
                // Recreate a new instance of your first fragment here.
            }
        } else {
            /*
             getFragmentManager() == null 
             I have never faced an issue when getFragmentManager() == null, but I would restart the activity if that is the case
            */

        }
        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {

            }
        }, 250);
    }
});

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