从firebase删除一个项目，而不会崩溃的应用程序 [英] Delete an item from firebase without crashing the app

问题描述

我是Android新手，我一直在玩这个来自Firebase的示例应用程序。我想在 PostDetailActivity ，它通过将DatabaseReference mPostReference设置为null来删除从数据库中查看的当前帖子。然而，这会导致一个空值指针例外的ValueEventListener添加到mPostReference，导致应用程序崩溃。

  intent 
 mPostKey = getIntent（）。getStringExtra（EXTRA_POST_KEY）; 
 if（mPostKey == null）{
 throw new IllegalArgumentException（必须通过EXTRA_POST_KEY）; 
 
 $ b $ //初始化数据库
 mPostReference = FirebaseDatabase.getInstance（）.getReference（）
 .child（posts）。child（mPostKey）; 
 
 ValueEventListener postListener = new ValueEventListener（）{
 @Override $ b $ public void onDataChange（DataSnapshot dataSnapshot）{
 //获取Post对象并使用值更新UI 
 Post post = dataSnapshot.getValue（Post.class）; 
 //活动中的文本视图
 mAuthorView.setText（post.author）; 
 mTitleView.setText（post.title）; 
 mBodyView.setText（post.body）; 
 
 $ b @Override 
 public void onCancelled（DatabaseError databaseError）{
 //获取发布失败，记录消息
 Log.w（TAG， loadPost：onCancelled，databaseError.toException（））; 
 // [START_EXCLUDE] 
 Toast.makeText（PostDetailActivity.this，Failed to load post，
 Toast.LENGTH_SHORT）.show（）; 
 // [END_EXCLUDE] 
} 
}; 
 mPostReference.addValueEventListener（postListener）; 
 
 // delete button clicked 
 deleteButton.setOnClickListener（new View.OnClickListener（）{
 mPostReference.setValue（null）; 
} 
  

如果mPostReference被删除或设置为null，有没有办法让应用程序崩溃？有没有更好的方法来实现删除按钮？
任何想法/建议将不胜感激。谢谢



以下是日志

 致命例外：main 
进程：com.google.firebase.quickstart.database，PID：2861 
 java.lang.NullPointerException：尝试从字段'java .lang.String com.google.firebase.quickstart.database.models.Post.author'在null对象引用
上com.google.firebase.quickstart.database.PostDetailActivity $ 1.onDataChange（PostDetailActivity.java:93 ）
在com.google.android.gms.internal.zzbmz.zza（未知来源）
在com.google.android.gms.internal.zzbnz.zzYj（未知来源）
 at com.google.android.gms.internal.zzboc $ 1.run（Unknown Source）
 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:6145）
 at java.lang.reflect.Method.invoke（Native Method）$ b $在java.lang.reflect.Method.invoke（Method.java:372）
在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller .run（ZygoteInit.java:1399）
 at com.android.internal.os.ZygoteInit.main（ZygoteInit.java:1194）
  




解决方案

如果语句只需要添加来检查 dataSnapshot 有一个值或 null

  public void onDataChange（DataSnapshot dataSnapshot）{
 if（dataSnapshot.exists（））{
 //获取Post对象并使用值更新UI 
 Post post = d ataSnapshot.getValue（Post.class）; 
 //活动中的文本视图
 mAuthorView.setText（post.author）; 
 mTitleView.setText（post.title）; 
 mBodyView.setText（post.body）; 
} 
} 
  




I'm new to android and I've been playing around with this sample app from firebase. I want to add a delete button on PostDetailActivity which deletes the current post being viewed from the database by setting the DatabaseReference mPostReference to null. However this causes a null pointer exception to a ValueEventListener added to mPostReference, causing the app to crash.

    // Get post key from intent
    mPostKey = getIntent().getStringExtra(EXTRA_POST_KEY);
    if (mPostKey == null) {
        throw new IllegalArgumentException("Must pass EXTRA_POST_KEY");
    }

    // Initialize Database
    mPostReference = FirebaseDatabase.getInstance().getReference()
            .child("posts").child(mPostKey);

    ValueEventListener postListener = new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            // Get Post object and use the values to update the UI
            Post post = dataSnapshot.getValue(Post.class);
            // Text Views in activity
            mAuthorView.setText(post.author);
            mTitleView.setText(post.title);
            mBodyView.setText(post.body);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            // Getting Post failed, log a message
            Log.w(TAG, "loadPost:onCancelled", databaseError.toException());
            // [START_EXCLUDE]
            Toast.makeText(PostDetailActivity.this, "Failed to load post.",
                    Toast.LENGTH_SHORT).show();
            // [END_EXCLUDE]
        }
    };
    mPostReference.addValueEventListener(postListener);

   //delete button clicked
   deleteButton.setOnClickListener(new View.OnClickListener() {
        mPostReference.setValue(null);
   }

Is there a way to keep the app from crashing if mPostReference has been deleted or set to null? Are there better ways in implementing a delete button? Any ideas/suggestions would be greatly appreciated. Thank you

Here's the log

FATAL EXCEPTION: main
   Process: com.google.firebase.quickstart.database, PID: 2861
   java.lang.NullPointerException: Attempt to read from field 'java.lang.String com.google.firebase.quickstart.database.models.Post.author' on a null object reference
   at com.google.firebase.quickstart.database.PostDetailActivity$1.onDataChange(PostDetailActivity.java:93)
   at com.google.android.gms.internal.zzbmz.zza(Unknown Source)
   at com.google.android.gms.internal.zzbnz.zzYj(Unknown Source)
   at com.google.android.gms.internal.zzboc$1.run(Unknown Source)
   at android.os.Handler.handleCallback(Handler.java:739)
   at android.os.Handler.dispatchMessage(Handler.java:95)
   at android.os.Looper.loop(Looper.java:145)
   at android.app.ActivityThread.main(ActivityThread.java:6145)
   at java.lang.reflect.Method.invoke(Native Method)
   at java.lang.reflect.Method.invoke(Method.java:372)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

解决方案

You just need to add an if statement to check if the dataSnapshot has a value or null

public void onDataChange(DataSnapshot dataSnapshot) {
    if (dataSnapshot.exists()) {
        // Get Post object and use the values to update the UI
        Post post = dataSnapshot.getValue(Post.class);
        // Text Views in activity
        mAuthorView.setText(post.author);
        mTitleView.setText(post.title);
        mBodyView.setText(post.body);
    }
}

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