我得到空屏幕与火碱UI回收器视图

Question

我无法在回收器视图中显示火力物品。数据库连接正常。

编辑：我添加了startListening((方法，如链接问题的答案建议的那样。但是现在我的应用程序崩溃了。看起来像这样

  @Override
    protected void onStart() {
        super.onStart();
        FirebaseRecyclerOptions<Coupon> options =
                new FirebaseRecyclerOptions.Builder<Coupon>()
                        .setQuery(mDatabase, Coupon.class)
                        .build();
        FirebaseRecyclerAdapter<Coupon, CouponViewHolder> firebaseRecyclerAdapter = new FirebaseRecyclerAdapter<Coupon, CouponsActivity.CouponViewHolder>
                (options) {
            @Override
            protected void onBindViewHolder(@NonNull CouponsActivity.CouponViewHolder holder, int position, @NonNull Coupon model) {
                holder.setTitle(model.getTitle());
                holder.setDesc(model.getDesc());
            }
            @Override
            public CouponsActivity.CouponViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
                View view = LayoutInflater.from(parent.getContext())
                        .inflate(R.layout.item_coupon_row, parent, false);
                return new CouponsActivity.CouponViewHolder(view);
            }
        };
        firebaseRecyclerAdapter.startListening();
        mRecyclerview.setAdapter(firebaseRecyclerAdapter);

    }
    @Override
    protected void onStop() {
        super.onStop();
        firebaseRecyclerAdapter.stopListening();
    }

以及日志猫中的崩溃信息： 优惠 券。优惠券是模型。

01-15 15:16:26.381 16526-16526/com.myapp E/AndroidRuntime: FATAL EXCEPTION: main
                                                                          Process: com.myapp, PID: 16526
                                                                          com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.myapp.coupons.Coupon
                                                                              at com.google.android.gms.internal.zzelw.zzb(Unknown Source)
                                                                              at com.google.android.gms.internal.zzelw.zza(Unknown Source)
                                                                              at com.google.firebase.database.DataSnapshot.getValue(Unknown Source)
                                                                              at com.firebase.ui.database.ClassSnapshotParser.parseSnapshot(ClassSnapshotParser.java:29)
                                                                              at com.firebase.ui.database.ClassSnapshotParser.parseSnapshot(ClassSnapshotParser.java:15)
                                                                              at com.firebase.ui.common.BaseCachingSnapshotParser.parseSnapshot(BaseCachingSnapshotParser.java:35)
                                                                              at com.firebase.ui.common.BaseObservableSnapshotArray.get(BaseObservableSnapshotArray.java:52)
                                                                              at com.firebase.ui.database.FirebaseRecyclerAdapter.getItem(FirebaseRecyclerAdapter.java:106)
                                                                              at com.firebase.ui.database.FirebaseRecyclerAdapter.onBindViewHolder(FirebaseRecyclerAdapter.java:122)
                                                                              at android.support.v7.widget.RecyclerView$Adapter.onBindViewHolder(RecyclerView.java:6508)
                                                                              at android.support.v7.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:6541)
                                                                              at android.support.v7.widget.RecyclerView$Recycler.tryBindViewHolderByDeadline(RecyclerView.java:5484)
                                                                              at android.support.v7.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:5750)
                                                                              at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:5589)
                                                                              at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:5585)
                                                                              at android.support.v7.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:2231)
                                                                              at android.support.v7.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1558)
                                                                              at android.support.v7.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1518)
                                                                              at android.support.v7.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:610)
                                                                              at android.support.v7.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:3719)
                                                                              at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:3436)
                                                                              at android.support.v7.widget.RecyclerView.consumePendingUpdateOperations(RecyclerView.java:1742)
                                                                              at android.support.v7.widget.RecyclerView$1.run(RecyclerView.java:354)
                                                                              at android.view.Choreographer$CallbackRecord.run(Choreographer.java:874)
                                                                              at android.view.Choreographer.doCallbacks(Choreographer.java:686)
                                                                              at android.view.Choreographer.doFrame(Choreographer.java:618)
                                                                              at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:860)
                                                                              at android.os.Handler.handleCallback(Handler.java:751)
                                                                              at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                              at android.os.Looper.loop(Looper.java:154)
                                                                              at android.app.ActivityThread.main(ActivityThread.java:6119)
                                                                              at java.lang.reflect.Method.invoke(Native Method)
                                                                              at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
                                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

---

这是我的build.gradle

apply plugin: 'com.android.application'
android {
    compileSdkVersion 27
    defaultConfig {
        applicationId "com.myapp"
        minSdkVersion 19
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}
dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:27.0.2'
    implementation 'com.android.support.constraint:constraint-layout:1.0.2'
    // firebase
    implementation 'com.google.firebase:firebase-core:11.8.0'
    implementation 'com.google.firebase:firebase-database:11.8.0'
    implementation 'com.firebaseui:firebase-ui-database:3.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.1'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
    compile 'com.android.support:cardview-v7:27.0.2'
    compile 'com.android.support:recyclerview-v7:27.0.2'
    compile 'com.android.support:design:27.0.2'
}
apply plugin: 'com.google.gms.google-services'

这是我的活动：

public class CouponsActivity extends AppCompatActivity {
    private DatabaseReference mDatabase;
    private RecyclerView mRecyclerview;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_coupons);
        mDatabase = FirebaseDatabase.getInstance().getReference().child("coupon");
        mDatabase.keepSynced(true);
        mRecyclerview = (RecyclerView) findViewById(R.id.coupon_recyclerView_list);
        mRecyclerview.setHasFixedSize(true);
        mRecyclerview.setLayoutManager(new LinearLayoutManager(this));
    }
    @Override
    protected void onStart() {
        super.onStart();

        FirebaseRecyclerOptions<Coupon> options =
                new FirebaseRecyclerOptions.Builder<Coupon>()
                        .setQuery(mDatabase, Coupon.class)
                        .build();
        FirebaseRecyclerAdapter<Coupon, CouponViewHolder> firebaseRecyclerAdapter = new FirebaseRecyclerAdapter<Coupon, CouponsActivity.CouponViewHolder>
                (options) {
            @Override
            protected void onBindViewHolder(@NonNull CouponsActivity.CouponViewHolder holder, int position, @NonNull Coupon model) {
                holder.setTitle(model.getTitle());
                holder.setDesc(model.getDesc());
            }
            @Override
            public CouponsActivity.CouponViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
                View view = LayoutInflater.from(parent.getContext())
                        .inflate(R.layout.item_coupon_row, parent, false);
                return new CouponsActivity.CouponViewHolder(view);
            }
        };
        mRecyclerview.setAdapter(firebaseRecyclerAdapter);
    }
    public static class CouponViewHolder extends RecyclerView.ViewHolder{
        TextView postTitle;
        TextView postDescription;
        public CouponViewHolder(View itemView) {
            super(itemView);
            postTitle = (TextView)itemView.findViewById(R.id.post_title);
            postDescription = (TextView)itemView.findViewById(R.id.post_description);
        }
        public void setTitle(String title) {
            postTitle.setText(title);
        }
        public void setDesc(String desc) {
            postDescription.setText(desc);
        }
    }
}

这就是我的 Firebase DB 的样子

Root
  |
  |__coupon
       |_____title: "This is title"
       |_____desc: "This is description"

这是活动的布局：

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.myapp.coupons.CouponsActivity">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/coupon_recyclerView_list"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@color/YELLOW">

    </android.support.v7.widget.RecyclerView>
</android.support.constraint.ConstraintLayout>

这是卡片项目布局：

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_margin="8dp"
    android:orientation="vertical"
    android:background="@color/GREEN"
    app:cardCornerRadius="5dp"
    android:elevation="90dp">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">

        <TextView
            android:id="@+id/post_title"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:padding="10dp"
            />
        <TextView
            android:id="@+id/post_description"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:paddingBottom="10dp"
            android:paddingLeft="10dp"
            android:paddingRight="10dp"
            />
    </LinearLayout>

</android.support.v7.widget.CardView>

Answer 1

这是因为您的数据库中存储了一个优惠券对象，而不是一个列表。

如果你想创建一个优惠券列表，我建议添加一个带有密钥Coupons的节点，并将每个优惠券存储在它下面。所以你的结构看起来像这样：

Root
  |
  |__Coupons
       |____coupon
               |_____title: "This is title"
               |_____desc: "This is description"

然后你的DatabaseReference看起来是：

mDatabase = FirebaseDatabase.getInstance().getReference().child("Coupons");