findViewById不适用于特定视图 [英] findViewById not working for specific view

问题描述

我有一个从XML加载的活动，其视图具有照常的ID:

I have an activity loaded from XML, with views having IDs as usual:

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="80dp"
    android:layout_height="110dp">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <FrameLayout
            android:layout_width="70dp"
            android:layout_height="70dp"
            android:layout_gravity="center_horizontal"
            android:background="@drawable/white_circle">

            <com.myapp.views.CircleImageView
                android:id="@+id/imageView"
                android:layout_width="match_parent"
                android:layout_height="match_parent" />
        </FrameLayout>

        <FrameLayout
            android:layout_width="wrap_content"
            android:layout_height="16dp"
            android:layout_gravity="center_horizontal"
            android:layout_marginTop="-12dp"
            android:background="@drawable/price_background">

            <TextView
                android:id="@+id/priceView"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"
                android:layout_marginLeft="4dp"
                android:layout_marginRight="4dp"
                android:text="0.1 BTC"
                android:textAllCaps="true"
                android:textColor="@color/white"
                android:textSize="10sp"
                android:textStyle="bold" />

        </FrameLayout>

        <TextView
            android:id="@+id/nameView"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="2dp"
            android:gravity="center_horizontal"
            android:lineSpacingMultiplier="0.8"
            android:lines="2"
            android:text="Bacon Cheeseburger"
            android:textSize="10sp" />
    </LinearLayout>
</FrameLayout>

我正在尝试在代码中引用三个视图:

I'm trying to reference three views in code:

public ItemViewHolder(View itemView) {
    super(itemView);
    nameView = itemView.findViewById(R.id.nameView);
    priceView = itemView.findViewById(R.id.priceView);
    imageView = itemView.findViewById(R.id.imageView);
}

正确引用了

nameView 和 priceView ，但是未引用 imageView ，并且为 null :

nameView and priceView are referenced correctly, however, imageView isn't being referenced and is null:

为什么我不能引用 imageView ?

(如果我遍历视图树，它在这里 .)

(If I traverse the view tree, it is there.)

更新:我同时执行了Clean Project和Invalidate Caches/Restart，问题仍然存在.

UPDATE: I did both Clean Project and Invalidate Caches/Restart, the problem persists.

更新2: ItemViewHolder 是从 RecyclerView.ViewHolder 派生的. CircleImageView 源自 FrameLayout (不是 ImageView ).该XML是我的视图持有者的布局.

UPDATE 2: ItemViewHolder is derived from RecyclerView.ViewHolder. CircleImageView is derived from FrameLayout (not ImageView). This XML is the layout of my view holder.

更新3:这是我的圈子视图的构造函数:

UPDATE 3: Here is my circle view's constructor:

public class CircleImageView extends FrameLayout {

    public CircleImageView(Context context) {
        super(context);
        init();
    }

    public CircleImageView(Context context, AttributeSet attrs) {
        super(context);
        init();
    }

    public CircleImageView(@NonNull Context context, @Nullable AttributeSet attrs, @AttrRes int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    ...
}

此外，正如Subzero指出的那样，我已经检查了视图的ID属性( mID 字段)，它是 -1 ，这似乎是导致问题.我不知道为什么它是-1.

Also, as Subzero noted, I've checked the ID property (mID field) of the view, and it's -1, which seems to be the cause of the problem. I have no idea why it is -1 though.

推荐答案

将两参数构造函数中的 super 调用更改为:

Change the super call in your two-parameter constructor to:

super(context, attrs);

当从布局中扩展 View 时，XML属性及其值将通过 AttributeSet 传递到两参数构造函数中.如果不将其传递给超类，则在XML中指定的 id 永远不会在 View 上设置，因此 findViewById()找不到具有给定ID的文件.

When a View is inflated from a layout, the XML attributes and their values are passed into the two-parameter constructor via the AttributeSet. If you don't pass that to the superclass, the id you've specified in the XML is never set on the View, so findViewById() won't find it with the given ID.

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