Android：我的自定义图像视图继承有什么问题？

Question

我刚刚开始使用Android，并认为创建一个新的图像类很容易，我将在后面移动等等，只是基本上玩Android。好吧，它似乎并不像预期的那么容易。我得到一份NPE：

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageResource(int)' on a null object reference

xml layout

 

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#0099cc"
    tools:context=".Start">

    <!-- The primary full-screen view. This can be replaced with whatever view
         is needed to present your content, e.g. VideoView, SurfaceView,
         TextureView, etc. -->

    <!-- This FrameLayout insets its children based on system windows using
         android:fitsSystemWindows. -->

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/imageView"
        android:layout_gravity="center" />
</FrameLayout>

主要活动

package com.andreiv.snake;

import com.andreiv.snake.util.Picture;
import com.andreiv.snake.util.SystemUiHider;

import android.annotation.TargetApi;
import android.app.Activity;
import android.os.Build;
import android.os.Bundle;
import android.os.Handler;
import android.view.MotionEvent;
import android.view.View;
import android.widget.ImageView;

public class Start extends Activity {

    ImageView img;
    Picture pic;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        img = (ImageView)findViewById(R.id.imageView);
        pic = new Picture(img.getContext());
        pic.drawPic();

        setContentView(R.layout.activity_start);

    }
}

图片类

package com.andreiv.snake.util;

import android.content.Context;
import android.util.AttributeSet;
import android.view.View;
import android.widget.ImageView;

import com.andreiv.snake.R;

/**
 * Created by andrei on 7/2/2014.
 */
public class Picture extends ImageView{

    public Picture(Context context) {
        super(context);
    }
    public Picture(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    public Picture(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    public void drawPic() {
        setImageResource(R.drawable.pic);
    }
}

Answer 1

首先 - 您需要在尝试使用FindViewById之前设置内容视图。

第二 - 你不需要从图像中获取上下文，你可以给活动本身（活动扩展上下文），在你的情况下即使这是没有必要的，因为视图将在框架时自动获取上下文会膨胀你的xml。

第三 - 如果您希望扩展视图在xml中膨胀，则必须将其类名放在xml（而不是ImageView类）中

public class Start extends Activity {

    Picture img;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_start);

    img = (Picture)findViewById(R.id.imageView);
    img.drawPic();

   }
}

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#0099cc"
tools:context=".Start">

<!-- The primary full-screen view. This can be replaced with whatever view
     is needed to present your content, e.g. VideoView, SurfaceView,
     TextureView, etc. -->

<!-- This FrameLayout insets its children based on system windows using
     android:fitsSystemWindows. -->

<com.andreiv.snake.util.Picture
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView"
    android:layout_gravity="center" />