Android自定义View(一) 画线段

最新推荐文章于 2023-11-23 08:50:43 发布

原创最新推荐文章于 2023-11-23 08:50:43 发布 · 2.7k 阅读

3 ·

CC 4.0 BY-SA版权

文章标签：

#自定义view

自定义view 专栏收录该内容

13 篇文章

订阅专栏

本文介绍了一个自定义Android View时遇到的空指针异常问题及其解决方案。通过调整构造函数逻辑，确保资源获取正确执行，避免了空指针异常，并成功绘制出线段。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

public class MusiceView extends View {
    Paint mPaint;
    public MusiceView(Context context) {
       super(context);
       mPaint = new Paint();

       mPaint.setColor(getResources().getColor(android.R.color.holo_blue_dark));

       mPaint.setStrokeWidth(20);

    }
    public MusiceView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }
    public MusiceView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }
    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawLine(0,0,0,100,mPaint);
    }
}

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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.example.superdy.test.MainActivity">

    <com.example.superdy.test.MusiceView
        android:layout_centerInParent="true"
        android:layout_width="100dp"
        android:layout_height="100dp"
     />

</RelativeLayout>

报错

     
        java.lang.NullPointerException: Attempt to invoke virtual method 'long

android.graphics.Paint.getNativeInstance()' on a null object reference

为什么

我们翻开源码看看注释

/**
 * Simple constructor to use when creating a view from code.
 *
 * @param context The Context the view is running in, through which it can
 *        access the current theme, resources, etc.
 */
public View(Context context) {
。。。

/**
 * Constructor that is called when inflating a view from XML. This is called
 * when a view is being constructed from an XML file, supplying attributes
 * that were specified in the XML file. This version uses a default style of
 * 0, so the only attribute values applied are those in the Context's Theme
 * and the given AttributeSet.
 *
 * <p>
 * The method onFinishInflate() will be called after all children have been
 * added.
 *
 * @param context The Context the view is running in, through which it can
 *        access the current theme, resources, etc.
 * @param attrs The attributes of the XML tag that is inflating the view.
 * @see #View(Context, AttributeSet, int)
 */
public View(Context context, @Nullable AttributeSet attrs) {
    this(context, attrs, 0);
}

===========================

原因明了了

第一个构造方法时在代码中创建view的时候可以使用的

而第二个构造方法则是在xml中创建view的时候使用的。

我们修改下代码

public class MusiceView extends View {
    Paint mPaint;
    public MusiceView(Context context) {
        super(context);
    }
    public MusiceView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        mPaint = new Paint();
        mPaint.setColor(getResources().getColor(android.R.color.holo_blue_dark));
        mPaint.setStrokeWidth(20);
    }
    public MusiceView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }
    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawLine(0,0,0,100,mPaint);
    }
}

这次成功了，画出了一条线段。