Android自定义View(一) 画线段

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。