Question

我正在尝试将图像绘制到画布上，并且当程序成功编译并按照我的预期运行时，错误日志显示存在与下面的drawBitmap（）方法关联的NullPointerException。奇怪的是，我的图像仍然被绘制在画布上。究竟是什么问题，我应该如何解决它？

我的代码：

public class ProgressBar extends View
{
    String packageName;

    public ProgressBar(Context context)
    {
        super(context);
        packageName = context.getPackageName();
    }

    public ProgressBar(Context context, AttributeSet attribs)
    {
        super(context, attribs);
        packageName = context.getPackageName();
    }

    @Override
    protected void onDraw(Canvas canvas)
    {
        super.onDraw(canvas);

        int resourceId = getResources().getIdentifier("bar1", "drawable", packageName);
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), resourceId);
        canvas.drawBitmap(bitmap, 35, 35, null);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
    {
        // super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        mWidth = View.MeasureSpec.getSize(widthMeasureSpec);
        mHeight = View.MeasureSpec.getSize(heightMeasureSpec);
        setMeasuredDimension(mWidth, mHeight);
    }
}

错误日志：

java.lang.NullPointerException
at android.graphics.Canvas.throwIfRecycled(Canvas.java:1057)
at android.graphics.Canvas.drawBitmap(Canvas.java:1097)
at com.myapp.ProgressBar.onDraw(ProgressBar.java:50)
at android.view.View.draw(View.java:13944)
at android.view.View.draw(View.java:13825)
at android.view.ViewGroup.drawChild(ViewGroup.java:3083)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2920)
at android.view.View.draw(View.java:13823)
at android.view.ViewGroup.drawChild(ViewGroup.java:3083)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2920)
at android.view.View.draw(View.java:13823)
at android.view.ViewGroup.drawChild(ViewGroup.java:3083)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2920)
at android.view.View.draw(View.java:13823)
at android.view.ViewGroup.drawChild(ViewGroup.java:3083)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2920)
at android.view.View.draw(View.java:13947)
at android.view.View.draw(View.java:13825)
at android.view.ViewGroup.drawChild(ViewGroup.java:3083)
at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2920)
at android.view.View.draw(View.java:13947)

Answer 1

你永远不应该从onDraw中的资源中提取位图，因为onDraw每秒被调用很多次，所以每次调用onDraw都没有时间来解码该资源。

您应该在构造函数中解码位图，将其保存在类变量中并在onDraw中使用。

所以，基本上，你需要做的就是：

//add bitmap to class variable
private Bitmap bitmap;

//move these to constructor
int resourceId = getResources().getIdentifier("bar1", "drawable", packageName);
bitmap = BitmapFactory.decodeResource(getResources(), resourceId);

一切都应该有效。

在画布上绘制位图的麻烦

1 个答案: