无法在安卓4上的自定义视图中从TypedArray获取可绘制内容

public class YourCustomView extends View {
    private Drawable drawable;

    public YourCustomView(Context context, AttributeSet attrs) {
        super(context, attrs);
        TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.YourCustomView);
        drawable = typedArray.getDrawable(R.styleable.YourCustomView_customDrawable);
        typedArray.recycle();
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if (drawable != null) {
            drawable.setBounds(0, 0, getWidth(), getHeight());
            drawable.draw(canvas);
        }
    }
}