Android / Java：将Canvas作为类构造函数参数传递失败

Question

在GraphicsView类的onDraw方法中，我创建了Plotter类的实例，但构造函数未设置类内Canvas字段

我们从这里开始

protected void onDraw(Canvas canvas) {
    Plotter plotter = new Plotter(1, canvas);
}

..并转到Plotter创建它的实例：

Plotter (int dotsPerPx, Canvas canvas_) {
    this.dotsPerPx = dotsPerPx;
    this.canvas = canvas_;
}

int dotsPerPx;  //This one gets set to 1, as supposed
Canvas canvas;  //And this remains null, though canvas_==openGLCanvas{...}
int screenWidth = canvas.getWidth();  //NullPointerException!

怎么了？

Answer 1

画布尚未实例化。 用这个：

Plotter (int dotsPerPx, Canvas canvas_) {
    this.dotsPerPx = dotsPerPx;
    this.canvas = canvas_;
    screenWidth = canvas.getWidth();
}

int dotsPerPx;  //This one sets to 1, as supposed
Canvas canvas;  //And this remains null, though canvas_==openGLCanvas...
int screenWidth;

基本上，构造函数和方法之外的所有代码（字段创建，初始化块）都在调用构造函数之前执行。 所以int screenWidth = canvas.getWidth(); 尝试在null字段上调用方法（默认情况下，除非您指定其他内容），因为尚未调用构造函数。