Question

我正在尝试在游戏中使用touchdown（）。动机很简单，我只想基于屏幕上的触摸检测来移动播放器，但它给出了以下错误：

    Exception in thread "LWJGL Application" java.lang.NullPointerException
at com.mygdx.game.sprites.Ron.touchDown(Ron.java:39)
at com.badlogic.gdx.backends.lwjgl.LwjglInput.processEvents(LwjglInput.java:329)
at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:215)
at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:124)

，代码是：

package com.mygdx.game.sprites;

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.Input;
import com.badlogic.gdx.InputAdapter;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.math.Vector3;
import com.badlogic.gdx.utils.viewport.ExtendViewport;
import com.badlogic.gdx.utils.viewport.Viewport;
import com.mygdx.game.Constants;

/**
 * Created by Vamsi Rao on 11/9/2016.
 */

public class Ron extends InputAdapter {
    private Vector2 position;
    private Vector2 velocity;
    Vector2 worldClick;
    boolean right;
    boolean left;

    private ExtendViewport viewport;

    private Texture ron;

    public Ron(int x, int y, ExtendViewport viewport) {
        super();
        position = new Vector2(x, y);
        velocity = new Vector2(Constants.VELOCITY_X, 0);
        this.viewport = viewport;

        ron = new Texture("ron.png");
    }

    public boolean touchDown(int screenX, int screenY, int pointer, int button) {
        worldClick = viewport.unproject(new Vector2(screenX, screenY));
        if (worldClick.x >= viewport.getWorldWidth() / 4) {
            right = true;
        }
        if (worldClick.x < viewport.getWorldWidth() / 4) {
            left = true;
        }
        return true;
    }




    public Vector2 getPosition() {
        return position;
    }

    public Texture getTexture() {
        return ron;
    }

    public void update(float delta) {
        left=false;
        right=false;


        if (right) {
            position.x += delta * velocity.x;
        }

        if (left) {
            position.x -= delta * velocity.x;
        }


    }

}

我已经在另一个类中设置了Input处理器，该类正在使用Ron类的对象ron（上面显示的类），如下所示：

Gdx.input.setInputProcessor(ron);

Answer 1

39行是

worldClick = viewport.unproject(new Vector2(screenX, screenY));

所以NullPointerException更像是viewport变量 - 你将它传递给构造函数

public Ron(int x, int y, ExtendViewport viewport) {
    ...
    this.viewport = viewport;
    ...

因此视口可能会提供null值。

查看What is a NullPointerException, and how do I fix it?

第二件事是，在这种情况下，使用标志可能不是最好的主意 - 你的代码肯定不太可读，而且你有一个冗余的不必要的代码。

//these lines are almost the same
position.x += delta * velocity.x;
position.x -= delta * velocity.x;

更好的是设置速度的方向向量然后将它与update中的delta相乘，就像你做的那样

//in your touchdown
if (worldClick.x >= viewport.getWorldWidth() / 4) {
    velocity.x *= (velocity.x >= 0) ? 1 : -1;
}
//also add else here - there is always one or another here
else if (worldClick.x < viewport.getWorldWidth() / 4) {
    velocity.x *= (velocity.x < 0) ? 1 : -1;
}

//in update
position.x += delta * velocity.x;

你也可以通过这种方式轻松实现变速逻辑（比如当角色改变方向时他慢下来然后开始向后移动...... 等）

在libGDX中使用触地得分

1 个答案: