onDraw（）方法只有當我在onDraw中創建我的對象（）

Question

我有問題。我不知道爲什麼我不能在我的iniGame()方法中創建對象。當我從GameObject類創建一個對象並使用onDraw()方法中的方法render()不起作用時。只有當我在onDraw()方法中創建所有相對對象時才起作用。

我的佈局XML：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" 
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin" 
    android:paddingBottom="@dimen/activity_vertical_margin" 
    tools:context="de.fhflensburg.manichja.barrier51.GameActivity"> 

    <de.fhflensburg.manichja.barrier51.GameView 
     android:id="@+id/gameView" 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     /> 

    <Button 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@id/gameView" 
     /> 
</RelativeLayout> 

我gameactivity：

package de.fhflensburg.manichja.barrier51; 

import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.Menu; 
import android.view.MenuItem; 

public class GameActivity extends AppCompatActivity { 


    private GameView gameView; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_game); 
     gameView = new GameView(this); 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.menu_game, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 

     //noinspection SimplifiableIfStatement 
     if (id == R.id.action_settings) { 
      return true; 
     } 

     return super.onOptionsItemSelected(item); 
    } 
} 

我GameView

package de.fhflensburg.manichja.barrier51; 

    import android.content.Context; 
    import android.graphics.Canvas; 
    import android.graphics.Color; 
    import android.graphics.Paint; 
    import android.os.CountDownTimer; 
    import android.util.AttributeSet; 
    import android.util.Log; 
    import android.view.View; 


    public class GameView extends View { 

     private Canvas canvas; 
     private Context context; 
     private Paint peter; 
     private GameObject ball; 

     public GameView(Context context) { 
      super(context); 
      this.context = context; 
      initGame(); 
     } 

     public void initGame(){ 


      //ball.setSprite(R.drawable.menu_background); 
     } 

     public GameView(Context context, AttributeSet attrs) { 
      super(context, attrs); 
     } 

     public GameView(Context context, AttributeSet attrs, int defStyleAttr) { 
      super(context, attrs, defStyleAttr); 
     } 

     @Override 
     public void onDraw(Canvas canvas){ 
      ball = new GameObject(this.context); 
      this.canvas = canvas; 
      peter = new Paint(); 
      peter.setColor(Color.BLUE); 
      canvas.drawRect(100, 100, 200, 200, peter); 
      ball.render(canvas,peter); 
     } 
} 

我的遊戲對象代碼

package de.fhflensburg.manichja.barrier51; 

    import android.content.Context; 
    import android.content.res.ColorStateList; 
    import android.graphics.Bitmap; 
    import android.graphics.BitmapFactory; 
    import android.graphics.Canvas; 
    import android.graphics.Color; 
    import android.graphics.Paint; 
    import android.graphics.Point; 
    import android.graphics.drawable.Drawable; 
    import android.util.Log; 
    import android.view.View; 
    import android.widget.ImageView; 
    import android.util.Size; 


public class GameObject { 
    private Size size;  // A color as stated as a resource in the XML file (thus, it is an int) 
    private Point position; // A color as stated as a resource in the XML file (thus, it is an int) 
    private int fillColor;  // A color as stated as a resource in the XML file (thus, it is an int) 
    private int strokeColor;// A color as stated as a resource in the XML file (thus, it is an int) 
    private int sprite;// A color as stated as a resource in the XML file (thus, it is an int) 


private Bitmap ourSprite; 
    private Context conny; 

    public GameObject(Context context) 
    { 
     this.conny = context; 
     /*Paint myPaint = new Paint(); 
     myPaint.setColor(Color.rgb(0, 0, 0)); 
     myPaint.setStrokeWidth(10); 
     c.drawRect(100, 100, 200, 200, myPaint);*/ 
    } 

    /** 
    * Sets the position of the GameObject on the screen. 
    * @param position A Point instance with x and y 
    */ 
    public void setPosition(Point position) 
    { 
     this.position = position; 
    } 

    /** 
    * Sets the fill color of the game object. If it has a Sprite image, the assigned color will be ignored. 
    * @param color A color resource, defined in the XML file (e.g. R.color.limegreen) 
    */ 
    public void setFillColor(int color) 
    { 
     this.fillColor = color; 
    } 

    /** 
    * Sets the stroke color of the game object. If it has a Sprite image, the assigned color will be ignored. 
    * @param color A color resource, defined in the XML file (e.g. R.color.limegreen) 
    */ 
    public void setStrokeColor(int color) 
    { 
     this.strokeColor = color; 
    } 

    /** 
    * @return Delivers the current size of the GameObject 
    */ 
    public Size getSize() 
    { 
     return size; 
    } 

    /** 
    * @return Delivers the current position of the GameObject 
    */ 
    public Point getPosition() 
    { 
     return position; 
    } 

    /** 
    * @return Delivers the current position of the GameObject as stated in the XML file. 
    */ 
    public int getFillColor() 
    { 
     return fillColor; 
    } 

    /** 
    * @return Delivers the current stroke color of the GameObject as stated in the XML file. 
    */ 
    public int getStrokeColor() 
    { 
     return strokeColor; 
    } 

    /** 
    * Checks for a collision of this object with another instance of a GameObject. 
    * @param obj The object to be checked for collision 
    * @return Returns true if the objects' coordinates overlap at any pixel. 
    */ 
    public boolean overlaps(GameObject obj) 
    { 
     if (getPosition().x < obj.getPosition().x + obj.getSize().getWidth() && 
       getPosition().x + getSize().getWidth() > obj.getPosition().x && 
       getPosition().y < obj.getPosition().y + obj.getSize().getHeight() && 
       getSize().getHeight() + getPosition().y > obj.getPosition().y) 
     { 
      return true; 
     } 
     else 
     { 
      return false; 
     } 
    } 

    /** 
    * Sets the sprite image of the GameObject. 
    * @param sprite A bitmap resource as defined in the XML file. 
    */ 
    public void setSprite(int sprite) { 

     Log.i("Peter", "Inigame get started"+sprite); 
     this.sprite = sprite; 
     ourSprite = BitmapFactory.decodeResource(this.conny.getResources(),R.drawable.menu_background); 
    } 

    /** 
    * @return Delivers the sprite resource id as an Integer. 
    */ 
    public Bitmap getSprite() { 

     return ourSprite; 
    } 

    /** 
    * Renders the object with the current attributes on the screen. 
    */ 
    public void render(Canvas c,Paint paint) 
    { 
     c.drawRect(0,0,120,120,paint); 
    } 

} 

Answer 1

您正在收到NullPointerException，因爲您的initGame可能未被調用。至少不在調用onDraw的對象上。
我的意思是你的GameView有兩個實例。第一個實例是您創建手動使用一個：

gameView = new GameView(this); 

第二個實例從XML的文件創建的，因爲你要定義：

<de.fhflensburg.manichja.barrier51.GameView 
    android:id="@+id/gameView" 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    /> 

所以這第二個實例是自動創建應用。而且你在開始時並不知道三個構造函數中的哪一個被調用。有兩件事情你應該改變：

1. 在您的活動，取代gameView = new GameView(this);與gameView = (GameView) findViewById(R.id.gameView);。這將爲您提供GameView的正確實例。請參閱View-Doc瞭解更多詳情。
2. 在您的GameView類中，在所有三個構造函數中添加對initGame的調用。這確保元素被初始化，而不管構造函數是什麼。