我正在通过阅读本书来学习android编程，由于某种原因，当代码到达“ if（ourHolder.getSurface（）。isValid（）”时，它将转到else语句。代码尚未完成，但根据我的书，当我运行代码时，看到屏幕上的蛇头。根据本书的前一游戏，“ if（ourHolder.getSurface（）。isValid（）”工作正常，并且绘制了Canvas。我也手动调用Draw （）;“ public SnakeAnimView（Context context）”中的方法。为什么“ if（ourHolder.getSurface（）。isValid（）”转到else语句？
谢谢您的帮助**
码：
package com.packetpub.snake;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
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.Rect;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.Display;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
//public class MainActivity extends AppCompatActivity {
public class MainActivity extends Activity
{
Canvas canvas;
SnakeAnimView snakeAnimView;
//The snake head sprite sheet
Bitmap headAnimBitmap;
//The portion of the bitmap to be drawn in the current frame
Rect rectToBeDrawn;
//The dimensions of a single frame
int frameHeight = 64;
int frameWidth = 64;
int numFrames = 6;
int frameNumber;
int screenWidth;
int screenHeight;
//stats
long lastFrameTime;
int fps;
int hi;
//To start the game from onTouchEvent
Intent i;
@Override
protected void onCreate(Bundle savedInstanceState)
{
Log.i("Step 1 onCreate: ", " start");
super.onCreate(savedInstanceState);
//find out the width and height of the screen
Display display = getWindowManager().getDefaultDisplay();
Point size = new Point();
display.getSize(size);
screenWidth = size.x;
screenHeight = size.y;
headAnimBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.head_sprite_sheet);
snakeAnimView = new SnakeAnimView(this);
setContentView(snakeAnimView);
//i = new Intent(this, GameActivity.class);
Log.i("Step 1 onCreate: ", " display : " + display);
Log.i("Step 1 onCreate: ", " screenWidth : " + screenWidth);
Log.i("Step 1 onCreate: ", " screenHeight : " + screenHeight);
Log.i("Step 1 onCreate: ", " headAnimBitmap : " + headAnimBitmap);
Log.i("Step 1 onCreate: ", " snakeAnimView : " + snakeAnimView);
}
class SnakeAnimView extends SurfaceView implements Runnable
{
Thread ourThread = null;
SurfaceHolder ourHolder;
volatile boolean playingSnake = true ;
Paint paint;
////Here is the constructor that gets the frameWidth value by dividing the
// bitmap width by the number of frames, and the frameHeight value using
// the getHeight method. Type this code after the code from the previous step:
public SnakeAnimView(Context context)
{
super(context);
ourHolder = getHolder();
paint = new Paint();
frameWidth = headAnimBitmap.getWidth() / numFrames;
frameHeight = headAnimBitmap.getHeight();
Log.i("Step 2 SnakeAnimView: ", "(Context context)");
Log.i("Step 2 SnakeAnimView: ", "headAnimBitmap:" + headAnimBitmap);
Log.i("Step 2 SnakeAnimView: ", "numFrames:" + numFrames);
Log.i("Step 2 SnakeAnimView: ", "frameWidth:" + frameWidth);
Log.i("Step 2 SnakeAnimView: ", "frameHeight:" + frameHeight);
Log.i("Step 2 SnakeAnimView: ", "playingSnake:" + playingSnake);
draw();
}
@Override
public void run()
{
Log.i("Step 3 Run: ", "while (playingSnake): " + playingSnake );
while (playingSnake)
{
Log.i("Step 3 Run: ", "start");
//update();
draw();
//controlFPS();
Log.i("Step 3 Run: ", "end");
}
}
public void update()
{
Log.i("Step 4 update: ", "public void update()");
//which frame should we draw
rectToBeDrawn = new Rect((frameNumber * frameWidth) - 1,
0, (frameNumber * frameWidth + frameWidth) - 1,
frameHeight);
//now the next frame
frameNumber++;
//don't try and draw frames that don't exist
if (frameNumber == numFrames)
{
frameNumber = 0;//back to the first frame
}
}
public void draw()
{
Log.i("Step 5 draw: ", "public void draw()");
Log.i("Step 5 draw: ", "ourHolder : " + ourHolder);
Log.i("Step 5 draw: ", "ourHolder.getSurface() = " + ourHolder.getSurface());
boolean test = ourHolder.getSurface().isValid();
Log.i("Step 5 draw: ", "ourHolder.getSurface().isValid() : " + test);
if (ourHolder.getSurface().isValid())
{
canvas = ourHolder.lockCanvas();
//Paint paint = new Paint();
canvas.drawColor(Color.BLACK);//the background
paint.setColor(Color.argb(255, 255, 255, 255));
paint.setTextSize(150);
canvas.drawText("Snake", 10, 150, paint);
paint.setTextSize(25);
canvas.drawText(" Hi Score:" + hi, 10, screenHeight - 50, paint);
//Draw the snake head
//make this Rect whatever size and location you like
//(startX, startY, endX, endY)
Rect destRect = new Rect(screenWidth/2-100,
screenHeight/2-100, screenWidth/2+100,
screenHeight/2+100);
canvas.drawBitmap(headAnimBitmap,
rectToBeDrawn, destRect, paint);
ourHolder.unlockCanvasAndPost(canvas);
Log.i("Step 5 draw: ", "ourHolder : " + ourHolder);
Log.i("Step 5 draw: ", "canvas : " + canvas);
Log.i("Step 5 draw: ", "paint : " + paint);
Log.i("Step 5 draw: ", "screenWidth : " + screenWidth);
Log.i("Step 5 draw: ", "screenHeight : " + screenHeight);
Log.i("Step 5 draw: ", "headAnimBitmap : " + headAnimBitmap);
}
else
{
Log.i("Step 5 draw: ", "Went to Else");
}
}
public void controlFPS()
{
Log.i("Step 6 controlFPS: ", "public void controlFPS()");
long timeThisFrame =
(System.currentTimeMillis() -
lastFrameTime);
long timeToSleep = 500 - timeThisFrame;
if (timeThisFrame > 0) {
fps = (int) (1000 / timeThisFrame);
}
if (timeToSleep > 0)
{
try
{
ourThread.sleep(timeToSleep);
}
catch (InterruptedException e)
{
}
}
lastFrameTime = System.currentTimeMillis();
}
public void pause()
{
Log.i("Step 7 pause: ", "public void pause())");
playingSnake = false;
try
{
ourThread.join();
}
catch (InterruptedException e)
{
}
}
@Override
public boolean onTouchEvent(MotionEvent motionEvent)
{
Log.i("Step 8 onTouchEvent: ", "public boolean onTouchEvent)");
startActivity(i);
return true;
}
}
@Override
protected void onStop()
{
Log.i("Step 9 onStop: ", "onStop");
super.onStop();
while (true) {
snakeAnimView.pause();
break;
}
finish();
}
// @Override
// protected void onResume() {
// super.onResume();
// snakeAnimView.resume();
// }
@Override
protected void onPause()
{
Log.i("Step 10 onPause: ", "onPause");
super.onPause();
snakeAnimView.pause();
}
public boolean onKeyDown(int keyCode, KeyEvent event)
{
Log.i("Step 11 onKeyDown: ", "onKeyDown");
if (keyCode == KeyEvent.KEYCODE_BACK) {
snakeAnimView.pause();
finish();
return true;
}
return false;
}`enter code here`
}
最佳答案
在创建表面之前，该表面是无效的，请查看该表面创建/销毁时的各个回调。请注意，销毁它时无效。