J2ME 游戏线程

Question

I'm trying to make a game for J2ME something like the game Zuma, the structure of classes that you need to know is as follow :

-Level class which holds info about a level such as a vector of points (class I made called Point, the path that the balls will follow), vector of balls (class I made called Ball) to hold different balls object, speed (the speed of the balls in that specific level).. (There're more fields but nothing that you need to worry about)

-Frog class (just so you'd know it exists) if you're not familiar wit the game zuma it serves as a controllable turret that can shoot balls.). (No need to worry about the fields in that class)

-MyGameCanvas which obviously extends GameCanvas and implement runnable, holds instance of frog and a level (the currently played level).. (There're more fields but nothing that you need to worry about). The run method serve as a listener for user input if user pressed right rotate the frog (essentially a sprite) by X amount if pressed left rotate by -X amount, pressed OK shoot a ball. Beside taking care of input the thread calls a render method which renders what going on, on the screen and updates balls position from the balls vector (by using the current level instance) using the vector of points (from the level instance aswell), now the problem is that I wanted each level to have different speed, by speed I mean that balls will "roll" (=move) on screen slower, so I can do it with just using the speed value in the Thread.sleep method and increase the sleep time because the run method is taking input from user making the frog movements and input reaction to be slowed down aswell, I thought maybe doing each ball a seperate thread but thats not really good in my opinion because there'll be alot of threads + when I update each ball location on the screen I actually use the WHOLE ball vector from the level instance for things like if a ball need to gove backwards if there's a gap between the ball and the one behind him, or when to render the ball depending if the ball after him is already rendered and on the "track" (the points vector), so I don't really know how I should do it, any advices guidance would be highly appriciated ! Thanks in advance and hopefully you could understand what I wrote.. Also I don't think I need to give code examples really because I don't wanna use what I want to CHANGE what I wrote so it wouldn't really help providing the run/render methods, all you need to know is the structure of the classes I gave you and the fields they have that I told you about.

Answer 1

Various developers use various methods for controlling the speed of sprite-movement. JavaME is one of the many platforms that requires you to also consider different resolutions and CPU speed. So it's never as simple as just incrementing the x value 1 pixel in each cycle of the main game loop.

Assuming 2 devices with different screen resolution, but otherwise running the same speed. Incremending x value by 1 in the cycle of the game loop, would obviously result in the sprite reaching the end of the screen faster on the small resolution. Assuming 2 device with the same screen resolkution, but with different CPU's. Incrementing x value by 1 in the cycle of the game loop, would obviously result in the sprite reaching the end of the screen faster on the device with the fastest CPU.

Here is one way of working around that:

Create a variable, call it itemWait (where "item" is the name of the object you want to control the speed for). The itemWait variable holds a value that tells the game loop how long should pass before next movement. For example, say that the sprite should wait 40 milliseconds for each pixel it moves. The code would look somewhat like this:

// Variables defined outside the game loop
long lastTime, thisTime;
int cyclems;
int itemTime; // Replace "item" with the name of the object, like e.g. ballTime.
int itemWait = 40; // Wait 40 ms for each movement
int itemPixelsToMove = 1;

while (running == true) { // Game loop
 lastTime = thisTime; // Set lastTime to the time-stamp of the previous cycle
 thisTime = System.currentTimeMillis(); // Get the current time-stamp
 cyclems = (int) (thisTime - lastTime); // How long did it take to execute previous loop

 itemTime += cyclems; // How long has passed since last move
 while (itemTime > itemWait) { // If it's time to move
  itemTime -= itemWait;
  moveItem(itemPixelsToMove);
 }

 // Do other game logic

 // Draw everything

 // Handle input, unless you handle it with keyPressed()
}

Using that method, the code doesn't care how fast the device is. And if you want to port the game to a different screen resolution, you can simply change either itemPixelsToMove or itemWait . For example, say you set itemWait = 40; for 240x320 resolutions. Then you would set it to 40/240*480 = 80 for 480x640 resolutions. (This requires of course, that you also scale the graphics).

We've used this approach in www.PirateDiamonds.com - in case you're curious. The same code runs on whatever screen resolution you want.