Could someone please help me with making an object move when you click and drag it along.
behappy2 wrote ...
2012/12/30
Dragging with the mouse
First make a variable that determines if it is dragging or not.
This'll work.
boolean drag = false;
if(Greenfoot.mousePressed(this)) drag = true;
if(Greenfoot.mouseClicked(null)) drag = false;
if(drag) {
// what ever you want to put in here
}
The following code will allow an object to be dragged by the mouse without having it go under other objects during the dragging procedure.
/* add an instance boolean field to the class of the Actor object to be dragged */
private boolean isGrabbed;
/* in the 'act' method in the class of the Actor object to be dragged */
// check for initial pressing down of mouse button
if (Greenfoot.mousePressed(this) && !isGrabbed)
{
// grab the object
isGrabbed = true;
// the rest of this block will avoid this object being dragged UNDER other objects
World world = getWorld();
MouseInfo mi = Greenfoot.getMouseInfo();
world.removeObject(this);
world.addObject(this, mi.getX(), mi.getY());
return;
}
// check for actual dragging of the object
if ((Greenfoot.mouseDragged(this)) && isGrabbed)
{
// follow the mouse
MouseInfo mi = Greenfoot.getMouseInfo();
setLocation(mi.getX(), mi.getY());
return;
}
// check for mouse button release
if (Greenfoot.mouseDragEnded(this) && isGrabbed)
{
// release the object
isGrabbed = false;
return;
}aha, so the world recreates the object, but where the mouse is!
BUT
it says --> if (Greenfoot.mousePressed(this) && !isGrabbed)
is 'an illegal start of type'
You will have to show your class code (you probably put it in the wrong place and do not have your bracketing set proper).
import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
/**
* Write a description of class something here.
*
* @author (your name)
* @version (a version number or a date)
*/
public class something extends Actor
{
/* add an instance boolean field to the class of the Actor object to be dragged */
private boolean isGrabbed;
/* in the 'act' method in the class of the Actor object to be dragged */
// check for initial pressing down of mouse button
if (Greenfoot.mousePressed(this) && !isGrabbed)
{
// grab the object
isGrabbed = true;
// the rest of this block will avoid this object being dragged UNDER other objects
World world = getWorld();
MouseInfo mi = Greenfoot.getMouseInfo();
world.removeObject(this);
world.addObject(this, mi.getX(), mi.getY());
return;
}
// check for actual dragging of the object
if ((Greenfoot.mouseDragged(this)) && isGrabbed)
{
// follow the mouse
MouseInfo mi = Greenfoot.getMouseInfo();
setLocation(mi.getX(), mi.getY());
return;
}
// check for mouse button release
if (Greenfoot.mouseDragEnded(this) && isGrabbed)
{
// release the object
isGrabbed = false;
return;
}
}You need to enclose everything from line 14 on inside the 'act' method:
public void act()
{
// lines 14 through 41
}import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
/**
* Write a description of class something here.
*
* @author (your name)
* @version (a version number or a date)
*/
public class something extends Actor
{
/* add an instance boolean field to the class of the Actor object to be dragged */
private boolean isGrabbed;
/**
*
*/
public void act()
{
/* in the 'act' method in the class of the Actor object to be dragged */
// check for initial pressing down of mouse button
if (Greenfoot.mousePressed(something) && !isGrabbed)
{
// grab the object
isGrabbed = true;
// the rest of something block will avoid something object being dragged UNDER other objects
World world = getWorld();
MouseInfo mi = Greenfoot.getMouseInfo();
world.removeObject(something);
world.addObject(something, mi.getX(), mi.getY());
return;
}
// check for actual dragging of the object
if ((Greenfoot.mouseDragged(something)) && isGrabbed)
{
// follow the mouse
MouseInfo mi = Greenfoot.getMouseInfo();
setLocation(mi.getX(), mi.getY());
return;
}
// check for mouse button release
if (Greenfoot.mouseDragEnded(something) && isGrabbed)
{
// release the object
isGrabbed = false;
return;
}
}
}do you have a dragging scenario that i could try out and edit into my own scen.?
Ok i tryed to follow this as i kinda needed something similar. But i get a nullpointer exception on line 27. Any help and especialy explenation as to why would be realy appreciated.
import greenfoot.*;
public class Cannon extends Towers
{
private String cannonType;
private Boolean isGrabbed;
public Cannon()
{
this("0");
}
public Cannon(String newCannon)
{
setCannon(newCannon);
}
private void setCannon(String newCannon)
{
cannonType = newCannon;
}
public void act()
{
super.act();
loadImage();
if (Greenfoot.mousePressed(this) && !isGrabbed)
{
// grab the object
isGrabbed = true;
// the rest of this block will avoid this object being dragged UNDER other objects
World world = getWorld();
MouseInfo mi = Greenfoot.getMouseInfo();
world.removeObject(this);
world.addObject(this, mi.getX(), mi.getY());
return;
}
// check for actual dragging of the object
if (Greenfoot.mouseDragged(this) && isGrabbed)
{
// follow the mouse
MouseInfo mi = Greenfoot.getMouseInfo();
setLocation(mi.getX(), mi.getY());
return;
}
// check for mouse button release
if (Greenfoot.mouseDragEnded(this) && isGrabbed)
{
// release the object
isGrabbed = false;
return;
}
}
private void loadImage()
{
GreenfootImage myCannon = new GreenfootImage("towers/cannon" + cannonType + ".png");
setImage(myCannon);
}
}The field type on line 6 should be 'boolean' -- with a lowercase 'b'. Using an uppercase 'B' will have the field treated as an Object field reference of type Boolean instead of as a primitive type boolean field. As an object reference, its default value is 'null'; as a primitive field, it is assigned a value of 'false' by default.
Also, all your multi-line comments should begin with '/**', using two asterisks. That way, at least in the postings here, some of your code lines will not look like comment lines because of their text color.
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