attempt to call method 'applyForce' (a nil value)

Question

function spawnRBird( x,y )  
    print("\n entered into spawnred");
    rBird = display.newSprite (sheet, seqData);
    rBird.x = x;
    rBird.y = y;
    rBird:setSequence("rBird")
    rBird.anchorX = 0;
    rBird.anchorY = 1;
    physics.addBody(rBird, "dynamic", {density=1, bounce=0.5, friction=0.5})
    rBird:play();
    rBird:addEventListener( "collision", birdCollision );

I am getting:

attempt to call method 'applyForce' (a nil value)

when I click on the screen after spawning red bird rBird object inside rBirdJump is becoming nil please explain in corona lua

function rBirdJump( event )
    if (event.phase == "began") then
            rBird:applyForce(0, -50, redBird.x, redBird.y)
            rBird:setLinearVelocity( 0, -10 )
            rBird.isFixedRotation = true;
    end
end
Runtime:addEventListener("touch", rBirdJump);
--timer.performWithDelay( 1000, function Runtime:addEventListener("touch", rBirdJump) end, 1 )
end

Answer 1

Do you know for a fact that spawnRBird is being called before you touch the screen? Also the message indicates that rBird is not nil but that applyForce is not a method on rBird . This implies that rBird has been removed from physics after being spawned, or removed as a display object (which does not make it nil, just removes certain methods from it).

If this doesn't help you find the problem you will have to post the file as you have it instead of just portions.

Answer 2

The error occurs because rBird is not a physics object when the function is called.
In your spawnBird function, you are initializing rBird and adding it to physics. You are declaring rBird to be a global variable. This is generally a bad programming practice and should not be adapted because it might lead to major memory leaks.

You can initialize rBird in the function as a local variable and use a local variable to the scene / current file to avoid using globals. Set the variable same as rBird after you have added it in physics so that your jump function wont return an error. This would also help you in making sure that your variable to display object is being used after it is added in the physics engine.

 -- forward reference to your created redBird. Initialize as nil to make sure the variable is assigned a value after its added in physics. local redBird = nil function spawnRBird( x,y ) print("\\n entered into spawnred"); local rBird = display.newSprite (sheet, seqData); rBird.x = x; rBird.y = y; rBird:setSequence("rBird") rBird.anchorX = 0; rBird.anchorY = 1; physics.addBody(rBird, "dynamic", {density=1, bounce=0.5, friction=0.5}) redBird = rBird rBird:play(); rBird:addEventListener( "collision", birdCollision ); 

By using the variable redBird in the later code, you can make sure that you are using a phusics object as you are assigning it a value after it is added in physics.

 function rBirdJump( event ) if (event.phase == "began" and redBird ~= nil) then redBird:applyForce(0, -50, redBird.x, redBird.y) redBird:setLinearVelocity( 0, -10 ) redBird.isFixedRotation = true; end end Runtime:addEventListener("touch", rBirdJump); --timer.performWithDelay( 1000, function Runtime:addEventListener("touch", rBirdJump) end, 1 ) 

Answer 3

确保您已开始物理：

physics.start();