我正在从Corona的示例代码中分析Fisheries项目，我无法理解这项任务。
background = ( backgroundLandscape == background and backgroundPortrait ) or backgroundLandscape
以下是完整代码：
-- Seed randomizer
local seed = os.time();
math.randomseed( seed )
display.setStatusBar( display.HiddenStatusBar )
-- Preload the sound file (theoretically, we should also dispose of it when we are completely done with it)
local soundID = audio.loadSound( "bubble_strong_wav.wav" )
-- Background
local halfW = display.viewableContentWidth / 2
local halfH = display.viewableContentHeight / 2
-- Create a table to store all the fish and register this table as the
-- "enterFrame" listener to animate all the fish.
local bounceAnimation = {
container = display.newRect( 0, 0, display.viewableContentWidth, display.viewableContentHeight ),
reflectX = true,
}
local backgroundPortrait = display.newImage( "aquariumbackgroundIPhone.jpg", 0, 0 )
local backgroundLandscape = display.newImage( "aquariumbackgroundIPhoneLandscape.jpg", -80, 80 )
backgroundLandscape.isVisible = false
local background = backgroundPortrait
-- Handle changes in orientation for the background images
local backgroundOrientation = function( event )
-- TODO: This requires some setup, i.e. the landscape needs to be centered
-- Need to add a centering operation. For now, the position is hard coded
local delta = event.delta
if ( delta ~= 0 ) then
local rotateParams = { rotation=-delta, time=500, delta=true }
if ( delta == 90 or delta == -90 ) then
local src = background
-- toggle background to refer to correct dst
background = ( backgroundLandscape == background and backgroundPortrait ) or backgroundLandscape
background.rotation = src.rotation
transition.dissolve( src, background )
transition.to( src, rotateParams )
else
assert( 180 == delta or -180 == delta )
end
transition.to( background, rotateParams )
audio.play( soundID ) -- play preloaded sound file
end
end
-- Add a global listener
Runtime:addEventListener( "orientation", backgroundOrientation )
--
-- Fishies
local numFish = 10
local file1 = "fish.small.red.png"
local file2 = "fish.small.blue.png"
--
-- Define touch listener for fish so that fish can behave like buttons.
-- The listener will receive an 'event' argument containing a "target" property
-- corresponding to the object that was the target of the interaction.
-- This eliminates closure overhead (i.e. the need to reference non-local variables )
local buttonListener = function( event )
if "ended" == event.phase then
local group = event.target
-- tap only triggers change from original to different color
local topObject = group[1]
if ( topObject.isVisible ) then
local bottomObject = group[2]
-- Dissolve to bottomObject (different color)
transition.dissolve( topObject, bottomObject, 500 )
-- Restore after some random delay
transition.dissolve( bottomObject, topObject, 500, math.random( 3000, 10000 ) )
end
-- we handled it so return true to stop propagation
return true
end
end
--
--
--
-- Add fish to the screen
for i=1,numFish do
-- create group which will represent our fish, storing both images (file1 and file2)
local group = display.newGroup()
local fishOriginal = display.newImage( file1 )
group:insert( fishOriginal, true ) -- accessed in buttonListener as group[1]
local fishDifferent = display.newImage( file2 )
group:insert( fishDifferent, true ) -- accessed in buttonListener as group[2]
fishDifferent.isVisible = false -- make file2 invisible
-- move to random position in a 200x200 region in the middle of the screen
group:translate( halfW + math.random( -100, 100 ), halfH + math.random( -100, 100 ) )
-- connect buttonListener. touching the fish will cause it to change to file2's image
group:addEventListener( "touch", buttonListener )
-- assign each fish a random velocity
group.vx = math.random( 1, 5 )
group.vy = math.random( -2, 2 )
-- add fish to animation group so that it will bounce
bounceAnimation[ #bounceAnimation + 1 ] = group
end
--
-- Function to animate all the fish
function bounceAnimation:enterFrame( event )
local container = self.container
container:setFillColor( 0, 0, 0, 0) -- make invisible
local containerBounds = container.contentBounds
local xMin = containerBounds.xMin
local xMax = containerBounds.xMax
local yMin = containerBounds.yMin
local yMax = containerBounds.yMax
local orientation = self.currentOrientation
local isLandscape = "landscapeLeft" == orientation or "landscapeRight" == orientation
local reflectX = nil ~= self.reflectX
local reflectY = nil ~= self.reflectY
-- the fish groups are stored in integer arrays, so iterate through all the
-- integer arrays
for i,v in ipairs( self ) do
local object = v -- the display object to animate, e.g. the fish group
local vx = object.vx
local vy = object.vy
if ( isLandscape ) then
if ( "landscapeLeft" == orientation ) then
local vxOld = vx
vx = -vy
vy = -vxOld
elseif ( "landscapeRight" == orientation ) then
local vxOld = vx
vx = vy
vy = vxOld
end
elseif ( "portraitUpsideDown" == orientation ) then
vx = -vx
vy = -vy
end
-- TODO: for now, time is measured in frames instead of seconds...
local dx = vx
local dy = vy
local bounds = object.contentBounds
local flipX = false
local flipY = false
if (bounds.xMax + dx) > xMax then
flipX = true
dx = xMax - bounds.xMax
elseif (bounds.xMin + dx) < xMin then
flipX = true
dx = xMin - bounds.xMin
end
if (bounds.yMax + dy) > yMax then
flipY = true
dy = yMax - bounds.yMax
elseif (bounds.yMin + dy) < yMin then
flipY = true
dy = yMin - bounds.yMin
end
if ( isLandscape ) then flipX,flipY = flipY,flipX end
if ( flipX ) then
object.vx = -object.vx
if ( reflectX ) then object:scale( -1, 1 ) end
end
if ( flipY ) then
object.vy = -object.vy
if ( reflectY ) then object:scale( 1, -1 ) end
end
object:translate( dx, dy )
end
end
-- Handle orientation of the fish
function bounceAnimation:orientation( event )
print( "bounceAnimation" )
for k,v in pairs( event ) do
print( " " .. tostring( k ) .. "(" .. tostring( v ) .. ")" )
end
if ( event.delta ~= 0 ) then
local rotateParameters = { rotation = -event.delta, time=500, delta=true }
Runtime:removeEventListener( "enterFrame", self )
self.currentOrientation = event.type
for i,object in ipairs( self ) do
transition.to( object, rotateParameters )
end
local function resume(event)
Runtime:addEventListener( "enterFrame", self )
end
timer.performWithDelay( 500, resume )
end
end
Runtime:addEventListener( "enterFrame", bounceAnimation );
Runtime:addEventListener( "orientation", bounceAnimation )
-- This function is never called,
-- but shows how we would unload the sound if we wanted to
function unloadSound()
audio.dispose(soundID)
soundID = nil
end
答案 0 :(得分：5)
对于and和or来说，Lua有一种奇怪的行为。
如果a and b被视为假（仅a且a被视为假，则nil表达式评估为false，其他所有值（包括0）均被视为如果a被认为是真的，则表达式的计算结果为b。
如果a or b被视为真，则a表达式评估为a;如果b被视为假，则表达式a。
注意：在两种情况下，如果表达式求值为a，则甚至不评估b的值。这称为短路逻辑。在and如果a为false，则and不可能为真，因此浪费计算时间来评估b没有意义。同样，如果a中a or b为真，则无法评估b，因为or不可能是假的。
构造cond and valiftrue or valiffalse（或等效的，由于运算符优先级，(cond and valiftrue) or valiffalse）等同于其他语言的三元if语句，但有一点需要注意： valiftrue不得评估为假。如果是这种情况，整个表达式将始终评估为valiffalse。 （尝试并推理出来，这是我发现抓住这个结构的最好方法。）
答案 1 :(得分：1)
即将详细发布，但@JPvdMerwe明确了。
准确地说，
background = ( backgroundLandscape == background and backgroundPortrait ) or backgroundLandscape
转换为
if backgroundLandscape == background then
background = backgroundPortrait
else
background = backgroundLandscape
end
修改
正如@JPvdMerwe指出的那样，在这种情况下，如果backgroundPortrait为false，那么
background = backgroundLandscape
将一直执行。