measure elapsed time of image loading in Corona SDK

问题

I'm trying to benchmark the loading of large images in Corona SDK.

local startTime = system.getTimer()
local myImage = display.newImageRect( "someImage.jpg", 1024, 768 )
local endTime = system.getTimer()
print( endTime - startTime ) -- prints 8.4319999999998

This returns values of around 8 ms. I know it takes longer to load an display an image because if it really took 8 ms I wouldn't notice the delay, but I do notice it. I'd say it takes about 300 ms.

Also the FPS drop drastically when loading a large image. I'm monitoring this using an enterFrame event and when loading the image it prints values of around 0.3 for 1 frame.

Runtime:addEventListener( "enterFrame", myListener )

function onEnterFrame (event)
    print( display.fps )
end

The frame takes a long time to render when loading, even when the loading of the image takes less than 1/60 of a second. I guess it means the rendering is happening asynchronously somewhere else.

So, how can I measure the time it takes to really load and display an image?

回答1:

Since Corona SDK is closed source, we'll have to use the docs and imagination.

I see three possibilities here:

Corona is doing what it says, and your subjective experience is wrong.
Corona is loading the images in a background thread, so the call to display.newImageRect is non-blocking: it "starts" loading the image, and then continues. When this happens in other SDKs (mostly javascript-based ones) you get a "ready callback" that you can use on the image object, but I could not find such thing in the docs.
Corona loads the image quickly, but requires "extra work afterwards". For example, it generates lots of garbage which has to be garbage-collected. So the image gets loaded fast, but then this "extra work" slows down the app.

My bet is on 3. But this doesn't really matter. Independently of which one of these options is causing the slowdowns, they can be solved the same way: instead of loading the images right before you draw them, you have to preload them.

I don't use Corona SDK, but a quick google pointed me to the storyboard module, in particular to storyboard.loadScene.

Create a new scene, list all the images that you need on it, and load it before showing it - that way image loading will be done in advance, not slowing down your app.

回答2:

Most likely the image is rendered during the scene's rendering loop. There is no event to indicate that an image has been rendered. However if you create the display object in the scene's create event handler or a button click handler, and register an enterFrame event handler, you can measure the time between that and the first frame event. I can't try this here but my guess is this will give you an estimate of the time to render the image. Dont use FPS. Larger image will probably give you a larger measurement. If you measure the time between enterFrame events you will probably find that it is much smaller than the time between create/click event and the first frame event, or between the first two frame events after the create/click event. Post a comment if you would like to see some example code.

来源：https://stackoverflow.com/questions/26333531/measure-elapsed-time-of-image-loading-in-corona-sdk

标签

image

lua

corona