Detecting Connection Speed For Use In Preloader

I had an "A-Ha!" moment, and I don't think I've ever seen it on this board, so I thought I would share.

It is possible to detect (estimate) connection speeds during the preloader. This would be useful when deciding how much of the movie to load before proceeding. For example, if you know that your user is on a cable modem, you know that the download could keep ahead of the playhead if 50% of the movie is loaded in the preloader. However, a 56k modem might require 80% of the movie to be preloaded.

In your preloader, prior to the loop, initialize a variable, and set it to the number of bytes loaded. Create a constant that stores a sample speed in seconds. Create a new function that reads the current bytesloaded, compares it to the old bytesloaded, and calculates the DL-speed based on the sample rate. Last, create an interval that calls the function every samplerate*1000 (since intervals go off of milliseconds).

Note: This must all be done BEFORE your preloader frame loop, so that multiple intervals aren't being created.

Here is how I coded it:code: var kSampleRate = 0.5; // This is the Sample Rate in Seconds.
var kDownloadSpeed; // This will be where the DL Speed gets stored
var iLastBytesLoaded; // a Public variable to store the initial bytes in

function GetDLSpeed(){
clearInterval(tDownloadSpeed); // Create your interval for sampling
// DL speed = (finalBytes - initialBytes) / time
kDownloadSpeed = (this.getBytesLoaded()-iLastBytesLoaded)/kSampleRate;
}

function SampleDLSpeed(){
iLastBytesLoaded = this.getBytesLoaded(); // Set your initial sample
// Create your interval to check ever specified time (Determined by sample rate)
var tDownloadSpeed = setInterval(GetDLSpeed, kSampleRate*1000);
}

SampleDLSpeed();Keep in mind that this is an estimate. Connection speeds vary, and will be affected by what else the user has downloading, but this is a good thing since we want to know the stream speed that this SWF is getting, not if they have DSL.
Also, this is not exact because intervals are not precise. A 1.0s interval may fire at 1.05s (See Flash's ActionScript Dictionary on setInterval for more info.) Using a larger sample rate will be more precise but take more time to sample.

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