[TIMOB-25941] Android: Optimized HTTPClient download performance

[jquick-axway]

JIRA:
https://jira.appcelerator.org/browse/TIMOB-25941

Summary:

• Download performance improved by about 25%. (Depends on network speed/quality.)
• Increased response buffer size from 4kb to 8kb.
• Removed undocumented event property blob from callback ondatastream().
  • Improves performance by avoiding extra buffer copy.
  • This undocumented property does not exist on iOS.
• Modified responseText property handling only create its string when accessed.
  • Was creating and transcoding the string for every response segment received. Was a huge performance hit.
  • Fixed responseText to provide total text downloaded like iOS instead of the buffered response segment's text.
• Synchronized file write to storage upon download completion.
  • Avoids race condition where reading responseText or responseData in onload() callback may provide an incomplete response.

Test:
@lokeshchdhry, I recommend that you test this at home. The office network is too congested and download speed varies wildly.

1. Acquire an Android device with Internet access.
2. Build and run the below code with Titanium 7.3.0.
3. Tap the "Download" button.
4. Wait for the download to complete.
5. Note down the download duration time. (You may want to do this a few times and record the average.)
6. Build and run with this PR and repeat steps 3-5.
7. The download time should have improved.

var url = "https://github.com/appcelerator-modules/ti.admob/releases/download/ios-2.2.0/ti.admob-iphone-2.2.0.zip";
var downloadFilePath = Ti.Filesystem.applicationDataDirectory + "downloaded.zip";

var window = Ti.UI.createWindow();
var button = Ti.UI.createButton({ title: "Download" });
var progressBar = Ti.UI.createProgressBar({
	min: 0,
	max: 100,
	bottom: "40dp",
	left: "5dp",
	right: "5dp",
});
button.addEventListener("click", function(e) {
	var startTime = new Date();
	var httpClient = Ti.Network.createHTTPClient({
		ondatastream: function(e) {
			progressBar.value = e.progress * 100;
		},
		onload: function(e) {
			var duration =  (new Date()) - startTime;
			Ti.API.info("@@@ onload() duration: " + duration + "ms");
			alert("Download Time: " + duration + "ms");
			progressBar.value = 100;
			button.enabled = true;
		},
		onerror: function(e) {
			Ti.API.info("@@@ onerror()");
			alert("Download Failed");
			button.enabled = true;
		},
	});
	progressBar.value = 0;
	button.enabled = false;
	httpClient.open("GET", url);
	httpClient.setRequestHeader("Accept-Encoding", "identity");
	httpClient.file = downloadFilePath;
	httpClient.send();
});
window.add(button);
window.add(progressBar);
window.open();

[sgtcoolguy]

Technically this is a breaking Api change (and should go into 8.0.0) since this is removed. Although, it's hard to see how many people would be using this undocumented event callback property...

[jquick-axway]

It is. But from searching our repo's and JIRA's history, it doesn't appear to be used. We've discussed it internally in the office and decided to remove it.

Also note that we have a customer that wants a solution to this performance issue before Titanium 8.0.0. You may want to talk to Eric about this.

[sgtcoolguy]

Extract the magic constant as a final static at top so it's easier to tweak the buffer size?

(in fact looking at this file it looks like we have a number of magic constants around buffers/sizes: 512 bytes for progress notifications/entity data buffer; 8MB for blob/file I/O buffer; 512Kb for dumping response to file first rather than keep in-memory)

[jquick-axway]

It's not worth changing to a constant if it's only used in 1 place in the code. It's better to keep it simple.

Yes, there are other place in this class where there is duplication, but that's a refactoring for another day.

[lokeshchdhry]

FR Passed.

An improved performance is seen as compared to 7.3.0 SDK.

Studio Ver: 5.1.0.201808080937
SDK Ver: 7.5.0 local build
OS Ver: 10.13.5
Xcode Ver: Xcode 9.4.1
Appc NPM: 4.2.13
Appc CLI: 7.0.6-master.12
Daemon Ver: 1.1.3
Ti CLI Ver: 5.1.1
Alloy Ver: 1.13.2
Node Ver: 8.9.1
NPM Ver: 5.5.1
Java Ver: 10.0.2
Devices: ⇨ google Nexus 5 (Android 6.0.1)
⇨ google Nexus 6P (Android 8.1.0)

[build]

	Messages
📖	💾 Here's the generated SDK zipfile.

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