Preloading WebView libraries on Android 5.0+

[Kostya Vasilyev]

Hi,

Since 5.0, Android WebView is provided by a separate "app".

The first time an app inflates or creates a WebView, there is a delay (WebView code loading native libraries, that sort of thing).

This delay can be as large as 200 ms (Nexus 5, 5.1) or even 300 ms (Samsung S6, 7.0) or larger - which is a very noticeable "freeze" of the app's UI, and in particular can ruin animations.

Is there a way to preload (pre-initialize) WebView on 5.0+ without blocking the UI thread?

I've tried doing "new WebView(appContext)" on a worker thread, and although that appeared to work...

... as soon as my code tried to inflate another WebView object back on the UI thread, the app crashed (stack trace below).

The error message in the exception is misleading too - this code was running on the UI thread, so apparently the original "new WebView(appContext)" on worker thread kept a reference to that worker thread, and assumed it was the UI thread, which it wasn't.

But in any case, this is a crash, so preloading by "new WebView" on a worker thread does not work.

Is there a better, working, technique for this?

---

 Caused by: android.view.InflateException: Binary XML file line #2: Error inflating class org.kman.AquaMail.view.MessageWebView

     at android.view.LayoutInflater.createView(LayoutInflater.java:645)

     at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:764)

     at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:704)

     at android.view.LayoutInflater.inflate(LayoutInflater.java:492)

     ... 15 more

 Caused by: java.lang.reflect.InvocationTargetException

     at java.lang.reflect.Constructor.newInstance(Native Method)

     at android.view.LayoutInflater.createView(LayoutInflater.java:619)

     ... 18 more

 Caused by: java.lang.IllegalStateException: Calling View methods on another thread than the UI thread.

     at com.android.webview.chromium.WebViewChromium.createThreadException(WebViewChromium.java:66)

     at com.android.webview.chromium.WebViewChromium.checkThread(WebViewChromium.java:76)

     at com.android.webview.chromium.WebViewChromium.init(WebViewChromium.java:31)

     at android.webkit.WebView.<init>(WebView.java:606)

     at android.webkit.WebView.<init>(WebView.java:542)

     at android.webkit.WebView.<init>(WebView.java:525)

     at android.webkit.WebView.<init>(WebView.java:512)

     at org.kman.AquaMail.view.MessageWebView.<init>(MessageWebView.java:34)

---

-- K

[Bo Liu]

Nothing great. Webview itself has strict thread checks as you've found out. And there is no explicit API to "warm up" webview.

Maybe a workaround is to create the CookieManager which is documented to be thread safe, and currently it loads the native library without other initialization. So it preloads some things, but everything.

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[Toby Sargeant]

WebView.findAddress would also work (causes native code to load, and does JNI init - which is slightly less than CookieManager).

I think that CookieManager is unsafe in some circumstances? Torne would remember the details.

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[Bo Liu]

findAddress (or any webview static method) calls ensureChromiumStartedLocked(true), which does the full initialization on the main thread, and blocks the calling thread if need be. That's not much different from new-ing a webview on the main thread.

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[Torne (Richard Coles)]

Calling static methods on a background thread does do the code loading on the calling thread, which includes much of the disk IO; while Bo's correct that it does ultimately post tasks to the main thread, it should still reduce the time the main thread is blocked quite a bit. We normally suggest WebSettings.getDefaultUserAgentString() as it doesn't have any other side effects besides causing webview to initialise.

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[Kostya Vasilyev]

Richard,

WebSettings.getDefaultUserAgent(appContext)

on a worker thread worked beautifully, moved about 3/4 of the total delay to the worker thread, and everything normal back on the UI thread.

Thanks much!

Is it considered safe with any WebView version assuming Android 5.0 - or would you recommend checking WebView package version to be safe, to only do this for say version 60 (or whatever) and newer?

-- K

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[Torne (Richard Coles)]

It's safe to do this on any version, even before 5.0. On earlier versions it'll just be a no-op, since webview is already loaded.

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