Is there a listener for when the WebView displays it's content?

Question

Using WebViewClient and/or the WebChromeClient you can get a listener for when the page has loaded, however this is sometimes called before the WebView has any content in it, before it has displayed anything.

What would be a efficient method for determining when the WebView has displayed it's content?

Edit: (Trying to be more clear)

When I load a page in a WebView, I want to set the scroll to a specific position. It seems that the scroll position cannot be set until the page is loaded and it has an actual content height. So, I have tried two different approaches to determining when the page has finished loading, onPageFinished() from the WebViewClient and also onProgressChanged() from the WebChromeClient. Both of these tell me when the page has finished loading.

However, the problem is that sometimes it is called before the page has been displayed and therefore the page has no height and the scroll call does nothing.

I am trying to find a solid way to determine when the page is ready to be scrolled, ie. when it has its content height.

I imagine I could setup a checking loop after it finished loading to keep looking for when the height is available but that seemed like quite the hack. Hoping there is a cleaner way.

Solution 1

I successfully used Richard's answer with a PictureListener for a few years, but I no longer recommend this as the best solution.

This is for two reasons:

1. webView.setPictureListener and PictureListener are both deprecated.
2. Using this listener will cause the WebView to allocate a new Picture often. This allocation is expensive and this can have some significant performance impacts or even cause native crashes on JellyBean MR1.

Instead I recommend creating a subclass of WebView and overriding invalidate() like so:

@Override
public void invalidate() {
    super.invalidate();

    if (getContentHeight() > 0) {
        // WebView has displayed some content and is scrollable.
    }
}

If you still want to use the PictureListener method, you will get better performance if you setPictureListener back to null after you are done with it.

Solution 2

EDIT: Since I first posted this, this method has been deprecated in Android API 12. Thanks, Google! I will leave the original answer in tact:

---

Yes-- there IS a listener for knowing when the content has finished loading. I had to create one on my project so I could do a splash screen which stayed up until the webview had loaded the page.

It's called .setPictureListener.

For example:

mWebView.setPictureListener(new MyPictureListener());
//... and then later on....
class MyPictureListener implements PictureListener {

    @Override
    public void onNewPicture(WebView view, Picture arg1) {
      // put code here that needs to run when the page has finished loading and
      // a new "picture" is on the webview.      
    }    
} 

Solution 3

The suggested solutions here are hacky at best, while there is a perfectly clean solution, in accordance with the Android docs:

WebViewClient client = new WebViewClient() {
    @Override
    public void onPageFinished(WebView view, String url) {
        Log.d("MYAPP", "Page loaded");
    }
};
webView.setWebViewClient(client);

Solution 4

I read this discussion, and I'm facing the same problem too.

I searched a lot, but they're not the solution they promise to be.

And I don't want to use the deprecated `onNewPicture` callback.

In my own situation I only need to restore the `WebView` scroll position when the activity is started, and I think this is the most cases since when the activity task stack goes to background, Android automatically save the `WebView` state, so when it pops to the foreground, it will looks the same. If it gets killed when under background, I'm sure you'll manage to save the state in Activity lifecycle methods like `onPause`.

So, instead of extending `WebView` and overriding blah, blah, blah..., I use a Handler and post model.

private final Handler mHandler = new Handler();
// in onCreate
mHandler.postDelayed(new Runnable() {

                @Override
                public void run() {
                    if (mWebView.getContentHeight() > 0) {
                        mWebView.scrollTo(0, mLastPosition);
                        Log.d("scrolling", "true");
                        mHandler.removeCallbacks(this);
                    } else {
                        mHandler.postDelayed(this, 100);
                    }
                }
            }, 100);

This code block will peridically check if the WebView page can be manipulated, if so, it does the scrolling and remove the callback, otherwise it loops. I test this and find out it ususally won't take very long.

Hope this helps!

Solution 5

I created a custom view extends webview, which overrides the "onSizeChanged" method. The onSizeChanged happens after the webview loads everything in it.

View more solutions

Comments

[cottonBallPaws]

Thanks for the reply but doesn't that just tell the app when the specific resource is about to begin loading not when it has just been displayed? onLoadResource is called for each resource on the on page, these are all called before onPageFinished. What I am looking for is something that would be called after onPageFinished, right after the page has actually be displayed to the user.

[Aaron Saunders]

you should update the question to make it clearer... maybe someone will give you a more suitable answer?

[cottonBallPaws]

sorry, for some reason I didn't see you responded. Thanks for your patience. I edited my question to try to make it clearer. Also, I tried the doUpdateVisitedHistory() idea and it was called while the page was loading so it won't work. Thanks again.

[cottonBallPaws]

This works perfectly the first time a page is loaded, but after the first one is loaded this is never called again so it doesn't work if the user will be moving onto other pages.

[cottonBallPaws]

Awesome. This works exactly as I had hoped. A warning to others using this though... It does get called fairly often as you move around the page so make sure whatever happens in the onNewPicture method is quick.

[cottonBallPaws]

I wish that was the answer, but in my experience onPageFinished can be, and is a great deal of the time, called before the content is displayed/scrollable/measurable. So if you try to scroll in the onPageFinished method, the content height might still be 0.

[Bahadır Yıldırım]

Ah, right. The docs actually suggest using onNewPicture(WebView, Picture), but from my experience, it doesn't always fire.

[Bahadır Yıldırım]

Follow-up: onNewPicture is only fired if the WebView is visible and displaying within the boundaries of the device. So if you want to use it to calculate the dimensions before displaying the WebView, it seems like this won't help.

[Richard]

@Paul: My understanding was that this listener fired when the webview updated it's "picture", that is, what it was showing on screen, regardless of what it was showing. But if it's true that you need to have at least one image on the page, you could always add a simple solid-color .gif or .jpg somewhere which would blend into the background and not be visible to the user, but would cause this listener to fire off. But I can't confirm that is even needed, as I've never tested a webview with just text. Can someone else reply to clarify?

[Bahadır Yıldırım]

No, I was mistaken. The "picture" the WebKit docs are referring to is the rendering of the browser contents. I was initially confused as to why the above technique wasn't working for me, because the event isn't fired unless the rendering actually appears on the screen.

[Richard]

@Paul: I see, though I use this technique myself to know when to make a splash screen disappear and show the webview (I want it to show after the content is loaded). And while that is going on, the webview's visibility is set to "invisible". So I don't think it actually needs to be on the phone's screen to fire. If you have your webview set to "gone", perhaps that is the issue?

[Bahadır Yıldırım]

Perhaps that's the problem. I'll check if that's the case when I have the opportunity.

[Johan S]

This doesn't work. The height might be null in onPageFinished.

[Gu1234]

This method was deprecated in API level 12

[kevin]

onNewPicture(WebView, Picture) is deprecated long time ago (for now).

[Martin]

This is the best solution I have found. However, there are some occasions when invalidate() is not called when WebView content in a WebView is updated. One unusual case is when the previous content is positioned right at the top when loadDataWithBaseURL is called so I also added a delayed call to invalidate after calling loadDataWithBaseURL.

[Michal Vician]

This approach is relevant only if you are interested in the fact that web page data has been loaded. It has nothing to do with drawing/painting the data.

[arlomedia]

I tried WebViewClient.onPageFinished, WebChromeClient.onProgressChanged, and even the deprecated PictureListener.onNewPicture and they were all unreliable, but this approach is working well. However, if you need to clear the WebView content, setting the URL to about:blank does not return the contentHeight to 0 as you might expect. The only way I could find to clear the content and know when the WebView is ready to accept new content is to create a new WebView object.

[Torsten Römer]

Indeed the best solution. Adding getProgress() == 100 helped me to make sure the content is completely rendered.

[sandalone]

This works in my case, but instead of mWebView.getContentHeight() > 0, I calculated the height of the device screen. Just to see that there are options.

[user4951834]

I know this is a bit late, but newbie here: where do I put the above mentioned code? Everywhere I put it, Android Console says not recognized.

[cottonBallPaws]

@user4951834 create a subclass of WebView and put it in there. Then use your custom WebView class instead of the original one. The WebView has a method called invalidate() and this will override its original implementation.

[user4951834]

@cottonBallPaws Sorry, this may sound like a dumb question, but can I get an example of the code? What do you mean create a subclass?

[Fizzix]

Can you please elaborate on this answer a little more? For example, what would you call within the if statement?

[cottonBallPaws]

@Fizzix if the content height is > 0, then the webview has displayed content. so inside that if statement represents whatever you want to do when that has occurred.

[Antwan]

Hey i tried this solution Actually its better than other but the problem that i always get the height is like half of the actual height any help with this?

[Papa Smurf]

It's the only device that worked in my case, where I navigate between pages I build myself and preserve the scroll position when navigating to a page that has been visited. Similarly to @sandalone, I note the contentHeight (WebView.getContentHeight) of a page prior to moving away from it, and then I run code similar to the above to scroll to the saved position only when the result returned by getContentHeight matches the stored value. I run it in onPageFinished.