[AndroidDev] WebView

[WebView]

• The basis upon which you can do followings within your Activity:
• Roll your own web browser.
• Simply display some online content.
• Uses the same rendering and JavaScript engine(WebKit) as the browser, but it runs under the control of your application.
• By default:
• A WebView provides no browser-like widgets.
• No zoom in and out.
• WebSettings.setBuiltInZoomControls(boolean) (introduced in API 3)
• Does not enable JavaScript.
• WebSettings.setJavaScriptEnabled()
• No navigation controls.
• Need to handle the BACK button on the device
• Overrides URL loading, it automatically accumulates a history of visited web pages.
• Web page errors are ignored.
• Perform text searches. (?)
• No address bar.
• As you click a link, by default WebView will ask Activity Manager to choose the proper handler for the url.
• Instead of adding an intent filter to view web pages, you can override the WebViewClient class and enable this Activity to handle its own URL requests.

Load content

• Can download content from the web.
• Can come from local files stored in your assets directory.
• Can even be dynamically generated by your application code.
• ex:

webview.loadUrl("http://www.google.com.tw/");  

// or

String summary = "You scored 192 points.";
webview.loadData(summary, "text/html", null);

Note:

• public void loadData (String data, String mimeType, String encoding)
• This API has JavaScript's same origin policy restriction.
• 此頁面中的 script 此能用 "data" 的 schema 去存取。
• ex: data 的 schema 為 "http"，則不能使用 "https"
• 因此提供了另一個 API
• public void loadDataWithBaseURL (String baseUrl, String data, String mimeType, String encoding, String historyUrl)

A WebView has several customization points where you can add your own behavior. These are:

• Creating and setting a WebChromeClient subclass. 
• This class is called when something that might impact a browser UI happens, ex:
• Progress update.
• JavaScript alerts are sent here.
• Creating and setting a WebViewClient subclass.
• It will be called when things happen that impact the rendering of the content, ex:
• Errors or form submissions.
• Will load any URL selected from this WebView into the same WebView.
• Can intercept URL loading here (via shouldOverrideUrlLoading()).
• 預設點擊頁面上的 link 會由系統處理，開啟相對應的 app。
• ex: 若是 URL 則開啟系統 Browser。
• 但若 override shouldOverrideUrlLoading()
• 則能依照自己意思判斷 URL 決定要在 WebView 開啟或是在 Browser 中開啟。ex:
• return true
• The method has handled the URL and the event should not propagate.
• ex: an Intent would be created that's handled by the Browser application
• 表示會自己處理 url。
• 因此若沒有任何處理動作，點擊後不會有反應。
• return false
• The current WebView handles the url
• 仍由系統處理，但會開啟在當前的 WebView。
• 因此若有設定自己的執行動作又 return false，則兩種執行動作都會被觸發。
• 所以如果想要畫面都在 WebView 中開啟，則表示一定要 override shouldOverrideUrlLoading()。
• Related JavaScript
• Enabling JavaScript with setJavaScriptEnabled() (Modifying the WebSettings)
• addJavascriptInterface(Object, String)
• Adding JavaScript-to-Java interfaces.
• Has security issue.
• Bind Java objects into the WebView so they can be controlled from the web pages JavaScript.
• ex: Your JavaScript code can call a method in your Android code to display a Dialog, instead of using JavaScript's alert() function.

Data

• For obvious security reasons, your application has its own cache, cookie store etc.—it does not share the Browser application's data. 
• Cookies are managed on a separate thread, so operations like index building don't block the UI thread. 
• CookieSyncManager
• Is used to synchronize the browser cookie store between RAM and permanent storage.
• To get the best performance, browser cookies are saved in RAM. A separate thread saves the cookies between, driven by a timer.
• Note that even sync() happens asynchronously, so don't do it just as your activity is shutting down.

Note

• Must claim this permission in AndroidManifest.xml or you won't open page and do not get any exception.
• <uses-permission android:name="android.permission.INTERNET">

Related
[AndroidLayout] WebView Performance


* Reference
- WebView | Android Developers
- WebViewClient | Android Developers
- WebChromeClient
- WebSettings | Android Developers
- CookieSyncManager | Android Developers
- Android Developers Blog: Using WebViews
- Building Web Apps in WebView | Android Developers
- WebView 1 - 4 ***
- SOP - Same Origin Policy (Browser Security Policy)