WebView Class Overview
A View that displays web pages. This class is the basis upon which you can roll your own web browser or simply display some online content within your Activity. It uses the WebKit rendering engine to display web pages and includes methods to navigate forward and backward through a history, zoom in and out, perform text searches and more.
WebView是用來顯示網頁的。它是你開發自己的web瀏覽器的基礎，也可以用來在Activity中只是顯示一些在線內容。它使用WebKit作爲底層引擎，包括向前先後的導航，縮放，文字搜索等等。
To enable the built-in zoom, set WebSettings.setBuiltInZoomControls(boolean) (introduced
in API version 3).
Note that, in order for your Activity to access the Internet and load web pages in a WebView, you must add the INTERNET permissions to your Android
Manifest file:
<uses-permission android:name="android.permission.INTERNET" />
This must be a child of the element.
See the Web View tutorial.
Basic usage
By default, a WebView provides no browser-like widgets, does not enable JavaScript and web page errors are ignored. If your goal is only to display some HTML as a part of your UI, this is probably fine; the user won't need to interact with the web page beyond reading it, and the web page won't need to interact with the user. If you actually want a full-blown web browser, then you probably want to invoke the Browser application with a URL Intent rather than show it with a WebView. For example:
缺省狀態下，WebView是一個不像瀏覽器的控件，不能使用JavaScript、忽略網頁錯誤。如果你的目標只是在UI中展示HTML是很好的；用戶不必和網頁交互操作，只是讀內容。如果你想要一個全功能的web瀏覽器，你需要使用URL intent啓動一個瀏覽器，而不是在WebView中顯示。
Uri uri = Uri.parse("http://www.example.com");
Intent intent = new Intent(Intent.ACTION_VIEW, uri);
startActivity(intent);
See Intent for more information.
To provide a WebView in your own Activity, include a in your layout, or set the entire Activity window as a WebView during onCreate():
在layout中包括WebView 或 設置整個window只有一個WebView 在onCreate()中：
WebView webview = new WebView(this);
setContentView(webview);
Then load the desired web page:
然後加載想要的頁面：
// Simplest usage: note that an exception will NOT be thrown
// if there is an error loading this page (see below).
webview.loadUrl("http://slashdot.org/");
// OR, you can also load from an HTML string:
String summary = "<html><body>You scored <b>192</b> points.</body></html>";
webview.loadData(summary, "text/html", null);
// ... although note that there are restrictions on what this HTML can do.
// See the JavaDocs for loadData() and loadDataWithBaseURL() for more info.
A WebView has several customization points where you can add your own behavior. These are:
WebView有幾個客戶化的點，你可以在那裏增加你的行爲。
-
Creating and setting a
WebChromeClientsubclass. This class is called when something that might impact a browser UI happens, for instance, progress updates and JavaScript alerts are sent here (see Debugging Tasks). - 創建和設置WebChromeClient的子類，當瀏覽器的UI發生變化時調用，如：進度條更改、JavaScript的窗口。
-
Creating and setting a
WebViewClientsubclass. It will be called when things happen that impact the rendering of the content, eg, errors or form submissions. You can also intercept URL loading here (viashouldOverrideUrlLoading()). - 創建和設置WebViewClient子類，當底層內容變化時調用，如：錯誤或表單提交。你也可以再次攔截URL調用。
-
Modifying the
WebSettings, such as enabling JavaScript withsetJavaScriptEnabled(). - 修改WebSettings,例如：讓JavaScript可用--setJavaScriptEnabled().
-
Adding JavaScript-to-Java interfaces with the
addJavascriptInterface(Object, String)method. This lets you bind Java objects into the WebView so they can be controlled from the web pages JavaScript. - 增加JavaScript-to-Java接口addJavascriptInterface(Object,String)。可以將java對象綁定到WebView,這樣對象可以被網頁的JavaScript控制。
Here's a more complicated example, showing error handling, settings, and progress notification:
// Let's display the progress in the activity title bar, like the
// browser app does.
getWindow().requestFeature(Window.FEATURE_PROGRESS);
webview.getSettings().setJavaScriptEnabled(true);
final Activity activity = this;
webview.setWebChromeClient(new WebChromeClient() {
public void onProgressChanged(WebView view, int progress) {
// Activities and WebViews measure progress with different scales.
// The progress meter will automatically disappear when we reach 100%
activity.setProgress(progress * 1000);
}
});
webview.setWebViewClient(new WebViewClient() {
public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
Toast.makeText(activity, "Oh no! " + description, Toast.LENGTH_SHORT).show();
}
});
webview.loadUrl("http://slashdot.org/");
Cookie and window management
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. Follow the instructions in CookieSyncManager if
you want to use cookies in your application.
由於安全的原因，你的應用有自己的緩存和cookie存儲。Cookies在一個單獨的線程中管理，像建立索引這樣的操作不會阻塞UI線程。如果想使用cookies，可以查看CookieSyncManager 。
By default, requests by the HTML to open new windows are ignored. This is true whether they be opened by JavaScript or by the target attribute on a link. You can customize yourWebChromeClient to
provide your own behaviour for opening multiple windows, and render them in whatever manner you want.
缺省情況，打開一個新窗口的HTML請求將被忽略，無論是JavaScript或連接屬性的請求，都被忽略。可以客戶化WebChromeClient ，提供打開多個窗口的功能，提供任何你想要的功能。
The standard behavior for an Activity is to be destroyed and recreated when the device orientation or any other configuration changes. This will cause the WebView to reload the current page. If you don't want that, you can set your
Activity to handle the orientation and keyboardHidden changes, and then just leave the WebView alone.
It'll automatically re-orient itself as appropriate. Read Handling Runtime Changes for more information about how to handle configuration changes
during runtime.
當設備的方向或其他屬性改變時，Activity的缺省表現是銷燬再重建。這回導致WebView重載當前頁面。如果不想重建，可以設置Activity自己處理orientation 和 keyboardHidden 改變。閱讀 Handling
Runtime Changes 獲得如何處理運行時改變的更多信息。
Building web pages to support different screen densities
The screen density of a device is based on the screen resolution. A screen with low density has fewer available pixels per inch, where a screen with high density has more — sometimes significantly more — pixels per inch. The density of a screen is important because, other things being equal, a UI element (such as a button) whose height and width are defined in terms of screen pixels will appear larger on the lower density screen and smaller on the higher density screen. For simplicity, Android collapses all actual screen densities into three generalized densities: high, medium, and low.
屏幕密度建立在屏幕分辨率上。低密度在每英寸上有較少的可用像素，高分辨率有更多像素--每英寸像素數PPI。一個UI元素，在低密度屏幕上比在高密度屏幕要大。爲了簡化，Android將所有屏幕精度分爲三種：高、中和低。
By default, WebView scales a web page so that it is drawn at a size that matches the default appearance on a medium density screen. So, it applies 1.5x scaling on a high density screen (because its pixels are smaller) and 0.75x scaling on a low density screen (because its pixels are bigger). Starting with API Level 5 (Android 2.0), WebView supports DOM, CSS, and meta tag features to help you (as a web developer) target screens with different screen densities.
缺省情況下，WebView縮放網頁以適應中密度的屏幕。所以，在高密度屏上放大1.5倍放大，在低密度屏上，0.75倍縮小。從API Leve 5(Android 2.0)WebView支持DOM,CSS和meta標籤，以幫助適應不同密度屏幕。
Here's a summary of the features you can use to handle different screen densities:
支持不同密度的特性彙總：
-
The
window.devicePixelRatioDOM property. The value of this property specifies the default scaling factor used for the current device. For example, if the value ofwindow.devicePixelRatiois "1.0", then the device is considered a medium density (mdpi) device and default scaling is not applied to the web page; if the value is "1.5", then the device is considered a high density device (hdpi) and the page content is scaled 1.5x; if the value is "0.75", then the device is considered a low density device (ldpi) and the content is scaled 0.75x. However, if you specify the"target-densitydpi"meta property (discussed below), then you can stop this default scaling behavior. -
The
-webkit-device-pixel-ratioCSS media query. Use this to specify the screen densities for which this style sheet is to be used. The corresponding value should be either "0.75", "1", or "1.5", to indicate that the styles are for devices with low density, medium density, or high density screens, respectively. For example:<link rel="stylesheet" media="screen and (-webkit-device-pixel-ratio:1.5)" href="hdpi.css" />The
hdpi.cssstylesheet is only used for devices with a screen pixel ration of 1.5, which is the high density pixel ratio. -
The
target-densitydpiproperty for theviewportmeta tag. You can use this to specify the target density for which the web page is designed, using the following values:-
device-dpi- Use the device's native dpi as the target dpi. Default scaling never occurs. -
high-dpi- Use hdpi as the target dpi. Medium and low density screens scale down as appropriate. -
medium-dpi- Use mdpi as the target dpi. High density screens scale up and low density screens scale down. This is also the default behavior. -
low-dpi- Use ldpi as the target dpi. Medium and high density screens scale up as appropriate. -
- Specify a dpi value to use as the target dpi (accepted values are 70-400).
Here's an example meta tag to specify the target density:
<meta name="viewport" content="target-densitydpi=device-dpi" /> -
If you want to modify your web page for different densities, by using the -webkit-device-pixel-ratio CSS media query and/or the window.devicePixelRatio DOM
property, then you should set the target-densitydpi meta property to device-dpi. This stops Android
from performing scaling in your web page and allows you to make the necessary adjustments for each density via CSS and JavaScript.