Android之WebView简易应用

WebView allows you to create your own window for viewing web pages (or even develop a complete browser). In this tutorial, you'll create a simple Activity that can view and navigate web pages.

Create a new project named HelloWebView.

Open the res/layout/main.xml file and insert the following:

<?xml version="1.0" encoding="utf-8"?>
<WebView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/webview"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
/>

Now open the HelloWebView.java file. At the top of the class, declare a WebView object:

WebView mWebView;

Then use the following code for the onCreate() method:

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    mWebView = (WebView) findViewById(R.id.webview);
    mWebView.getSettings().setJavaScriptEnabled(true);	//调用方法getSettings返回WebSetting对象去管理WebView
    mWebView.loadUrl("http://www.google.com");		//需要访问的地址
}

This initializes the member WebView with the one from the Activity layout; requests a WebSettings object with getSettings(); and enables JavaScript for the WebView withsetJavaScriptEnabled(boolean). Finally, an initial web page is loaded with loadUrl(String).

Because this application needs access to the Internet, you need to add the appropriate permissions to the Android manifest file. Open the AndroidManifest.xml file and add the following as a child of the <manifest> element:
```
<uses-permission android:name="android.permission.INTERNET" />
```

While you're in the manifest, give some more space for web pages by removing the title bar, with the "NoTitleBar" theme:

<activity android:name=".HelloGoogleMaps" android:label="@string/app_name"
     android:theme="@android:style/Theme.NoTitleBar">

Now run the application.
You now have a simplest web page viewer. It's not quite a browser yet because as soon as you click a link, the default Android Browser handles the Intent to view a web page, because thisActivity isn't technically enabled to do so. 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.
在第六步这里运行意味着本程序是需要依靠别的浏览器打开指定网页，说明程序不能自动打开网页；第七第八步是可以在本程序打开网页啦

In the HelloAndroid Activity, add this nested class:

private class HelloWebViewClient extends WebViewClient {
    @Override
    public boolean shouldOverrideUrlLoading(WebView view, String url) {
        view.loadUrl(url);
        return true;
    }
}

Then towards the end of the onCreate(Bundle) method, set an instance of the HelloWebViewClient as the WebViewClient:
```
mWebView.setWebViewClient(new HelloWebViewClient());
```
This line can go anywhere following the initialization of the WebView object.
This creates a WebViewClient that will load any URL selected from this WebView into the same WebView. The shouldOverrideUrlLoading(WebView, String) method is passed the current WebView and the URL requested, so all it needs to do is load the URL in the given view. Returning true says that the method has handled the URL and the event should not propagate (in which case, an Intent would be created that's handled by the Browser application).
If you run the application again, new pages will now load in this Activity. However, you can't navigate back to previous pages. To do this, you need to handle the BACK button on the device, so that it will return to the previous page, rather than exit the application.
第八步以上的步骤按返回键的时候程序会直接退出而并不会返回上一页，以下第九步就是解决这一功能的实现
To handle the BACK button key press, add the following method inside the HelloWebView Activity:
```
 
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if ((keyCode == KeyEvent.KEYCODE_BACK) && mWebView.canGoBack()) {
        mWebView.goBack();
        return true;
    }
    return super.onKeyDown(keyCode, event);
}
```
This onKeyDown(int, KeyEvent) callback method will be called anytime a button is pressed while in the Activity. The condition inside uses the KeyEvent to check whether the key pressed is the BACK button and whether the WebView is actually capable of navigating back (if it has a history). If both are true, then the goBack() method is called, which will navigate back one step in the WebView history.Returning true indicates that the event has been handled. If this condition is not met, then the event is sent back to the system.