Android之WebView简易应用

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

Create a new project namedHelloWebView.

Open theres/layout/main.xmlfile and insert the following:

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

Now open theHelloWebView.javafile. At the top of the class, declare aWebViewobject:

WebView mWebView;

Then use the following code for theonCreate()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 memberWebViewwith the one from theActivitylayout; requests aWebSettingsobject withgetSettings(); and enables JavaScript for theWebViewwithsetJavaScriptEnabled(boolean). Finally, an initial web page is loaded withloadUrl(String).

Because this application needs access to the Internet, you need to add the appropriate permissions to the Android manifest file. Open theAndroidManifest.xmlfile 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 thisActivityisn't technically enabled to do so. Instead of adding an intent filter to view web pages, you can override theWebViewClientclass and enable thisActivityto 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 theHelloAndroidActivity, 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 theonCreate(Bundle)method, set an instance of theHelloWebViewClientas theWebViewClient:
```
mWebView.setWebViewClient(new HelloWebViewClient());
```
This line can go anywhere following the initialization of theWebViewobject.
This creates aWebViewClientthat will load any URL selected from thisWebViewinto the sameWebView. TheshouldOverrideUrlLoading(WebView, String)method is passed the currentWebViewand the URL requested, so all it needs to do is load the URL in the given view. Returningtruesays 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 theHelloWebViewActivity:
```
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
  if ((keyCode == KeyEvent.KEYCODE_BACK) && mWebView.canGoBack()) {
    mWebView.goBack();
    return true;
  }
  return super.onKeyDown(keyCode, event);
}
```
ThisonKeyDown(int, KeyEvent)callback method will be called anytime a button is pressed while in the Activity. The condition inside uses theKeyEventto check whether the key pressed is the BACK button and whether theWebViewis actually capable of navigating back (if it has a history). If both are true, then thegoBack()method is called, which will navigate back one step in theWebViewhistory.Returningtrueindicates that the event has been handled. If this condition is not met, then the event is sent back to the system.

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