手机 简易浏览器 WebView的基本使用 返回 缓存 进度条

public class MainActivity extends AppCompatActivity {
    private WebView webView;
    private String url = "http://2014.qq.com/";
    private ProgressDialog dialog;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        init();
    }

    private void init() {

        webView = (WebView)findViewById(R.id.webview);
        //加载网页地址
        webView.loadUrl(url);
        //重写shouldOverrideUrlLoading方法使网页可以在应用中显示 而不是系统浏览器
        webView.setWebViewClient(new WebViewClient() {
            @Override
            public boolean shouldOverrideUrlLoading(WebView view, String url) {
//                return super.shouldOverrideUrlLoading(view, url);
                view.loadUrl(url);
                return true;
            }
        });

        //启用支持JavaScript
        WebSettings settings = webView.getSettings();
        settings.setJavaScriptEnabled(true);
        //WebView加载页面优先使用缓存
        settings.setCacheMode(WebSettings.LOAD_CACHE_ELSE_NETWORK);

        webView.setWebChromeClient(new WebChromeClient() {
            @Override
            public void onProgressChanged(WebView view, int newProgress) {
                if (newProgress == 100) {
                    //加载完毕 关闭ProgressDialog
                    closeDialog();
                } else {
                    //正在加载 打开ProgressDialog
                    openDialog(newProgress);
                }
            }
        });

    }

    private void openDialog(int newProgress) {
        if(dialog == null){
            dialog = new ProgressDialog(MainActivity.this);
            dialog.setTitle("正在拼命加载...");                         //设置标题
            dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);   //设置样式
            dialog.setProgress(newProgress);                            //更新内容
            dialog.show();                                              //显示出来
        }
        else{
            dialog.setProgress(newProgress);
        }
    }

    private void closeDialog() {
        if(dialog != null && dialog.isShowing()){
            dialog.dismiss();       //取消显示
            dialog = null;
        }
    }

    //改写手机物理按键 返回逻辑
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if(keyCode ==  KeyEvent.KEYCODE_BACK){

            if(webView.canGoBack()) {
                webView.goBack();   //返回上一页面
                return true;
            }
            else {
                System.exit(0);     //推出程序n b
            }
        }
        return super.onKeyDown(keyCode, event);

    }