private?void?init()?{
		//?TODO?Auto-generated?method?stub
		webview=(WebView)?findViewById(R.id.WebView);
		webview.loadUrl("http://www.baidu.com");??
//		webview.loadUrl("file:///android_assets/xxx.xml");
		//覆蓋WEBVIE默認通過第三方軟件或系統自帶瀏覽器打開網頁
		webview.setWebViewClient(new?WebViewClient(){
		?@Override
		public?boolean?shouldOverrideUrlLoading(WebView?view,?String?url)?{
			//?TODO?Auto-generated?method?stub
			?webview.loadUrl(url);
			return?true;
		}
			?
		});
		WebSettings?settings=webview.getSettings();
		settings.setJavaScriptEnabled(true);
		//webview?加載優先使用緩存加載
		settings.setCacheMode(WebSettings.LOAD_CACHE_ELSE_NETWORK);
		webview.setWebChromeClient(new?WebChromeClient(){
			@Override
			public?void?onProgressChanged(WebView?view,?int?newProgress)?{
				//?TODO?Auto-generated?method?stub
				if(newProgress==100){
					closeDialog();
				}else{
					openDialog(newProgress);
				}
			}
		});
	}
	protected?void?openDialog(int?newProgress)?{
		//?TODO?Auto-generated?method?stub
		if(progressDialog==null){
			progressDialog=new?ProgressDialog(MainActivity.this);
			progressDialog.setTitle("正在加載");
			progressDialog.setProgressStyle(progressDialog.STYLE_HORIZONTAL);
			progressDialog.setProgress(newProgress);
			progressDialog.show();
		}else{
			progressDialog.setProgress(newProgress);
		}
	}
	protected?void?closeDialog()?{
		//?TODO?Auto-generated?method?stub
		if(progressDialog!=null&&progressDialog.isShowing()){
			progressDialog.dismiss();
			progressDialog=null;
		}
	}
	@Override
	public?boolean?onKeyDown(int?keyCode,?KeyEvent?event)?{
		//?TODO?Auto-generated?method?stub
		if(keyCode==KeyEvent.KEYCODE_BACK){
			if(webview.canGoBack()){
//				Toast.makeText(this,?webview.getUrl(),?Toast.LENGTH_SHORT).show();
				webview.goBack();
				return?true;
			}else{
				System.exit(0);
			}
		}
		return?super.onKeyDown(keyCode,?event);
	}

代碼是按照視頻里面寫的，但是有時返回不了。不知道怎么了，求大神指點。