Fragment中加载网页（WebView）监听Back键 ，实现返回上一页的功能

首先创建一个抽象类BackHandledFragment，该类有一个抽象方法onBackPressed()，所有BackHandledFragment的子类在onBackPressed方法中处理各自对Back事件的消费逻辑。onBackPressed返回布尔值，宿主Activity将会根据该方法的返回值判断子Fragment是否有消费Back事件。此外，宿主FragmentActivity还会保持一份当前Fragment的引用，当用户按下Back键时，宿主Activity会判断当前Fragment是否需要消费该事件，如果没有Fragment消费才会自己消费。

public abstract class BackHandledFragment extends Fragment{

protected BackHandledInterface mBackHandledInterface;

/**
* 所有继承BackHandledFragment的子类都将在这个方法中实现物理Back键按下后的逻辑
* FragmentActivity捕捉到物理返回键点击事件后会首先询问Fragment是否消费该事件
* 如果没有Fragment消息时FragmentActivity自己才会消费该事件
*/
public abstract boolean onBackPressed();

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if(!(getActivity() instanceof BackHandledInterface)){
throw new ClassCastException("Hosting Activity must implement BackHandledInterface");
}else{
this.mBackHandledInterface = (BackHandledInterface)getActivity();
}
}

@Override
public void onStart() {
super.onStart();
//告诉FragmentActivity，当前Fragment在栈顶
mBackHandledInterface.setSelectedFragment(this);
}
}

宿主FragmentActivity需要实现BackHandledIntegerface，子Fragment会通过该接口告诉宿主FragmentActivity自己是当前屏幕可见的Fragment。

public interface BackHandledInterface {
void setSelectedFragment(BackHandledFragment selectedFragment);
}

所以在Fragment的onCreate中会判断宿主FragmentActivity是否已继承了该接口。在Fragment的onStart()方法中就会调用该接口告诉宿主FragmentActivity自己是当前屏幕可见的Fragment。

宿主FragmentActivity就可以在onBackPressed()方法中对Back事件进行判断处理了。

public class MainActivity extends FragmentActivity implements BackHandledInterface{

private BackHandledFragment mBackHandedFragment;
private boolean hadIntercept;

@Override
public void setSelectedFragment(BackHandledFragment selectedFragment) {
this.mBackHandedFragment = selectedFragment;
}

@Override
public void onBackPressed() {
if(mBackHandedFragment == null || !mBackHandedFragment.onBackPressed()){
if(getSupportFragmentManager().getBackStackEntryCount() == 0){
super.onBackPressed(); //退出
}else{
getSupportFragmentManager().popBackStack(); //fragment 出栈
}
}
}
}

Fragment需要继承抽象类BackHandledFragment，并覆写抽象方法onBackPressed()，在抽象方法中判断WebView是否可以返回到之前打开的网页

public class WorkFragment extends BackHandledFragment {

private WebView webView;
private WebSettings webSettings;

private String url = "https://www.baidu.com/";
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

View view = inflater.inflate(R.layout.work_fragment,container,false);
webView = (WebView)view.findViewById(R.id.workWebView);

webSettings = webView.getSettings();
webSettings.setJavaScriptEnabled(true);
webSettings.setUseWideViewPort(true);
webSettings.setLoadWithOverviewMode(true);

webView.setWebViewClient(new MyWebViewClient());
webView.loadUrl(url);

return view;
}

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

@Override
public void onReceivedError(WebView view, int errorCode,
String description, String failingUrl) {
//     super.onReceivedError(view, errorCode, description, failingUrl);
//  Toast.makeText(this,"网页加载错误！",0).show();
}
}
@Override
public  boolean onBackPressed(){

if(webView.canGoBack()){
webView.goBack();
Log.v("webView.goBack()", "webView.goBack()");
return true;

}else{
Log.v("Conversatio退出","Conversatio退出");
return false;
}

}

}