在用移动设备加载H5页面时，一些公共资源如css，js，图片等如果比较大时，就需要通过拦截网络，改从本地资源加载。在Android原生WebView中，我们可以在WebViewClient中的shouldInterceptRequest方法来拦截替换资源。
然而在Flutter中的WebView插件，不管是官方的webview_flutter,还是flutter_webview_plugin都不支持加载本地资源。
庆幸的是webview_flutter底层实现是基于WebView(Android)和WKWebView(iOS)。只要在官方webview_flutter上稍作修改，就可以实现离线资源加载。
项目地址
github: github.com/iamyours/we…
pub: iwebview_flutter
Android端实现
首先我们从webview_flutter中下载最新Archive(当前使用0.3.15+1)。
解压后，使用AndroidStudio打开，右键工程目录，使用Android模式打开
如果要实现WebView请求拦截，就必须给webView设置WebViewCilent，全局搜索setWebViewClient找到只有一处实现:
//FlutterWebView.java
private void applySettings(Map<String, Object> settings) {
for (String key : settings.keySet()) {
switch (key) {
...
case "hasNavigationDelegate":
final boolean hasNavigationDelegate = (boolean) settings.get(key);
final WebViewClient webViewClient =
flutterWebViewClient.createWebViewClient(hasNavigationDelegate);
webView.setWebViewClient(webViewClient);
break;
...
}
}
}
修改WebViewClient
通过以上代码我们知道具体逻辑在createWebViewClient方法中：
WebViewClient createWebViewClient(boolean hasNavigationDelegate) {
this.hasNavigationDelegate = hasNavigationDelegate;
if (!hasNavigationDelegate || android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
return internalCreateWebViewClient();
}
return internalCreateWebViewClientCompat();
}
然后在internalCreateWebViewClient和internalCreateWebViewClientCompat添加shouldInterceptRequest方法，然后参照onPageFinished在FlutterWebViewClient加入shouldInterceptRequest方法：
private WebViewClient internalCreateWebViewClient() {
return new WebViewClient() {
...
@Override
public void onPageFinished(WebView view, String url) {
FlutterWebViewClient.this.onPageFinished(view, url);
}
...//参照
@Override
public WebResourceResponse shouldInterceptRequest(WebView view, String url) {
WebResourceResponse response = FlutterWebViewClient.this.shouldInterceptRequest(view, url);
if (response != null) return response;
return super.shouldInterceptRequest(view, url);
}
};
}
异步变同步，MethodChannel接收Flutter层数据
我们在shouldInterceptRequest中接收来自Flutter世界的数据，如assets中的二进制数据。但是要注意的是通过MethodChannel接收数据是通过异步回调的形式，但是shouldInterceptRequest方法需要同步接收数据，因此需要一个异步变同步的执行器，同时MethodChannel调用必须在主线呈调用。方法有很多，我这里通过CountDownLatch实现。
public class SyncExecutor {
private final CountDownLatch countDownLatch = new CountDownLatch(1);
Handler mainHandler = new Handler(Looper.getMainLooper());
WebResourceResponse res = null;
public WebResourceResponse getResponse(final MethodChannel methodChannel, final String url) {
res = null;
mainHandler.post(new Runnable() {
@Override
public void run() {
methodChannel.invokeMethod("shouldInterceptRequest", url, new MethodChannel.Result() {
@Override
public void success(Object o) {
if (o instanceof Map) {
Map<String, Object> map = (Map<String, Object>) o;
byte[] bytes = (byte[]) map.get("data");
String type = (String) map.get("mineType");
String encode = (String) map.get("encoding");
res = new WebResourceResponse(type, encode, new ByteArrayInputStream(bytes));
}
countDownLatch.countDown();
}
@Override
public void error(String s, String s1, Object o) {
res = null;
countDownLatch.countDown();
}
@Override
public void notImplemented() {
res = null;
countDownLatch.countDown();
}
});
}
});
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
return res;
}
}
注意到这里success中接收到Map数据，我们会在接下来Flutter层传过来。
Flutter层传递数据
在webview_method_channel.dart中我们找到了onPageFinished接收来自Android或iOS的调用。参照onPageFinished方法，我们加入shouldInterceptRequest方法，同样的在_platformCallbacksHandler对应的类中加入shouldInterceptRequest方法。依次向上层类推。
Future<dynamic> _onMethodCall(MethodCall call) async {
switch (call.method) {
case 'onPageFinished':
_platformCallbacksHandler.onPageFinished(call.arguments['url']);
return null;
case 'shouldInterceptRequest':
String url = call.arguments;
var response = await _platformCallbacksHandler.shouldInterceptRequest(url);
if (response != null) {
return {"data": response.data, "mineType": response.mineType, "encoding": response.encoding};
}
return null;
}
//webview_method_channel.dart
abstract class WebViewPlatformCallbacksHandler {
...
void onPageFinished(String url);
/// iamyours:Invoked by [WebViewPlatformController] when a request url intercepted.
Future<Response> shouldInterceptRequest(String url);
}
//webview_method_channel.dart
class Response {
final String mineType;
final String encoding;
final Uint8List data;
Response(this.mineType, this.encoding, this.data);
}
typedef void PageFinishedCallback(String url);
/// iamyours Signature for when a [WebView] interceptRequest .
typedef Future<Response> ShouldInterceptRequestCallback(String url);
class WebView extends StatefulWidget {
...
const WebView({
...
this.onPageFinished,
this.shouldInterceptRequest,
...,
})
class _WebViewState extends State<WebView> {
...
@override
void onPageFinished(String url) {
if (_widget.onPageFinished != null) {
_widget.onPageFinished(url);
}
}
...
@override
Future<Response> shouldInterceptRequest(String url) async{
if (_widget.shouldInterceptRequest != null) {
return _widget.shouldInterceptRequest(url);
}
return null;
}
}
然后我们在example中实现一个简单的logo替换效果
WebView(
initialUrl: "https://wap.sogou.com/",
javascriptMode: JavascriptMode.unrestricted,
debuggingEnabled: true,
onProgressChanged: (int p){
setState(() {
progress = p/100.0;
});
},
backgroundColor: Colors.red,
shouldInterceptRequest: (String url) async {//替换搜狗搜索logo为baidu
var googleLogo = "https://wap.sogou.com/resource/static/index/images/logo_new.6f31942.png";
print("============url:$url");
if (url == googleLogo) {
ByteData data = await rootBundle.load("assets/baidu.png");
Uint8List bytes = Uint8List.view(data.buffer);
return Response("image/png", null, bytes);
}
return null;
},
),
最终效果
iOS端实现
webview_flutteriOS端是基于WKWebview实现的，拦截请求通过NSURLProtocol实现,可以参照iOS WKWebView (NSURLProtocol)拦截js、css，图片资源一文。此法拦截是全局拦截的，所以需要一个全局变量存储所有的FlutterMethodChannel,这里定义一个单例存储这些数据。
//FlutterInstance.h
#import <Foundation/Foundation.h>
#import <Flutter/Flutter.h>
NS_ASSUME_NONNULL_BEGIN
@interface FlutterInstance : NSObject
@property(nonatomic,retain)NSMutableDictionary *channels;
+(FlutterInstance*)get;
+(FlutterMethodChannel*)channelWithAgent:(NSString*)agent;
+(void)removeChannel:(int64_t)viewId;
@end
NS_ASSUME_NONNULL_END
这里为了区分对应的请求是在哪个channel下的，我们在给相应的WKWebview的agent最后加入#_viewId
//
// FlutterInstance.m
#import "FlutterInstance.h"
@implementation FlutterInstance
static FlutterInstance *instance = nil;
+(FlutterInstance *)get
{
@synchronized(self)
{
if(instance==nil)
{
instance= [FlutterInstance new];
instance.channels = [NSMutableDictionary dictionary];
}
}
return instance;
}
+(FlutterMethodChannel*)channelWithAgent:(NSString*)agent{
NSRange range = [agent rangeOfString:@"#" options:NSBackwardsSearch];
NSLog(@"range:%d,%d",range.length,range.location);
NSString *key = [agent substringFromIndex:range.location+1];
NSDictionary *channels = [self get].channels;
FlutterMethodChannel *channel = (FlutterMethodChannel*)[channels objectForKey:key];
return channel;
}
+(void)removeChannel:(int64_t)viewId{
NSMutableDictionary *channels = [self get].channels;
NSString *key = [NSString stringWithFormat:@"%lld",viewId];
[channels removeObjectForKey:key];
}
@end
userAgent区分MethodChannel
我们在WKWebview的loadUrl中修改userAgent区分各个WebView对应的viewId。
//FlutterWebView.m
- (bool)loadUrl:(NSString*)url withHeaders:(NSDictionary<NSString*, NSString*>*)headers {
NSURL* nsUrl = [NSURL URLWithString:url];
if (!nsUrl) {
return false;
}
NSMutableURLRequest* request = [NSMutableURLRequest requestWithURL:nsUrl];
NSString *vid = [NSString stringWithFormat:@"%lld",_viewId];
[_webView evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id result, NSError *error) {
NSString *fixAgent = [NSString stringWithFormat:@"%@#%d",result,_viewId];
[_webView setCustomUserAgent:fixAgent];
}];
[_webView loadRequest:request];
return true;
}
NSURLProtocol实现请求拦截
然后在NSURLProtocol协议中的startLoading方法实现请求拦截
// FlutterNSURLProtocol.h
#import <Foundation/Foundation.h>
#import <Flutter/Flutter.h>
NS_ASSUME_NONNULL_BEGIN
@interface FlutterNSURLProtocol : NSURLProtocol
@end
NS_ASSUME_NONNULL_END
// FlutterNSURLProtocol.m
- (void)startLoading
{
NSMutableURLRequest *mutableReqeust = [[self request] mutableCopy];
//给我们处理过的请求设置一个标识符, 防止无限循环,
[NSURLProtocol setProperty:@YES forKey:KFlutterNSURLProtocolKey inRequest:mutableReqeust];
NSString *agent = [mutableReqeust valueForHTTPHeaderField:@"User-Agent"];
FlutterMethodChannel *channel = [FlutterInstance channelWithAgent:agent];
if(channel==nil){
NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:nil];
self.task = [session dataTaskWithRequest:self.request];
[self.task resume];
return;
}
[channel invokeMethod:@"shouldInterceptRequest" arguments:url result:^(id _Nullable result) {
if(result!=nil){
NSDictionary *dic = (NSDictionary *)result;
FlutterStandardTypedData *fData = (FlutterStandardTypedData *)[dic valueForKey:@"data"];
NSString *mineType = dic[@"mineType"];
NSString *encoding = dic[@"encoding"];
if([encoding isEqual:[NSNull null]])encoding = nil;
NSData *data = [fData data];
NSURLResponse* response = [[NSURLResponse alloc] initWithURL:self.request.URL MIMEType:mineType expectedContentLength:data.length textEncodingName:encoding];
[self.client URLProtocol:self didReceiveResponse:response cacheStoragePolicy:NSURLCacheStorageAllowed];
[self.client URLProtocol:self didLoadData:data];
[self.client URLProtocolDidFinishLoading:self];
}else{
NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:nil];
self.task = [session dataTaskWithRequest:self.request];
[self.task resume];
}
}];
}
通过之前设置的userAgent获取相应的FlutterMethodChannel,调用shouldInterceptRequest方法获取Flutter数据，通过Xcode调试，我们知道相应的byte数据类型为FlutterStandardTypedData。或者参照下图:
具体效果
实现google搜索logo替换
Webview黑夜模式Flutter端实践
因为之前做了玩Android客户端kotlin版，适配各个站点文章黑夜模式，需要替换相关css资源，因此才会想到改造webview_flutter插件，下面是flutter版本替换掘金文章css文件产生的黑夜模式效果
