Merge pull request #58 from MetaMask/fix/ignore_unnessessary_blob_download_requests

fix: Download file requested when connecting to any Dapp.
Previous JS injection was catching a Blob file creation and triggered downloading immediately. Turned out some Dapps may create Blob files without user actually trying to download these files. To prevent the issue, we try to download Blobs only on clicks on download elements in the website or when Android receives a download callback.

To test Blob file downloading I use two websites:
https://eligrey.com/demos/FileSaver.js/
https://tyschenko.github.io/download_blob_file.html

New solution was also tested on Uniswap.org and pancakeswap.finance Dapps.

Author: Tyschenko

android/src/main/java/com/reactnativecommunity/webview/RNCWebViewManagerImpl.kt

@@ -123,7 +123,7 @@ class RNCWebViewManagerImpl(private val newArch: Boolean = false) {
 			      	  return@DownloadListener
 			      }
 			      if (url.startsWith("blob:")) {
-                      // Handled in RNCWebView.injectBlobFileDownloaderScript()
+                webView.evaluateJavascriptWithFallback(BlobFileDownloader.getDownloadBlobInterceptor(url))
 			      	  return@DownloadListener
 			      }
             webView.setIgnoreErrFailedForThisURL(url)

android/src/main/java/com/reactnativecommunity/webview/extension/file/BlobFileDownloader.kt

@@ -28,21 +28,55 @@ internal class BlobFileDownloader(
 	companion object {
 		const val JS_INTERFACE_TAG: String = "BlobFileDownloader"
 
-    fun getBlobFileInterceptor(): String =
-      """
-        (function() {
-          const originalCreateObjectURL = URL.createObjectURL;
-          URL.createObjectURL = function(blob) {
-            const url = originalCreateObjectURL.call(URL, blob);
-            const reader = new FileReader();
-            reader.readAsDataURL(blob);
-            reader.onloadend = function() {
-              const base64 = reader.result;
-              ${JS_INTERFACE_TAG}.getBase64FromBlobData(base64);
-            };
-            return url;
-          };
+    /**
+     * Invokes JS method downloadBlob from [getBlobFileInterceptor]
+     */
+    fun getDownloadBlobInterceptor(url: String): String = "downloadBlob('$url');"
+
+    /**
+     * This script handles Blob downloading in two ways:
+     * 1) It intercepts clicks on elements with a Blob: href.
+     * 2) It defines a method, downloadBlob, which is manually invoked from Android [com.reactnativecommunity.webview.RNCWebViewManagerImpl] in cases where the href is undefined.
+     */
+    fun getBlobFileInterceptor(): String = """
+       (function() {
+            if (window.blobDownloadInjected) return;
+            window.blobDownloadInjected = true;
+            
+            function blobToBase64(blob, callback) {
+                const reader = new FileReader();
+                reader.onloadend = function() {
+                    const base64 = reader.result;
+                    callback(base64);
+                };
+                reader.readAsDataURL(blob);
+            }
+            
+            function downloadBlobUrl(url) {
+                const xhr = new XMLHttpRequest();
+                xhr.open('GET', url, true);
+                xhr.responseType = 'blob';
+                xhr.onload = function() {
+                    if (this.status === 200) {
+                        const blob = this.response;
+                        blobToBase64(blob, function(base64) {
+                            ${JS_INTERFACE_TAG}.getBase64FromBlobData(base64);
+                        });
+                    }
+                };
+                xhr.send();
+            }
+            
+            document.addEventListener('click', function(e) {
+                const target = e.target;
+                if (target.tagName === 'A' && target.href && target.href.startsWith('blob:')) {
+                    e.preventDefault();
+                    downloadBlobUrl(target.href);
+                }
+            }, true);
+            
+            window.downloadBlob = downloadBlobUrl;
         })();
-    """.trimIndent()
+      """.trimIndent()
 	}
 }