Recognise and run BC bundles

Reviewed By: javache

Differential Revision: D4067425

fbshipit-source-id: fade9adebfa8a59dc49aeadfd01a782f7b686082

React/Base/RCTJavaScriptLoader.h

@@ -9,7 +9,40 @@
 
 #import <UIKit/UIKit.h>
 
-extern uint32_t const RCTRAMBundleMagicNumber;
+#import "RCTDefines.h"
+
+/**
+ * RCTScriptTag
+ *
+ * Scripts given to the JS Executors to run could be in any of the following
+ * formats. They are tagged so the executor knows how to run them.
+ */
+typedef NS_ENUM(NSInteger) {
+  RCTScriptString = 0,
+  RCTScriptRAMBundle,
+  RCTScriptBCBundle,
+} RCTScriptTag;
+
+/**
+ * RCTMagicNumber
+ *
+ * RAM bundles and BC bundles begin with magic numbers. For RAM bundles this is
+ * 4 bytes, for BC bundles this is 8 bytes. This structure holds the first 8
+ * bytes from a bundle in a way that gives access to that information.
+ */
+typedef union {
+  uint64_t allBytes;
+  uint32_t first4;
+  uint64_t first8;
+} RCTMagicNumber;
+
+/**
+ * RCTParseMagicNumber
+ *
+ * Takes the first 8 bytes of a bundle, and returns a tag describing the
+ * bundle's format.
+ */
+RCT_EXTERN RCTScriptTag RCTParseMagicNumber(RCTMagicNumber magic);
 
 extern NSString *const RCTJavaScriptLoaderErrorDomain;
 
@@ -42,7 +75,7 @@ typedef void (^RCTSourceLoadBlock)(NSError *error, NSData *source, int64_t sourc
  * @experimental
  * Attempts to synchronously load the script at the given URL. The following two conditions must be met:
  *   1. It must be a file URL.
- *   2. It must point to a RAM bundle.
+ *   2. It must not point to a text/javascript file.
  * If the URL does not meet those conditions, this method will return nil and supply an error with the domain
  * RCTJavaScriptLoaderErrorDomain and the code RCTJavaScriptLoaderErrorCannotBeLoadedSynchronously.
  */