update NFT standard and ExampleNFT implementation

cadence/contracts/example-assets/ExampleNFT.cdc

@@ -123,7 +123,7 @@ access(all) contract ExampleNFT: NonFungibleToken {
     access(all) resource Collection: NonFungibleToken.Collection {
         /// dictionary of NFT conforming tokens
         /// NFT is a resource type with an `UInt64` ID field
-        access(contract) var ownedNFTs: @{UInt64: ExampleNFT.NFT}
+        access(all) var ownedNFTs: @{UInt64: {NonFungibleToken.NFT}}
 
         access(all) var storagePath: StoragePath
         access(all) var publicPath: PublicPath
@@ -187,7 +187,7 @@ access(all) contract ExampleNFT: NonFungibleToken {
 
         /// Borrow the view resolver for the specified NFT ID
         access(all) view fun borrowViewResolver(id: UInt64): &{ViewResolver.Resolver}? {
-            if let nft = &self.ownedNFTs[id] as &ExampleNFT.NFT? {
+            if let nft = &self.ownedNFTs[id] as &{NonFungibleToken.NFT}? {
                 return nft as &{ViewResolver.Resolver}
             }
             return nil

cadence/contracts/standards/NonFungibleToken.cdc

@@ -36,7 +36,7 @@ Collection to complete the transfer.
 
 */
 
-import ViewResolver from "ViewResolver"
+import "ViewResolver"
 
 /// The main NFT contract. Other NFT contracts will
 /// import and implement the interfaces defined in this contract
@@ -63,7 +63,7 @@ access(all) contract interface NonFungibleToken: ViewResolver {
     /// and query the updated metadata from the owners' collections.
     ///
     access(all) event Updated(type: String, id: UInt64, uuid: UInt64, owner: Address?)
-    access(contract) view fun emitNFTUpdated(_ nftRef: auth(Update | Owner) &{NonFungibleToken.NFT})
+    access(all) view fun emitNFTUpdated(_ nftRef: auth(Update | Owner) &{NonFungibleToken.NFT})
     {
         emit Updated(type: nftRef.getType().identifier, id: nftRef.id, uuid: nftRef.uuid, owner: nftRef.owner?.address)
     }
@@ -85,9 +85,6 @@ access(all) contract interface NonFungibleToken: ViewResolver {
     ///
     access(all) event Deposited(type: String, id: UInt64, uuid: UInt64, to: Address?, collectionUUID: UInt64)
 
-    /// Included for backwards-compatibility
-    access(all) resource interface INFT: NFT {}
-
     /// Interface that the NFTs must conform to
     ///
     access(all) resource interface NFT: ViewResolver.Resolver {
@@ -105,6 +102,7 @@ access(all) contract interface NonFungibleToken: ViewResolver {
         access(all) fun createEmptyCollection(): @{Collection} {
             post {
                 result.getLength() == 0: "The created collection must be empty!"
+                result.isSupportedNFTType(type: self.getType()): "The created collection must support this NFT type"
             }
         }
 
@@ -174,6 +172,7 @@ access(all) contract interface NonFungibleToken: ViewResolver {
         access(all) fun deposit(token: @{NFT})
         access(all) view fun getLength(): Int
         access(all) view fun getIDs(): [UInt64]
+        access(all) fun forEachID(_ f: fun (UInt64): Bool): Void
         access(all) view fun borrowNFT(_ id: UInt64): &{NFT}?
     }
 
@@ -182,6 +181,10 @@ access(all) contract interface NonFungibleToken: ViewResolver {
     ///
     access(all) resource interface Collection: Provider, Receiver, CollectionPublic, ViewResolver.ResolverCollection {
 
+        /// Cadence allows implementing types to specify less restrictive access
+        /// so implementing contracts can have this as `access(all)` with no problem
+        access(contract) var ownedNFTs: @{UInt64: {NonFungibleToken.NFT}}
+
         /// deposit takes a NFT as an argument and stores it in the collection
         /// @param token: The NFT to deposit into the collection
         access(all) fun deposit(token: @{NonFungibleToken.NFT}) {
@@ -196,7 +199,16 @@ access(all) contract interface NonFungibleToken: ViewResolver {
 
         /// Gets the amount of NFTs stored in the collection
         /// @return An integer indicating the size of the collection
-        access(all) view fun getLength(): Int
+        access(all) view fun getLength(): Int {
+            return self.ownedNFTs.length
+        }
+
+        /// Allows a given function to iterate through the list
+        /// of owned NFT IDs in a collection without first
+        /// having to load the entire list into memory
+        access(all) fun forEachID(_ f: fun (UInt64): Bool): Void {
+            self.ownedNFTs.forEachKey(f)
+        }
 
         /// Borrows a reference to an NFT stored in the collection
         /// If the NFT with the specified ID is not in the collection,
@@ -231,4 +243,4 @@ access(all) contract interface NonFungibleToken: ViewResolver {
             result.getIDs().length == 0: "The created collection must be empty!"
         }
     }
-}
+}