objective c - Enforcing order with ALAssetsLibrary enumeration blocks -


i’m doing iterations using apple’s alassetslibrary framework. need iteration finish before start playing videos.

here reducted sample code shows problem i’m having. 

@property (nonatomic) nsmutablearray *assetstoplay;  - (void)viewdidload {     alassetslibrary *library = [[alassetslibrary alloc] init];      [library enumerategroupswithtypes:alassetsgroupall usingblock:^(alassetsgroup *group, bool *stop) {         [group enumerateassetsusingblock:^(alasset *result, nsuinteger index, bool *stop) {             avasset *asset = <#get asset#>             [self.assetstoplay addobject:asset];         }];     } failureblock:nil];      [self playem]; // bug: code invoked immediately. run after above block iteration finished. }  - (void)playem {     for(avasset *asset in self.assetstoplay) {         nslog(@"%@", asset);     } }

to elad's point, enumeration method runs asynchronously, , group nil when done.

so, asynchronous situations, should move call playem inside completion block rather after it. want call playem when group nil (i.e. you're done enumerating):

[library enumerategroupswithtypes:alassetsgroupall usingblock:^(alassetsgroup *group, bool *stop) {     if (group) {         // if `group` non-`nil`, still enumerating groups          [group enumerateassetsusingblock:^(alasset *result, nsuinteger index, bool *stop) {             if (result) {                  // note, if `result` not `nil` (`nil` signifies end of asset enumeration)                  avasset *asset = <#get asset#>                 [self.assetstoplay addobject:asset];             }         }];     } else {         // if `group` `nil`, done enumeration, call method          [self playem]; // called when enumeration done.     } } failureblock:^(nserror *error) {     // handle user's rejection of permission here }];

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