如何使用URL检查ALAsset是否仍然存在

Question

我有一个包含ALAsset网址的数组(不是完整的ALAsset对象)因此,每次启动我的应用程序时,我都要检查我的数组,看看它是否仍然是最新的...

所以我试过了

NSData *assetData = [[NSData alloc] initWithContentsOfFile:@"assets-library://asset/asset.PNG?id=1000000001&ext=PNG"];

但是资产数据总是零

thx的帮助

Answer 1

使用assetForURL:resultBlock:failureBlock:ALAssetsLibrary的方法来代替从其URL获取资产.

// Create assets library
ALAssetsLibrary *library = [[[ALAssetsLibrary alloc] init] autorelease];

// Try to load asset at mediaURL
[library assetForURL:mediaURL resultBlock:^(ALAsset *asset) {
    // If asset exists
    if (asset) {
        // Type your code here for successful
    } else {
        // Type your code here for not existing asset
    }
} failureBlock:^(NSError *error) {
    // Type your code here for failure (when user doesn't allow location in your app)
}];

Answer 2

拥有资源路径,您可以使用此功能检查图像是否存在:

-(BOOL) imageExistAtPath:(NSString *)assetsPath
{  
    __block BOOL imageExist = NO; 
    ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
    [library assetForURL:[NSURL URLWithString:assetsPath] resultBlock:^(ALAsset *asset) {
        if (asset) {
            imageExist = YES;
            }
    } failureBlock:^(NSError *error) {
        NSLog(@"Error %@", error);
    }];
    return imageExist;
}

请记住,检查图像是否存在是检查异步.如果你想等到新线程在主线程中完成他的生命调用函数"imageExistAtPath":

dispatch_async(dispatch_get_main_queue(), ^{
    [self imageExistAtPath:assetPath];
});

或者你可以使用信号量,但这不是一个很好的解决方案:

-(BOOL) imageExistAtPath:(NSString *)assetsPath
{  
    __block BOOL imageExist = YES; 
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);   
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);   
    dispatch_async(queue, ^{  
        ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
        [library assetForURL:[NSURL URLWithString:assetsPath] resultBlock:^(ALAsset *asset) {
            if (asset) {
                dispatch_semaphore_signal(semaphore); 
            } else {
                imageExist = NO;
                dispatch_semaphore_signal(semaphore); 
                }
        } failureBlock:^(NSError *error) {
             NSLog(@"Error %@", error);
        }];
    });
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER); 
    return imageExist;
}

Answer 3

对于 iOS 8 或更高版本，有一个同步方法来检查一个是否ALAsset存在。

@import Photos;

if ([PHAsset fetchAssetsWithALAssetURLs:@[assetURL] options:nil].count) {
    // exist
}

迅速：

import Photos

if PHAsset.fetchAssetsWithALAssetURLs([assetURL], options: nil).count > 0 {
   // exist
}

斯威夫特 3：

import Photos

if PHAsset.fetchAssets(withALAssetURLs: [assetURL], options: nil).count > 0 {
   // exist
}