iOS : To get selected photos in sequence with PHImageManager

Question

我正在使用 PHImageManager 来选择多个图像并这样写。问题是它是 block 并且顺序不正确(当用户选择照片 1、2、3 时，它可能返回 3、1、2)。我应该怎么写才能井井有条？

PHImageManager *manager = [PHImageManager defaultManager];

[manager requestImageForAsset:asset targetSize:PHImageManagerMaximumSize
                  contentMode:PHImageContentModeDefault
                      options:self.requestOptions
                resultHandler:^(UIImage *image, NSDictionary *info){

                    if (self.isToSelectSingleImage) {
                    self.mediaCollection = [NSMutableArray array];
                    self.thumbnailCollection = [NSMutableArray array];
                    }
                    [self addImageToMediaContainerWithImage:image.normalizedImage];
                }];

Answer 1

您可以创建一个下载助手类，我们将其命名为AssetsDownloader。基本思想是获取一组 Assets 并使用递归函数下载它们。

数组已经是一个有序的集合，所以我们从下载第一个 Assets 开始。下载 Assets 后，我们将获取数组中的下一项并开始下载该项目。

AssetsDownloader.h

@import Foundation;
@import Photos;

@interface AssetsDownloader : NSObject

- (void)downloadAssets:(NSArray<PHAsset *> *)assets;

@end

AssetsDownloader.m

#import "AssetsDownloader.h"

@implementation AssetsDownloader {
    NSMutableArray<PHAsset *> *_assets; 
}

- (void)downloadAssets:(NSArray<PHAsset *> *)assets {
    _assets = [[NSMutableArray alloc] initWithArray: assets];
    PHAsset *firstAsset = assets.firstObject;
    [self downloadAsset:firstAsset];
}

- (void)downloadAsset:(PHAsset *)asset {
    [[PHImageManager defaultManager] requestImageForAsset:asset targetSize:PHImageManagerMaximumSize
                                              contentMode:PHImageContentModeDefault
                                                  options:nil
                                            resultHandler:^(UIImage *image, NSDictionary *info){

                                                // Do what you have to with the asset

                                                // Remove first object from your assets
                                                [_assets removeObjectAtIndex:0];
                                                // Get the next asset from your assets
                                                PHAsset *nextAsset = _assets.firstObject;
                                                // Check if it exists
                                                if(nextAsset) {
                                                    // Use the same function to dowloand the asset 
                                                    [self downloadAsset:asset];
                                                } else {
                                                    // No more assets to download
                                                }
                                            }];
}

@end