從Samples中入門IOS開發（六）—— Photo和Album瀏覽

MyImagePicker展示了如何調用系統Photo和Album，並在自訂的UI中進行瀏覽，而這個也是很福士化的需求。

先看看這個例子的使用流程：

對於這個Sample，我主要關注兩個關鍵點：

• 調用系統Photo和Album
• UI組件的嵌套和組合

調用系統Photo和Album

IOS提供了類ALAssetsLibrary來取得Photo下面的資源（photo, album, video等），它的主要調用方式是遍曆和block回調：

NSUInteger groupTypes = ALAssetsGroupAlbum | ALAssetsGroupEvent | ALAssetsGroupFaces;[assetsLibrary enumerateGroupsWithTypes:groupTypes usingBlock:listGroupBlock failureBlock:failureBlock];

以上代碼就是要遍曆ALAssetsGroupAlbum | ALAssetsGroupEvent | ALAssetsGroupFaces這三種資源，並在遍曆過程中回調listGroupBlock和failureBlock這兩個block，下面看看這兩個block的實現：

ALAssetsLibraryGroupsEnumerationResultsBlock listGroupBlock = ^(ALAssetsGroup *group, BOOL *stop) {        if (group) {        [groups addObject:group];    } else {        [self.tableView performSelectorOnMainThread:@selector(reloadData) withObject:nil waitUntilDone:NO];    }};ALAssetsLibraryAccessFailureBlock failureBlock = ^(NSError *error) {    AssetsDataIsInaccessibleViewController *assetsDataInaccessibleViewController = [[AssetsDataIsInaccessibleViewController alloc] initWithNibName:@"AssetsDataIsInaccessibleViewController" bundle:nil];        NSString *errorMessage = nil;    switch ([error code]) {        case ALAssetsLibraryAccessUserDeniedError:        case ALAssetsLibraryAccessGloballyDeniedError:            errorMessage = @"The user has declined access to it.";            break;        default:            errorMessage = @"Reason unknown.";            break;    }        assetsDataInaccessibleViewController.explanation = errorMessage;    [self presentModalViewController:assetsDataInaccessibleViewController animated:NO];    [assetsDataInaccessibleViewController release];};

邏輯很簡單，就是在遍曆過程中把資源放入groups數組並重新整理tableview。

UI組件的嵌套和組合

本例子的UI由各種UI組件嵌套組合而成，而這也體現了IOS標準化和定製化有效結合。首先看看中間的page，此page是展示某個album下所有photos，每一行展示4個photo縮微圖，點擊某個縮微圖能調轉到下個page，本例子採用的是tableview嵌套自訂的tableViewCell，並在此cell裡嵌套四個自訂的ImageView。代碼如下所示：

// Customize the appearance of table view cells.- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {        static NSString *CellIdentifier = @"Cell";        AlbumContentsTableViewCell *cell = (AlbumContentsTableViewCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];    if (cell == nil) {        [[NSBundle mainBundle] loadNibNamed:@"AlbumContentsTableViewCell" owner:self options:nil];        cell = tmpCell;        tmpCell = nil;    }        cell.rowNumber = indexPath.row;    cell.selectionDelegate = self;        // Configure the cell...    NSUInteger firstPhotoInCell = indexPath.row * 4;    NSUInteger lastPhotoInCell  = firstPhotoInCell + 4;        if (assets.count <= firstPhotoInCell) {        NSLog(@"We are out of range, asking to start with photo %d but we only have %d", firstPhotoInCell, assets.count);        return nil;    }        NSUInteger currentPhotoIndex = 0;    NSUInteger lastPhotoIndex = MIN(lastPhotoInCell, assets.count);    for ( ; firstPhotoInCell + currentPhotoIndex < lastPhotoIndex ; currentPhotoIndex++) {                ALAsset *asset = [assets objectAtIndex:firstPhotoInCell + currentPhotoIndex];        CGImageRef thumbnailImageRef = [asset thumbnail];        UIImage *thumbnail = [UIImage imageWithCGImage:thumbnailImageRef];                switch (currentPhotoIndex) {            case 0:                [cell photo1].image = thumbnail;                break;            case 1:                [cell photo2].image = thumbnail;                break;            case 2:                [cell photo3].image = thumbnail;                break;            case 3:                [cell photo4].image = thumbnail;                break;            default:                break;        }    }        return cell;}

在最右邊的那個page中，展示某個photo，最外層是個UIScollView，此view再嵌套一個自訂的imageView，在自訂的imageview中實現多點觸控等功能。代碼如下所示：

- (void)viewDidLoad {        self.title = @"Photo";    UIScrollView *imageScrollView = (UIScrollView *)self.view;        [imageScrollView setBackgroundColor:[UIColor blackColor]];    [imageScrollView setDelegate:self];    [imageScrollView setBouncesZoom:YES];    ALAssetRepresentation *assetRepresentation = [asset defaultRepresentation];        UIImage *fullScreenImage = [UIImage imageWithCGImage:[assetRepresentation fullScreenImage] scale:[assetRepresentation scale] orientation:(UIImageOrientation)[assetRepresentation orientation]];    // add touch-sensitive image view to the scroll view    TapDetectingImageView *imageView = [[TapDetectingImageView alloc] initWithImage:fullScreenImage];    [imageView setDelegate:self];    [imageView setTag:ZOOM_VIEW_TAG];    [imageScrollView setContentSize:[imageView frame].size];    [imageScrollView addSubview:imageView];    [imageView release];        // calculate minimum scale to perfectly fit image width, and begin at that scale    float minimumScale = [imageScrollView frame].size.width  / [imageView frame].size.width;    [imageScrollView setMinimumZoomScale:minimumScale];        [imageScrollView zoomToRect:CGRectMake(0.0, 0.0, imageView.frame.size.width, imageView.frame.size.height) animated:NO];    }

所以從這個例子中能學習到如何靈活地嵌套和組合多種UI組件，並把事件偵聽delegate出來。