从系统相册选择多张图片

    

从系统相册选择多张图片             

        分类：             iPhone开发              2012-04-17 17:25     3920人阅读     评论(3)     收藏     举报    

相册 ios5 library 照片 api 手机

项目要用到图片上传功能，要求读取相册中的全部图片加以展示，由用户选择进行上传。从网上找了大家比较通用的方法，稍加修改，实现了需求。

参考代码:ELCImagePickerDemo

一、读取相册

// Load Albums into assetGroups
dispatch_async(dispatch_get_main_queue(), ^
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    // Group enumerator Block
    void (^assetGroupEnumerator)(ALAssetsGroup *, BOOL *) = ^(ALAssetsGroup *group, BOOL *stop)
    {
        if (group == nil)
        {
            return;
        }

        [self.assetGroups addObject:group];

        // Reload albums
        [self performSelectorOnMainThread:@selector(reloadTableView) withObject:nil waitUntilDone:YES];
    };

    // Group Enumerator Failure Block
    void (^assetGroupEnumberatorFailure)(NSError *) = ^(NSError *error) {

        UIAlertView * alert = [[UIAlertView alloc] initWithTitle:@"Error" message:[NSString stringWithFormat:@"Album Error: %@ - %@", [error localizedDescription], [error localizedRecoverySuggestion]] delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil];
        [alert show];
        [alert release];

        NSLog(@"A problem occured %@", [error description]);
    };

    // Enumerate Albums
    [library enumerateGroupsWithTypes:ALAssetsGroupAll
                           usingBlock:assetGroupEnumerator
                         failureBlock:assetGroupEnumberatorFailure];

    [pool release];
});

    // Load Albums into assetGroups
    dispatch_async(dispatch_get_main_queue(), ^
    {
        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        
        // Group enumerator Block
        void (^assetGroupEnumerator)(ALAssetsGroup *, BOOL *) = ^(ALAssetsGroup *group, BOOL *stop) 
        {
            if (group == nil) 
            {
                return;
            }
            
            [self.assetGroups addObject:group];

            // Reload albums
            [self performSelectorOnMainThread:@selector(reloadTableView) withObject:nil waitUntilDone:YES];
        };
        
        // Group Enumerator Failure Block
        void (^assetGroupEnumberatorFailure)(NSError *) = ^(NSError *error) {
            
            UIAlertView * alert = [[UIAlertView alloc] initWithTitle:@"Error" message:[NSString stringWithFormat:@"Album Error: %@ - %@", [error localizedDescription], [error localizedRecoverySuggestion]] delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil];
            [alert show];
            [alert release];
            
            NSLog(@"A problem occured %@", [error description]);	                                 
        };	
                
        // Enumerate Albums
        [library enumerateGroupsWithTypes:ALAssetsGroupAll
                               usingBlock:assetGroupEnumerator 
                             failureBlock:assetGroupEnumberatorFailure];
        
        [pool release];
    });

上面的代码就是用于读取相册内容的。几点说明：

1. 因为图片中包含位置信息，因此访问图片资源时，系统会弹出询问框“是否允许访问位置信息”？如果选择否，就会进入这一块assetGroupEnumberatorFailure，提示读取图片失败。据说ios5.0以后系统才增加了这个提示，之前是没有这种情况的。

2. ios5.0以后用户可以自己新建相簿，因此这个ALAssetsGroupAll这个值是表示读出所有相簿，你可以把每个相簿列出来，再进一步选择展示哪个相簿中的图片。开源例子中就是这个效果。不过因为我的需求是只展示系统默认的“相机胶卷”这个相簿中的图片，因此，把这里改为ALAssetsGroupSavedPhotos这个值，就可以直接读取相机胶卷中的照片，不需要进一步选择了。

二、读取相册中的图片

ELCAssetTablePicker.m中的preparePhotos函数

-(void)preparePhotos {

    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];


    NSLog(@"enumerating photos");
    [self.assetGroup enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop)
     {
         if(result == nil)
         {
             return;
         }

         ELCAsset *elcAsset = [[[ELCAsset alloc] initWithAsset:result] autorelease];
         [elcAsset setParent:self];
         [self.elcAssets addObject:elcAsset];
     }];
    NSLog(@"done enumerating photos");

    [self.tableView reloadData];
    [self.navigationItem setTitle:@"Pick Photos"];

    [pool release];

}

-(void)preparePhotos {
    
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

	
    NSLog(@"enumerating photos");
    [self.assetGroup enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) 
     {         
         if(result == nil) 
         {
             return;
         }
         
         ELCAsset *elcAsset = [[[ELCAsset alloc] initWithAsset:result] autorelease];
         [elcAsset setParent:self];
         [self.elcAssets addObject:elcAsset];
     }];    
    NSLog(@"done enumerating photos");
	
	[self.tableView reloadData];
	[self.navigationItem setTitle:@"Pick Photos"];
    
    [pool release];

}

在次之前，可以对assetGroup设置过滤，如下设置为只读取图片。因为其实用手机拍摄的视频也会存入“相机胶卷”中，所以可以根据自己的需要进行过滤。

[picker.assetGroup setAssetsFilter:[ALAssetsFilter allPhotos]];

每张图片对应的就是一个ALAsset对象。

三、读取出的图片信息

[self.asset thumbnail]

   [self.asset thumbnail]

ALAsset的thumbnail是图片的缩略图，如果你需要以九宫格的形式把图片都展示出来，肯定要展示的是缩略图，而不能是巨大的原图。

-(void)selectedAssets:(NSArray*)_assets {

    NSMutableArray *returnArray = [[[NSMutableArray alloc] init] autorelease];

    for(ALAsset *asset in _assets) {

        NSMutableDictionary *workingDictionary = [[NSMutableDictionary alloc] init];
        [workingDictionary setObject:[asset valueForProperty:ALAssetPropertyType] forKey:@"UIImagePickerControllerMediaType"];
        [workingDictionary setObject:[UIImage imageWithCGImage:[[asset defaultRepresentation] fullScreenImage]] forKey:@"UIImagePickerControllerOriginalImage"];
        [workingDictionary setObject:[[asset valueForProperty:ALAssetPropertyURLs] valueForKey:[[[asset valueForProperty:ALAssetPropertyURLs] allKeys] objectAtIndex:0]] forKey:@"UIImagePickerControllerReferenceURL"];

        [returnArray addObject:workingDictionary];

        [workingDictionary release];
    }

    [self popToRootViewControllerAnimated:NO];
    [[self parentViewController] dismissModalViewControllerAnimated:YES];

    if([delegate respondsToSelector:@selector(elcImagePickerController:didFinishPickingMediaWithInfo:)]) {
        [delegate performSelector:@selector(elcImagePickerController:didFinishPickingMediaWithInfo:) withObject:self withObject:[NSArray arrayWithArray:returnArray]];
    }
}

-(void)selectedAssets:(NSArray*)_assets {

	NSMutableArray *returnArray = [[[NSMutableArray alloc] init] autorelease];
	
	for(ALAsset *asset in _assets) {

		NSMutableDictionary *workingDictionary = [[NSMutableDictionary alloc] init];
		[workingDictionary setObject:[asset valueForProperty:ALAssetPropertyType] forKey:@"UIImagePickerControllerMediaType"];
        [workingDictionary setObject:[UIImage imageWithCGImage:[[asset defaultRepresentation] fullScreenImage]] forKey:@"UIImagePickerControllerOriginalImage"];
		[workingDictionary setObject:[[asset valueForProperty:ALAssetPropertyURLs] valueForKey:[[[asset valueForProperty:ALAssetPropertyURLs] allKeys] objectAtIndex:0]] forKey:@"UIImagePickerControllerReferenceURL"];
		
		[returnArray addObject:workingDictionary];
		
		[workingDictionary release];	
	}
	
    [self popToRootViewControllerAnimated:NO];
    [[self parentViewController] dismissModalViewControllerAnimated:YES];
    
	if([delegate respondsToSelector:@selector(elcImagePickerController:didFinishPickingMediaWithInfo:)]) {
		[delegate performSelector:@selector(elcImagePickerController:didFinishPickingMediaWithInfo:) withObject:self withObject:[NSArray arrayWithArray:returnArray]];
	}
}

这个函数，将用户选择的图片的信息记录下来，在主界面上弄个scrollView来一一展示，这里就要用到原图了，开源的例子有问题，居然把图片都加载进去了，选多了必然崩溃呀，反正就是示例吧，没考虑那么细。

这个函数中可以看到，ALAsset对象中包括媒体类型、原图、各种的句柄URL之类的（我还没用过），应该足够满足你的需求了。因为我要做的功能是图片的上传功能，肯定涉及将图片转为NSData进行上传的处理，因此还获取了图片的类型，是png还是jpg，以便调用相应的api进行转码。下面代码能获取，然后比较是kUTTypePNG还是kUTTypeJPEG。

[[asset defaultRepresentation]UTI]

总结：上述方法算是大家都采用的常规方法了吧，开源的例子挺清楚的，相关界面的实现也有一定参考价值，最终我实现的效果是这样。

PS:顶部的navibar仿照系统的相册，系统提供的方法就足够了，稍微尝试一下，找到最接近系统相册的效果就好了。