通过在iOS中连接表格从Firebase获取数据 [英] Fetch data from Firebase by joining tables in iOS

问题描述

我试图从两个不同的Firebase表中获取数据。这里是表格的结构：

pre $
$ {
pImages {
i1 ：true
i2：true
}
}
2 {
pImages {
i3：true

}


$ {
i1 {
iUrl：....
pId：1
}
i2 {
iUrl：...
pId：1
}
i3 {
iUrl：....
pId：2
}
}

我需要检索与id = 1的post对应的图像。图片：
$ b $ pre $ func retrieveImagesForPost（postId：String，completion：（result：AnyObject ?, error：NSError？） - >（ ））{
var imgArray：[Image] = []
let postsRef = self.ref.child（post）
let imagesRef = self.ref.child（image）
let postImagesRef = postsRef.child（pos TID）.child（ pImages）;
postImagesRef.observeEventType（FIRDataEventType.Value，withBlock：{（snapshot）in
for snapshot.children中的项目{
imagesRef.child（item.key）.observeSingleEventOfType（.Value，withBlock： {（snap）in
let image = Image（snapshot：snap）
print（image）
imgArray.append（image）
}）
}
print（snapshot.key）
print（called）
completion（result：imgArray，error：nil）
}）
}
imgArray 中的所有图像到 code> $ b>能够发送到完成处理程序。以下是调用 retrieveImagesForPost 的帖子id = 1的输出。

  pImages 
叫
 < TestProject.Image：0x7f955466a150> 
  

在完成处理程序叫做。我尝试了调度组和信号量方法，如下所示帖子。但结果还是一样的。我怎样才能让完成处理程序等待从Firebase获取所有图片？

解决方案保持一个计数器，你增加每个图像加载。一旦计数器达到 snapshot.children 列表的长度，就完成并调用完成处理程序。

  let postImagesRef = postsRef.child（postId）.child（pImages）; 
 postImagesRef.observeEventType（FIRDataEventType.Value，withBlock：{（snapshot）in 
 var counter = 0 
用于snapshot.children中的物品{
 imagesRef.child（item.key） .bserveSingleEventOfType（.Value，withBlock：{（snap）in 
 let image = Image（snapshot：snap）
 print（image）
 imgArray.append（image）
 counter =计数器+ 1 
 if（counter == snapshot.childrenCount）{
 completion（result：imgArray，error：nil）
} 
}）
} 
}）
  

您应该在上面添加一些错误处理，但总的来说，并经过测试。

I am trying to fetch data from two different Firebase tables. Here is the structure of table:

Post {
    1{
       pImages{
           i1:true
           i2:true
       }
    }
    2{
       pImages{
           i3:true

       }
    }
}
Images{
       i1{
          iUrl : ....
          pId : 1
         }
       i2{
          iUrl :...
          pId : 1
         }
       i3{
         iUrl:....
          pId : 2
         }
 }

I need to retrieve images corresponding to post with id = 1. The following is my implementation to retrieve images:

 func retrieveImagesForPost(postId: String,completion: (result: AnyObject?, error: NSError?)->()){
        var imgArray:[Image]=[]
        let postsRef = self.ref.child("post")
        let imagesRef = self.ref.child("image")
        let postImagesRef = postsRef.child(postId).child("pImages");
        postImagesRef.observeEventType(FIRDataEventType.Value, withBlock: { (snapshot) in
            for item in snapshot.children{
                imagesRef.child(item.key).observeSingleEventOfType(.Value, withBlock: { (snap) in
                    let image = Image(snapshot: snap)
                    print(image)
                    imgArray.append(image)
                })
            }
            print(snapshot.key)
            print("called")
            completion(result:imgArray, error:nil)
        })
    }

But, the problem is I am not able to get all images in imgArray to be able to send to completion handler. Below is the output of calling retrieveImagesForPost with post id ==1.

pImages
called
<TestProject.Image: 0x7f9551e82000>
<TestProject.Image: 0x7f955466a150>

The images are retrieved after the completion handler is called. I tried the dispatch groups and the semaphores approach as described in the following post. But the results are still the same. How can I make completion handler to wait for all images to be fetched from Firebase?

解决方案

Keep a counter that you increase as each image is loaded. Once the counter reaches the length of the snapshot.children list, you're done and call your completion handler.

let postImagesRef = postsRef.child(postId).child("pImages");
postImagesRef.observeEventType(FIRDataEventType.Value, withBlock: { (snapshot) in
    var counter = 0
    for item in snapshot.children{
        imagesRef.child(item.key).observeSingleEventOfType(.Value, withBlock: { (snap) in
            let image = Image(snapshot: snap)
            print(image)
            imgArray.append(image)
            counter = counter + 1
            if (counter == snapshot.childrenCount) {
                completion(result:imgArray, error:nil)
            }
        })
    }
})

You should probably add some error handling in the above, but in general this approach is tried and tested.

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