How to download multiple pictures in uniapp

In recent years, with the rapid development of mobile Internet, various applications have emerged one after another. Among them, uniapp is a cross-platform application development framework that allows developers to use one code to run programs on multiple platforms at the same time. In uniapp development, how to download multiple pictures is a common problem. This article will introduce how uniapp downloads multiple pictures, hoping to be helpful to developers.

1. How to download a single picture with uniapp

Before introducing how to download multiple pictures with uniapp, let’s first learn how to download a single picture. In uniapp, we can use the uni.downloadFile() method to download network images. This method needs to pass in an object as a parameter, where the url attribute represents the link address of the image to be downloaded. After the download is completed, you can obtain the local path of the image through the tempFilePath attribute in the success callback function.

For example, the following code snippet demonstrates how to download an online image:

uni.downloadFile({
    url: 'http://example.com/image.jpg',
    success: function(res) {
        console.log('下载成功', res.tempFilePath);
    },
    fail: function(res) {
        console.log('下载失败', res.errMsg);
    }
});

In the above code, we set the link address of the image to be downloaded to 'http://example. com/image.jpg', if the download is successful, the console will output the download success message and print out the local path.

2. How to download multiple pictures with uniapp

With the experience of downloading a single picture, downloading multiple pictures becomes much easier. We can use the Promise.all() method to download multiple images at the same time. The specific steps are as follows.

The first step is to define a method for downloading a single image. This method receives an image link address as a parameter and returns a Promise object to represent the download task status.

function downloadSingleImage(url) {
  return new Promise((resolve, reject) => {
    uni.downloadFile({
      url: url,
      success: (res) => {
        if (res.statusCode === 200) {
          resolve(res.tempFilePath)
        } else {
          reject(new Error('下载失败'))
        }
      },
      fail: (err) => {
        reject(new Error('下载失败'))
      }
    })
  })
}

The second step is to define a method for downloading multiple images. This method receives an array of image link addresses as a parameter and returns a Promise object to represent the status of all image download tasks.

function downloadMultipleImages(urls) {
  let tasks = []
  urls.forEach((url) => {
    tasks.push(downloadSingleImage(url))
  })
  return Promise.all(tasks)
}

In the downloadMultipleImages() method, we traverse the urls array, pass each image link address to the downloadSingleImage() method, and add the returned Promise object to the tasks array. Finally, we call the Promise.all() method and wait for all download tasks to complete.

With the downloadMultipleImages() method, we can download multiple images at the same time. For example, we have the following image link address array:

let urls = ['http://example.com/1.png', 'http://example.com/2.png', 'http://example.com/3.png']

We only need to call the downloadMultipleImages() method and pass in the array. After all downloads are completed, the Promise.all() method will return an array containing the local paths of all images:

downloadMultipleImages(urls).then((imagePaths) => {
  console.log(imagePaths)
}).catch((err) => {
  console.error(err)
})

In the above code, we output the image local path array returned by the downloadMultipleImages() method to the control If there is any download failure, the catch() method is used to capture and output error information.

3. Summary

In uniapp development, downloading multiple pictures is a relatively common requirement. We can use the Promise.all() method to download multiple images at the same time. The code size is small and easy to expand. At the same time, we can also perform some optimizations on download tasks as needed, such as setting the number of concurrent downloads, adding download progress, etc.

I hope this article will be helpful to uniapp developers and make it easier and more enjoyable for everyone to download multiple pictures.

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