Phonegap插件：如何在Android中将Base64字符串转换为PNG图像

Question

Android does not allow native apps like Phonegap-based apps to write binary files. A common application is converting Base64 Strings to Images. So, how do you go about this problem?

Answer 1

The solution; This plugin that converts a Base64 PNG String and generates an image to the sdCard. Let's go!

1. The Base64 Decoder

Get this blazing fast Base64 encode/decoder class called MiGBase64. Download it from SourceForge . Create a folder called 'util' within your project's src/ folder. Place the downloaded class there.

2. The java

Create a folder called 'org/apache/cordova' within your project's src/ folder. Create in it a Java file called "Base64ToPNG.java" with the following source code.

package org.apache.cordova;

/**
* A phonegap plugin that converts a Base64 String to a PNG file.
*
* @author mcaesar
* @lincese MIT.
*/

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;

import org.apache.cordova.api.Plugin;
import org.apache.cordova.api.PluginResult;
import org.json.JSONArray;

import android.os.Environment;
import java.io.*;
import org.json.JSONException;
import org.json.JSONObject;
import util.Base64;

public class Base64ToPNG extends Plugin {

    @Override
    public PluginResult execute(String action, JSONArray args, String callbackId) {

        if (!action.equals("saveImage")) {
            return new PluginResult(PluginResult.Status.INVALID_ACTION);
        }

        try {

            String b64String = "";
            if (b64String.startsWith("data:image")) {
                b64String = args.getString(0).substring(21);
            } else {
                b64String = args.getString(0);
            }
            JSONObject params = args.getJSONObject(1);

            //Optional parameter
            String filename = params.has("filename")
                    ? params.getString("filename")
                    : "b64Image_" + System.currentTimeMillis() + ".png";

            String folder = params.has("folder")
                    ? params.getString("folder")
                    : Environment.getExternalStorageDirectory() + "/Pictures";

            Boolean overwrite = params.has("overwrite") 
                    ? params.getBoolean("overwrite") 
                    : false;

            return this.saveImage(b64String, filename, folder, overwrite, callbackId);

        } catch (JSONException e) {

            e.printStackTrace();
            return new PluginResult(PluginResult.Status.JSON_EXCEPTION, e.getMessage());

        } catch (InterruptedException e) {
            e.printStackTrace();
            return new PluginResult(PluginResult.Status.ERROR, e.getMessage());
        }

    }

    private PluginResult saveImage(String b64String, String fileName, String dirName, Boolean overwrite, String callbackId) throws InterruptedException, JSONException {

        try {

            //Directory and File
            File dir = new File(dirName);
            if (!dir.exists()) {
                dir.mkdirs();
            }
            File file = new File(dirName, fileName);

            //Avoid overwriting a file
            if (!overwrite && file.exists()) {
                return new PluginResult(PluginResult.Status.OK, "File already exists!");
            }

            //Decode Base64 back to Binary format
            byte[] decodedBytes = Base64.decode(b64String.getBytes());

            //Save Binary file to phone
            file.createNewFile();
            FileOutputStream fOut = new FileOutputStream(file);
            fOut.write(decodedBytes);
            fOut.close();


            return new PluginResult(PluginResult.Status.OK, "Saved successfully!");

        } catch (FileNotFoundException e) {
            return new PluginResult(PluginResult.Status.ERROR, "File not Found!");
        } catch (IOException e) {
            return new PluginResult(PluginResult.Status.ERROR, e.getMessage());
        }

    }
}

3. The Javascript

Write this JavaScript as Base64ToPNG.js to your project's www folder. DONT forget to include a reference to it in your html files.

/**Works on all versions prior and including Cordova 1.6.1 
* by mcaesar
*  MIT license
*  
*/

(function() {
    /* This increases plugin compatibility */
    var cordovaRef = window.PhoneGap || window.Cordova || window.cordova; // old to new fallbacks

    /**
    * The Java to JavaScript Gateway 'magic' class 
    */
    function Base64ToPNG() { }

    /**
    * Save the base64 String as a PNG file to the user's Photo Library
    */
    Base64ToPNG.prototype.saveImage = function(b64String, params, win, fail) {
        cordovaRef.exec(win, fail, "Base64ToPNG", "saveImage", [b64String, params]);
    };

    cordovaRef.addConstructor(function() {
        if (!window.plugins) {
            window.plugins = {};
        }
        if (!window.plugins.base64ToPNG) {
            window.plugins.base64ToPNG = new Base64ToPNG();
        }
    });

})(); 

4. The plugins.xml file

Add the following to res/xml/plugins.xml file

<plugin name="Base64ToPNG" value="org.apache.cordova.Base64ToPNG"/>

5. Finally, HTML examples and the parameters

<button onclick="test();">No optional params required, Cowboy.</button> </br>
<button onclick="test2();">Make PNG with some parameters</button>

<script src="Base64ToPNG.js" type="text/javascript"></script>

<script type="text/javascript">

//May have a mime-type definition or not 
var myBase64 = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg=="//a red dot


function test(){

    //Illustrates how to use plugin with no optional parameters. Just the base64 Image.
    window.plugins.base64ToPNG.saveImage(myBase64, {}, 
       function(result) {
          alert(result);
       }, function(error) {
          alert(error);
       });
 }

 //No mimetype definition example
 var myOtherBase64 = "iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg=="

 function test2(){

    //Shows how to use optional parameters
    window.plugins.base64ToPNG.saveImage(myBase64, {filename:"dot.png", overwrite: true}, 
       function(result) {
          alert(result);
       }, function(error) {
          alert(error);
    });

 }
 </script>

Parameters

1. filename : Name of the file to be generated. By default the same as the one in url.
2. folder : Name of the directory to generate the file to. By default "sdcard/Pictures"

3. overwrite : If the file already exists, replace it. By default false.

   I hope this answers some bothering questions. Happy coding!

Answer 2

Fo anybody wanting to use this with kineticjs, the following works a treat:

function saveCanvas() {
    $('#save').bind( $bind, function(){
        stage.toDataURL({
        callback: function(dataUrl){
            window.plugins.base64ToPNG.saveImage(dataUrl.substr(22,dataUrl.length), {}, 
                function(result) {
                    alert(result);
                }, function(error) {
                    alert(error);
                }
            );
        },
            mimeType: 'image/png',
            quality: 0.5
        });
    });
}

Answer 3

This solution only works when feeding it a CLEAN Base64 string. In other words, the "data:image/png;base64," part should be removed or the Base64 decoder fill fail, causing a nullpointer error when writing the file.

Also I noticed that the image does not show up in the Gallery but it is stored correctly on the SD card. When I download it to my PC I can open it just fine. Not sure what that is about.

Thanks for the work!