ExecUtil.qml

import QtQuick 2.0
import org.kde.plasma.core 2.0 as PlasmaCore

Item {
    property variant execQueue: []

    PlasmaCore.DataSource {
        id: executeSource
        engine: "executable"
        connectedSources: []
        onNewData: {
            // for (var key in executeSource) {
            //     console.log(key, executeSource[key])
            // } 
            // for (var key in executeSource.data) {
            //     console.log(key, executeSource.data[key])
            // }
            // console.log(executeSource.data[sourceName]["exit code"])
            // console.log("cmd", sourceName)
            // console.log("exit code", executeSource.data[sourceName]["exit code"])
            // console.log("exit status", executeSource.data[sourceName]["exit status"])
            // console.log("stdout", executeSource.data[sourceName]["stdout"])
            // console.log("stderr", executeSource.data[sourceName]["stderr"])
            execItemFinished(sourceName, executeSource.data[sourceName])
            
        }
    }
    function callExecItem() {
        var execItem = execQueue[0]
        executeSource.connectSource(execItem.cmd)
    }
    function execItemFinished(cmd, data) {
        var execItem = execQueue[0]
        var exitCode =  data["exit code"]
        var exitStatus =  data["exit status"]
        var stdout =  data["stdout"]
        var stderr =  data["stderr"]
        executeSource.disconnectSource(cmd)
        if (execItem.callback) {
            execItem.callback(stdout, exitCode, exitStatus, stderr)
        }
        execQueue.shift()
    }
    function exec(cmd, callback) {
        var execItem = {
            cmd: cmd,
            callback: callback,
        };
        execQueue.push(execItem)
        callExecItem()
    }
}

test.qml

Item {
	ExecUtil { id: executable }

	function downloadWallpaper(url, callback) {
		// var localUrl = config.localDir + '/wallhaven-' + item.wallpaperId + '.jpg'
		var fileName = url.substr(url.lastIndexOf('/')+1)
		var localUrl = config.localDir + '/' + fileName
		var command = '[ -f ' + localUrl + ' ] || (cd ' + config.localDir + ' && wget ' + url + ')'
		executable.exec(command, function(stdout, exitCode, exitStatus, stderr) {
			// console.log('(stdout, exitCode, exitStatus, stderr)', stdout, exitCode, exitStatus, stderr)
			callback()
		})
	}
}