##$q的使用

var q = $q.defer(),
    p = q.promise;

chrome.fileSystem.chooseEntry({type: "openWritableFile", accepts: [
        {extensions: ["md"]}
    ]},
    function (entry) {

        q.resolve(entry.fullPath);

    });

p.then(function (greeting) {
    $scope.setMsg(greeting, 1000);
});