我一整天都在努力让 Firefox 服从我的意愿......
我想要:
int c = SELECT COUNT(*) FROM ...
我尝试过 executeAsync({...});
,但我认为这是错误的范例,因为我希望立即得到结果。 (mozIStoragePendingStatement
会导致错误)
var count = 0;
var conn = Services.storage.openDatabase(dbfile); // Will also create the file if it does not exist
let statement = conn.createStatement("SELECT COUNT(*) FROM edges LIMIT 42;");
console.log("columns: " + statement.columnCount); // prints "1";
console.log("col name:" + statement.getColumnName(0)); // is "COUNT(*)"
while (statement.executeStep())
count = statement.row.getResultByIndex(0); // "illegal value"
count = statement.row.getString(0); // "illegal value", too
count = statement.row.COUNT(*); // hahaha. still not working
count = statement.row[0]; // hahaha. "undefinded"
count = statement.row[1]; // hahaha. "undefinded"
}
statement.reset();
它基本上可以工作,但我不明白它的值(value)。所有语句(循环内的语句)有什么问题。
感谢您的任何提示...
最佳答案
I've tried
executeAsync({...});
, but I believe it's the wrong paradigm, as I want the result immediately.
您不应该希望这样,存储 API 是异步的是有原因的。对数据库的同步访问可能会导致随机延迟(例如,如果硬盘驱动器繁忙)。由于您的代码在主线程(为用户界面提供服务的同一线程)上执行,因此当您的代码等待数据库响应时,整个用户界面将挂起。 Mozilla 开发人员在 Firefox 3 中尝试了同步数据库访问,很快注意到它会降低用户体验 - 因此采用异步 API,数据库处理发生在后台线程上,不会阻塞任何内容。
您应该更改代码以异步工作。例如,类似这样的事情应该做:
Components.utils.import("resource://gre/modules/Services.jsm");
var conn = Services.storage.openDatabase(dbfile);
if (conn.schemaVersion < 1)
{
conn.createTable("edges", "s INTEGER, t INTEGER");
conn.schemaVersion = 1;
}
var statement = conn.createStatement("SELECT COUNT(*) FROM edges");
statement.executeAsync({
handleResult: function(resultSet)
{
var row = resultSet.getNextRow();
var count = row.getResultByIndex(0);
processResult(count);
},
handleError: function(error) {},
handleCompletion: function(reason) {}
});
// Close connection once the pending operations are completed
conn.asyncClose();
另请参阅:mozIStorageResultSet , mozIStorageRow .
关于javascript - 如何获取 SELECT COUNT(*) 的值?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/11040200/