Components.isSuccessCode 编辑

Summary

Determines whether a given XPCOM return code (that is, an nsresult value) indicates the success or failure of an operation, returning true or false respectively.

Syntax

var succeeded = Components.isSuccessCode(returnCode);

Parameters

returnCode

The return code (of type nsresult) to be checked.

Description

Components.isSuccessCode() may be used to determine whether an XPCOM return code (an nsresult) indicates success or failure. An XPCOM return code indicates success if its high-order bit is 0, and it indicates failure if its high-order bit is 1.

Note: A non-zero XPCOM return code (that is, not NS_OK) does not necessarily indicate failure.

Components.isSuccessCode() is functionally equivalent to the following JavaScript:

function isSuccessCode(returnCode) {
  return (returnCode & 0x80000000) === 0;
}

Since failure error codes are turned into exceptions when encountered in JavaScript, this function usually is not necessary. However, if you are using asynchronous APIs, it may be essential. For example, if you ask a component or service to asynchronously perform some task, you must usually pass in an object which will be notified when the task is completed. If the task is sufficiently complex that it can fail, the notification will include a status code indicating the success or failure of the operation (see, for example, nsIRequestObserver.onStopRequest()). To determine the success or failure of the complex task, you would call Components.isSuccessCode() upon the status code included in the notification.

Examples

Checking whether copying a stream's data succeeded

The following example demonstrates copying data from a buffered nsIInputStream to an nsIOutputStream, checking for whether the copy succeeded using Components.isSuccessCode().

Note: nsIAsyncStreamCopier.init() has changed in Gecko 1.9.2, omit last 2 boolean parameters if you're using Gecko 1.9.1 and earlier.

const Cc = Components.classes;
const Ci = Components.interfaces;
const Cr = Components.results;

// global flags polled externally
var copyFailed = false;
var copyInProgress = false;

function copyBufferedStream(inStream, outStream)
{
  var copyObserver =
    {
      onStartRequest: function(request, context)
      {
        copyInProgress = true;
      },
      onStopRequest: function(request, context, statusCode)
      {
        copyInProgress = false;

        // did the copy fail?
        if (!Components.isSuccessCode(statusCode))
          copyFailed = true;
      },
      QueryInterface: function(aIID)
      {
        if (aIID.equals(Ci.nsIRequestObserver) ||
            aIID.equals(Ci.nsISupports))
          return this;

        throw Cr.NS_ERROR_NO_INTERFACE;
      }
    };

  var copier = Cc["@mozilla.org/network/async-stream-copier;1"]
                 .createInstance(Ci.nsIAsyncStreamCopier);
  copier.init(inStream, outStream, null, true, false, 8192, true, true);
  copier.asyncCopy(copyObserver, null);
}