我正在尝试实现“实时”输入验证。我现在面临的问题是如何检查用户名是否已存在于数据库中。
对于所有数据库请求,我使用Volley POST到PHP API,导致:
private boolean checkTaken(String username, Context context){
boolean taken;
Volley.getInstance(context).addToRequestQueue(new StringRequest(Request.Method.POST, url, output -> taken = output.contains("taken"), error -> {}) {
@Override
protected Map<String, String> getParams() {
Map<String, String> params = new HashMap<>();
params.put("username", username);
return params;
}
@Override
public Map<String, String> getHeaders() {
Map<String, String> params = new HashMap<>();
params.put("Content-Type", "application/x-www-form-urlencoded");
return params;
}
});
return taken;
}
哪个会很好但不起作用,因为在API响应到达之前很久就已达到return语句。就在那时我发现了RequestFutures并试图实现它们,因为我看到了here:
private boolean checkTaken(String username, Context context){
RequestFuture<String> taken = RequestFuture.newFuture();
Volley.getInstance(context).addToRequestQueue(new StringRequest(Request.Method.POST, url, taken, error -> {}) {
@Override
protected Map<String, String> getParams() {
Map<String, String> params = new HashMap<>();
params.put("username", username);
return params;
}
@Override
public Map<String, String> getHeaders() {
Map<String, String> params = new HashMap<>();
params.put("Content-Type", "application/x-www-form-urlencoded");
return params;
}
});
try {
return taken.get(500, TimeUnit.MILLISECONDS).contains("taken");
} catch (InterruptedException | ExecutionException | TimeoutException e) {
return false;
}
}
不幸的是,即使我将超时增加到难以承受的数量,它也无效,taken.get()
永远不会得到解决。我认为这是因为请求中使用的taken
被解释为输出的新变量而不是实际的FutureRequest。
我会非常感谢提示为什么这不起作用或我的问题的另一个解决方案。
方法调用:
if (!email.matches(regex)){
return "Username has an invalid format";
} else if (checkTaken(username, context)){
return "Username already taken";
} else{
return null;
}
private void checkTaken(String username, Context context, Callback callback){
boolean taken;
Volley.getInstance(context).addToRequestQueue(new StringRequest(Request.Method.POST, url,
output -> callback.done(output.contains("taken")),
error -> {
// TODO handle this error
}) {
@Override
protected Map<String, String> getParams() {
Map<String, String> params = new HashMap<>();
params.put("username", username);
return params;
}
@Override
public Map<String, String> getHeaders() {
Map<String, String> params = new HashMap<>();
params.put("Content-Type", "application/x-www-form-urlencoded");
return params;
}
});
}
定义你的Callback接口,并从你调用checkTaken
的任何地方传入一个lambda表达式:
// This method wherever this is defined also needs to accept a callback so it can be asynchronous
public void whatever(StringCallback callback) {
// ...
if (!email.matches(regex)){
callback.done("Username has an invalid format");
} else {
checkTaken(username, context, taken ->
callback.done(taken ? "Username already taken" : null));
}
然后你在哪里打电话:
whatever(error -> {
if(error == null) {
handleSuccess();
}
else {
handleError(error);
}
);
这可能看起来像是一个烦人且复杂的变化链,但是当你从同步变为异步时,通常会产生很多连锁效应,而且由于你不能在Android的主线程上进行网络操作,你将会经常发现自己编写异步方法。