如何解决为什么 MediaStream 对象是 {} JSON？
我有一个指向实时流的 MediaStream 对象。它存储在一个名为 Create Table Veterinarians
(vetName varchar2(20),vid Number(5) primary key);
Create Table Dogs
(dogName varchar2(20),did Number(5) primary key);
Create Table Location
(lid Number(5) primary key,locName varchar2(20),);
Create Table examine
(vid int foreign key references Veterinarians(vid),did int foreign key references Dogs(did),lid int foreign key references Location(lid),fee Number(5));
INSERT INTO Veterinarians VALUES ('Alice',112);
INSERT INTO Veterinarians VALUES ('Mary',211);
INSERT INTO Veterinarians VALUES ('Jim',111);
INSERT INTO Dogs VALUES ('Spot',324);
INSERT INTO Dogs VALUES ('Fido',582);
INSERT INTO Dogs VALUES ('Tiger',731);
INSERT INTO Location VALUES (1001,'St.Cloud');
INSERT INTO Location VALUES (1002,'Minneapolis');
INSERT INTO Location VALUES (1003,'Duluth');
INSERT INTO examine VALUES (111,324,1001,10);
INSERT INTO examine VALUES (111,731,1003,20);
INSERT INTO examine VALUES (112,30);
INSERT INTO examine VALUES (112,582,50);
INSERT INTO examine VALUES (112,1002,35);
INSERT INTO examine VALUES (211,25);
INSERT INTO examine VALUES (211,20);
INSERT INTO examine VALUES (211,65);
--Creating a stored procedure
Create PROCEDURE display_Avg(vid int) AS
BEGIN
select v.vid,v.vetName,Avg(e.fee) as AverageFee
from Veterinarians v
INNER JOIN examine e
ON v.vid=e.vid
Where v.vid=display_Avg.vid
Group By v.vid,v.vetName;
END;
--Executing stored procedure
Execute display_Avg(112);
CREATE PROCEDURE display_DogNames
AS BEGIN
select dogName
from Dogs
INNER JOIN examine
ON examine.did=Dogs.did
INNER JOIN Location
On examine.lid=Location.lid
Where Location.lid=1001
AND Location.lid=1002
AND Location.lid=1003
END;
的变量中。这是我的控制台：
vsMediaStream 对象为什么是 JSON 中的 > vs
< MediaStream {id: "MEDIASTREAMID",active: true,...}
> JSON.stringify({Livestream:vs})
< "{"Livestream":{}}"
？
编辑：
这是一个例子：
{}var canvas = document.querySelector("canvas");
var context = canvas.getContext("2d");
const video = document.querySelector('#myVidplayer');
var w,h;
canvas.style.display = "none";
function snapshot(){
context.fillRect(0,w,h);
context.drawImage(video,h);
canvas.style.display = "block";
}
var vs;
window.navigator.mediaDevices.getUserMedia({ video: true,audio: true })
.then(function(stream){
video.srcObject = stream;
vs=stream;
console.log(JSON.stringify({Livestream:stream}));
video.onloadedMetadata = (e) => {
video.play();
w = video.videoWidth;
h = video.videoHeight
canvas.width = w;
canvas.height = h;
};
}).catch(function(e){
alert('You have to enable the microphone and the camera');
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