ASP.NET Core SignalR JavaScript client
By Rachel Appel
The ASP.NET Core SignalR JavaScript client library enables developers to call server-side SignalR hub code.
Install the SignalR client package
The SignalR JavaScript client library is delivered as an npm package. The following sections outline different ways to install the client library.
Install with npm
Run the following commands from Package Manager Console:
npm init -y
npm install @microsoft/signalr
npm installs the package contents in the node_modules\@microsoft\signalr\dist\browser folder. Create the wwwroot/lib/signalr folder. Copy the signalr.js
file to the wwwroot/lib/signalr folder.
Reference the SignalR JavaScript client in the <script>
element. For example:
<script src="~/lib/signalr/signalr.js"></script>
Use a Content Delivery Network (CDN)
To use the client library without the npm prerequisite, reference a CDN-hosted copy of the client library. For example:
<script src="https://cdnjs.cloudflare.com/ajax/libs/microsoft-signalr/6.0.1/signalr.js"></script>
The client library is available on the following CDNs:
Install with LibMan
LibMan can be used to install specific client library files from the CDN-hosted client library. For example, only add the minified JavaScript file to the project. For details on that approach, see Add the SignalR client library.
Connect to a hub
The following code creates and starts a connection. The hub's name is case insensitive:
const connection = new signalR.HubConnectionBuilder()
.withUrl("/chathub")
.configureLogging(signalR.LogLevel.Information)
.build();
async function start() {
try {
await connection.start();
console.log("SignalR Connected.");
} catch (err) {
console.log(err);
setTimeout(start, 5000);
}
};
connection.onclose(async () => {
await start();
});
// Start the connection.
start();
Cross-origin connections (CORS)
Typically, browsers load connections from the same domain as the requested page. However, there are occasions when a connection to another domain is required.
When making cross domain requests, the client code must use an absolute URL instead of a relative URL. For cross domain requests, change .withUrl("/chathub")
to .withUrl("https://{App domain name}/chathub")
.
To prevent a malicious site from reading sensitive data from another site, cross-origin connections are disabled by default. To allow a cross-origin request, enable CORS:
using SignalRChat.Hubs;
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddRazorPages();
builder.Services.AddSignalR();
builder.Services.AddCors(options =>
{
options.AddDefaultPolicy(
builder =>
{
builder.WithOrigins("https://example.com")
.AllowAnyHeader()
.WithMethods("GET", "POST")
.AllowCredentials();
});
});
var app = builder.Build();
if (!app.Environment.IsDevelopment())
{
app.UseExceptionHandler("/Error");
app.UseHsts();
}
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();
app.UseAuthorization();
// UseCors must be called before MapHub.
app.UseCors();
app.MapRazorPages();
app.MapHub<ChatHub>("/chatHub");
app.Run();
UseCors must be called before calling MapHub.
Call hub methods from the client
JavaScript clients call public methods on hubs via the invoke method of the HubConnection. The invoke
method accepts:
- The name of the hub method.
- Any arguments defined in the hub method.
In the following highlighted code, the method name on the hub is SendMessage
. The second and third arguments passed to invoke
map to the hub method's user
and message
arguments:
try {
await connection.invoke("SendMessage", user, message);
} catch (err) {
console.error(err);
}
Calling hub methods from a client is only supported when using the Azure SignalR Service in Default mode. For more information, see Frequently Asked Questions (azure-signalr GitHub repository).
The invoke
method returns a JavaScript Promise
. The Promise
is resolved with the return value (if any) when the method on the server returns. If the method on the server throws an error, the Promise
is rejected with the error message. Use async
and await
or the Promise
's then
and catch
methods to handle these cases.
JavaScript clients can also call public methods on hubs via the send method of the HubConnection
. Unlike the invoke
method, the send
method doesn't wait for a response from the server. The send
method returns a JavaScript Promise
. The Promise
is resolved when the message has been sent to the server. If there is an error sending the message, the Promise
is rejected with the error message. Use async
and await
or the Promise
's then
and catch
methods to handle these cases.
Using send
doesn't wait until the server has received the message. Consequently, it's not possible to return data or errors from the server.
Call client methods from the hub
To receive messages from the hub, define a method using the on method of the HubConnection
.
- The name of the JavaScript client method.
- Arguments the hub passes to the method.
In the following example, the method name is ReceiveMessage
. The argument names are user
and message
:
connection.on("ReceiveMessage", (user, message) => {
const li = document.createElement("li");
li.textContent = `${user}: ${message}`;
document.getElementById("messageList").appendChild(li);
});
The preceding code in connection.on
runs when server-side code calls it using the SendAsync method:
using Microsoft.AspNetCore.SignalR;
namespace SignalRChat.Hubs;
public class ChatHub : Hub
{
public async Task SendMessage(string user, string message)
{
await Clients.All.SendAsync("ReceiveMessage", user, message);
}
}
SignalR determines which client method to call by matching the method name and arguments defined in SendAsync
and connection.on
.
A best practice is to call the start method on the HubConnection
after on
. Doing so ensures the handlers are registered before any messages are received.
Error handling and logging
Use console.error
to output errors to the browser's console when the client can't connect or send a message:
try {
await connection.invoke("SendMessage", user, message);
} catch (err) {
console.error(err);
}
Set up client-side log tracing by passing a logger and type of event to log when the connection is made. Messages are logged with the specified log level and higher. Available log levels are as follows:
signalR.LogLevel.Error
: Error messages. LogsError
messages only.signalR.LogLevel.Warning
: Warning messages about potential errors. LogsWarning
, andError
messages.signalR.LogLevel.Information
: Status messages without errors. LogsInformation
,Warning
, andError
messages.signalR.LogLevel.Trace
: Trace messages. Logs everything, including data transported between hub and client.
Use the configureLogging method on HubConnectionBuilder to configure the log level. Messages are logged to the browser console:
const connection = new signalR.HubConnectionBuilder()
.withUrl("/chathub")
.configureLogging(signalR.LogLevel.Information)
.build();
Reconnect clients
Automatically reconnect
The JavaScript client for SignalR can be configured to automatically reconnect using the WithAutomaticReconnect method on HubConnectionBuilder. It won't automatically reconnect by default.
const connection = new signalR.HubConnectionBuilder()
.withUrl("/chathub")
.withAutomaticReconnect()
.build();
Without any parameters, WithAutomaticReconnect configures the client to wait 0, 2, 10, and 30 seconds respectively before trying each reconnect attempt. After four failed attempts, it stops trying to reconnect.
Before starting any reconnect attempts, the HubConnection
:
- Transitions to the
HubConnectionState.Reconnecting
state and fires itsonreconnecting
callbacks. - Doesn't transition to the
Disconnected
state and trigger itsonclose
callbacks like aHubConnection
without automatic reconnect configured.
The reconnect approach provides an opportunity to:
- Warn users that the connection has been lost.
- Disable UI elements.
connection.onreconnecting(error => {
console.assert(connection.state === signalR.HubConnectionState.Reconnecting);
document.getElementById("messageInput").disabled = true;
const li = document.createElement("li");
li.textContent = `Connection lost due to error "${error}". Reconnecting.`;
document.getElementById("messageList").appendChild(li);
});
If the client successfully reconnects within its first four attempts, the HubConnection
transitions back to the Connected
state and fire its onreconnected
callbacks. This provides an opportunity to inform users the connection has been reestablished.
Since the connection looks entirely new to the server, a new connectionId
is provided to the onreconnected
callback.
The onreconnected
callback's connectionId
parameter is undefined if the HubConnection
is configured to skip negotiation.
connection.onreconnected(connectionId => {
console.assert(connection.state === signalR.HubConnectionState.Connected);
document.getElementById("messageInput").disabled = false;
const li = document.createElement("li");
li.textContent = `Connection reestablished. Connected with connectionId "${connectionId}".`;
document.getElementById("messageList").appendChild(li);
});
withAutomaticReconnect
won't configure the HubConnection
to retry initial start failures, so start failures need to be handled manually:
async function start() {
try {
await connection.start();
console.assert(connection.state === signalR.HubConnectionState.Connected);
console.log("SignalR Connected.");
} catch (err) {
console.assert(connection.state === signalR.HubConnectionState.Disconnected);
console.log(err);
setTimeout(() => start(), 5000);
}
};
If the client doesn't successfully reconnect within its first four attempts, the HubConnection
transitions to the Disconnected
state and fires its onclose callbacks. This provides an opportunity to inform users:
- The connection has been permanently lost.
- Try refreshing the page:
connection.onclose(error => {
console.assert(connection.state === signalR.HubConnectionState.Disconnected);
document.getElementById("messageInput").disabled = true;
const li = document.createElement("li");
li.textContent = `Connection closed due to error "${error}". Try refreshing this page to restart the connection.`;
document.getElementById("messageList").appendChild(li);
});
In order to configure a custom number of reconnect attempts before disconnecting or change the reconnect timing, withAutomaticReconnect
accepts an array of numbers representing the delay in milliseconds to wait before starting each reconnect attempt.
const connection = new signalR.HubConnectionBuilder()
.withUrl("/chathub")
.withAutomaticReconnect([0, 0, 10000])
.build();
// .withAutomaticReconnect([0, 2000, 10000, 30000]) yields the default behavior
The preceding example configures the HubConnection
to start attempting reconnects immediately after the connection is lost. The default configuration also waits zero seconds to attempt reconnecting.
If the first reconnect attempt fails, the second reconnect attempt also starts immediately instead of waiting 2 seconds using the default configuration.
If the second reconnect attempt fails, the third reconnect attempt start in 10 seconds which is the same as the default configuration.
The configured reconnection timing differs from the default behavior by stopping after the third reconnect attempt failure instead of trying one more reconnect attempt in another 30 seconds.
For more control over the timing and number of automatic reconnect attempts, withAutomaticReconnect
accepts an object implementing the IRetryPolicy
interface, which has a single method named nextRetryDelayInMilliseconds
.
nextRetryDelayInMilliseconds
takes a single argument with the type RetryContext
. The RetryContext
has three properties: previousRetryCount
, elapsedMilliseconds
and retryReason
which are a number
, a number
and an Error
respectively. Before the first reconnect attempt, both previousRetryCount
and elapsedMilliseconds
will be zero, and the retryReason
will be the Error that caused the connection to be lost. After each failed retry attempt, previousRetryCount
will be incremented by one, elapsedMilliseconds
will be updated to reflect the amount of time spent reconnecting so far in milliseconds, and the retryReason
will be the Error that caused the last reconnect attempt to fail.
nextRetryDelayInMilliseconds
must return either a number representing the number of milliseconds to wait before the next reconnect attempt or null
if the HubConnection
should stop reconnecting.
const connection = new signalR.HubConnectionBuilder()
.withUrl("/chathub")
.withAutomaticReconnect({
nextRetryDelayInMilliseconds: retryContext => {
if (retryContext.elapsedMilliseconds < 60000) {
// If we've been reconnecting for less than 60 seconds so far,
// wait between 0 and 10 seconds before the next reconnect attempt.
return Math.random() * 10000;
} else {
// If we've been reconnecting for more than 60 seconds so far, stop reconnecting.
return null;
}
}
})
.build();
Alternatively, code can be written that reconnects the client manually as demonstrated in the following section.
Manually reconnect
The following code demonstrates a typical manual reconnection approach:
- A function (in this case, the
start
function) is created to start the connection. - Call the
start
function in the connection'sonclose
event handler.
async function start() {
try {
await connection.start();
console.log("SignalR Connected.");
} catch (err) {
console.log(err);
setTimeout(start, 5000);
}
};
connection.onclose(async () => {
await start();
});
Production implementations typically use an exponential back-off or retry a specified number of times.
Browser sleeping tab
Some browsers have a tab freezing or sleeping feature to reduce computer resource usage for inactive tabs. This can cause SignalR connections to close and may result in an unwanted user experience. Browsers use heuristics to figure out if a tab should be put to sleep, such as:
- Playing audio
- Holding a web lock
- Holding an
IndexedDB
lock - Being connected to a USB device
- Capturing video or audio
- Being mirrored
- Capturing a window or display
Browser heuristics may change over time and can differ between browsers. Check the support matrix and figure out what method works best for your scenarios.
To avoid putting an app to sleep, the app should trigger one of the heuristics that the browser uses.
The following code example shows how to use a Web Lock to keep a tab awake and avoid an unexpected connection closure.
var lockResolver;
if (navigator && navigator.locks && navigator.locks.request) {
const promise = new Promise((res) => {
lockResolver = res;
});
navigator.locks.request('unique_lock_name', { mode: "shared" }, () => {
return promise;
});
}
For the preceding code example:
- Web Locks are experimental. The conditional check confirms that the browser supports Web Locks.
- The promise resolver,
lockResolver
, is stored so that the lock can be released when it's acceptable for the tab to sleep. - When closing the connection, the lock is released by calling
lockResolver()
. When the lock is released, the tab is allowed to sleep.
Additional resources
By Rachel Appel
The ASP.NET Core SignalR JavaScript client library enables developers to call server-side hub code.
View or download sample code (how to download)
Install the SignalR client package
The SignalR JavaScript client library is delivered as an npm package. The following sections outline different ways to install the client library.
Install with npm
For Visual Studio, run the following commands from Package Manager Console while in the root folder. For Visual Studio Code, run the following commands from the Integrated Terminal.
npm init -y
npm install @microsoft/signalr
npm installs the package contents in the node_modules\@microsoft\signalr\dist\browser folder. Create a new folder named signalr under the wwwroot\lib folder. Copy the signalr.js
file to the wwwroot\lib\signalr folder.
Reference the SignalR JavaScript client in the <script>
element. For example:
<script src="~/lib/signalr/signalr.js"></script>
Use a Content Delivery Network (CDN)
To use the client library without the npm prerequisite, reference a CDN-hosted copy of the client library. For example:
<script src="https://cdnjs.cloudflare.com/ajax/libs/microsoft-signalr/3.1.7/signalr.js"></script>
The client library is available on the following CDNs:
Install with LibMan
LibMan can be used to install specific client library files from the CDN-hosted client library. For example, only add the minified JavaScript file to the project. For details on that approach, see Add the SignalR client library.
Connect to a hub
The following code creates and starts a connection. The hub's name is case insensitive:
const connection = new signalR.HubConnectionBuilder()
.withUrl("/chathub")
.configureLogging(signalR.LogLevel.Information)
.build();
async function start() {
try {
await connection.start();
console.log("SignalR Connected.");
} catch (err) {
console.log(err);
setTimeout(start, 5000);
}
};
connection.onclose(async () => {
await start();
});
// Start the connection.
start();
Cross-origin connections
Typically, browsers load connections from the same domain as the requested page. However, there are occasions when a connection to another domain is required.
Important
The client code must use an absolute URL instead of a relative URL. Change .withUrl("/chathub")
to .withUrl("https://myappurl/chathub")
.
To prevent a malicious site from reading sensitive data from another site, cross-origin connections are disabled by default. To allow a cross-origin request, enable it in the Startup
class:
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using SignalRChat.Hubs;
namespace SignalRChat
{
public class Startup
{
public void ConfigureServices(IServiceCollection services)
{
services.AddRazorPages();
services.AddSignalR();
services.AddCors(options =>
{
options.AddDefaultPolicy(builder =>
{
builder.WithOrigins("https://example.com")
.AllowCredentials();
});
});
}
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
else
{
app.UseExceptionHandler("/Error");
}
app.UseStaticFiles();
app.UseRouting();
app.UseCors();
app.UseEndpoints(endpoints =>
{
endpoints.MapRazorPages();
endpoints.MapHub<ChatHub>("/chathub");
});
}
}
}
Call hub methods from the client
JavaScript clients call public methods on hubs via the invoke method of the HubConnection. The invoke
method accepts:
- The name of the hub method.
- Any arguments defined in the hub method.
In the following example, the method name on the hub is SendMessage
. The second and third arguments passed to invoke
map to the hub method's user
and message
arguments:
try {
await connection.invoke("SendMessage", user, message);
} catch (err) {
console.error(err);
}
Note
Calling hub methods from a client is only supported when using the Azure SignalR Service in Default mode. For more information, see Frequently Asked Questions (azure-signalr GitHub repository).
The invoke
method returns a JavaScript Promise
. The Promise
is resolved with the return value (if any) when the method on the server returns. If the method on the server throws an error, the Promise
is rejected with the error message. Use async
and await
or the Promise
's then
and catch
methods to handle these cases.
JavaScript clients can also call public methods on hubs via the send method of the HubConnection
. Unlike the invoke
method, the send
method doesn't wait for a response from the server. The send
method returns a JavaScript Promise
. The Promise
is resolved when the message has been sent to the server. If there is an error sending the message, the Promise
is rejected with the error message. Use async
and await
or the Promise
's then
and catch
methods to handle these cases.
Note
Using send
doesn't wait until the server has received the message. Consequently, it's not possible to return data or errors from the server.
Call client methods from the hub
To receive messages from the hub, define a method using the on method of the HubConnection
.
- The name of the JavaScript client method.
- Arguments the hub passes to the method.
In the following example, the method name is ReceiveMessage
. The argument names are user
and message
:
connection.on("ReceiveMessage", (user, message) => {
const li = document.createElement("li");
li.textContent = `${user}: ${message}`;
document.getElementById("messageList").appendChild(li);
});
The preceding code in connection.on
runs when server-side code calls it using the SendAsync method:
public async Task SendMessage(string user, string message)
{
await Clients.All.SendAsync("ReceiveMessage", user, message);
}
SignalR determines which client method to call by matching the method name and arguments defined in SendAsync
and connection.on
.
Note
As a best practice, call the start method on the HubConnection
after on
. Doing so ensures your handlers are registered before any messages are received.
Error handling and logging
Use try
and catch
with async
and await
or the Promise
's catch
method to handle client-side errors. Use console.error
to output errors to the browser's console:
try {
await connection.invoke("SendMessage", user, message);
} catch (err) {
console.error(err);
}
Set up client-side log tracing by passing a logger and type of event to log when the connection is made. Messages are logged with the specified log level and higher. Available log levels are as follows:
signalR.LogLevel.Error
: Error messages. LogsError
messages only.signalR.LogLevel.Warning
: Warning messages about potential errors. LogsWarning
, andError
messages.signalR.LogLevel.Information
: Status messages without errors. LogsInformation
,Warning
, andError
messages.signalR.LogLevel.Trace
: Trace messages. Logs everything, including data transported between hub and client.
Use the configureLogging method on HubConnectionBuilder to configure the log level. Messages are logged to the browser console:
const connection = new signalR.HubConnectionBuilder()
.withUrl("/chathub")
.configureLogging(signalR.LogLevel.Information)
.build();
Reconnect clients
Automatically reconnect
The JavaScript client for SignalR can be configured to automatically reconnect using the withAutomaticReconnect
method on HubConnectionBuilder. It won't automatically reconnect by default.
const connection = new signalR.HubConnectionBuilder()
.withUrl("/chathub")
.withAutomaticReconnect()
.build();
Without any parameters, withAutomaticReconnect()
configures the client to wait 0, 2, 10, and 30 seconds respectively before trying each reconnect attempt, stopping after four failed attempts.
Before starting any reconnect attempts, the HubConnection
will transition to the HubConnectionState.Reconnecting
state and fire its onreconnecting
callbacks instead of transitioning to the Disconnected
state and triggering its onclose
callbacks like a HubConnection
without automatic reconnect configured. This provides an opportunity to warn users that the connection has been lost and to disable UI elements.
connection.onreconnecting(error => {
console.assert(connection.state === signalR.HubConnectionState.Reconnecting);
document.getElementById("messageInput").disabled = true;
const li = document.createElement("li");
li.textContent = `Connection lost due to error "${error}". Reconnecting.`;
document.getElementById("messageList").appendChild(li);
});
If the client successfully reconnects within its first four attempts, the HubConnection
will transition back to the Connected
state and fire its onreconnected
callbacks. This provides an opportunity to inform users the connection has been reestablished.
Since the connection looks entirely new to the server, a new connectionId
will be provided to the onreconnected
callback.
Warning
The onreconnected
callback's connectionId
parameter will be undefined if the HubConnection
was configured to skip negotiation.
connection.onreconnected(connectionId => {
console.assert(connection.state === signalR.HubConnectionState.Connected);
document.getElementById("messageInput").disabled = false;
const li = document.createElement("li");
li.textContent = `Connection reestablished. Connected with connectionId "${connectionId}".`;
document.getElementById("messageList").appendChild(li);
});
withAutomaticReconnect()
won't configure the HubConnection
to retry initial start failures, so start failures need to be handled manually:
async function start() {
try {
await connection.start();
console.assert(connection.state === signalR.HubConnectionState.Connected);
console.log("SignalR Connected.");
} catch (err) {
console.assert(connection.state === signalR.HubConnectionState.Disconnected);
console.log(err);
setTimeout(() => start(), 5000);
}
};
If the client doesn't successfully reconnect within its first four attempts, the HubConnection
will transition to the Disconnected
state and fire its onclose callbacks. This provides an opportunity to inform users the connection has been permanently lost and recommend refreshing the page:
connection.onclose(error => {
console.assert(connection.state === signalR.HubConnectionState.Disconnected);
document.getElementById("messageInput").disabled = true;
const li = document.createElement("li");
li.textContent = `Connection closed due to error "${error}". Try refreshing this page to restart the connection.`;
document.getElementById("messageList").appendChild(li);
});
In order to configure a custom number of reconnect attempts before disconnecting or change the reconnect timing, withAutomaticReconnect
accepts an array of numbers representing the delay in milliseconds to wait before starting each reconnect attempt.
const connection = new signalR.HubConnectionBuilder()
.withUrl("/chathub")
.withAutomaticReconnect([0, 0, 10000])
.build();
// .withAutomaticReconnect([0, 2000, 10000, 30000]) yields the default behavior
The preceding example configures the HubConnection
to start attempting reconnects immediately after the connection is lost. This is also true for the default configuration.
If the first reconnect attempt fails, the second reconnect attempt will also start immediately instead of waiting 2 seconds like it would in the default configuration.
If the second reconnect attempt fails, the third reconnect attempt will start in 10 seconds which is again like the default configuration.
The custom behavior then diverges again from the default behavior by stopping after the third reconnect attempt failure instead of trying one more reconnect attempt in another 30 seconds like it would in the default configuration.
If you want even more control over the timing and number of automatic reconnect attempts, withAutomaticReconnect
accepts an object implementing the IRetryPolicy
interface, which has a single method named nextRetryDelayInMilliseconds
.
nextRetryDelayInMilliseconds
takes a single argument with the type RetryContext
. The RetryContext
has three properties: previousRetryCount
, elapsedMilliseconds
and retryReason
which are a number
, a number
and an Error
respectively. Before the first reconnect attempt, both previousRetryCount
and elapsedMilliseconds
will be zero, and the retryReason
will be the Error that caused the connection to be lost. After each failed retry attempt, previousRetryCount
will be incremented by one, elapsedMilliseconds
will be updated to reflect the amount of time spent reconnecting so far in milliseconds, and the retryReason
will be the Error that caused the last reconnect attempt to fail.
nextRetryDelayInMilliseconds
must return either a number representing the number of milliseconds to wait before the next reconnect attempt or null
if the HubConnection
should stop reconnecting.
const connection = new signalR.HubConnectionBuilder()
.withUrl("/chathub")
.withAutomaticReconnect({
nextRetryDelayInMilliseconds: retryContext => {
if (retryContext.elapsedMilliseconds < 60000) {
// If we've been reconnecting for less than 60 seconds so far,
// wait between 0 and 10 seconds before the next reconnect attempt.
return Math.random() * 10000;
} else {
// If we've been reconnecting for more than 60 seconds so far, stop reconnecting.
return null;
}
}
})
.build();
Alternatively, you can write code that will reconnect your client manually as demonstrated in Manually reconnect.
Manually reconnect
The following code demonstrates a typical manual reconnection approach:
- A function (in this case, the
start
function) is created to start the connection. - Call the
start
function in the connection'sonclose
event handler.
async function start() {
try {
await connection.start();
console.log("SignalR Connected.");
} catch (err) {
console.log(err);
setTimeout(start, 5000);
}
};
connection.onclose(async () => {
await start();
});
Production implementations typically use an exponential back-off or retry a specified number of times.
Browser sleeping tab
Some browsers have a tab freezing or sleeping feature to reduce computer resource usage for inactive tabs. This can cause SignalR connections to close and may result in an unwanted user experience. Browsers use heuristics to figure out if a tab should be put to sleep, such as:
- Playing audio
- Holding a web lock
- Holding an
IndexedDB
lock - Being connected to a USB device
- Capturing video or audio
- Being mirrored
- Capturing a window or display
Note
These heuristics may change over time or differ between browsers. Check your support matrix and figure out what method works best for your scenarios.
To avoid putting an app to sleep, the app should trigger one of the heuristics that the browser uses.
The following code example shows how to use a Web Lock to keep a tab awake and avoid an unexpected connection closure.
var lockResolver;
if (navigator && navigator.locks && navigator.locks.request) {
const promise = new Promise((res) => {
lockResolver = res;
});
navigator.locks.request('unique_lock_name', { mode: "shared" }, () => {
return promise;
});
}
For the preceding code example:
- Web Locks are experimental. The conditional check confirms that the browser supports Web Locks.
- The promise resolver (
lockResolver
) is stored so that the lock can be released when it's acceptable for the tab to sleep. - When closing the connection, the lock is released by calling
lockResolver()
. When the lock is released, the tab is allowed to sleep.
Additional resources
ASP.NET Core