Package context

func DoSomething(ctx context.Context, arg Arg) error {
	// ... use ctx ...
}

Variables

Canceled is the error returned by Context.Err when the context is canceled.

var Canceled = context.Canceled

DeadlineExceeded is the error returned by Context.Err when the context's deadline passes.

var DeadlineExceeded = context.DeadlineExceeded

type CancelFunc ¶

A CancelFunc tells an operation to abandon its work. A CancelFunc does not wait for the work to stop. After the first call, subsequent calls to a CancelFunc do nothing.

type CancelFunc func()

type Context ¶

A Context carries a deadline, a cancelation signal, and other values across API boundaries.

Context's methods may be called by multiple goroutines simultaneously.

type Context interface {
    // Deadline returns the time when work done on behalf of this context
    // should be canceled. Deadline returns ok==false when no deadline is
    // set. Successive calls to Deadline return the same results.
    Deadline() (deadline time.Time, ok bool)

    // Done returns a channel that's closed when work done on behalf of this
    // context should be canceled. Done may return nil if this context can
    // never be canceled. Successive calls to Done return the same value.
    //
    // WithCancel arranges for Done to be closed when cancel is called;
    // WithDeadline arranges for Done to be closed when the deadline
    // expires; WithTimeout arranges for Done to be closed when the timeout
    // elapses.
    //
    // Done is provided for use in select statements:
    //
    //  // Stream generates values with DoSomething and sends them to out
    //  // until DoSomething returns an error or ctx.Done is closed.
    //  func Stream(ctx context.Context, out chan<- Value) error {
    //  	for {
    //  		v, err := DoSomething(ctx)
    //  		if err != nil {
    //  			return err
    //  		}
    //  		select {
    //  		case <-ctx.Done():
    //  			return ctx.Err()
    //  		case out <- v:
    //  		}
    //  	}
    //  }
    //
    // See http://blog.golang.org/pipelines for more examples of how to use
    // a Done channel for cancelation.
    Done() <-chan struct{}

    // Err returns a non-nil error value after Done is closed. Err returns
    // Canceled if the context was canceled or DeadlineExceeded if the
    // context's deadline passed. No other values for Err are defined.
    // After Done is closed, successive calls to Err return the same value.
    Err() error

    // Value returns the value associated with this context for key, or nil
    // if no value is associated with key. Successive calls to Value with
    // the same key returns the same result.
    //
    // Use context values only for request-scoped data that transits
    // processes and API boundaries, not for passing optional parameters to
    // functions.
    //
    // A key identifies a specific value in a Context. Functions that wish
    // to store values in Context typically allocate a key in a global
    // variable then use that key as the argument to context.WithValue and
    // Context.Value. A key can be any type that supports equality;
    // packages should define keys as an unexported type to avoid
    // collisions.
    //
    // Packages that define a Context key should provide type-safe accessors
    // for the values stores using that key:
    //
    // 	// Package user defines a User type that's stored in Contexts.
    // 	package user
    //
    // 	import "golang.org/x/net/context"
    //
    // 	// User is the type of value stored in the Contexts.
    // 	type User struct {...}
    //
    // 	// key is an unexported type for keys defined in this package.
    // 	// This prevents collisions with keys defined in other packages.
    // 	type key int
    //
    // 	// userKey is the key for user.User values in Contexts. It is
    // 	// unexported; clients use user.NewContext and user.FromContext
    // 	// instead of using this key directly.
    // 	var userKey key = 0
    //
    // 	// NewContext returns a new Context that carries value u.
    // 	func NewContext(ctx context.Context, u *User) context.Context {
    // 		return context.WithValue(ctx, userKey, u)
    // 	}
    //
    // 	// FromContext returns the User value stored in ctx, if any.
    // 	func FromContext(ctx context.Context) (*User, bool) {
    // 		u, ok := ctx.Value(userKey).(*User)
    // 		return u, ok
    // 	}
    Value(key interface{}) interface{}
}

func Background ¶

func Background() Context

Background returns a non-nil, empty Context. It is never canceled, has no values, and has no deadline. It is typically used by the main function, initialization, and tests, and as the top-level Context for incoming requests.

func TODO ¶

func TODO() Context

TODO returns a non-nil, empty Context. Code should use context.TODO when it's unclear which Context to use or it is not yet available (because the surrounding function has not yet been extended to accept a Context parameter). TODO is recognized by static analysis tools that determine whether Contexts are propagated correctly in a program.

func WithCancel ¶

func WithCancel(parent Context) (ctx Context, cancel CancelFunc)

WithCancel returns a copy of parent with a new Done channel. The returned context's Done channel is closed when the returned cancel function is called or when the parent context's Done channel is closed, whichever happens first.

Canceling this context releases resources associated with it, so code should call cancel as soon as the operations running in this Context complete.

func WithDeadline ¶

func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc)

WithDeadline returns a copy of the parent context with the deadline adjusted to be no later than d. If the parent's deadline is already earlier than d, WithDeadline(parent, d) is semantically equivalent to parent. The returned context's Done channel is closed when the deadline expires, when the returned cancel function is called, or when the parent context's Done channel is closed, whichever happens first.

Canceling this context releases resources associated with it, so code should call cancel as soon as the operations running in this Context complete.

func WithTimeout ¶

func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc)

WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).

Canceling this context releases resources associated with it, so code should call cancel as soon as the operations running in this Context complete:

func slowOperationWithTimeout(ctx context.Context) (Result, error) {
	ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
	defer cancel()  // releases resources if slowOperation completes before timeout elapses
	return slowOperation(ctx)
}

// Pass a context with a timeout to tell a blocking function that it
// should abandon its work after the timeout elapses.
ctx, _ := context.WithTimeout(context.Background(), 100*time.Millisecond)
select {
case <-time.After(200 * time.Millisecond):
    fmt.Println("overslept")
case <-ctx.Done():
    fmt.Println(ctx.Err()) // prints "context deadline exceeded"
}

context deadline exceeded

func WithValue ¶

func WithValue(parent Context, key interface{}, val interface{}) Context

WithValue returns a copy of parent in which the value associated with key is val.

Use context Values only for request-scoped data that transits processes and APIs, not for passing optional parameters to functions.

Subdirectories