Refactor a bit.

service/memcache/context.go

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package memcache
+
+import (
+        "golang.org/x/net/context"
+)
+
+type key int
+
+var (
+        memcacheKey       key
+        memcacheFilterKey key = 1
+)
+
+// Factory is the function signature for factory methods compatible with
+// SetFactory.
+type Factory func(context.Context) Interface
+
+// Filter is the function signature for a filter MC implementation. It
+// gets the current MC implementation, and returns a new MC implementation
+// backed by the one passed in.
+type Filter func(context.Context, Interface) Interface
+
+// GetUnfiltered gets gets the Interface implementation from context without
+// any of the filters applied.
+func GetUnfiltered(c context.Context) Interface {
+        if f, ok := c.Value(memcacheKey).(Factory); ok && f != nil {
+                return f(c)
+        }
+        return nil
+}
+
+// Get gets the current memcache implementation from the context.
+func Get(c context.Context) Interface {
+        ret := GetUnfiltered(c)
+        if ret == nil {
+                return nil
+        }
+        for _, f := range getCurFilters(c) {
+                ret = f(c, ret)
+        }
+        return ret
+}
+
+// SetFactory sets the function to produce Interface instances, as returned by
+// the Get method.
+func SetFactory(c context.Context, mcf Factory) context.Context {
+        return context.WithValue(c, memcacheKey, mcf)
+}
+
+// Set sets the current Interface object in the context. Useful for testing
+// with a quick mock. This is just a shorthand SetFactory invocation to set
+// a factory which always returns the same object.
+func Set(c context.Context, mc Interface) context.Context {
+        return SetFactory(c, func(context.Context) Interface { return mc })
+}
+
+func getCurFilters(c context.Context) []Filter {
+        curFiltsI := c.Value(memcacheFilterKey)
+        if curFiltsI != nil {
+                return curFiltsI.([]Filter)
+        }
+        return nil
+}
+
+// AddFilters adds Interface filters to the context.
+func AddFilters(c context.Context, filts ...Filter) context.Context {
+        if len(filts) == 0 {
+                return c
+        }
+        cur := getCurFilters(c)
+        newFilts := make([]Filter, 0, len(cur)+len(filts))
+        newFilts = append(newFilts, getCurFilters(c)...)
+        newFilts = append(newFilts, filts...)
+        return context.WithValue(c, memcacheFilterKey, newFilts)
+}