Clear up distinction between previewing vs saving values in a snapshot

[westonruter]

There seems to be somewhat of a gray area in Customize Snapshots, and in the Customizer in general for previewing vs saving values. In the Customize Snapshots plugin the can_preview method is used for both of these conditions, it seems going for the save check by seeing if is_admin(). I think perhaps they should be separated out a but more to make it more black and white?

diff --git a/php/class-customize-snapshot-manager.php b/php/class-customize-snapshot-manager.php
index dab955d..0c1208d 100644
--- a/php/class-customize-snapshot-manager.php
+++ b/php/class-customize-snapshot-manager.php
@@ -305,7 +305,7 @@ class Customize_Snapshot_Manager {
        }

        foreach ( $this->customize_manager->settings() as $setting ) {
-           if ( $this->can_preview( $setting, $this->unsanitized_snapshot_post_data ) ) {
+           if ( $this->can_preview( $setting, $this->unsanitized_snapshot_post_data ) && $setting->check_capabilities() ) {
                $post_data = $this->unsanitized_snapshot_post_data[ $setting->id ];
                $this->snapshot->set( $setting, $post_data['value'], $post_data['dirty'] );
            }
@@ -518,9 +518,6 @@ class Customize_Snapshot_Manager {
        if ( ! ( $setting instanceof \WP_Customize_Setting ) ) {
            return false;
        }
-       if ( ! $setting->check_capabilities() && is_admin() ) {
-           return false;
-       }
        if ( ! array_key_exists( $setting->id, $values ) ) {
            return false;
        }
@@ -538,7 +535,7 @@ class Customize_Snapshot_Manager {
            $this->customize_manager->add_dynamic_settings( array_keys( $values ) );

            foreach ( $this->snapshot->settings() as $setting ) {
-               if ( $this->can_preview( $setting, $values ) ) {
+               if ( $this->can_preview( $setting, $values ) && $setting->check_capabilities() ) {
                    $this->customize_manager->set_post_value( $setting->id, $values[ $setting->id ] );
                }
            }

I think maybe the can_preview method should be removed entirely?

diff --git a/php/class-customize-snapshot-manager.php b/php/class-customize-snapshot-manager.php
index dab955d..6ef1a7d 100644
--- a/php/class-customize-snapshot-manager.php
+++ b/php/class-customize-snapshot-manager.php
@@ -305,7 +305,7 @@ class Customize_Snapshot_Manager {
        }

        foreach ( $this->customize_manager->settings() as $setting ) {
-           if ( $this->can_preview( $setting, $this->unsanitized_snapshot_post_data ) ) {
+           if ( array_key_exists( $setting->id, $this->unsanitized_snapshot_post_data ) && $setting->check_capabilities() ) {
                $post_data = $this->unsanitized_snapshot_post_data[ $setting->id ];
                $this->snapshot->set( $setting, $post_data['value'], $post_data['dirty'] );
            }
@@ -508,26 +508,6 @@ class Customize_Snapshot_Manager {
    }

    /**
-    * Check if the setting can be previewed.
-    *
-    * @param \WP_Customize_Setting $setting A WP_Customize_Setting derived object.
-    * @param array                 $values  All settings' values in the snapshot.
-    * @return bool
-    */
-   public function can_preview( $setting, $values ) {
-       if ( ! ( $setting instanceof \WP_Customize_Setting ) ) {
-           return false;
-       }
-       if ( ! $setting->check_capabilities() && is_admin() ) {
-           return false;
-       }
-       if ( ! array_key_exists( $setting->id, $values ) ) {
-           return false;
-       }
-       return true;
-   }
-
-   /**
     * Set the snapshot settings post value.
     */
    public function set_post_values() {
@@ -538,7 +518,7 @@ class Customize_Snapshot_Manager {
            $this->customize_manager->add_dynamic_settings( array_keys( $values ) );

            foreach ( $this->snapshot->settings() as $setting ) {
-               if ( $this->can_preview( $setting, $values ) ) {
+               if ( $setting->check_capabilities() ) {
                    $this->customize_manager->set_post_value( $setting->id, $values[ $setting->id ] );
                }
            }
@@ -566,7 +546,8 @@ class Customize_Snapshot_Manager {
                $values = $this->snapshot->values();

                foreach ( $this->snapshot->settings() as $setting ) {
-                   if ( $this->can_preview( $setting, $values ) ) {
+                   $is_dirty = array_key_exists( $setting->id, $values );
+                   if ( $is_dirty ) {
                        $setting->preview();
                        $setting->dirty = true;
                    }