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1 <?php
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2
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3 namespace Drupal\Core\Render;
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4
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5 use Drupal\Core\Cache\CacheFactoryInterface;
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6 use Drupal\Core\Cache\Context\CacheContextsManager;
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7 use Symfony\Component\HttpFoundation\RequestStack;
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8
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9 /**
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10 * Adds automatic placeholdering to the RenderCache.
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11 *
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12 * This automatic placeholdering is performed to ensure the containing elements
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13 * and overarching response are as cacheable as possible. Elements whose subtree
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14 * bubble either max-age=0 or high-cardinality cache contexts (such as 'user'
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15 * and 'session') are considered poorly cacheable.
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16 *
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17 * @see sites/default/default.services.yml
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18 *
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19 * Automatic placeholdering is performed only on elements whose subtree was
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20 * generated using a #lazy_builder callback and whose bubbled cacheability meets
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21 * the auto-placeholdering conditions as configured in the renderer.config
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22 * container parameter.
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23 *
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24 * This RenderCache implementation automatically replaces an element with a
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25 * placeholder:
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26 * - on render cache hit, i.e. ::get()
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27 * - on render cache miss, i.e. ::set() (in subsequent requests, it will be a
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28 * cache hit)
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29 *
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30 * In either case, the render cache is guaranteed to contain the to-be-rendered
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31 * placeholder, so replacing (rendering) the placeholder will be very fast.
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32 *
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33 * Finally, in case the render cache item disappears between the time it is
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34 * decided to automatically placeholder the element and the time where the
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35 * placeholder is replaced (rendered), that is guaranteed to not be problematic.
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36 * Because this only automatically placeholders elements that have a
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37 * #lazy_builder callback set, which means that in the worst case, it will need
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38 * to be re-rendered.
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39 */
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40 class PlaceholderingRenderCache extends RenderCache {
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41
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42 /**
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43 * The placeholder generator.
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44 *
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45 * @var \Drupal\Core\Render\PlaceholderGeneratorInterface
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46 */
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47 protected $placeholderGenerator;
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48
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49 /**
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50 * Stores rendered results for automatically placeholdered elements.
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51 *
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52 * This allows us to avoid talking to the cache twice per auto-placeholdered
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53 * element, or in case of an uncacheable element, to render it twice.
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54 *
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55 * Scenario A. The double cache read would happen because:
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56 * 1. when rendering, cache read, but auto-placeholdered
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57 * 2. when rendering placeholders, again cache read
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58 *
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59 * Scenario B. The cache write plus read would happen because:
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60 * 1. when rendering, cache write, but auto-placeholdered
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61 * 2. when rendering placeholders, cache read
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62 *
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63 * Scenario C. The double rendering for an uncacheable element would happen because:
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64 * 1. when rendering, not cacheable, but auto-placeholdered
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65 * 2. when rendering placeholders, rendered again
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66 *
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67 * In all three scenarios, this static cache avoids the second step, thus
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68 * avoiding expensive work.
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69 *
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70 * @var array
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71 */
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72 protected $placeholderResultsCache = [];
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73
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74 /**
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75 * Constructs a new PlaceholderingRenderCache object.
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76 *
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77 * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
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78 * The request stack.
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79 * @param \Drupal\Core\Cache\CacheFactoryInterface $cache_factory
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80 * The cache factory.
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81 * @param \Drupal\Core\Cache\Context\CacheContextsManager $cache_contexts_manager
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82 * The cache contexts manager.
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83 * @param \Drupal\Core\Render\PlaceholderGeneratorInterface $placeholder_generator
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84 * The placeholder generator.
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85 */
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86 public function __construct(RequestStack $request_stack, CacheFactoryInterface $cache_factory, CacheContextsManager $cache_contexts_manager, PlaceholderGeneratorInterface $placeholder_generator) {
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87 parent::__construct($request_stack, $cache_factory, $cache_contexts_manager);
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88 $this->placeholderGenerator = $placeholder_generator;
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89 }
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90
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91 /**
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92 * {@inheritdoc}
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93 */
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94 public function get(array $elements) {
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95 // @todo remove this check when https://www.drupal.org/node/2367555 lands.
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96 if (!$this->requestStack->getCurrentRequest()->isMethodCacheable()) {
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97 return FALSE;
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98 }
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99
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100 // When rendering placeholders, special case auto-placeholdered elements:
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101 // avoid retrieving them from cache again, or rendering them again.
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102 if (isset($elements['#create_placeholder']) && $elements['#create_placeholder'] === FALSE) {
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103 $cached_placeholder_result = $this->getFromPlaceholderResultsCache($elements);
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104 if ($cached_placeholder_result !== FALSE) {
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105 return $cached_placeholder_result;
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106 }
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107 }
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108
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109 $cached_element = parent::get($elements);
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110
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111 if ($cached_element === FALSE) {
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112 return FALSE;
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113 }
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114 else {
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115 if ($this->placeholderGenerator->canCreatePlaceholder($elements) && $this->placeholderGenerator->shouldAutomaticallyPlaceholder($cached_element)) {
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116 return $this->createPlaceholderAndRemember($cached_element, $elements);
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117 }
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118
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119 return $cached_element;
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120 }
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121 }
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122
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123 /**
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124 * {@inheritdoc}
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125 */
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126 public function set(array &$elements, array $pre_bubbling_elements) {
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127 $result = parent::set($elements, $pre_bubbling_elements);
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128
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129 // @todo remove this check when https://www.drupal.org/node/2367555 lands.
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130 if (!$this->requestStack->getCurrentRequest()->isMethodCacheable()) {
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131 return FALSE;
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132 }
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133
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134 if ($this->placeholderGenerator->canCreatePlaceholder($pre_bubbling_elements) && $this->placeholderGenerator->shouldAutomaticallyPlaceholder($elements)) {
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135 // Overwrite $elements with a placeholder. The Renderer (which called this
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136 // method) will update the context with the bubbleable metadata of the
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137 // overwritten $elements.
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138 $elements = $this->createPlaceholderAndRemember($this->getCacheableRenderArray($elements), $pre_bubbling_elements);
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139 }
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140
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141 return $result;
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142 }
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143
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144 /**
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145 * Create a placeholder for a renderable array and remember in a static cache.
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146 *
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147 * @param array $rendered_elements
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148 * A fully rendered renderable array.
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149 * @param array $pre_bubbling_elements
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150 * A renderable array corresponding to the state (in particular, the
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151 * cacheability metadata) of $rendered_elements prior to the beginning of
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152 * its rendering process, and therefore before any bubbling of child
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153 * information has taken place. Only the #cache property is used by this
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154 * function, so the caller may omit all other properties and children from
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155 * this array.
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156 *
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157 * @return array
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158 * Renderable array with placeholder markup and the attached placeholder
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159 * replacement metadata.
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160 */
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161 protected function createPlaceholderAndRemember(array $rendered_elements, array $pre_bubbling_elements) {
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162 $placeholder_element = $this->placeholderGenerator->createPlaceholder($pre_bubbling_elements);
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163 // Remember the result for this placeholder to avoid double work.
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164 $placeholder = (string) $placeholder_element['#markup'];
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165 $this->placeholderResultsCache[$placeholder] = $rendered_elements;
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166 return $placeholder_element;
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167 }
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168
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169 /**
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170 * Retrieves an auto-placeholdered renderable array from the static cache.
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171 *
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172 * @param array $elements
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173 * A renderable array.
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174 *
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175 * @return array|false
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176 * A renderable array, with the original element and all its children pre-
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177 * rendered, or FALSE if no cached copy of the element is available.
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178 */
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179 protected function getFromPlaceholderResultsCache(array $elements) {
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180 $placeholder_element = $this->placeholderGenerator->createPlaceholder($elements);
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181 $placeholder = (string) $placeholder_element['#markup'];
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182 if (isset($this->placeholderResultsCache[$placeholder])) {
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183 return $this->placeholderResultsCache[$placeholder];
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184 }
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185 return FALSE;
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186 }
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187
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188 }
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