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33 lines
1.7 KiB
Markdown
Executable File
33 lines
1.7 KiB
Markdown
Executable File
---
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title: Placeholders
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layout: docs
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permalink: /docs/placeholder-fade-duration.html
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prevPage: image-modification-block.html
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nextPage: accessibility.html
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---
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## ASDisplayNodes may Implement Placeholders
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Any `ASDisplayNode` subclass may implement the `-placeholderImage` method to provide a placeholder that covers content until a node's contents are finished displaying. To use placeholders, set `.placeholderEnabled = YES` and optionally set a `.placeholderFadeDuration`;
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For image drawing, use the node's `.calculatedSize` property.
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<div class = "note">
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The `-placeholderImage` function may be called on a background thread, so it is important that this function is thread safe. Note that `-[UIImage imageNamed:]` is not thread safe when using image assets. Instead use `-[UIImage imageWithContentsOfFile:]`.
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</div>
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An ideal resource for creating placeholder images, including rounded rect solid colored ones or simple square corner ones is the `UIImage+ASConvenience` category methods in Texture.
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See our ancient <a href="https://github.com/texturegroup/texture/tree/master/examples_extra/Placeholders">Placeholders sample app</a> to see this concept, first invented by the Facebook Paper team, in action.
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## `.neverShowPlaceholders`
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Hear <a href="https://youtu.be/RY_X7l1g79Q">Scott Goodson explain</a> placeholders, `.neverShowPlaceholders` and why UIKit doesn't have them.
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## ASNetworkImageNode also have Default Images
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In _addition_ to placeholders, `ASNetworkImageNode`s also have a `.defaultImage` property. While placeholders are meant to be transient, default images will persist if the image node's `.URL` property is `nil` or if the URL fails to load.
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We suggest using default images for avatars, while using placeholder images for photos.
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