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(⏸︎) Replace TileLayer.errorImage with TileLayer.tilePlaceholder #1587

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(⏸︎) Replace TileLayer.errorImage with TileLayer.tilePlaceholder #1587

wants to merge 5 commits into from
+372 −131

Commits on Aug 3, 2023

  1. Replace TileLayer's error image option with a placeholder image option 

    When a tile fails to load and there are no existing loaded tiles which
overlay with it from another zoom level we are left with a blank portion
on the map. When there are no loaded tiles visible, for example because
of a network problem, there is no visual feedback when
zooming/panning/rotating because the map is just a single solid color.
This prompted me to add a placeholder image.

We already have an error image implementation but the error image will
not show up until loading fails which, in the case of a poor network
connection, can take quite a while.

The placeholder implementation gives visual feedback both whilst loading
and when loading fails.

Note that the current implementation just fills the viewport with the
placeholder tile behind all other tiles. It would be more efficient to
avoid adding placeholder tiles which are completely obscured by other
visual tiles but my attempts to do this were unsuccessful due to the
complex nature of tile pruning.
    @rorystephenson
    rorystephenson committed Aug 3, 2023
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  2. Only create placeholder tiles where tiles are missing or not fully tr… 

    …ansitioned at the current zoom

This reduces the number of placeholder widgets. The algorithm used to
determine which coordinates to show placeholders for is a tradeoff
between speed and the number of redundant placeholders created. It
creates a placeholder for every tile at the current zoom which failed to
load or is still transitioning. This means that if a tile from a lower
zoom obscures the placeholder or multiple tiles from a higher zoom
collectively obscure the placeholder it will be unnecessarily created.
It would be possible to avoid this but it would require a more complex
data structure or iterating all of the tiles for every potential
placeholder.
    @rorystephenson
    rorystephenson committed Aug 3, 2023
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  3. Display tile placeholders by default and use a builder instead of Ima… 

    …geProvider for placeholders.
    @rorystephenson
    rorystephenson committed Aug 3, 2023
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  4. Simplify placeholder creation 

    Now that placeholder is a widget instead of an ImageProvider we can
simplify the construction by making it a CustomPaint.
    @rorystephenson
    rorystephenson committed Aug 3, 2023
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  5. Simplify tile placeholder options

    @rorystephenson
    rorystephenson committed Aug 3, 2023
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