Flutter Tutorial - Layout Widgets #15 Flow

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A widget that sizes and positions children efficiently, according to the logic in a FlowDelegate.

Flow layouts are optimized for repositioning children using transformation matrices.

The flow container is sized independently from the children by the FlowDelegate.getSize function of the delegate. The children are then sized independently given the constraints from the FlowDelegate.getConstraintsForChild function.

Rather than positioning the children during layout, the children are positioned using transformation matrices during the paint phase using the matrices from the FlowDelegate.paintChildren function. The children can be repositioned efficiently by simply repainting the flow, which happens without the children being laid out again (contrast this with a Stack, which does the sizing and positioning together during layout).

The most efficient way to trigger a repaint of the flow is to supply an animation to the constructor of the FlowDelegate. The flow will listen to this animation and repaint whenever the animation ticks, avoiding both the build and layout phases of the pipeline.


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