APIcomponents

类型 MessageScrollState

API 文档由 Go 源码生成，内容为英文。

type MessageScrollState struct {
	// AutoScroll keeps the transcript at its end while the reader is at the end:
	// the official autoScroll. Off, the offset only moves when told to.
	AutoScroll bool
	// PreviousPeek is how much of the item before an anchored turn stays visible
	// above it; MessageScrollPreviousPeek is the official 64.
	PreviousPeek float32

	// content height including the room an anchored turn grows into

}

MessageScrollState is the scroll policy of a chat transcript, the part of the official MessageScroller provider that is not a DOM observer: it follows the live edge while the reader is there, anchors a new turn near the top of the viewport, and keeps the reader's place when older rows are prepended. It owns one number, the offset, and moves it only from what the caller tells it, so it needs no timer and schedules no frame; the caller paints the transcript at Offset and asks for a frame itself when its content changes.

The caller feeds it three kinds of input:

  • the reader's scrolling (ScrollBy, ScrollTo), which releases the live edge when it moves away from it and takes hold of it again on reaching it;

  • commands (ScrollToEnd, ScrollToStart, AnchorTurn, Prepended);

  • the sizes of the viewport and the content, once per paint, in Resolve, which returns the offset to paint at.

The zero value sits at the start, follows nothing and keeps no peek.

state := components.MessageScrollState{AutoScroll: true}
state.ScrollToEnd()                      // open at the newest message
offset := state.Resolve(viewport.Height, content) // every paint
state.ScrollBy(wheelDelta)               // on a wheel event

方法

AnchorTurn#

func (state *MessageScrollState) AnchorTurn(top float32)

AnchorTurn puts the item whose top is at top (in content space) near the top of the viewport, PreviousPeek below it, and holds it there while its reply streams in below. With AutoScroll the transcript follows the end once the reply has grown to fill the viewport; without it the turn stays where it is until the reader moves.

AtEnd#

func (state *MessageScrollState) AtEnd() bool

AtEnd reports whether the viewport is within MessageScrollerEdgeThreshold of the end of the content as of the last Resolve.

Extent#

func (state *MessageScrollState) Extent() float32

Extent is the content height Resolve last settled on: the caller's, or more while a turn is anchored, so that there is room below the anchor for the reply to grow into. Give it to BeginMessageScroller as the content extent.

Following#

func (state *MessageScrollState) Following() bool

Following reports whether the transcript is pinned to its end and follows new content.

Offset#

func (state *MessageScrollState) Offset() float32

Offset is the offset resolved by the last Resolve, after the reader's scrolling since.

Prepended#

func (state *MessageScrollState) Prepended(height float32)

Prepended tells the state that height of content was inserted above what it was showing, older history loaded while the reader looks at the transcript. The offset moves by the same amount at the next Resolve so the rows on screen stay where they are. A transcript that follows its end needs no correction.

Resolve#

func (state *MessageScrollState) Resolve(viewport, extent float32) float32

Resolve settles the offset for a viewport and content of the given heights, applying the commands given since the last call, and returns it. Call it once per paint.

ScrollBy#

func (state *MessageScrollState) ScrollBy(delta float32)

ScrollBy moves the offset by delta, positive toward the end, as the reader's wheel, keys or scrollbar do. Moving toward the start releases the live edge; reaching the end takes hold of it again. Any reader scrolling releases an anchored turn.

ScrollTo#

func (state *MessageScrollState) ScrollTo(offset float32)

ScrollTo moves the offset to an absolute position the reader chose, such as a dragged scrollbar thumb. It follows the same rules as ScrollBy.

ScrollToEnd#

func (state *MessageScrollState) ScrollToEnd()

ScrollToEnd jumps to the end at the next Resolve and, with AutoScroll, follows the content from there: the jump button, a sent message, opening at the newest message.

ScrollToStart#

func (state *MessageScrollState) ScrollToStart()

ScrollToStart jumps to the start and releases the live edge.