Jetpack compose学习笔记之ConstraintLayout（布局）

一，简介

Jetpack compose中没有提供ConstraintLayout支持，所以需要添加下面的依赖来导入。

// build.gradle
implementation "androidx.constraintlayout:constraintlayout-compose:1.0.0-alpha07"

在Compose中，ConstraintLayout需要通过DSL来使用。

• createRefs()或者createRef()创建references，ConstraintLayout中的每一个composable都需要创建（guidelines，barriers不需要）
• constrainAs可以把reference当作参数，然后在lambda中设置约束关系
• linkTo来表明约束关系
• parent是已经存在的reference，标识ConstraintLayout本身

二，示例

效果一：约束Button和Text的关系

@Composable
fun ConstraintLayoutContent() {
    ConstraintLayout {
        // 创建references
        val (button, text) = createRefs()

        Button(
            onClick = {},
            // reference "button"相当于原来的id
            // 约束在constraintlayout的top，margin为16dp
            modifier = Modifier.constrainAs(button) {
                top.linkTo(parent.top, margin = 16.dp)
            }
        ) {
            Text("Button")
        }
        // reference "text"
        // 约束Text在Button的下面，margin为16dp
        Text("Text", Modifier.constrainAs(text) {
            top.linkTo(button.bottom, margin = 16.dp)
        })
    }
}

效果二：Text在parent中居中显示

@Composable
fun ConstraintLayoutContent() {
    ConstraintLayout {
        // 和上面的例子一致
         ...

        // reference "text"
        // 约束Text在Button的下面，margin为16dp
        Text("Text", Modifier.constrainAs(text) {
            top.linkTo(button.bottom, margin = 16.dp)
            // Text在parent居中
            // ConstraintLayout默认是wrap_content的
            centerHorizontallyTo(parent)
        })
    }
}

效果三：创建Barrier

@Composable
fun ConstraintLayoutContent() {
    ConstraintLayout {
        // 创建references
        val (button1, button2, text) = createRefs()

        Button(
            onClick = {},
            modifier = Modifier.constrainAs(button1) {
                top.linkTo(parent.top, margin = 16.dp)
            }
        ) {
            Text("Button1")
        }
        // Text显示在Button1的下方，Text的中线和Button1的end对齐
        Text("Text", Modifier.constrainAs(text) {
            top.linkTo(button1.bottom, margin = 16.dp)
            centerAround(button1.end)
        })
        // 根据button1和text的end（取其长）创建barrier
        val barrier = createEndBarrier(button1, text)
        // Button2显示在barrier开始处
        Button(
            onClick = {},
            modifier = Modifier.constrainAs(button2) {
                top.linkTo(parent.top, margin = 16.dp)
                start.linkTo(barrier)
            }
        ) {
            Text("Button2")
        }
    }
}

效果四：创建guideline

@Composable
fun LargeConstraintLayout() {
    ConstraintLayout {
        val text = createRef()
        // 设置为屏幕宽度的一半
        val guideline = createGuidelineFromStart(fraction = 0.5f)
        Text(
            "This is a very very very very very very long text",
            Modifier.constrainAs(text) {
                linkTo(start = guideline, end = parent.end)
            }
        )
    }
}

效果五：设置Text的宽度为wrap_content

@Composable
fun LargeConstraintLayout() {
    ConstraintLayout {
        val text = createRef()

        val guideline = createGuidelineFromStart(fraction = 0.5f)
        Text(
            "This is a very very very very very very long text",
            Modifier.constrainAs(text) {
                linkTo(start = guideline, end = parent.end)
                width = Dimension.preferredWrapContent
            }
        )
    }
}

除了preferredWrapContent，还有其他几个设置项

preferredWrapContent	the layout is wrap content, subject to the constraints in that dimension.
wrapContent	the layout is wrap content even if the constraints would not allow it.
fillToConstraints	the layout will expand to fill the space defined by its constraints in that dimension.
preferredValue	the layout is a fixed dp value, subject to the constraints in that dimension.
value	the layout is a fixed dp value, regardless of the constraints in that dimension.

也可以设置在固定值范围内，如

width = Dimension.preferredWrapContent.atLeast(100.dp)

效果六：横屏和纵屏的情况下，margin不一样

在ConstraintSet中通过createRefFor创建reference，创建不同的ConstraintSet，传递给ConstraintLayout。

@Composable
fun DecoupledConstraintLayout() {
    BoxWithConstraints {
        val constraints = if (maxWidth < maxHeight) {
            // 竖屏
            decoupledConstraints(margin = 16.dp)
        } else {
            // 横屏
            decoupledConstraints(margin = 32.dp)
        }

        ConstraintLayout(constraints) {
            Button(
                onClick = {},
                modifier = Modifier.layoutId("button")
            ) {
                Text("Button")
            }
            Text("Text", Modifier.layoutId("text"))
        }
    }
}

private fun decoupledConstraints(margin: Dp): ConstraintSet {
    return ConstraintSet {
        val button = createRefFor("button")
        val text = createRefFor("text")

        constrain(button) {
            top.linkTo(parent.top, margin = margin)
        }
        constrain(text) {
            top.linkTo(button.bottom, margin)
        }
    }
}

更多内容请参看Layouts in Jetpack Compose (google.cn)