react native实现拖拽/拖放

首先初始化一个react native的项目，cd到你的目录下运行

react-native init rndd

创建一个新的组件./app/components/Draggable.js

import React, {
    Component,
} from 'react';
import {
    StyleSheet,
    Image,
    PanResponder,
    Animated,
} from 'react-native';
class Draggable extends Component{
    constructor(props){
        super(props);
    }
    render(){
        return (
            <Animated.View style={styles.container}>
                <Image style={{width:80,height:80}} source={require('../assets/react-native.jpg')}/>
            </Animated.View>
        )
    }
}
export default Draggable;
const styles = StyleSheet.create({
    container: {
        position: 'absolute',
        left: 100,
        top: 100,
    }
});

加上PanResponder

PanResponder的api可以上官网查看PanResponder

componentWillMount() {
    this._panResponder = PanResponder.create({
        onMoveShouldSetResponderCapture: () => true,
        onMoveShouldSetPanResponderCapture: () => true,
        onPanResponderGrant: (e, gestureState) => {
        },
        onPanResponderMove: Animated.event([
        ]),
        onPanResponderRelease: (e, {vx, vy}) => {
        }
    });
}
render(){
    return (
        <Animated.View style={styles.container} {...this._panResponder.panHandlers}>
            <Image style={{width:80,height:80}} source={require('../assets/react-native.jpg')}/>
        </Animated.View>
    )
}

响应拖拽事件

先在state中创建一个对象来记录拖拽的记录。

constructor(props) {
    super(props);
    this.state = {
        pan: new Animated.ValueXY()
    };
}

更新panHandler：

componentWillMount() {
    this._panResponder = PanResponder.create({
        onMoveShouldSetResponderCapture: () => true,
        onMoveShouldSetPanResponderCapture: () => true,
        // 设置初始位置
        onPanResponderGrant: (e, gestureState) => {
            this.state.pan.setValue({x: 0, y: 0});
        },
        // 使用拖拽的偏移量来定位
        onPanResponderMove: Animated.event([
            null, {dx: this.state.pan.x, dy: this.state.pan.y},
        ]),
        onPanResponderRelease: (e, {vx, vy}) => {
        }
    });
}

更新render方法

render(){
    // 从state中取出pan
    const { pan } = this.state;
    // 从pan里计算出偏移量
    const [translateX, translateY] = [pan.x, pan.y];
    // 设置transform为偏移量
    const imageStyle = {transform: [{translateX}, {translateY}]};
    return (
        <Animated.View style={[styles.container,imageStyle]} {...this._panResponder.panHandlers}>
            <Image style={{width:80,height:80}} source={require('../assets/react-native.jpg')}/>
        </Animated.View>
    )
}

这个时候我们刷新用CMD+R刷新，发现已经可以拖动这个图片了，但是有个问题，第二次拖拽的时候又从原点开始移动，我们接下来解决这个问题。
我们有几个选择：

1. 让拖动的元素在释放的时候回到原来的位置：
   修改PanResponder的onPanResponderRelease方法

   onPanResponderRelease: (e, {vx, vy}) => {
       Animated.spring(
           this.state.pan,
           {toValue: {x: 0, y: 0}}
       ).start();
   }
   

2. 让拖动的元素停留在拖拽到的位置：
   修改PanResponder的onPanResponderGrant和onPanResponderRelease方法

   this._panResponder = PanResponder.create({
       onMoveShouldSetResponderCapture: () => true,
       onMoveShouldSetPanResponderCapture: () => true,
       // 设置初始位置
       onPanResponderGrant: (e, gestureState) => {
           this.state.pan.setOffset({
               x: this.state.pan.x._value,
               y: this.state.pan.y._value
           });
           this.state.pan.setValue({x: 0, y: 0});
       },
       // 使用拖拽的偏移量来定位
       onPanResponderMove: Animated.event([
           null, {dx: this.state.pan.x, dy: this.state.pan.y},
       ]),
       onPanResponderRelease: (e, {vx, vy}) => {
           this.state.pan.flattenOffset();
       }
   });
   

我们再来加上拖拽时候放大的效果

先在state里加上一个scale用来记录放大的倍数

this.state = {
    pan: new Animated.ValueXY(),
    scale: new Animated.Value(1)
};

我们要在render里使用放大的值

render(){
    // 从state中取出pan和scale
    const { pan, scale } = this.state;
    // 从pan里计算出偏移量
    const [translateX, translateY] = [pan.x, pan.y];
    // 设置transform为偏移量
    const imageStyle = {transform: [{translateX}, {translateY},  {scale}]};
    return (
        <Animated.View style={[styles.container,imageStyle]} {...this._panResponder.panHandlers}>
            <Image style={{width:80,height:80}} source={require('../assets/react-native.jpg')}/>
        </Animated.View>
    )
}

在拖拽的时候变大：

onPanResponderGrant: (e, gestureState) => {
    this.state.pan.setOffset({
        x: this.state.pan.x._value,
        y: this.state.pan.y._value
    });
    this.state.pan.setValue({x: 0, y: 0});
    Animated.spring(
        this.state.scale,
        { toValue: 1.3, friction: 3 }
    ).start();
},

在释放的时候缩小到原来的样子：

onPanResponderRelease: (e, {vx, vy}) => {
    this.state.pan.flattenOffset();
    Animated.spring(
        this.state.scale,
        { toValue: 1, friction: 3 }
    ).start();
}

加上拖拽的旋转效果：

先在state中加上一个rotate用来记录旋转的角度

this.state = {
    pan: new Animated.ValueXY(),
    scale: new Animated.Value(1),
    rotate: new Animated.Value(0)
};

在拖拽的时候旋转，需要在onPanResponderGrant里加上

Animated.timing(this.state.rotate, {
    toValue: 25, // 旋转25%，render里interpolate出deg的值
    duration: 300
}).start();

在释放的时候恢复原状，需要在onPanResponderRelease中加上

Animated.timing(this.state.rotate, {
    toValue: 0,
    duration: 300
}).start();

render函数里设置transform：

render(){
    // 从state中取出pan
    const { pan, scale } = this.state;
    // 从pan里计算出偏移量
    const [translateX, translateY] = [pan.x, pan.y];
    // 计算旋转
    const rotate = this.state.rotate.interpolate({
        inputRange: [0, 100],
        outputRange: ['0deg', '360deg']
    });
    // 设置transform为偏移量
    const imageStyle = {transform: [{translateX}, {translateY},  {scale}, {rotate}]};
    return (
        <Animated.View style={[styles.container,imageStyle]} {...this._panResponder.panHandlers}>
            <Image style={{width:80,height:80}} source={require('../assets/react-native.jpg')}/>
        </Animated.View>
    )
}

完整代码：

https://github.com/shengoo/rndd

import React, {
    Component,
} from 'react';
import {
    StyleSheet,
    Image,
    PanResponder,
    Animated,
} from 'react-native';
class Draggable extends Component{
    constructor(props){
        super(props);
        this.state = {
            pan: new Animated.ValueXY(),
            scale: new Animated.Value(1),
            rotate: new Animated.Value(0)
        };
    }
    componentWillMount() {
        this._panResponder = PanResponder.create({
            onMoveShouldSetResponderCapture: () => true,
            onMoveShouldSetPanResponderCapture: () => true,
            // 设置初始位置
            onPanResponderGrant: (e, gestureState) => {
                this.state.pan.setOffset({
                    x: this.state.pan.x._value,
                    y: this.state.pan.y._value
                });
                this.state.pan.setValue({x: 0, y: 0});
                Animated.spring(this.state.scale, {
                    toValue: 1.3,
                    friction: 3 }
                ).start();
                Animated.timing(this.state.rotate, {
                    toValue: 25,
                    duration: 300
                }).start();
            },
            // 使用拖拽的偏移量来定位
            onPanResponderMove: Animated.event([
                null, {dx: this.state.pan.x, dy: this.state.pan.y},
            ]),
            onPanResponderRelease: (e, {vx, vy}) => {
                this.state.pan.flattenOffset();
                // Animated.spring(
                //     this.state.pan,
                //     {toValue: {x: 0, y: 0}}
                // ).start();
                Animated.spring(
                    this.state.scale,
                    { toValue: 1, friction: 3 }
                ).start();
                Animated.timing(this.state.rotate, {
                    toValue: 0,
                    duration: 300
                }).start();
            }
        });
    }
    render(){
        // 从state中取出pan
        const { pan, scale } = this.state;
        // 从pan里计算出偏移量
        const [translateX, translateY] = [pan.x, pan.y];
        // 计算旋转
        const rotate = this.state.rotate.interpolate({
            inputRange: [0, 100],
            outputRange: ['0deg', '360deg']
        });
        // 设置transform为偏移量
        const imageStyle = {transform: [{translateX}, {translateY},  {scale}, {rotate}]};
        return (
            <Animated.View style={[styles.container,imageStyle]} {...this._panResponder.panHandlers}>
                <Image style={{width:80,height:80}} source={require('../assets/react-native.jpg')}/>
            </Animated.View>
        )
    }
}
export default Draggable;
const styles = StyleSheet.create({
    container: {
        position: 'absolute',
        left: 100,
        top: 100,
    }
});