348 lines
13 KiB
Markdown
348 lines
13 KiB
Markdown
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本篇主要通过一个简单例子,讨论一下 Dart 代码里一个有趣的现象。
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我们都知道 Dart 里一切都是对象,就连基础类型 `int` 、`double` 、`bool` 也都是 `class` 。
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当我们对于 `int` 、 `double` 这些 `class` 进行的 `+` 、`-` 、`*` 、 `\` 等操作时,其实是执行了这个 `class` 的 `operator` 操作符的操作, 然后返回了新的 `num` 对象。
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![](http://img.cdn.guoshuyu.cn/20211223_DEMO-INTEREST/image1)
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对于这些 `operator` 操作最终会通过 `VM` 去进行实现返回,而本质上 dart 代码也只是文本,需要最终编译成二进制去运行。
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![](http://img.cdn.guoshuyu.cn/20211223_DEMO-INTEREST/image2)
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> **以下例子基于 dart 2.12.3 测试**
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那这里想要讨论什么呢?
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首先我们看一段代码,如下代码所示,可以看到:
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- 首先我们定义了一个叫 `idx` 的 `int` 型参数;
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- 然后在 `for` 循环里添加了三个 `InkWell` 可点击控件;
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- 最后在 `onTap` 里面将 `idx` 打印出来;
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```dart
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class MyHomePage extends StatelessWidget {
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var images = ["RRR", "RRR", "RRR",];
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@override
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Widget build(BuildContext context) {
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List<Widget> contents = [];
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int idx = 0;
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for (var imgUrl in images) {
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contents.add(InkWell(
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onTap: () {
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print("######## $idx");
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},
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child: Container(
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height: 100,
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width: 100,
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color: Colors.red,
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child: Text(imgUrl),
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)));
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idx++;
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}
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return Scaffold(
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appBar: AppBar(),
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body: Center(
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child: Column(
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children: [
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...contents,
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],
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)));
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}
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}
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```
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- 问题来了,**你觉得点击这三个 `InkWell` 打印出来的会是什么结果?**
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- **答案是打印出来的都是 3。**
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为什么呢?让我们看这段代码编译后的逻辑,如下所示代码,可以看到上述代码编译后, **`print` 函数里指向的永远是 `idx` 这个 `int*` 指针,当我们点击时,最终打印出来的都是最后的 `idx` 的值**。
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```c++
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@#C475
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method build(fra2::BuildContext* context) → fra2::Widget* {
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core::List<fra2::Widget*>* contents = core::_GrowableList::•<fra2::Widget*>(0);
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core::int* idx = 0;
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{
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core::Iterator<core::String*>* :sync-for-iterator = this.{main::MyHomePage::images}.{core::Iterable::iterator};
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for (; :sync-for-iterator.{core::Iterator::moveNext}(); ) {
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core::String* imgUrl = :sync-for-iterator.{core::Iterator::current};
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{
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[@vm.call-site-attributes.metadata=receiverType:dart.core::List<library package:flutter/src/widgets/framework.dart::Widget*>*] contents.{core::List::add}(new ink5::InkWell::•(onTap: () → Null {
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core::print("######## ${idx}");
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}, child: new con7::Container::•(height: 100.0, width: 100.0, color: #C40086, $creationLocationd_0dea112b090073317d4: #C66610), $creationLocationd_0dea112b090073317d4: #C66614));
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idx = idx.{core::num::+}(1);
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}
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}
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}
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```
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**那如果我们需要打印出来的是每个 `InkWell` 自己的 `index` 呢?**
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如下代码所示,我们在 for 循环里增加了一个 `index` 参数,把每次 `idx` 都赋值给 `index` ,这样点击打印出来的结果,就会是点击对应的 `index` 。
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```dart
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class MyHomePage extends StatelessWidget {
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var images = ["RRR", "RRR", "RRR",];
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@override
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Widget build(BuildContext context) {
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List<Widget> contents = [];
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int idx = 0;
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for (var imgUrl in images) {
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int index = idx;
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contents.add(InkWell(
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onTap: () {
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print("######## $index");
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},
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child: Container(
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height: 100,
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width: 100,
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color: Colors.red,
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child: Text(imgUrl),
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)));
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idx++;
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}
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return Scaffold(
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appBar: AppBar(),
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body: Center(
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child: Column(
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children: [
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...contents,
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],
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)));
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}
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}
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```
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为什么呢?
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让我们看新编译出来的代码,如下所示,可以看到对了 `core::int* index = idx;` 这段代码,然后回忆下前面所说的,**Dart 里基本类型都是对象,而 `operator` 操作符运算后返回新的对象。**
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这样就等于用 `index` 把每次的操作到保存下来,而 `print` 打印的自然就是每次被保存下来的 `idx` 。
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```c++
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@#C475
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method build(fra2::BuildContext* context) → fra2::Widget* {
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core::List<fra2::Widget*>* contents = core::_GrowableList::•<fra2::Widget*>(0);
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core::int* idx = 0;
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{
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core::Iterator<core::String*>* :sync-for-iterator = this.{main::MyHomePage::images}.{core::Iterable::iterator};
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for (; :sync-for-iterator.{core::Iterator::moveNext}(); ) {
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core::String* imgUrl = :sync-for-iterator.{core::Iterator::current};
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{
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core::int* index = idx;
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[@vm.call-site-attributes.metadata=receiverType:dart.core::List<library package:flutter/src/widgets/framework.dart::Widget*>*] contents.{core::List::add}(new ink5::InkWell::•(onTap: () → Null {
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core::print("######## ${index}");
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}, child: new con7::Container::•(height: 100.0, width: 100.0, color: #C40086, $creationLocationd_0dea112b090073317d4: #C66610), $creationLocationd_0dea112b090073317d4: #C66614));
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idx = idx.{core::num::+}(1);
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}
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}
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}
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```
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**那再来个不一样的写法**。
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如下代码所示,把 `InkWell` 放到一个 `getItem` 函数里返回,然后 `index` 通过函数参数传递进来,可以看到运行后的结果,也是点击对应 `InkWell` 打印对应的 `index` 。
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```dart
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class MyHomePage extends StatelessWidget {
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var images = ["RRR", "RRR", "RRR",];
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@override
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Widget build(BuildContext context) {
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List<Widget> contents = [];
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int idx = 0;
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getItem(int index, String imgUrl) {
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return InkWell(
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onTap: () {
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print("######## $index");
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},
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child: Container(
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height: 100,
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width: 100,
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color: Colors.red,
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child: Text(imgUrl)));
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}
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for (var imgUrl in images) {
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contents.add(getItem(idx, imgUrl));
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idx++;
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}
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return Scaffold(
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appBar: AppBar(),
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body: Center(
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child: Column(
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children: [
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...contents,
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],
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)));
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}
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}
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```
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为什么呢?
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我们继续看编译后的代码,如下代码所示,其实就是每次的 `idx` 都通过 ` getItem.call(idx)` 被 `getItem` 的 `index` 引用,然后下次又再次传递一个对应的 `idx` 进去,原理其实和上面的情况一样,所以每次点击也会打印对应的 `index` 。
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```c++
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@#C475
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method build(fra2::BuildContext* context) → fra2::Widget* {
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core::List<fra2::Widget*>* contents = core::_GrowableList::•<fra2::Widget*>(0);
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core::int* idx = 0;
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function getItem(core::int* index) → ink5::InkWell* {
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return new ink5::InkWell::•(onTap: () → Null {
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core::print("######## ${index}");
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}, child: new con7::Container::•(height: 100.0, width: 100.0, color: #C40086, $creationLocationd_0dea112b090073317d4: #C66610), $creationLocationd_0dea112b090073317d4: #C66614);
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}
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{
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core::Iterator<core::String*>* :sync-for-iterator = this.{main::MyHomePage::images}.{core::Iterable::iterator};
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for (; :sync-for-iterator.{core::Iterator::moveNext}(); ) {
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core::String* imgUrl = :sync-for-iterator.{core::Iterator::current};
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{
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[@vm.call-site-attributes.metadata=receiverType:dart.core::List<library package:flutter/src/widgets/framework.dart::Widget*>*] contents.{core::List::add}([@vm.call-site-attributes.metadata=receiverType:library package:flutter/src/material/ink_well.dart::InkWell* Function(dart.core::int*)*] getItem.call(idx));
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idx = idx.{core::num::+}(1);
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}
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}
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}
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```
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**最后我们再换种写法。**
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如下代码所示,直接用最基本的 `for` 循环添加 `InkWell` 并打印 `idx` ,结果会怎么样呢?
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```dart
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class MyHomePage extends StatelessWidget {
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var images = [ "RRR","RRR", "RRR"];
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@override
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Widget build(BuildContext context) {
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List<Widget> contents = [];
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for (int idx = 0; idx < images.length; idx++) {
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contents.add(InkWell(
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onTap: () {
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print("######## $idx");
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},
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child: Container(
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height: 100,
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width: 100,
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color: Colors.red,
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child: Text(images[idx]),
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)));
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}
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return Scaffold(
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appBar: AppBar(),
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body: Center(
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child: Column(
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children: [
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...contents,
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],
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)));
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}
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}
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```
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答案就是:**点击对应 `InkWell` 打印对应的 `index`**。
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为什么呢?
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我们继续看编译后的代码,可以看到都是打印的 `idx` ,为什么这样就可以正常呢?
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**这里最大的不同就是`idx` 被声明的位置不同**。
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```c++
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@#C475
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method build(fra2::BuildContext* context) → fra2::Widget* {
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core::List<fra2::Widget*>* contents = core::_GrowableList::•<fra2::Widget*>(0);
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for (core::int* idx = 0; idx.{core::num::<}(this.{main::MyHomePage::images}.{core::List::length}); idx = idx.{core::num::+}(1)) {
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[@vm.call-site-attributes.metadata=receiverType:dart.core::List<library package:flutter/src/widgets/framework.dart::Widget*>*] contents.{core::List::add}(new ink5::InkWell::•(onTap: () → Null {
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core::print("######## ${idx}");
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}, child: new con7::Container::•(height: 100.0, width: 100.0, color: #C40086, child: new text::Text::•(this.{main::MyHomePage::images}.{core::List::[]}(idx), $creationLocationd_0dea112b090073317d4: #C66607), $creationLocationd_0dea112b090073317d4: #C66613), $creationLocationd_0dea112b090073317d4: #C66617));
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}
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```
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那这时候我们重新调整下,**把 `idx` 放到 for 外面,点击测试会发现,打印的结果又都是 3**。
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```
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class MyHomePage extends StatelessWidget {
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var images = [ "RRR", "RRR","RRR"];
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@override
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Widget build(BuildContext context) {
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List<Widget> contents = [];
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int idx = 0;
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for (; idx < images.length; idx++) {
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contents.add(InkWell(
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onTap: () {
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print("######## $idx");
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},
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child: Container(
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height: 100,
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width: 100,
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color: Colors.red,
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child: Text(images[idx]),
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)));
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}
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return Scaffold(
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appBar: AppBar(),
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body: Center(
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child: Column(
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children: [
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...contents,
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],
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)));
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}
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}
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```
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这是为什么呢?
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看编译后的代码,唯一不同的就是 `core::int* idx` 的声明位置,那原因究竟是什么呢?
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```c++
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@#C475
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method build(fra2::BuildContext* context) → fra2::Widget* {
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core::List<fra2::Widget*>* contents = core::_GrowableList::•<fra2::Widget*>(0);
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core::int* idx = 0;
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for (; idx.{core::num::<}(this.{main::MyHomePage::images}.{core::List::length}); idx = idx.{core::num::+}(1)) {
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[@vm.call-site-attributes.metadata=receiverType:dart.core::List<library package:flutter/src/widgets/framework.dart::Widget*>*] contents.{core::List::add}(new ink5::InkWell::•(onTap: () → Null {
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core::print("######## ${idx}");
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}, child: new con7::Container::•(height: 100.0, width: 100.0, color: #C40086, child: new text::Text::•(this.{main::MyHomePage::images}.{core::List::[]}(idx), $creationLocationd_0dea112b090073317d4: #C66607), $creationLocationd_0dea112b090073317d4: #C66613), $creationLocationd_0dea112b090073317d4: #C66617));
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}
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```
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因为 `onTap` 是在点击后才输出参数的,而对于 `for (core::int* idx = 0;` 来说,`idx` 的作用域是在 `for` 循环之内,所以编译后在 `onTap` 内要有对应持有一个值,来保存需要输出的结果。
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而对于 `for` 循环外定义的 `core::int* idx` , 循环内的所有 `onTap` 都可以指向它这个地址,所以导致点击时都输出了同一个 `idx` 的值。
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至于为什么会有这样的逻辑,在深入的运行时逻辑就没有去探索了(懒),**推测应该是编译后的二进制文件在运行时,针对循环外的参数和循环内的参数优化有关系**。
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理论上,应该是属于变量捕获:
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* 对于全局变量,不会捕获,通过全局变量访问。
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* 对于局部变量,自动变量将会捕获,且是值传递。
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最后,如果你也想查看 `dill` 内容,可以通过 mac 下的 xxd 命令:
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```
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xxd /Users/xxxxxxx/workspace/flutter-wrok/flutter_app_test/.dart_tool/flutter_build/bf7ed8e7e7b3e64f28f0af8a89a29ca9/app.dill
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```
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也可以通过 `dump_kernel.dart` (在完整版 `dart-sdk` 的`/Users/guoshuyu/workspace/dart-sdk/pkg/vm` 目录下)执行如下命令,生成 `app.dill.txt` 查看,比如你可以查看 `final` 和 `const` 编译后的区别。
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```
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dart dump_kernel.dart /Users/xxxxxxx/workspace/flutter-wrok/flutter_app_test/.dart_tool/flutter_build/bf7ed8e7e7b3e64f28f0af8a89a29ca9/app.dill /Users/xxxxxxx/workspace/flutter-wrok/flutter_app_test/.dart_tool/flutter_build/bf7ed8e7e7b3e64f28f0af8a89a29ca9/app.dill.txt
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```
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