可复用 & 组合： 响应性计算和侦听

test: {{msg}}

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1. computed(() this is the title

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2. computed(() 的 getter 和 setter this is another title

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3. watchEffect this is the 3rd title

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4. watch: 侦听一个 getter

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5. watch: 直接侦听ref

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6. 侦听多个数据源: 侦听器还可以使用数组同时侦听多个源（逐个看数组的变化，返回数组前后的值）

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7. 侦听响应式对象