history对象 back() forward() go() 和pushState() replaceState()

History(Window.history对象)对象保存着用户上网的历史记录。处于安全方面的考虑，开发人员无法得知用户浏览过的URL，但是借由用户访问过的页面列表，同样可以在不知道实际URL的情况下实现后退和前进。

History对象概况：

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Manipulating the browser history

Manipulating the browser history

 The DOM window object provides access to the browser's history through the history object. It exposes useful methods and properties(方法和属性) that let you move back and forth through the user's history, as well as -- starting with HTML5 -- manipulate the contents of the history stack(操作历史堆栈).

Traveling through history

Moving backward and forward through the user's history is done using the back(), forward(), and go() methods.

1 Moving forward and backward

To move backward through history, just do:

 window.history.back();

This will act exactly like the user clicked on the Back button in their browser toolbar.(就像用户点击了浏览器上工具栏的后退按钮)

Similarly, you can move forward (as if the user clicked the Forward button), like this:

window.history.forward();

2 Moving to a specific point in history

You can use the go() method to load a specific page from session history, identified by its relative position to the current page (with the current page being, of course, relative index 0).

To move back one page (the equivalent of calling back()):

window.history.go(-1);

To move forward a page, just like calling forward():

window.history.go(1);

Similarly, you can move forward 2 pages by passing 2, and so forth.

You can determine the number of pages in the history stack(历史堆栈中的“历史数量”) by looking at the value of the length property:

var numberOfEntries = window.history.length;

Adding and modifying history entries

HTML5 introduced the history.pushState() and history.replaceState() methods, which allow you to add and modify history entries, respectively. These methods work in conjunction with the window.onpopstate event.(HTML5添加了history.pushState()和history.replaceState()方法，两个方法和window.onpopstate()连在一起使用)

Using history.pushState() changes the referrer that gets used in the HTTP header for XMLHttpRequest objects created after you change the state. The referrer will be the URL of the document whose window is this at the time of creation of the XMLHttpRequest object.

The pushState() method

pushState() takes three parameters: a state object, a title (which is currently ignored), and (optionally) a URL. Let's examine each of these three parameters in more detail:

• state object — The state object is a JavaScript object which is associated with the new history entry created by pushState(). Whenever the user navigates to the new state, a popstate event is fired, and the state property of the event contains a copy of the history entry's state object.

  The state object can be anything that can be serialized(可以被序列化的JS对象). Because Firefox saves state objects to the user's disk so they can be restored after the user restarts the browser, we impose a size limit of 640k characters on the serialized representation of a state object. If you pass a state object whose serialized representation is larger than this to pushState(), the method will throw an exception. If you need more space than this, you're encouraged to use sessionStorage and/or localStorage.

• title — Firefox currently ignores this parameter, although it may use it in the future. Passing the empty string here should be safe against future changes to the method. Alternatively, you could pass a short title for the state to which you're moving.

• URL — The new history entry's URL is given by this parameter. Note that the browser won't attempt to load this URL after a call topushState(), but it might attempt to load the URL later, for instance after the user restarts the browser. The new URL does not need to be absolute; if it's relative, it's resolved relative to the current URL. The new URL must be of the same origin(必须同源) as the current URL; otherwise,pushState() will throw an exception. This parameter is optional; if it isn't specified, it's set to the document's current URL.

In a sense, calling pushState() is similar to setting window.location = "#foo", in that both will also create and activate another history entry associated with the current document.(某种意义上，使用pushState()和使用window.location是相同的，两者都会生成并激活和当前文档有关的“历史记录实体”) But pushState() has a few advantages:

• The new URL can be any URL in the same origin as the current URL. In contrast, setting window.location keeps you at the same documentonly if you modify only the hash(只有单单更新hash部分，window.location才会保持在同一个document；反之pushState()中的URL参数可以设置任何url，只要是符合同源).
• You don't have to change the URL if you don't want to. In contrast, setting window.location = "#foo"; only creates a new history entry if the current hash isn't #foo.
• You can associate arbitrary data with your new history entry. With the hash-based approach, you need to encode all of the relevant data into a short string.
• If title is subsequently used by browsers, this data can be utilized (independent of, say, the hash).

Note that pushState() never causes a hashchange event to be fired, even if the new URL differs from the old URL only in its hash.(即使是hash部分不一样，pushState()事件不会触发hashchange事件)

In a XUL document, it creates the specified XUL element.

In other documents, it creates an element with a null namespace URI.

The replaceState() method

history.replaceState() operates exactly like history.pushState() except that replaceState() modifies the current history entry instead of creating a new one. Note that this doesn't prevent the creation of a new entry in the global browser history.

replaceState() is particularly useful when you want to update the state object or URL of the current history entry in response to some user action.

The popstate event

onpopstate

A popstate event is dispatched to the window every time the active history entry changes. If the history entry being activated was created by a call to pushState or affected by a call to replaceState, the popstate event's state property contains a copy of the history entry's state object(一份“历史对象”状态对象的copy).

Note that just calling history.pushState() or history.replaceState() won't trigger(不会触发) apopstate event. The popstate event is only triggered by doing a 1、browser action such as clicking on the back button (or 2 、calling history.back() in JavaScript). And the event is only triggered when the user navigates between two history entries for the same document(同一个document).

Reading the current state

When your page loads, it might have a non-null state object.  This can happen, for example, if the page sets a state object (using pushState() orreplaceState()) and then the user restarts their browser.  When your page reloads, the page will receive an onload event, but no popstate event(页面刚载入，只有onload事件，并没有popstate事件).  However, if you read the history.state property, you'll get back the state object you would have gotten if a popstate had fired.

You can read the state of the current history entry without waiting for a popstate event using the history.state property like this:

var currentState = history.state;

Example

Suppose http://mozilla.org/foo.html executes the following JavaScript:

var stateObj = { foo: "bar" };

history.pushState(stateObj, "page 2", "bar.html");

This will cause the URL bar to display http://mozilla.org/bar.html, but won't cause the browser to load bar.html or even check that bar.html exists(浏览器不会去加载页面或检验页面是否存在).

Suppose now that the user now navigates to http://google.com, then clicks back. At this point, the URL bar will display http://mozilla.org/bar.html, and the page will get a popstate event whose state object contains a copy of stateObj. The page itself will look like foo.html, although the page might modify its contents during the popstate event.

If we click back again, the URL will change to http://mozilla.org/foo.html, and the document will get another popstate event, this time with a null state object. Here too, going back doesn't change the document's contents from what they were in the previous step, although the document might update its contents manually upon receiving the popstate event.

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理解浏览器历史记录（2）-hashchange、pushState

前端路由的两种实现原理