Question

在我的角度项目中，我试图通过点击下一个/上一个按钮来浏览列表。我有一个复选框，可以将显示的小时数从几小时更改为几天。我希望能够在员工中来回走动，让价值观正确。我被告知可以通过ngOnChanges并跟踪上一个和新值来完成。

这就是我所拥有的，即更改新值，但我将如何更改之前的值？

 ngOnChanges(changes: { [propName: string]: SimpleChange }) {
    for (let propName in changes) {
        if (propName == "selectedEmployee") {
            if (this.timeVar == "days") {
                this.empInfo[this.selectedEmployee].STDLTD = this.empInfo[this.selectedEmployee].STDLTD * 8;
                this.empInfo[this.selectedEmployee].Uncharged = this.empInfo[this.selectedEmployee].Uncharged * 8;

                this.empInfo[this.selectedEmployee].PTOBase = this.empInfo[this.selectedEmployee].PTOBase * 8;
                this.empInfo[this.selectedEmployee].PTOCarry = this.empInfo[this.selectedEmployee].PTOCarry * 8;
                this.empInfo[this.selectedEmployee].PTOBorrowed = this.empInfo[this.selectedEmployee].PTOBorrowed * 8;
                this.empInfo[this.selectedEmployee].PTOBalance = this.empInfo[this.selectedEmployee].PTOBalance * 8;
                this.empInfo[this.selectedEmployee].PTORequests = this.empInfo[this.selectedEmployee].PTORequests * 8;
                this.empInfo[this.selectedEmployee].PTORemaining = this.empInfo[this.selectedEmployee].PTORemaining * 8;

                this.empInfo[this.selectedEmployee].ETOEarned = this.empInfo[this.selectedEmployee].ETOEarned * 8;
                this.empInfo[this.selectedEmployee].ETORequests = this.empInfo[this.selectedEmployee].ETORequests * 8;
                this.empInfo[this.selectedEmployee].ETORemaining = this.empInfo[this.selectedEmployee].ETORemaining * 8;  
            }
        }
    }
}

Answer 1

来自angular documentation：

ngOnChanges(changes: SimpleChanges) {
  for (let propName in changes) {
    let chng = changes[propName];
    let cur  = JSON.stringify(chng.currentValue);
    let prev = JSON.stringify(chng.previousValue);
  }
}

因此，您需要.currentValue和.previousValue属性才能访问当前和以前的值。

Answer 2

使用ngOnChanges的更安全方式：

nameof.ts

export const nameof = <T>(name: keyof T) => name;

my.component.ts

selectedEmployee: string[];

  ngOnChanges(changes: SimpleChanges): void {
    // won't compile if 'selectedEmployee' is no a property of this component
    const change = changes[nameof<MyComponent>('selectedEmployee')];

    if (change) {
      // casting it, to avoid dumb error that won't throw, like
      // doing `change.currentValue > 0` instead of `change.currentValue.length > 0`
      const previous: string[] = change.previousValue;
      const current: string[] = change.currentValue;

      // do something with it.
    }
  }

ngOnChanges跟踪上一个值和新值？

2 个答案: